Ultrashort Pulsed Laser Machining of Ti6Al4VAlloy

Transcription

1 Ultrashort Pulsed Laser Machining of Ti6Al4VAlloy A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy In the Faculty of Science and Engineering 2017 Ahmad Syamaizar Bin Ahmad Sabli School of Mechanical, Aerospace and Civil Engineering

2 Table of Contents TABLE OF CONTENTS... 2 LIST OF FIGURE... 6 LIST OF TABLES ABSTRACT DECLARATION COPYRIGHT STATEMENT ACKNOWLEDGEMENTS CHAPTER 1 INTRODUCTION INTRODUCTION CONVENTIONAL HOLE DRILLING METHODS LASER DRILLING RESEARCH MOTIVATION SCIENTIFIC CHALLENGES AIMS AND OBJECTIVES AIMS THESIS OUTLINE CHAPTER 2 LITERATURE REVIEW INTRODUCTION LASER PULSE PARAMETERS BEAM FOCUSING PARAMETERS LASER PROCESSES LASER ABLATION LASER DRILLING LASER CUTTING LASER MILLING ULTRA SHORT PULSED LASER MACHINING PREVIOUS WORK ON PICOSECOND PULSED LASERS CHALLENGES IN LASER PROCESSING TITANIUM ALLOYS LASER INTERACTION SUMMARY OF CHAPTER

3 CHAPTER 3 EQUIPMENT AND MATERIAL SPECIFICATION INTRODUCTION EQUIPMENT PICOSECOND LASER BEAM PROFILER LASER POWER METER OPTICAL SPECTROMETER WHITE-LIGHT OPTICAL PROFILING SYSTEM SAMPLE PREPARATION EQUIPMENT KEYENCE VHX-500F MICROSCOPE QUANTA 200 SCANNING ELECTRON MICROSCOPE MATERIALS TITANIUM ALLOY TI6AL4V EXPERIMENTAL METHODS MACHINING PROCEDURE EXPERIMENTAL SAMPLE MEASUREMENT PROCEDURE SUMMARY OF CHAPTER CHAPTER 4 THE CHARACTERISTICS OF HIGH PULSE REPETITION FREQUENCY 300W PICOSECOND LASER ABLATION OF TI6AL4V INTRODUCTION EXPERIMENTAL METHOD RESULTS KERF WIDTH AND ABLATION DEPTH ABLATION DEPTH CROSS-SECTIONAL EVALUATION LASER-MATERIAL INTERACTION ABLATION THRESHOLD, OPTICAL PENETRATION, ABSORPTION COEFFICIENT AND THERMAL LOADING DISCUSSION WIDTH AND DEPTH FURTHER ANALYSIS FOR ABLATION THRESHOLD AND ABSORPTION COEFFICIENTS FOR DIFFERENT P.P.S APPLICATION: 6 MM CUTTING OF HOLES ON 1 MM THICK TI6AL4V AND 3.5 MM THICK AL CONCLUSION SUMMARY OF CHAPTER

4 CHAPTER 5 FOCUS SHIFT IN HIGH POWER, HIGH REPETITION FREQUENCY PICOSECOND LASER MATERIALS PROCESSING INTRODUCTION EXPERIMENTAL METHODS RESULTS BEAM PROFILING AND DIAMETER DISCUSSION EFFECT OF Z ON MATERIAL REMOVAL WITH VARYING POWERS EFFECT OF Z ON MATERIAL REMOVAL RATE WHEN VARYING SCANNING SPEED EFFECT OF FAST SCANNING SPEED ON MATERIAL REMOVAL RATE WITH VARYING Z POSITION EFFECT OF SLOW SCANNING SPEED ON MATERIAL REMOVAL RATE WITH VARYING Z SEM BACKSCATTER RESULTS FOR GROOVE MACHINING AT DIFFERENT Z POSITIONS STUDY THE EFFECT OF 15 SECONDS PS LASER EXPOSURE OF 1MHZ P.R.F AT 44 W WITH PS LASER ON CERAMIC WITH VARYING Z SHIFTING MAXIMUM MATERIAL REMOVAL RATE: THERMAL LENSING? CONCLUSION SUMMARY OF CHAPTER CHAPTER 6 UNDERSTANDING THE TRANSIENT BEHAVIOUR OF PICOSECOND LASER ABLATION OF TI6AL4V USING HIGH-SPEED HOLOGRAPHIC IMAGING INTRODUCTION EXPERIMENTAL METHODS PICOSECOND LASER SYSTEM RESULTS AND DISCUSSION MICROGRAPHS OF ABLATED SITES RESULTS FOR DEPTH EFFECT OF USING ARTIFICIAL SHOT REPEAT AND BURSTS WITH VARYING Z POSITIONS PULSED DIGITAL HOLOGRAPHY CONCLUSION SUMMARY OF CHAPTER

5 CHAPTER 7 CONCLUSIONS AND FURTHER WORK CONCLUSIONS CHAPTER 4: THE CHARACTERISTICS OF HIGH PULSE REPETITION FREQUENCY 300 W PICOSECOND LASER ABLATION OF TI6AL4V CHAPTER 5: FOCUS SHIFT IN HIGH POWER, HIGH REPETITION FREQUENCY PICOSECOND LASER MATERIALS PROCESSING CHAPTER 6: UNDERSTANDING THE TRANSIENT BEHAVIOUR OF PICOSECOND LASER ABLATION OF TI6AL4V USING HIGH-SPEED HOLOGRAPHIC IMAGING FURTHER WORK REFERENCES

6 List of Figure Figure 1-1: 30 years of laser material processing vs. machine tools - market statistics (normalised with the 1986 data) German Machine Tool Builders Association [2] Figure 2-1: The focal length and the depth of focus Figure2-2: The interaction between laser and material during laser milling operation[39] Figure 2-3: Comparison between conventional and ultra-short-pulsed laser Oppenlander, W. et al. (2009)[40] Figure 2-4: Process time scales for absorption of target following a laser pulse (adapted Sugioka et al.[36]) Figure 2-5: Comparison between (a) short laser pulse and (b) ultrashort laser pulse interaction[43] Figure 2-6: General differences in pulse width duration for (a) microsecond pulse( 80 µs,1064 nm, 80W and 500 Hz), (b) nanosecond pulse ( 60 ns, 532 nm, 38 W and 120kHz), (c) picosecond pulse ( 10 ps, 1064nm, 15 W and 50 khz) and (d) femtosecond pulses ( 170 fs, 800 nm, 0.7 W and 1 khz)[43] Figure 2-7: Laser trepanning system[66] Figure 2-8: Influence of the laser drilling strategy on bore hole diameter [73] Figure 2-9:Overview of drilling strategies [36] Figure 2-10: Mask projection scanning technique [87] Figure 2-11: Nitinol cut edge without any post processing showing the dross free cut and sharp edge in different magnifications: (a) 60x and (b) 200x[105] Figure 2-12: Galvo versus polygon scanner scanning comparison [120] Figure 2-13: Sample beam scanning using rotating drum and acusto optics scanner[124] Figure 2-14: Ablation process and plasma plume development Figure 2-15: Illustration of different types of plasma[138] Figure 3-1: The Edgewave 400 W picosecond laser materials processing system

7 Figure 3-2: Experimental setup for picosecond pulsed laser Figure 3-3: Spiricon LBS 100 beam profiler Figure 3-4: Spiricon LBS 100 beam profiler schematics Figure 3-5: Sample measurement of intensity and beam shape Figure 3-6: Gentec power meter, Model UP55G-500F-H12-D0 and Gentec Electro- Optics DUO Figure 3-7: Gentec power meter placement schematics Figure 3-8:The Analytikjena Specord 250 spectrometer Figure 3-9: Sample reflectance measurement on a as received' Ti6Al4V sample Figure 3-10: Wyko surface profiler NT Figure 3-11: (a)2 D top profile (b) cross sectional Y bar profile (c) cross sectional X bar profile of ablation depth measurement Figure 3-12: (a)struers Labotom 5 (b) Struers Accutom 5 (c) OmegaPol Twin Metallurgical Polisher and (d) Struers LaboPol- 35 Polishing Figure 3-13: Keyence 3D optical microscope Figure 3-14: Sample micrograph Figure 3-15: Quanta 200 Scanning electron microscope Figure 3-16: Sample SEM image for analysing the cross-sectional depth and width of an ablated line Figure 3-17: Line ablation Figure 3-18: (a) laser machine profile, top view and (b) laser machined side profile view for measurement of cross-sectional area Figure 3-19: One pulse beam spot measurement for determining focus position Figure 3-20: Measurement of diameter (a) method of measurement and (b) results of example measurement on sample Figure 4-1: Edgewave 1064nm 300 W Ps laser configuration Figure 4-2: Kerf width measurement, laser machined profile top view, (a) 2d representative and (b) measurement of kerf width and heat affected zone

8 Figure 4-3: Titanium Alloy kerf width vs. No. of Pulses (a) 2.4 MHz and (b) 19.2 MHz Figure 4-4: Titanium alloy kerf width with different number of pulses per spot per pass vs. Fluence (a) for 2.4 MHz and (b) for 19.2 MHz Figure 4-5: (a) Laser machine side view and (b) Laser machine micrograph view Figure 4-6: Final ablation depth vs. total no. of pulses for p.r.f (a) 2.4 MHz and (b) 19.2 MHz Figure 4-7: Titanium Alloy depth per pass vs. fluence (a) for 2.4 MHz and (b) for 19.2 MHz Figure 4-8: Titanium alloy top view variation for different scan speeds and 12.9 µj pulses for Ti6AL4V alloy (a) 2400 mm/s, (b) 4800 mm/s and (c) 9500 mm/s Figure 4-9: Top view for parameter 132 W at (a) 9.2 MHz and (b) 2.4 MHz and scanner speed of 2400 mm/s Figure 4-10: Cross-sectional view with laser power 132W at (a) 9.2 MHz and (b) 2.4 MHz and scanner speed of 2400 mm/s Figure 4-11: Effect of fluence on overall ablation depth for different pulse per spot per pulse Figure 4-12:Ablation depth per pulse vs Fluence for Ti6Al4V machining using 132 W PS laser Figure 4-13: Effect of increasing number of pulses on fluence threshold for Ti6Al4V machining using 132 W PS laser system Figure 4-14: 1mm thick cpti Alloy ps laser 6 mm drilling Figure 4-15: 3.5 mm Al Alloy (Al7075) ps laser 6 mm drilling Figure 4-16: Effect of pulse repetition rates on depth Figure 4-17: Effect of number of passes on depth Figure 4-18: Effect of scanning speed on depth Figure 4-19: Titanium alloy cutting(a) at entrance and (b) exit side 1mm thick Figure 4-20: Al7075 alloy cutting(a) at entrance and (b) exit side Al7075 alloy 3.5 mm thick

9 Figure 4-21: SEM graph of cross-sectional view of the cpti 1mm thick sample cut with the ps laser Figure 4-22: AL7075Alloy cutting 3.5 mm thick SEM graph of cross-sectional view Figure 4-23: Depth for different spacing in 10 x multipath trepanning of cpti alloy Figure 4-24: Different spacings in 6 mm multipath trepanning of cpti alloy (a) 25 µm, (b) 50 µm, (c) 100 µm, (d) 150 µm and (e) 200 µm Figure 4-25: Pocket milling strategy to study the surface modifications Figure 4-26: (a) EDX result, untreated, Pocket milling strategy to study the surface modifications and (b) EDX result, with 9 W, 102 khz p.r.f and scan speed of 100 mm/s, Figure 4-27: Pocket milling strategy to study the surface modifications (a) untreated, (b)100 mm/s, (c) 250 mm/s, (d) 500 mm/s and (e) 750 mm/s while other laser parameters are kept constant with 9 W, 102 khz p.r.f and 1 pass only Figure 5-1: Illustration of focal position where Z= 0 was the standard laser beam focal plane on the workpiece surface, Z= -ve was when the focal lens moved towards the workpiece surface, and Z= +ve was moving the focal lens away from the workpiece surface Figure 5-2: Sample processing for measurement of width, depth and cross-sectional area Figure 5-3: (a) Laser machine top profile and (b) Laser machine cross-sectional profile Figure 5-4:2D Beam profiles for different z positions from z = +20 mm above to z = 0 mm and to z = -20 mm below -focused position (all units in mm) Figure 5-5: 3D Beam profiles with different Z positions from -20mm below, to z = 0 mm and to +20 mm with respect to the focused position Figure 5-6: The comparison between the theoretical beam diameter and experimentally measured diameter Figure 5-7: Results of (a) width, (b) depth and (c) removal rate in mm 3 /min measurements for Ti6Al4V against z-axis position in mm on the surface of the 9

10 sample. (Laser parameters are 24 W, 500 khz p.r.f and a scanning speed of 1000 mm/ repeated 100 times) Figure 5-8: (a) The cross-sectional micrograph for z = +5 mm and (b)the cross-sectional micrograph for z = +15 mm Figure 5-9: Effect of power on material removal rate for different positions of Z positions above the sample surface Figure 5-10: Effect of z positonwith maximum removal rate with power Figure 5-11: Effect of scanning speed and z position on the material removal rate Figure 5.12: The SEM and backscatter images for cross section and high magnification for different z. Laser parameters: 44 W, p.r.f. of 1MHz, 500 mm/s scanning speed and repeated 100 times Figure 5-13:Backscatter SEM comparison for z = + 4 and z = + 16 mm above sample surface Figure 5-14: Volume removal rate for different z positions at 44W, 1MHz repetition rate and 500 mm/s with 100 repetitions Figure 5-15:Micrographs of 15 seconds exposure with static ps laser pulses on ceramic tile Figure 5-16: Measurements of width and depth for 15 seconds 1 MHz p.r.f 44 W pulses exposure Figure 5-17: Illustration of thermal lensing of the lens resulting in the focus position effect Figure 6-1: Ablated particles and plasma stages in a high p.r.f ultrafast lasers interaction Figure 6-2: Beam focal position and position of z with respect to the sample surface Figure 6-3:Schematics of the burst mode Figure 6-4:Microscopic images of sites with 2000 pulses at 44 µj per pulseand one shot at (a) z = 0 mm above sample surface, (b) z = +7.5mm above sample surface and (c) z = mm above sample surface. Laser parameters are 44 W and 1 MHz p.r.f and 2000 pulse per burst

11 Figure 6-5:Microscopic images of sites with 2000 pulses at 44 µj and one, two and three shots at the corresponding focus position, +7.5 mm above focus position and mm above focus position. Laser parameters were 44 W and 1 MHz p.r.f Figure 6-6: SEM images of the craters of 2000 pulses per shot, one, two and three bursts at z = 0.0 mm, z = +7.5 mm and z = mm above the sample surface.laser parameters are 44W and 1 MHz p.r.f Figure 6-7: Illustration for comparison of time between executions for (a)un specified off time (long, about 5 seconds) and (b) 4ms offtime Figure 6-8: SEM images of 1,5, 10, 20 and 100 shot repeats and one, two and three bursts and varying z positions z = 0 mm above surface, z = +7.5 mm above surface and z = mm above surface. Laser parameters were 44 W and 1 MHz p.r.f and 2000 pulses per shot Figure 6-9: The depth with different number of shot each having 2000 pulses at z = 0 mm above sample surface, z = +7.5 mm above sample surface and z = mm above sample surface. Laser parameters 44 W, 1 MHz p.r.f. and each shot delivers 2000 pulses Figure 6-10: The (a) depth and (b) depth per pulse with different number of shot repeats and bursts for z= 0 mm above sample surface with laser paramaters of 44 W and 1 MHz p.r.f. and 2000 pulses per burst Figure 6-11: Holography setup for studying ps laser material interaction of Ti6Al4V Figure 6-12: Plume results for Z = 0 mm, Z = +7.5 and Z = +15(mm above sample surface) for 1, 5, 10, 20 and 100 pulses with one, two and three bursts. Each images are having field of view (FOV) of approximately x = 1.2 mm by y = 3.65 mm with sample surface and the focused laser plane placed at the bottom of the FOV. Laser parameters are 44 W and 1 MHz p.r.f. (Results courtesy from Dr Krste Pangovski) Figure 6-13: Particle eject a results for z = 0, z = +7.5 and z = +15 (mm above sample surface) for 1, 10, 20 and 100 shots with one, two and three bursts. Each images are having field of view (FOV) of approximately x = 1.2 mm by y = 3.65 mm with sample surface and the focused laser plane placed at the bottom of the FOV. Laser 11

12 parameters are 44 W and 1 MHz p.r.f..(results courtesy from Dr Krste Pangovski)

13 List of Tables Table 2-1: Comparing Rayleigh range for 355 nm and 1064 nm laser wavelengths Table 2-2: Comparison of different cutting methods[37] Table2-3: Qualitative comparison of the four Alternative cutting methods with laser cutting for metals[37] Table2-4: Penetration depth l1, l2, threshold fluence F0 1, F0 2 and critical fluence For for Al by Spiro et. al(2012)[20] Table 3-1: Laser specification chart[163] Table 3-2 : Technical Data of Specord 250 [164] Table 3-3: Specification Data of Wyko NT1100 [165] Table 3-4: Ti6Al4V thermo-physical properties[167, 168] Table 4-1: Table of parameters for 26 W, 55 W and 62 W Table 4-2: Table of parameters for 64 W, 90 W and 132 W Table 4-3: Depth and HAZ variation for different scan speeds and 12.9 µj pulses for Ti6Al4V alloy Table 4-4: Summary of Ti6Al4V ablation study with 1064 nm Ps laser Table 4-5: Comparison of ablation thresholds and optical penetration for varying pulse per spot Table 4-6: Properties of CpTi and Al7075[167] Table4-7: Processing parameters for analysing surface properties effect of scanning speed Table 4-8: Results for effect of scanning speed on surface properties Table 5-1: Summary of beam properties Table 5-2: Effect of z position at high-speed scanning of Ti6AL4V Table 5-3: Effect of Z position on low-speed scanning of Ti6AL4V Table 5-4: Properties of fused silica[197]

14 Word Count: words (Approximate) 14

15 Abstract Machining of hard metal alloys such as Ti6Al4V alloys with cutting tools incurs high cost particularly in the replacement of worn out tools. In light of this, lasers offer a non-contact processing method which could potentially reduce costs. Lasers can introduce undesirable processing effects, but with the emergence of high powered ultrashort lasers, these processing defects can be greatly reduced. To date, there have been limited studies conducted within the area of picosecond laser machining process. This research has two primary objectives. Firstly, using lasers as an alternative to mechanical processes. Secondly, using a picosecond laser in machining of Ti6Al4V alloy to maximise material removal rate and minimise defects. In this study, an Nd:YVO4 Edge wave picosecond pulsed laser was used for machining Ti6Al4V alloy in air and at room temperature and pressure to understand laser interaction with the Ti6Al4V alloy. The laser was rated at 300W with up to 20 MHz repetition rate and up to 10 m/s scanning rate. Design of experiments was used to understand the effects of varying laser parameters and establishing the ablation threshold. Once the process parameters were established, the next stage was aimed at improving the material removal rates through various strategies. To understand the material removal process, a state of the art holography method was utilised to visualise the laser material interaction. This research has produced three significant results. It was established that the ablation threshold was 45 mj/cm 2 for picosecond laser machining of Ti6Al4V alloy. For the first time in this field of research, the optimal material removal was achieved when the laser was focused at 15 mm above the sample surface resulting in an improvement from 0.1 to 0.6 mm 3 /min. The holography visualisation revealed that the material removal rate was significantly reduced as the number of pulses increased due to the presence of plasma. Findings of this research support the future of picosecond laser machining of hard metals for micro as well as macro scale applications. Some of the relevant industries for this area of research include aerospace manufacture, automotive parts manufacturing and even manufacture of personal items such as watches, eye wear and jewellery. 15

16 Declaration I hereby declare that no portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. Ahmad Syamaizar Bin Ahmad Sabli, April

17 Copyright Statement The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related right in it (the Copyright ) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property Rights and/or Reproductions. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see in any relevant Thesis restriction declarations deposited in the University Library, the University Library s regulations (see and in The University s policy on Presentation of Theses. 17

18 Acknowledgements I would like to dedicate this thesis and the completion of my PhD. study to my loving wife Waqiyatu Shahidah Haji Mail for having the dedication and perseverance for always being there for me through good times and hard times. I would also like to dedicate this thesis to my late mother Hajah Aiani Haji Abdul Hamid (Al-Fatihah and may Allah bless her soul) who passed away on the 24 th October I would also like to say many thanks to my family members for their support and love. Special thanks to my supervisor Professor Lin Li and Co- supervisor Dr David Whitehead for their guidance and unlimited patience. Also many thanks to staff Daniel Wilson, and my colleagues Abdeslam Mhich, Adel Salama, Omonigho Otonocha and Fatema Rajab for their help and precious time. Last but not least, I would like to thank Allah the Almighty God for guiding me through all times. 18

19 Chapter 1 Introduction 1.1 Introduction Lasers have gone a long way from concealed laboratories to useful industrial manufacturing tools. From their invention in 1960, the applications of lasers as industrial manufacturing processes slowly supersede traditional manufacturing methods. This is partly due to the need for the evolution of manufacturing processes by making products quicker, better quality while reducing overall cost. This is reflected by the following statement[1]: The global market of laser processing is expected to grow to $17.36 billion by 2020 from its 2013 market size of $11.24 billion, at an estimated CAGR of 6.18% between 2014 and The compound annual growth rate, CAGR estimated at 6.18% from 2014 to $17.36 billion in 2020 is a staggering figure. The processes concerned in this market review encompasses various laser processes such as welding, cutting, drilling and surface modifications. Moreover, this evaluation was accompanied by the comparison growth of laser processing systems and machine tool market as shown in Figure 1-1. Figure 1-1: 30 years of laser material processing vs. machine tools - market statistics (normalised with the 1986 data) German Machine Tool Builders Association [2] 19

20 Nowadays, high precision cannot be associated anymore with a slow process as laser micromachining has matured to high-speed machining[3]. This leads to laser processing applications in die making, a process that is accustomed to machine tools due to the nature of the type of material to be machined and the corresponding material removal rates. By using laser processing, one can be cost efficient than when using conventional machine tool systems such as milling machines. The main reason for the cost effectiveness was that material removal in laser processing is a non-contact method. Although their capital cost requirement is slightly higher than conventional processing machines, their non-contact material removal processing leads to savings such as in cutting tool costs over the long run, because there is no tool wear involved. Apart from die machining, laser processing was also utilised in many specific areas where high quality, high production rate was required, in particular, for the processing of sheet metals [4-8]. One important sheet metal processing is hole drilling Conventional hole drilling methods Hole drilling, specifically in the aerospace industry was currently carried out mainly using mechanical drilling followed by reaming. This led to high tooling cost from replacement requirements for both the drilling tools and reaming tools. Consequently, there was a need to reduce tooling cost by examining alternative drilling and/or machining techniques. In the light of this, there have been developments in machining strategies and techniques for the reduction of heat affected zones from conventional machining processes such as drilling, turning and milling are continuously researched [9-12]. The use of state of the art machining processes such as the electric discharge machining EDM[13, 14] offers a good prospect, but lasers offer better finish quality with minimal heat affected zones, such can be done with ultra short pulsed lasers especially when laser parameters are optimised. The apparent knowledge gap was the advancement in the application of ultra short pulsed lasers such as ps lasers for industrial macro processing of hard material such as the Ti6Al4V Laser drilling The development of laser machining and applications in the industry include effusion cooling hole drilling in aerospace industries, instrument panel profiling in the automotive industry to micro-fabrication of medical devices. Some of the advantages of laser machining for machining processes are listed below[15]: 20

21 1. Very narrow kerf width giving substantial savings in cut material 2. Cut edge is smooth and clean 3. Very narrow heat affected zone (HAZ) 4. Blind cuts can be made in some materials 5. Fast process 6. Non-contact process hence zero tool wear Recent developments of laser drilling/milling as a non-contact machining method [16-24] allow substantial reductions in tooling cost and tool replacements times and hence maximise the production rate. On the other hand, laser drilling has its sets of challenges. Challenges in laser drilling/milling include elimination or minimisation of laser machining defects such as dross, dimensional inaccuracy, taper, striations and most importantly heat affected zones (HAZs) [25-28]. Furthermore, laser drilling or machining involves a large number of process parameters such as scanning speed, pulse frequency, wavelength, pulse width, laser power, laser fluence (energy density), focal position, and the number of pulse per spot, pulse overlap, and processing sequence. Alternatively, earlier studies of ultra-short-pulsed laser machining have been limited to micro-components and micro-features at very low material removal rates [17, 20, 21, 29]. The emergence of high power, high repetition rates and high scan rates picosecond lasers, theoretically allow for high machining rates and application for macro-components, offer an opportunity for the transition to using ultra-fast lasers from micro-machining to macro-processing. 1.2 Research Motivation Conventional machining methods such as milling and turning are typically applied to machining macro-components. The component size limits were restricted to the width of 200 microns depending on the diameter of the cutting tool. Currently, lasers equipped with high-speed scanning galvo systems can machine complex geometry with high precision, and features smaller than 100 µm can be achieved through beam focusing and used in the applications such as texturing [30, 31]. Ultra-fast lasers (with pico- or femtosecond pulse widths) offer the advantage of low heat affected zones and high precision in machining. However, the existing use of ultrafast lasers was mainly in 21

22 processing micro-components or micro-features due to their very low removal rates. With the availability of picosecond lasers with over 100 W average powers, a new opportunity arises to use ultrafast lasers for macro processing. This opportunity also comes with its unfamiliar territories in experimental and scientific grounds specifically, to the effects of the pulse to pulse interactivity and their significant influence on material removal rate, surface morphology and thus their applicable applications. This was the knowledge gap that this research will hope to unravel. Furthermore, these high-power ultrafast lasers also come with them high pulse repetition rates of up 20 MHz. Although the pulse energy reduces as pulse repetition frequency increases, the interaction between hard metals such as titanium alloys on high powered and high pulse repetition rate lasers have yet to be extensively researched. On the other hand, Titanium alloy such as Ti6Al4V offers a higher strength to weight ratio compared to other alloys. Their resistance to corrosion and heat has also paved for their uses in healthcare, chemical industry, energy industry and sports industries [32]. Furthermore, titanium is one of the most abundant engineering materials [33]. It is however, very difficult to machine titanium alloys due to their shear strength, low heat conductivity which reduces tool effectiveness, and tooling cost is very high. This is where laser comes in, to be an evolution of the current manufacturing processes. The use of continuous wave lasers and longer pulsed lasers often result in poor machining quality and excess level of heat affected zones. Therefore, there was a need to develop a new tool for the machining of titanium alloys without tool contact and with high processing quality. High average powered (> 100 W) picosecond (ps) lasers, available only over the last five years, would offer a new opportunity macro machining of titanium alloys. The development of the understanding of the processing regime for sheet metal Ti6Al4V by ps laser, optimisation and the development of the machining techniques and strategies would be studied. Their implications and limitations for maximising material removal rates will be reviewed and reported. Ultimately, the interaction and effects of the laser pulses with ablated plume and thus plasma development during the ultrafast pulsed machining using high pulse repetition rate will be investigated, analysed and discussed. The success of the high-powered ps laser with high pulse repetition rate could be the pinnacle of enabling technology for future macro manufacturing capable of achieving industrial standards for macro components manufacturing. 22

23 1.3 Scientific Challenges Only recently that high averaged powered, and high repetition rate ps lasers are made available for laboratories. The characteristics of their interactions with engineering materials are not entirely understood and known. As the pulse repetition increases, the degree of heat accumulation and plasma/plume interactions with the oncoming laser pulses also increases. Therefore, it may not be necessary to achieve the high machining quality as seen in lower powered ps laser machining. The main challenges in laser drilling/milling include elimination or minimisation of laser machining defects such as recast, dimensional inaccuracy, circularity, taper, striations and most importantly heat affected zones (HAZs)[25-28]. Furthermore, ultra short pulses remove material in high precision. This means material removal was a slower process compared to longer pulsed lasers. This research will investigate the knowledge gap for strategies in maximising the material removal rates while keeping the laser processing defects to the minimum. 1.4 Aims and Objectives The underlying importance of this study was to gather scientific evidence and advancement in scientific knowledge for the improvement of the processing regime of ultra short pulsed laser systems processing of hard engineering materials such as the Ti6Al4V for macro machining applications such as die making, hole making and part fabrication Aims The research aims are to understand the specific characteristics of higher average powered ps laser interactions with titanium alloys in the macro-machining/drilling of the materials. The specific objectives of the research are: To investigate the effects of high power ps laser interactions with aerospace Ti alloy, such as Ti6Al4V, in macro-machining processes. To determine the ablation threshold, thermal loading, absorption coefficients and effect of laser power, scanning speed, pulse repetition frequencies and their resulting material removal rates. To investigate strategies for optimum ps laser cutting of holes in Ti alloy. 23

24 To investigate the effects of high average powers and p.r.f on the beam focal position shifts and materials processing variations. To investigate using state of the art holographic techniques to study transient ablation behaviour of picosecond laser on Ti6Al4V 1.5 Thesis Outline This thesis structure was presented in the manner tabulated below to illustrate the investigations for the purpose of macro machining of Titanium alloys by using ps pulse laser operated at an emission wavelength of 1064 nm. These sets of studies were taken vitally to fill the gap in understanding the mechanism of multiple pulse laser machining of the titanium alloys using 1064 nm wavelength ps pulsed lasers with high pulse repetition rates of over 100 khz. The outline of the thesis is tabulated in the following page: 24

25 Chapter Chapter 1 Chapter 2 Literature review Chapter 3 Equipment and material specifications Chapter 4 The characteristics of high pulse repetition frequency 300 W picosecond laser ablation of Ti6Al4V Chapter 5 Focus shift in high power, high repetition frequency picosecond laser materials processing Chapter 6 Understanding the transient behaviour of picosecond laser ablation of Ti6Al4V using high-speed holographic imaging Chapter 7 Conclusions and Further work Introduction Description Outlines the need to study Ps laser machining of Ti6Al4V This chapter presents a literature review on laser processing, their implication on analysis, pulse energy, pulse duration and the laser characteristics of focused beams. Review on laser machining, cutting, drilling by different lasers will be presented. A general review was made on the ultrashort pulsed laser machining development, procedures for their ablation thresholds and machining strategies. A focus will be on the development of machining strategies using ultrashort pulsed lasers and high pulse repetition frequency on metals and the current status and knowledge gap for the further implementation of macro machining applications. This chapter covers the details of the equipment used for this research. Their motivation for their uses, operating principles and their impending limitations are described. This chapter will include the details of the ps laser, microscopes and SEM, the metallurgical sample preparation equipment together with their procedure and measurements methods. These chapter reports the experimental work on the interaction of Ti6Al4V by a 300 W powered ps laser operating at 1064 nm wavelength. The identification of ablation thresholds, thermal loading coefficients and their absorption coefficients is presented, and their significance to drilling process applications is discussed. The experimental study on high pulse repetition frequency of 1 MHz machining of Ti6Al4V was reported focusing on maximising material removal rate by an unusual strategy of varying the focus positions. The results of material removal rate at different positions relative to the surface of the sample were presented and their implications discussed. This chapter reports on the experimental study of using bursts of multiple pulses for understanding the morphology and material removals. Furthermore, a joint experiment with Cambridge University on using holography techniques ensued where the effects of plasma plume interaction were investigated and reported to understand the underlying mechanisms of multiple pulse ps laser and Ti6Al4V ablation and material removal. This chapter outlines the output of the chapters and details on the further work needed to advance the scientific achievement ultra short pulsed laser processing systems and their applications. 25

26 Chapter 2 Literature Review 2.1 Introduction This chapter reviews the previous work and state of the art in ultrafast laser machining and laser drilling to identify the knowledge gaps and new scientific challenges in high-power picosecond laser machining of metallic materials. Focus was placed on the understanding of fundamental beam/material interactions and physical phenomena and technical challenges in achieving high quality, high production rate processes. Laser Beam Characteristics Laser pulse parameters The Laser Pulse Energy can be calculated using the equation (2.1) below: E = P av PrF (2.1) Where E is the calculated pulse energy in Joules (J), Pav is the average power in Watt (W) measured, and p.r.f is the pulse repetition frequency in Hertz (Hz). Additionally, the Fluence or Energy Density quantifies the amount of energy being deposited onto a focused beam area in cm 2 can be calculated from the equation (2.2) below. Fluence = E Beam Area (2.2) Fluence or the energy density (J/cm 2 ) can be calculated by dividing the E energy in Joules from equation (2.1) by the Beam Area in cm 2. The Peak Power Density, I, is the instantaneous power density acting on the area that will be transmitted to the substrate within each pulse duration. It can be calculated by the following equation (2.3): I = Fluence / τ p (2.3) p is the pulse length. The peak power density, I, per pulse has a unit of W/cm 2. Peak Power can be calculated using: 26

27 Power Peak = Energy / τ p (2.4) The Powerpeak is the pulse peak power. When the pulses are repeated at aspecific time interval, the pulses are delivered in a repeated manner at a constant rate. It influences the dosage of the amount of pulse energy transferred to the substrate. The Number of Pulses Per Spot being deposited on to an area on a sample for a pulsed laser travelling in a straight line as in machining a groove is determined by [34]: N.O.P = (p.r.f. x Beam diameter) / scanning speed (2.5) Then, once the number of pulses per spot is calculated, the theoretical accumulated pulse energy is then calculated by multiplying the number of pulses per spot received onto a particular area of the pulse energy E Beam focusing parameters The laser beam diverges and converges depending on the focusing optics used. The beam can be redirected, collimated and focused depending on the setup and applications. For a particular focusing optic, the distance between the lens and the focused spot is called the Focal Length. The distance and region near the focal point where the relative intensity stays the same is referred to as the Focal Depth. It is also quantified as the distance by the focal spot size changes by only ±5%[35]. Figure 2-1: the focal length and the depth of focus The focal depth Zf can be determined by the following (2.6) and (2.7), Where F = fl/d, hence: Z f = ±2.56F 2 M 2 λ (2.6) 27

28 Or DOF = ( 8λ π )(F L D ) (2.7) Where fl is the focusing lens focal length (mm), D is the laser beam diameter before focusing (mm), is the laser wavelength (mm), Zf is the depth of focus (mm), and F is the relative aperture number as defined in (2.6). Example: In the case of the Edge wave picoseconds laser, wavelength, λ= 1064 nm and a focal distance fl of 330 mm and beam diameter before focus is D = 3 mm as specified by manufacturer, the depth of focus is calculated as: Z f = ±2.56 ( ) = mm On top of that, theoretically, the Smallest Beam Diameter can be determined by using the following (2.8) when the focal length fl, the unfocused beam diameter d and the wavelength of the laser source are known, hence theoretical beam waist: d min = 4fλ = mm (2.8) πd Furthermore, typically for Gaussian beams, another quality of the beam would be the Rayleigh Range. Assuming the beam quality M 2 is 1, when the beam maintains a minimal change of just smaller than 2 of the beam focused diameter over a distance with respect to the focused position, the length can be calculated by using the equation (2.9): Z R = πd 0 2 4M 2 = 16.4 mm (2.9) λ Where D0 is the beam diameter at the focus, M is the beam mode, λ is the wavelength of the laser. The Rayleigh range ZR in mm. Example: The table below shows Rayleigh length calculation for the picoseconds at 355 nm and 1064 nm wavelengths with an approximate beam diameter calculated using equation 2.8. Table 2-1: Comparing Rayleigh range for 355 nm and 1064 nm laser wavelengths. 355 nm 1064 nm Rayleigh Range (mm)

29 Apart from that, as the beam propagates from the focal plane where the beam is focused, the radius w(z) will vary with the distance z from the focused position. This is shown in the 2.10 below: Laser beam profile w(z) = w λz πw /2 Sugioka et al.[36] state that beam size and shape are important parameters for laser machining. For most lasers, the beam profile influences the edge profiles of the machined features. For example, a Gaussian profile considerable beam waist that would lead to further exposure of the substrate. The profile of a Gaussian beam peaks at the centre. The intensity reduces, as the radius gets further away from the centre. This area where the intensity falls does not contribute much to the actual material removal. Irradiation distribution of a Gaussian beam is mathematically represented twodimensionally on intensity and the radial distance using the equation (2.11) below: (2.10) I(r) = I 0 e 2r 2 w 2 = P πw 2. e 2r 2 w 2 (2.11) Where I is the irradiance W/cm 2, P is the average laser power, w the beam radius and r is the distance in the radial direction from the centre of the beam. In fact, the middle part of the beam is where the intensity is highest and an effective region for laser machining. The primary requirement for material removal is that pulse energy needs to maintain above a threshold value. 2.2 Laser processes There are different mechanisms for laser processing. The laser processing completely depends on the type of laser, pulsed or continuous, type of processing and assisting accessories available such as assist gas, nozzle, galvo scanner, etc. On the other hand, laser processing depended on power to match the type of applications used. Examples of applications of laser processes are; laser cutting, laser drilling and laser milling. Each process have their best work strategies and methods such as galvo scanning for cutting glass, nozzle head for welding metals and so on. These different mechanisms will be elaborated throughout this chapter. 29

30 2.2.1 Laser ablation Material removal mechanism for these pulsed lasers would only be possible with the right combination of laser beam setups such as their pulse duration, wavelength, and optical configuration including depth of focus, focal position and beam spot size as well as the processing scanning speeds. The essential characteristics of the beam output such as the pulse energy and pulse overlap can be determined by knowing the above. The ability of a material to be removed by ablation would depend on the beam quality and the substrate absorption properties towards the laser being utilised. Every material has their absorbance qualities and is represented by the equation (2.12) below[37]. a = 4πk (2.12) λ Where a is the absorbance value, k is the extinction coefficient and λ is the wavelength of the laser used. The lower the value of the wavelength, the higher the absorbance value will be. Apart from the absorbance value, there is the penetration depth z that is the inverse of the absorbance and is depicted by the following equation (2.13). z = λ 4πk (2.13) Relying on light properties alone when dealing with lasers would not be sufficient as the thermophysical properties have to be taken into account. Another property that is of concern would be the thermal diffusivity within the following equation (2.14). α Th = k (2.14) ρc p Thermal diffusivity is a function of the thermal conductivity k and the specific heat capacity ρ of the substrate. This function will determine the amount of energy being converted to thermal energy for a particular density. Furthermore, there is the thermal depth diffusion l in the form of the equation (2.15) below. l = 2 κτ H (2.15) The thermal depth diffusion l where k is the thermal conductivity and τh is the pulse duration. When the rate of material removal can be characterised as a function of fluence, the Beer-Lamberts equation requirement is satisfied. A simplified version is shown in the two equations (2.16) and (2.17) below for gentle and thermal ablations respectively [29, 38]. 30

31 Gentle ablation: D = 1 α ln F F th (2.16) Thermal Ablation: D = l ln F F th (2.17) Here D is the ablation depth per pulse and α is the absorption coefficient, and l is the thermal penetration depth or the heat penetration depth. Plotting graph of the depth removed per pulse against the log natural of the fluence can be used to determine the threshold fluence. To add to that, another method for characterising the threshold fluence Fth, is by measuring the beam diameter from the given fluence for a particular material interaction with any laser yields the following equation (2.18) below. D 2 = 2w 2 0 ln F (2.18) Where D is the measured diameter of the ablation, w0 is the beam radius and F and the Fth corresponds to the fluence used and the threshold fluence respectively. Collectively, equations (2.16) and (2.17) acquire the values of the absorption coefficients and the thermal penetration depth. The absorption coefficients, when multiplied by the threshold fluence, would result in the thermal loading shown in the equation (2.19) below. F th Thermal Loading = F th. α (2.19) Then, apart from analysing the threshold fluence and the resulting thermal loading, another factor that needs serious consideration would be the reflectivity, R. The reflectivity of a material can be a function of absorptivity as R = (1 Absorptivity)[35]. When referring to first law of conservation of energy for a closed system of thermodynamics is: Q = U + W (2.20) Where Q is the total added heat, W is the work done, and U is the change in internal energy of the system. 31

32 For laser processing, there is no net workdone. Therefore,W is zero in this case and the energy required for the material removal Q is equal to the total energy added into and absorbed by the body, U is given by the following equation: W= 0 = Q U (2.21) Therefore,the equation becomes: Q = U (2.22) Where Q now is the laser pulse energy added to the material and U is the energy required for ablation to take place for the material. Q is: Q = Q laser = P av PrF (2.23) Moreover,U in perunit mass is shown below: U = Qablation = m. Cp. (T-T0) + m. Lmelt + m. Lvap (2.24) Where m is the mass ablated, T is the temperature rise, Lmelt latent heat of fusion and Lvap latent heat of evaporation. Furthermore, when taking into account the reflectivity of a material under the irradiation of light, the resulting pulse energy after reflection is the energy for material removal, the whole equation for pulse energy to removal energy then becomes: Q = (1-R)U ( Laser Power / Pulse repetition rate) =(1-R) ( m. Cp. (T-T0) + m. Lmelt + m. Lvap) (2.25) Figure 2-2 below shows the stages of a typical laser ablation process for lasers. First, the beam is incident on the material, and the absorption of the beam energy starts to take place by beam interactions and then vibrational energy transfer to the lattice by the energy leading to heat being transferred and conducted to the substrate. This energy transfers causing thermal conduction then leads to the substrate reaching the melting temperature. After that, the beam interaction diffuses internally and gain more heat from the laser which then leads to vapourisation of melts. Part of the vapour is then ionised leading to the formation of plasma. The formation of plasma would then partially absorbs and scatter the oncoming incident laser and thus limiting the laser energy from reaching the workpiece. Another part of the vapourised particles are expelled outwards away from the substrate. In ultra-short laser beam interaction with materials, the laser peak power is usually in several gigawatts, plasma generation and its blockage of the laser beam is an 32

33 important challenge to overcome. Furthermore, heat accumulation from plasma in time frames of tens of nanoseconds needs to be avoided and will be discussed further under the plasma interaction section. Figure2-2: the interaction between laser and material during laser milling operation[39] To the plasma challenge, lasers can be categorised into two distinct forms; firstly, the continuous waveform or conventional lasers, and secondly is pulsed from where the laser can be configured to stay on or switches off for a specified amount of time. In pulsed mode, the on time for the laser was termed the pulse duration. This pulse duration can be long pulses of seconds, millisecond, microsecond and ultra-short pulses can be from nanosecond, picosecond, etc., either will have different processing regimes. Figure 2-3 illustrates the difference between conventional laser cutting and ultra-short pulsed laser cutting. Figure 2-3: Comparison between conventional and ultra-short-pulsed laser Oppenlander, W. et al. (2009)[40] 33

34 According to Sugioka et al. [36] there are two types of material removal by ablation, the first being photothermal ablation (vapourisation) and the second being photochemical ablation (chemical dissociation). Photothermal ablation involves a process where the absorbed laser energy is converted into thermal energy in the material. On the other hand, the photochemical ablation involves photonic collisions that cause direct bond breaking of the molecular chains[41]. A condition for this to occur is, the photon energy should be greater than the bond energy between molecules of the substrate. Thus, usually an ultraviolet (UV) laser beam is suitable. For very short pulse (e.g. femtosecond lasers) laser beams, multi-photon absorption occurs. Furthermore, Sugioka state that at lower fluences, ablation is dominated by spallation and then phase explosion. Whereas, at higher fluences, the phenomenon would then shifts to fragmentation with vaporisation[42]. To add to that, the effect of multiple pulses on the ablation process is not entirely understood as it involves multiple phase interactions such as the vapourised particles, melt phase and solid phase. Moreover, the three phases would interact dynamically and form shockwaves due to different pressure and temperature gradients. Apart from multiphase interaction, not to mention the timescale dependencies for each of the processes, from the incidence of a laser pulse to the time it takes for the expulsion of the vapourised materials, etc. Below in figure 2-4 shows the processes and the similar time scales. Figure 2-4: Process time scales for absorption of target following a laser pulse (adapted Sugioka et al.[36]) 34

35 On above processes and time scales, femtosecond (fs) lasers are only available at relatively low average powers (1-10 W), thus, their practical applications in industry were limited. On the other hand, picosecond (ps) lasers (pulse length of second) can be produced at much higher average power (up to 400 W), thus, would offer an opportunity for large-scale industrial applications. Studies show the different rates of ablation of microsecond laser[43], nanosecond laser[44, 45], picosecond laser[46] as well as femtosecond laser[47] on different types of materials such as polymeric and composite materials [48-50] and metals[43, 51]. One study reports on the effect of pulse duration on fs pulses; ps pulses to longer ns pulses found that higher ablation rate was achieved with longer pulse durations albeit at a cost of quality. On the other hand, reasons for lower material removal rate for ultrashort pulsed laser is due to the lower penetration depth and the use of high repetition rates resulting in particle shielding effects[52, 53]. These particle shielding effects are also termed as incubation effect and can be characterised as coefficients[54]. The Figure 2-5 below shows another comparison between the short laser pulse and ultrashort laser pulse interaction. Figure 2-5: Comparison between (a) short laser pulse and (b) ultrashort laser pulse interaction[43]. Furthermore, ablation process for a single pulse is more straightforward than for multiple pulses. For multiple pulses, the non-linear effects of ablation mechanisms have yet to be studied thoroughly. Studies state that, the development and formation of plasma and their interaction with the oncoming laser pulses leads to a non-linear behaviour. The non linear behaviour was due to increased interaction and absorption of the oncoming subsequent laser pulses with the ablated particles. In ultra-short laser beam interaction with materials, the laser peak power was high,e.g., GW, thus contributing to plasma generation, lifetime plasma enhancement and blockage of the incident laser beam. 35

36 Challenge to overcome apart from avoiding heat accumulation especially when plasma life duration was in tens of ns to 1 µs [55-57]. Leitz et al. [43]reported on the comparison of the different rates of ablation of microsecond laser, nanosecond(ns) laser, picosecond(ps) laser as well as femtosecond(fs) laser. In his study, it was reported that due to the lower fluence of the ps laser used, lower ablation depth was observed as shown in Figure 2-6. Phase explosion dominates the material removal for the ps laser ablation pulses. Thermal effects are still apparent due to pulse accumulation effect and the laser fluence used were higher than the threshold fluence. Figure 2-6: General differences in pulse width duration for (a) microsecond pulse( 80 µs,1064 nm,80w and 500 Hz),(b) nanosecond pulse ( 60 ns,532 nm,38 W and 120kHz), (c) picosecond pulse ( 10 ps,1064nm, 15 W and 50 khz) and (d) femtosecond pulses ( 170 fs, 800 nm,0.7 W and 1 khz)[43] Literature also reports of temporal studies of ablation by pulses using high-speed imaging systems such as holography methods to observe the laser material interaction and the time frame at which the processes occur[58]. Breitling et al.[59]when studying plasma plume by silicon irradiated with ns pulses using different wavelengths categorised plasma plumes in three regions namely: shock front, ionisation front and an absorption region. In addition, he categorised two types of absorption namely the Inverse Bremsstrahlung and the photoionization type absorptions. One particular study on Al with ns laser reports of vapourisation expansion resulting in shockwave followed by plume formation and then melt ejecta with processes starting at ns and finishes at 1µs. The timeframe at which the processes resides was well within 1MHz pulse repetition rates.the interaction between the pulses and the plume dynamics will be a challenge for high pulse repetition rate lasers[60]. A detailed review of plasma behaviour and interaction will be discussed further in section Plasma Interaction. To further investigate the applications of lasers, we now start with covering aspects of research done for laser processing type: Drilling. 36

37 2.2.2 Laser drilling Drilling is one of the favoured laser processes due to the speed of drilling when compared to the conventional mechanical drilling operations on sheet metal. Laser drilling does not involve any tool contact hence ideally, would reduce the tool cost compared to the conventional drilling methods. For drilling, there are many strategies employed for laser processing namely percussion, trepanning, helical trepanning and twist trepanning[18]. Nath et al. reported the drilling of thin sheets using Nd:YAG microsecond pulsed lasers in the air as well under water[61, 62]. Their study indicates that the focal position for underwater drilling was above the focused focal position of the optics. In the air, the removal mechanism of drilling was dominated by recoil pressure of the vapour and the plasma. On the other hand, when drilling in water, the removal mechanism was dominated by recoil pressure of evaporation, Marangoni force induced by surface tension gradient (or viscous force) in melt pool as well as the cavitations effect of the bubbles. Furthermore, when the laser was used as an assisting tool for a study of laserassisted hole drilling which is presented by Okasha et al. [63] The study demonstrated that laser assisted micro-drilling of super hard alloys such as Inconel increases the efficiency of the process. The increase in efficiency is possible because the laser has to initiate a pilot hole then followed by a finishing process using mechanical drilling. One of the underlying reasons of this strategy was to prolong tool life. Another author reported the findings of process combinations of laser for a pilot hole and finished off by mechanical drilling to eliminate one of the face milling process used in preparation for mechanical drilling thus improving process reliability and productivity[64]. Moreover, the parameters that influence the success of the laser assisted drilling operation are assisted gas pressure, gas type and laser power. The right combination of the parameters leads to better quality drilling by controlling recast layer thickness, HAZ from microstructure change thickness and hole taper angle while also improving hole circularity. Besides, laser assisted hole drilling concept not only reduces tool consumption, but the process also allows for a reduction in cutting tool force as well. Kumar et al.[65] investigated laser aided micro machining and reported that up to 69% reduction in cutting force during material removal was assisted by a laser. 37

38 Taper and circularity When laser was used as a tool for drilling, the process requires the use of creative strategies to utilise the high energy and heat input of lasers. One study by Ashkenasi et al.[66] demonstrated a design and implementation of trepanning system for laser drilling. The trepanning method was done by motorising the optics setup shown in Figure 2-7, so the write paths of the laser corresponds to the specified diameter of the drilling as well as the required taper of the drilling. This led to the possibilities of having holes with no taper, positive taper and negative taper. The setup was experimented on the ceramics and metal substrates with success. The laser used was a 30 ns laser pulses at the maximum average power of 38 W with a pulsed repetition frequency of 20 khz at a wavelength of 1064 nm. However, having the setup primarily for hole drilling only would limit the actual potential for the multi-dimensional capability of scanning laser processing. Figure 2-7: Laser trepanning system[66]. Apart from optical setups and hole taper types, studies also reported the effects of laser parameters on hole qualities for taper and circularity by using different statistical approaches such as the Analysis of Variance and the Central Response Method [67]. Responses measured such as taper and circularity are measured against current, pulse frequency and assist gas pressure. They reported that pulse frequency and assist gas pressure were dominating the results. However, it is also reported that hole taper can be controlled by changing the focal plane, the pulse width, the peak power and a total number of pulses [27, 68, 69].A strategy implemented as part of their experimentation to 38

39 control taper was inter-pulse laser beam shaping technique by linearly increasing the peak power pulses. Maintaining the circularity was equally important as the taper angle. The repeatability of laser percussion drilled holes have been demonstrated and reported by Ng et al.[70]. In their findings, they reported that for drilling using Nd:YAG laser of 1.06 µm wavelengths with pulse width range of 1-3 ms and peak power of kw, the circularity of the hole was prominently influenced by pulse width variation apart from peak power. This experiment was done on steel of 2 mm thickness. The circularity was measured by calculating a ratio of the smallest diameter against the bigger diameter of the hole Spatter On the other hand, Ng et al.[71] suggest that for better repeatability, higher peak power and shorter pulse duration would reduce the effect of melt ejection on the direction of the expelled materials from the laser drilling. There were various literatures suggesting ways for reducing spatter especially for assist gas percussion type drilling [19, 28, 72]. Spatter was one of the underlying problems for percussion drilling. This was caused when the removed material was pushed out of the hole while drilling and cools down just outside of the hole circumference and adheres to the top surface of the substrate. One study reports for Nimonic 263 alloy sheets drilling of closely spaced holes using a fibreoptic 400W Nd:YAG laser. The experiment implements a layer of anti-spatter composite coating (ASCC) for a spatter-free drilling with different types of assist gas conditions either air, O2, N2 and Argon Comparison of drilling strategies Another approach to laser assisted drilling method was reported by Biermann et al. [73] as a study for different deep hole drilling strategies as shown in Figure

40 Figure 2-8: Influence of the laser drilling strategy on bore hole diameter [73] The strategies were percussion, helical drilling and single pulse drilling were made and results on the resulted diameter analysed and presented. The laser used was Nd: YAG with peak power 7000 W and a pulse frequency of 10 Hz. The substrate used was AISI 316L with an assisted compressed air gas. The results in Figure 2-8 shows that single pulse drilling have minimal melt formation but the largest diameter. Sugioka et al[36].categorised the different strategies for laser hole drilling in two general types, laser drilling without relative movement of laser spot and work piece and with movement between laser spot and work piece as shown in Figure 2-9. The trepanning and percussion work well for micro drilling, however, the current challenge would be which strategy would suit better for macro drilling. Furthermore, the next section deals with important aspects applied for laser cutting as compared to laser drilling. 40

41 Figure 2-9:Overview of drilling strategies [36]. In a more advanced manner, a closed loop method for monitoring laser affected depth was reported by Chang et al.[74]. This was made possible by using sensors for the detection of the light emitted by the laser through a substrate (in this case alumina). However, the first light emitted from the pulsed laser drilling may only be of piercing. This method would not be applicable if we were to fabricate a uniform in diameter hole or a hole with specific exit diameter larger than the exit diameter when the first laser pulse was detected Laser Cutting Laser cutting was one of the most major laser operations to date. Various industries have utilised the laser cutting services for their production, be it automotive industries, aerospace and also medical industries. Laser cutting operations usually start with a piercing hole. This can be done by introducing low-frequency percussion operations of the laser with a ratio of 1:5 of ON to OFF instances of the laser with time. The longer the off instance allows the substrate to cool down and hence limits the damage by heat accumulation. The effectiveness of varying the ratio of the on and off instances would be subject to the type of material and the interaction with the beam/pulses and products expelled. 41

42 Below is a table that shows a comparison of various cutting methods when compared with laser cutting. Table 2-2:Comparison of different cutting methods[37]. Method Oxyfuel cutting Plasma arc cutting Material Thick practical maxima mm Laser 20 Water jet 150 non-metals 25 metals Wire EDM 100 Advantages Low cost, portable, easy to use Lower capital cost, fairly portable High speed and accuracy, flexibility Cut any material, cut stacked material, no HAZ, no recast, no dross, no fumes Most accurate, high edge quality, noncontact Drawbacks Slow, accuracy limit, large kerf, large HAZ, thermal distortion, fumes, metals only High consumable cost, accuracy limit, large kerf, large HAZ, thermal distortion, noise, ultraviolet rays, dust and fumes, metals only High capital cost, material restrictions, thickness limitations, fumes High operating cost, disposal of metal contaminated abrasive, larger kerf, noise, tool wear, Slow, electrode wear, wire cost, metals only. A few of the advantages of using lasers for cutting are listed below[37]: Applicability to a wide range of materials and thickness Narrow kerf widths High speeds Very high repeatability Very high reliability Easily automated and programmable Flexibility in change overs and geometries Reduced tooling costs and reduced setup times Non-contact process (no tooling wear or breakage, minimal material distortion) Versatility (the same tool can also be used for laser drilling and laser welding) Capacity for high degree of beam manipulation (true 3-D cutting) Applicable for no post processing treatments. 42

43 In general, laser cutting can be categorised into three types when assist gas was used. First, melt shearing, where the beam is focused to the surface, the pressurised inert assist gas would then be placed incident coaxially to the laser beam to eject the melt produced by the cut. The other type would be oxidation cutting. A combination of the laser beam and chemically reacting gas can be used to eject the melts with the expelled materials to generate more heat. This method allows for increased cutting speeds. Finally, the vapourisation cutting, this will work with or without any assist gas. This process has the lowest cutting speed of all the three primary types of laser cutting[37] due to the fact the process would need multiple passes to cut through thick materials. Dubey et al.[75, 76] presented an overview of experimentation studies of the Nd:YAG laser beams machining. Other research focuses on machining using the Nd:YAG laser beams performing several procedures accounting cutting, drilling and micro-machining[77, 78]. Like drilling mentioned before, the method of design of experiments was implemented to validate and evaluate the parameters of laser beam and interaction of the materials. The methods for experimental optimisation were ANOVA, response surface methods, Taguchi and central composite design (CCD) [4, 25, 79-82]. Measurable results such as HAZ, kerf width, edge profile and cut surface roughness were measured and analysed against laser beam parameters such as beam energy, feed rate, pulse duration, pulse frequency, spot overlap, scan or cutting speed, the focal position of the focused beam, pulse energy. On the other hand, the operational conditions such as type of gas and gas pressure and finally type of material and their material thickness could also influence the laser material interactions Apart from laser cutting of metals, laser cutting of ceramics have been reported by Wee et al.[83]. The effect of time, irradiance and assist gas pressure were taken to analyse the variation on striation wavelength, striation angle and the depth of separation line during laser cutting of ceramics. Unfortunately, the striation formation cannot be reduced or even eliminated. Rajaram et al. [84] reported on four main quantitative comparisons of kerf width, surface roughness, striation frequency and HAZ (heat affected zones) for the changes in feed rate and power for the laser cut of 4130 type steel with a CO2 laser. They reported that striation was independent of power and that the increase in striation frequency is observed with increase in feed rate. Below in Table 2-3, laser cutting finishes when compared to other alternative cutting methods. 43

44 Table2-3: Qualitative comparison of the four Alternative cutting methods with laser cutting for metals[37]. Item Nd:YAG Plasma Water Jet Oxygen-Flame Capital Cost S G S VG Running Cost S S S G Speed P G VP P Quality S-G VP VG P Max thickness S VG VG VG HAZ S-G VP VG P Kerf Width S-G VP VP VP Key: N/A, not applicable, VG, very good, G, Good, S, similar, P, poor, VP, very poor From Table 2-3 above, there are significant studies to be made when using a laser as a cutting tool. The advantage of laser cutting over the other types of machining was the non-contacting tool method that reduces the need for post-processing. In conclusion, much of the phenomenon in laser cutting owes to the power capability, speed capability and the type of material used for the cutting. The accessories such as the assist gas add cost to the system, but it greatly increases the efficiency of the laser especially for cutting operations Laser milling For parts with complex shapes, laser milling caters for small accurate and precise machining. Gathered here are previous studies where the laser was used for laser milling processes. During the last decade, engineers and scientist have shown a keen interest in laser milling. The ability to produce complex parts by laser scanning without tools contacting the workpiece has led to vigorous studies in laser machining. The ability to machine almost any material has made laser processing favourable for industrial applications. Much of the success of the laser milling was attributed to the understanding of the physical interaction between the laser and material. Interactions between the laser beam and the substrate playvital role in determining the optimal process factors for the laser milling process [85]. Laser machining involves laser power density, the pulse repetition rates for pulse beams, focal position,etc. Pham et al. have reviewed some of the laser parameters that affect the material removal properties[86]. Work was done with ceramics. Their methods of determining the process parameters for the quality of part could be adapted and applied to any laser type be it continuous or pulsed beams. 44

45 Several authors have reviewed the developments of micromachining[87-89]. Rizviet al. introduced micromachining using mask projection. The mask projection method enables designs to be projected to be as accurate as the projection mask used as shown in Figure 2-10 below. Figure 2-10: Mask projection scanning technique [87]. Though more processes can be done using direct writing techniques, the effect of laser beam tilt especially for galvo fitted systems would have a significant impact on the quality of the cut component provided the machining of high aspect ratio or over a large area. Campanelli et al.[90] has reported an experimental analysis of the effect of laser milling parameters. In the report, a design of experiment was used to evaluate the interactions of the laser parameters. The parameters were laser power, pulse repetition rate, pulse to pulse overlap, pulse duration and scanning speed. This method was used for the implementation of manufacturing complex three-dimensional structures. The shortcomings of the study were that the output factors were mainly focused on the depth of material removal and the surface roughness only. There was no indication of the other desirable qualities such as elimination or minimisation of heat affected zones and thermal damages. On the other hand, Bartolo et al.[91] reports on the interaction of the laser parameters to the surface finish of the laser machined substrates and concludes that scanning strategy not only influences the removal rates but also the surface quality. In conclusion, laser milling is still one of the fundamental machining processes for lasers. Apart from the system speed and power, strategies for the laser path and material interaction have to be taken into consideration as well. The developments of 45

46 CADCAM would further enhance the capabilities when using lasers for machining purposes. 2.3 Ultra short pulsed laser machining Here review studies were done for ultra short pulse laser machining with particular emphasis on picosecond lasers. Throughout the review, pointers on laser material interaction of different pulse widths, different wavelengths and different machining strategies to discuss how these studies compared to picosecond pulsed laser material interaction Previous Work on Picosecond Pulsed Lasers Picosecond lasers (pulse length of second) can be produced at much higher average power (up to 400 W) using an INNOSLAB technology[92]. High power results in high fluence and this together with focusing optics designed for minimal beam spot size possible, relatively high pulse repetition frequencies and rapid motion control could offer an opportunity for large-scale industrial applications. However, much of the adaptation to industrial applications depends on how the ps lasers perform in comparison to established processes. Picosecond laser applications have gained popularity in micro and nano machining operations. They are placed in the middle range of the ultra fast pulsed laser category. Past studies demonstrated that there is a significant influence on the threshold fluence and penetration depth when varying the number of pulses as well as pulse durations [38, 93]. One study concluded that pulse repetition frequency of ps laser machining influence the rate of material removal [94]. On the other hand, when using 7.23 J/cm 2 peak fluence, at 100kHz repetition rate and a scanner speed of 160 mm/s, the volume ablation rate for copper was down from 0.25 mm 3 /min for 10 ps to 0.04 mm 3 /min for 100 ps[95]. A significant reduction was due to the reduction in peak power density and pulse fluence hence the decrease in the temperature attained. Decreased heat affected zones was also observed as less time will be present to allow for conduction of individual pulse energies to the rest of the lattice and material. Reduction in heat-affected zones resulted from machining by picoseconds lasers supercedes nano-pulsed and micro pulsed lasers [21, 96]. It was reported that depending on the type of material, the surface roughness of sub-micron quality up to Ra = 300 nm [97-100] can be achieved. 46

47 Besides heat affected zones, particular wavelengths for most ps lasers with high energies are in 532 nm or 355 nm to better take advantage of the higher spectral absorption on materials [17, 20, ]. Table2-4: Penetration depth l 1, l 2, threshold fluence F 01, F 0 2 and critical fluence F or for Al by Spiro et. al(2012)[20] Metal 1 st Regime 2 nd Regime Critical Fluence L1 L2 Fcr (nm) (J/cm 2 ) (nm) (J/cm 2 ) (J/cm 2 ) Al F0 1 F0 2 Separately, differences in the effect of pulse shapes in machining strategy were reported by Nebel et al.[103]. The study demonstrated that different pulse strategies in the vicinity of few ns for the ps pulse width results in improved machining with minimal thermal effect, with pulse shapes of groups of 20 pulses that are repeated with 400 khz of burst repetition frequency. Apart from strategies of pulse duration, wavelength and pulse shaping, one author reports on using acusto optics and revolving cylinder while scanning on the surface of the cylinder to maintain spectral separation of pulses[92] to be able to achieve high material removal rate while maintaining surface quality. Separately, using high pulse energy leads to the formation of metal vapour plume that can cause undesirable thermal damage [104]. Therefore, separations of pulses are crucial. However, high repetition rate conversely produces better throughput provided laser processing parameters, and machining strategies are optimised. One ps laser research by Muhammad et al. [105] demonstrated that a dross free cut is possible when using picoseconds laser at the UV wavelength as shown in Figure The surface roughness of the cut was found to be Ra = 1.34 µm for Nithinol (a nickeltitanium alloy) material that was used for the manufacture of medical coronary stents. The use of the argon assist gas helps improve the quality of cut of medical application. The metallurgical changes observed under a microscope are within 35 µm of the cut edge. That shows considerable heat affected zones. Much of the material that attributed to the HAZ was the recast layer of the material upon beam and substrate interaction. Thus the ongoing research is to minimise the HAZ. 47

48 Figure 2-11: Nitinol cut edge without any post processing showing the dross free cut and sharp edge in different magnifications: (a) 60x and (b) 200x[105]. Siegel et al. [106] reported on machining eight different metals and alloys of Al, Cu, Fe, Mg, Ni, Pb, Ti and W and have investigated their ablation rate per pulse, the geometry of ablation and formation of the melt phases. The strategy of cutting such as the pulse overlap and some repetitions have also been investigated for their influences in the machining. The study reported that throughput will be improved with the increase in pulse energy. The throughput will also improve with higher repetition rates. Furthermore, the study also states that besides the system configuration, the machining strategy could also improve throughput rates. Cheng et al.[107] on Aluminium, Titanium and Gold carried out ablation study using 10 ps laser. A single shot pulse was devised to study the effect of ablation depth against increasing fluence. Single shot ablation removes the complication of the multiphase phenomenon by pulse incubation effect. A critical point phase separation modelling technique was employed. Cheng et al. founded that aluminium has the shortest electron-phonon coupling time of 4 ps followed by titanium at 7ps and gold of over 100 ps. Ultra-fast pulsed lasers have higher precision of a few nanometre per pulse removal rate and low or no heat-affected zones [37, 108]. Unfortunately, laser material interaction phenomenon for ultra-short-pulsed lasers is complicated. The complication arises as a result of the non-linear effects of absorption of the material to high peak power laser radiation[109]. The cold machining characteristics of very short pulse width ultra-short pulsed lasers can be realised with the strict fulfilment of the process parameters. The pulse width, laser source wavelength, parameters such as beam spot size, fluence, repetition rate, pulse overlap and scan speed 48

49 to maximise on the quality of the machining. Conveniently, the strict quality requirements for manufacturing medical devices paves the need for high precision ultrafast pulsed lasers cutting and machining for stents and machining of dental items [110, 111]. Ultra-short pulsed laser machining of features in micro, and nano scale, high repetition with lower laser fluence would result in better surface finish and morphology[36]. Further improvement in the quality of machining with decreasing pulse width of nanoseconds to femtoseconds pulsed laser applications is reported by Meier et al. [112]. One particular observation that was highlighted was that short to ultra-short pulsed lasers have the ability to machine with little or no heat affected zones as the pulsed phenomenon enables for the instant vaporisation of substrates that leaves the substrate with minimal heat. Separately, a study on fs laser machining and texturing by Momma et al. [113] suggested that for better-machined surface qualities, low beam power, high beam scanning speeds and a high number of over scans will be needed. Some even claim that there was absence of conventional laser induced machining defects such as HAZ, spatter, recast layers and micro cracks at the machined region [31, ]. There were even reports on two ablation regimes have been reported to behave logarithmically at low fluence and linearly at higher fluence [117]. Gakovic et al.[118] presented work using Nd:YAG laser with 40ps pulse duration on Ti alloy and found the damage threshold to be 0.9 and 0.6 J/cm 2 fluence at 1064nm and 532 nm respectively, suggested that the difference is owing to the lower reflectivity of Ti alloy at 1064 nm than at 532 nm. The study also reported for morphological changes, the fluence of 4.0 J/cm 2 and 13.6 J/cm 2 at 1064 nm and 532 nm can generate hydrodynamic features on the surface and wavelike structure exclusive to 532 nm wavelength only. To summarise, there are many studies on ps laser machining for better-cut edges, smooth walls of cut edges and almost free of heat affected zones. These were achieved due to short pulse interaction with materials enabling most materials to be vaporised leaving very little energy left for heating thus easier to cool down before the next pulse. This may not always be the case, particularly when the pulse repetition frequency used is high. 49

50 2.3.2 Challenges in laser processing Introduction Since laser processing is a thermal process, studies always focuses on how to minimise heat input and heat accumulation onto the substrate. The reason for this was to limit the formation of heat affected zones. Heat affected zones will affect the service life of the component by inducing variation in hardness and thus the residual stress profiles [119]. This section deals with challenges in laser processing using ultra short pulsed lasers Scanning method First, one-way of reducing accumulated heat energy is to scan fast. For continuous beams, by scanning, the energy deposited per area will be less hence reducing heat accumulation. On the other hand for pulsed lasers, especially ultra short pulsed lasers, high scanning speed allows the separation of individual pulses. The requirement for pulse separation would include a suitable selection of pulse repetition frequency and scanning rate. High-speed scanning coupled with high fluence was the best method to achieve high removal rate. Figure 2-12: Galvo versus polygon scanner scanning comparison [120] 50

51 The main method of scanning were galvo scanners [121, 122], and polygon scanners [92, 104, 120] as shown in Figure Applications for these scanners vary such as micro-structuring of metal surfaces and surface modification. Change in surface roughness can be achieved by changing the laser parameters and the scanning speed Removal rate Another challenge for laser ablation machining would be material removal rate. The material removal rates reduce as laser ablation leads to the use of shorter pulse width for lower HAZ and high-quality machining. When laser pulses get shorter up to femtoseconds, the resulting penetration depth reduces. Hence the removal depth also reduces[21, 31, 44, 46, 62, 114, 115, 123]. However, developments in mechanical hardware to increase the throughput of machining were developed and studied. In another study, reports on the availability of higher power ultra short pulsed lasers able to cope with the material removal issues. The studies by Bruening et al. and Du et al.[92, 124] reported the improvements of the removal rates with the aid of acusto optic deflectors and high repetition rate high powered lasers. This was then coupled with a fast movement of scan or movement of the substrate produces combined speeds of up to 40 m/s as shown in Figure With these high-powered ultrafast lasers coming into the scene, it led the way to higher removal rates for ablation method. A commanding removal rate of up to 4 mm 3 / min can be achieved using their setup. Figure 2-13: Sample beam scanning using rotating drum and acusto optics scanner[124]. 51

52 Other studies have reported better material removal rates at positions of the focal plane away from the focused plane position. Chang et al.[125, 126] have reported for 100 fs at 1 khz repetition rate and wavelength of 800 nm. The maximum material removal rate occurs when the focal plane at 60 µm above the sample surface. The study states that the increase in material removal rate was due to the resultant intensity of the laser pulses that are still above the minimum threshold intensity. Furthermore, the diameter increases as z moves away from the focused position which contributed to the broadening of the ablation affected area. This phenomenon will take into effect provided the intensity was above the threshold. Another strategy for improving material removal rates was by the effect of pulse shaping[127] and multiple pulsing of pulsed lasers. Literature reports that multiple pulses increase the efficiency of material removal[128]. However this approach can be successful provided plasma plume shielding, interaction with ejecta, avoidance of shockwave can be fulfilled. In particular, a study by Forsman et al. [129] in studying enhancement of material removal rates by 532 nm wavelength laser with three ns pulse width on type 304 stainless steel reports that when pulses are separated by 150 ns, to avoid plasma coupling, drilling efficiency is increased. Another study reports of increased material removal rate when using multiple pulses of up to 5 pulses per burst at high fluences and low repetition rates of under 100 khz as reported by Hu et al. [130]. However, even when scanning strategies, multiple pulses and focal plane positions are optimised, the interaction of multiple pulses of ultrashort pulsed lasers with high peak intensity produce shockwaves, material vapour expulsion and moreover plasma. The next section deals with the plasma interaction of ps laser ablation Plasma interaction In laser machining, especially in ultra-short pulses, the actual damage by thermalisation occurs after the first pulse has passed before the next pulse, as this is when the material vapour and plasma expand away from the surface and is time dependent. The formation of the dense plasma with high intensities would result in high absorption of the beam energy[37]. On the other hand, when the material was vapourised from the bulk, the ablation plume evolution can be described by four development stages[131]. The first stage was when the laser pulse energy is absorbed by the focused area and volume that results in 52

53 strong heating. The second stage was when the focused area and volume gain enough energy to vapourize and detach itself from the bulk until a layer of ionised vapour is formed on the surface of the target. The third stage is when the pulse terminates and the ablation plume expands adiabatically depending on the surrounding pressure and species. Lastly, the last stage is when the plume interacts with surrounding gas as illustrated in Figure This is not considering the pulse to pulse separation for pulsed laser machining. Figure 2-14: Ablation process and plasma plume development. When pulsed laser machining is used, this leads to the possibility for the ablation plume interacting with the oncoming laser pulses and firstly induce laser energy attenuation or plasma shielding[37]. Then, the plasma can absorb and scatter the oncoming laser pulses due to backwards propagating plasma to the laser beam and then fades away in their internal energy until the irradiance becomes too low to support the plasma. However, plasma can also be sustained as long as the laser pulse supply is adequate and uninterrupted to induce higher temperatures and this is called laser-induced plasmas. Plasma shielding would occur within the power intensity range of 10 7 <Icr<10 10 W/cm 2. When this critical value is reached, the light is absorbed by the plasma and laser pulses will not reach the substrate. This would also depend on a couple of factors, firstly the laser wavelength and secondly the location of the plasma during the machining process. Furthermore, laser plasma can be brought to a usable processing range when it is left to attach itself to the surface of the substrate[132]. When the plasma frequency is lower than the laser frequency, the plasma is transparent to the oncoming beam. It is in this region that the laser beam focus position may be altered due to the different 53

54 refractive index of the plasma/plume from that of air and that the plasma/plume may have thermal and density gradients[35]. To increase the material removal rates, any beam attenuating should be avoided or minimised. A comparison of the frequency can be made between the plasma frequency and the laser frequency. The plasma frequency is shown in (2.26). (2.26) ε 0 m e Where wp is the plasma frequency, ne is electron number, e is the electric charge of the electron, εo is the permittivity of free space, and me is the mass of the electron. It is important that pulse duration τp should be less than the 1/wp. Furthermore, the equation 2.21 is used to calculate the laser frequency on the laser wavelength: (2.27) Where wl is the laser frequency, c is the velocity of light and λ is the wavelength of the laser. Therefore, when wp>wl then there is a possibility that the plasma absorbs the intensity of the laser. Hence, this explains the particle shielding effect and also the plasma absorption. However, another phenomenon is non-linear. The non-linear phenomenon involves refraction of optics due to high intensities [133]. The governing equation for refraction is governed by Index of refraction: n 1 = n 0 + n 2 I(r, t) (2.28) For self-focusing to occur, a critical power has to be achieved. The equation for critical power is shown in (2.29) below. w p = w l = 2πc λ n e e 2 P c = λ2 2πη 0η 2 (2.29) λ is the wavelength, no and n2 are refractive indexes of the medium. This significant power is the power required to overcome the natural diffraction limit of the laser pulses to achieve the Kerr effect or self-focusing phenomenon[134]. A difference in refractive index of plasma due to the density of electrons may also lead to another event called plasma defocusing. A combination of this two phenomenon; the self-focusing and the plasma defocusing may result in filamentation effects [ ]. 54

55 Other studies reports of different types of plasma as shown in Figure 2-15.The three types of plasma are the atmospheric plasma; this plasma hovers close to the surface of the material. It is caused by the gas breakdown of the vapour and the air. Second is the particle ignited atmospheric plasma and contains ablation plume but confined to the walls of the ablation depth[138]. Depending on the material type, multiple reflection and light scattering could also have an effect on this kind of plasma. Lastly, the material-vapor plasma and is found at the interface between the laser pulse and the target material. Figure 2-15: Illustration of different type of plasma[138]. A paper presented by Bulgakova et al. [139]shows the laser-induced plasma formation from nanosecond laser radiated substrates and taking graphite as an example for forming ultra-deep holes. The work concluded that the laser induced plasma formation could absorb the radiation from the laser depending on the energy of the laser beam and the pulse duration. The absorption, on the contrary, could lead to explosive ablation process leading to ultra-deep drilling phenomenon. Rather than eliminating plasma, plasma demonstrated here can be useful for enhancing material removal rate. There are two stages in plasma formation where the first stage of the ablation pulse forms a thin layer on the surface of the material through ablation by sublimation, the transition to a plasma state and proceeds to hydrodynamic state forming shockwaves that travels away from the target. The second stage is the diffusion of heat to the material and the resultant thermal effects at longer time scales. Furthermore, lifetimes of plasma typically for femtosecond lasers are 1 µs as reported by Drogoff et al. [140]. Plasma 55

56 reheating of multiple pulses may not be favourable for laser machining. Consequent laser pulses will be absorbed by the plasma as reported by Semerok et al. [141]. On the other hand, when one could understand the dynamics of the plasma evolution with respect to time, for a chosen laser parameters, the plasma could be used as a coupling reaction with the laser pulses thus enabling higher contact temperatures that could induce increased material removal through boiling and melt ejection. Mahmood, S., R. Rawat, et al.[142] reported that the velocity and the characteristics of the laser induced plasma was correlated to the type of the material being ablated, precisely their density and masses. Kumar et al. [143] reported that the modelling of the laser-induced plasma was determined as a compilation of combined effects of many processes such as radiative, excitation and ionisation. Not all phenomenon involving plasma interactions with the beam as well as the surface of the substrate could be analysed qualitatively because of the rapid disappearance after the beam stops. A method to get around this would be to model and simulate a laser substrate material interaction. As previously mentioned in section Laser Ablation, after the pulse that results in ablation, there was the possibility of plasma formation after the consequent pulses. On the other hand, the formation of the dense plasma upon high laser pulse intensities would result in high absorption of the beam energy resulting in energy coupling to the plasma instead of the target surface. The multi-pulse interaction would also act as a reheating of the plasma in the plasma development. This resulted in the prolonging of the plasma laser lifetime[37]. The phenomenon suggests that the plasma evolution has a significant effect on determining the role of the plasma between thermal coupling or plasma shielding. A study by Babushok et al.[144] reports that the main issue for multi-pass picosecond lasers was plasma shielding. The study reports that pulse width of 1 to < 10 ps are intermediate regime, whereas for 10 ps to 250 ps pulse width are characterised as total plasma shielding. For efficiency, the laser off time should be considerably longer than the laser on time to prevent from plasma formation in the first place[145]. The plasma interaction, material removal enhancement and the high-quality machining should also be suited to the material being used for this research, the Titanium 56

57 alloy, Ti6Al4V. The following section reviews the conventional machining and compares it with laser processing of Titanium alloy. 2.4 Titanium alloys laser interaction Titanium is the ninth most abundant element in the earth s surface and the fourth most abundant structural metal after aluminium, iron and magnesium. Various studies are conducted to minimise tool wear by tool coatings[146] and optimise the quality of the finished product through cooling procedures during conventional machining of titanium alloys[9, 10, 147]. Another study implemented the hybrid machining technique where the work piece was heated, and the cutting tool was cryogenically cooled to improve machinability of the Titanium as well as nickel alloys[148].the attractiveness of titanium is that it has low density, high strength and corrosion resistance hence the alloys of titanium are used in jet aircraft engines. Difficulties in machining titanium alloys come partly due to their surface integrity for any types of machining even when using conventional machining such as surface tearing and cracking [11, 12, 149]. A particular study by Crawford et al.reported the energy intensive process of precision turning of a near-alpha titanium alloy with regular machining reports that a tradeoff was concluded where thermal exposure is balanced against subsurface micro structural damage such as melt profiles and microstructure changes[150, 151]. When machining using lasers, titanium also exhibits a strong reaction to oxygen and nitrogen. The preferred assists gas for laser machining of Titanium would be argon gas at pressures up to 750 kpa. Inert gas cutting of titanium with lasers would give a smooth edge[37, 152, 153]. However, material removal rate for ultra-short pulsed lasers was known to be very low. The thermal penetration depth can be estimated using [154, 155]: δ therm = 2 kt p c p ρ, (2.30) where k, is the thermal conductivity, tp is the pulse duration, is the material density and cp is the specific heat capacity[156]. For a 10 picosecond (ps) laser and a titanium alloy (Ti6Al4V), the thermal penetration depth is only 9.27 nm for each pulse. On the other hand, in pulsed laser ablation, the ablation process can be affected by multiple laser parameters such as laser 57

58 power, pulse duration, repetition rates, fluence, beam spot size, number of pulses per spot and scanning speeds. These parameters are subject of scrutiny for process optimisation studies[48, 97, 157]. Yilbas et al. reported on the thermal stress analysis of the Titanium alloy when subjected to laser hole cutting using a 2kW CO2 laser with the pulsed mode. The residual stresses using XRD technique is compared with numerical simulation predictions from calculations and experimental results with that laser, the top circumference was found free from recasts, and dross still incurs attached at the bottom of the hole. The authors have also reported that the heat affected zones of the cut edges are small attributing to the low thermal conductivity of the titanium alloy[158]. The previous study by Yilbas et al. reported that in the drilling of titanium, the taper decreases as the thickness increases[24]. On a separate occasion, Yilbas et al. concluded that apart from the material interaction of the thermal diffusivity and thermal absorption coefficient, the laser drilling speed was a significant parameter for drilling[23]. Laser cutting parameters crucial for the high-quality cutting especially when HAZ is concerned during the cutting of Titanium are, cutting speed and pressure of assist gases. Shanjin et al.[159] reported that to minimise the HAZ, high cutting speed, high pulse rate repetitions and argon as an assist gas are used. Another study by Rahman et al. [160] determines that significant increase in cutting speed m/min and decreased cutting force was reported when using high laser power, for example 1600 W. Biffi et al. [16] reported on the nanosecond pulsed fibre laser micro drilling of titanium. Apart from spattering, microstructure changes and taper formation still evident, the control factors for better quality were concluded to be pulse energy and pulse frequency. 20 µm micro structural changes on the surface of the entrance was reported for this procedure. Kerf width and surface roughness were also reported by Kumar et al. [161] for pulsed Nd: YAG laser drilling of thin Titanium alloy sheet. This study concluded that for the better surface finish, control has to be made to the pulse frequency followed by cutting frequency. Lower values of pulse width and frequency, and high values of feed rate and moderate nitrogen assist gas pressure, will result in a better surface finish. Another study reports the influences in laser striation free cutting. The author indicated that for low striation roughness or the no striation effect, intermediate speeds have to be identified. This would depend on the type of laser and the substrate[162]. 58

59 To summarise the literature on titanium laser machining, parameters such as frequency, assist gas pressure, cutting speed and pulse energy would give a better cut and or drilling quality. In conclusion, due to the thermal conductivity of the titanium and its alloy together with the absorptivity of the alloy to laser beams, machining and drilling with laser beams are more beneficial compared to conventional machining. Finally, the research is purposely designed for the scaling up of the micro machining of ultra-short pulsed lasers for macro machining. Machining that we are interested in would be drilling and v groove machining. 2.5 Summary of Chapter 2 This chapter discusses the previous work done on laser systems and laser technology on different types of machining processes. The rise of the need for picosecond pulsed lasers and their distinct advantages over intermediate pulse widths such as femtosecond lasers and nanosecond lasers result in a fair trade off in material removal rate. Further study with regards to the effects of varying laser parameters of power, p.r.f and perhaps machining strategies could support scientific advancement in the field of picosecond pulsed laser machining for the analysis of the ablation threshold, optimisation of laser parameters for processing and to better understand the effects of plasma and ablation plume. The next chapter then reviews various methods and equipment that were used in this research. 59

60 Chapter 3 Equipment and Material Specification 3.1 Introduction This chapter describes the equipment, facilities, and materials used in this research. The equipment specification, operating principles, and operating parameters ranges are given. The basic material properties are listed. 3.2 Equipment Picosecond Laser Laser Side The picosecond pulsed laser is a 400 W diode-pumped Nd: YVO4 oscillator coupled with Innoslab amplifiers Model: PX(400W) from Edgewave shown in Figure 3-1. The laser pulse width is 10 ps and a Gaussian beam intensity profile, a pulsed repetition frequency ranging from 102 khz up to 20 MHz. This laser emits at 1064 nm wavelength, with an unfocused beam diameter of 3 mm. Laser Path Figure 3-1: The Edgewave 400 W picosecond laser materials processing system. 60

61 Table 3-1: Laser specification chart[163]. Edgewave PX laser Specifications Max Average Power[W] 400 Max repetition rate[mhz] 20 MHz Wavelength[nm] 1064 (IR) Highest beam quality M Pulse width[ps] 10 Energy stability [%] rms 1 Polarisation >100.1 Beam ellipticity (far field) [%] <10 Diameter at window [mm] 3 Full divergence angle [μrad] 500 Point stability [μrad] <100 Warm up time, from cold start [min] 15 Focusing the beam was achieved with an f-theta focusing optic with a 330 mm focal length resulting in a focused beam diameter of 125 µm as stated by the manufacturer. In the later chapters, the need for clarifying the beam diameter and the beam position gets crucial. It was then that the beam diameter was measured as function of the measured impressions of the ablated sites for different focus positions above and below the sample surface. The procedures for measurements of spherical diameter and thus placement of the focused position with respect to the sample surface were detailed in the following sections Processing side The laser was equipped with a galvo scanner from Scanlab Hurryscan 20 with a maximum scan rate of 10 m/s and an effective working area of 120 mm by 120 mm. The galvo scanner position in the beam path was shown in Figure 3-2. A PRO 165 Aerotech vertical Z-axis with 400 mm traverse range and 0.5-µm resolutions was used for the adjustment and positioning of focus onto the work piece. Another, an Aerotech linear motorised stage controls the X and Y directions with 400 mm x 400 mm traverse range and 2 µm accuracy coupled together with 20 nm resolution. The X Y stage was used for moving the samples and can be used simultaneously during processing enabling multiaxes machining. The maximum travel velocity of this X and Y stage was 500 mm/s with a maximum acceleration of 0.5 g and a maximum load of 75 kg. The laser and motion control uses an Aerotech CNC Windows based interface software using conventional G 61

62 & M codes. An Edgewave supplied control software controls the laser parameters such as the power and the pulse repetition frequency. The limitation of this setup was that it was not possible to change the power and pulse repetition rate parameters from the G & M code CNC interface that was inconveniently controlled by a separate Windows program. Furthermore, the manufacturer has installed a built-in CCD proximity camera used for locating and alignment of the laser beam point on the sample surface and aiding focusing. Figure 3-2 illustrates the beam path and the laser processing arrangement. Figure 3-2: Experimental setup for picosecond pulsed laser. A fume extraction system, the LMD 08 was used to collect the fumes generated in the laser processing. An enclosure with an integrated safety inter-lock mechanism was used to contain the laser processing away from the operator. All the operations were done remotely from an externally attached computer placed near the enclosure for controlling laser power, repetition rate, and CNC motion control (using G- codes). The processes can be viewed with a CCD camera. An emergency stop button is placed visibly should anything go wrong. The materials processing experiments carried out include line ablation, drilling and effect of laser beam focal plane position. 62

63 3.2.2 Beam Profiler A Spiricon LBS 100 beam profiler was used to determine the focused beam diameter, the beam profile and the focused position. This profiler consists of a camera sensor SCOR-20-FW from Point Grey coupled with a beam attenuator attachment with a silicon CCD imaging device as shown in Figure 3-3. f-theta lens laser beam path z - axis Sensor Figure 3-3: Spiricon LBS 100 beam profiler The attenuator attachment functions to lower down the peak intensity of the beam with different grades of attenuating filters. The maximum power limit of the profiler was 1 W. The beam profiler connects to a computer through a firewire connection. Data were collected by firing the laser beam onto the sensor and analysed by the Windows Operating system based LBA-FW beam profiler software. The scorpion firewire camera has a resolution of 1600 x 1200 pixels at 4.4 µm per pixel. The maximum circular beam detectable will be 5.3 mm and the area the sensor covers is 7 mm x 5.3 mm. Both the 2D and 3D profile can be detected and presented in the software. This profiler can determine the position of the beam waist and thus the focal plane position. Do take into account that the lowest power from the laser was 2W. To counter this, filters were used to reduce the power. Generally Figure 3-4 below shows the 63

64 positions along the beam path with respect to the galvo position and their corresponding beam intensity profile. Focal length Profiler moved along laser path in z axis Galvo Scanner Laser beam path Profiler placement Low peak wide beam High peak narrow beam(focus) Low peak wide beam Sketch of Beam Intensity Shapes for different z positions Figure 3-4: Spiricon LBS 100 beam profiler schematics x pixel y pixel Beam shape Beam intensity shape Figure 3-5: Sample measurement of intensity and beam shape. 64

65 Figure 3-5 was used to characterise the beam shape, low intensities and high intensities at y axis and their corresponding width in the x axis. The position with the highest intensity peak at y axis and narrowest width at x was taken as the focused position. The surface of the sample would then be placed at this position. However, please do note that due to the power restrictions, the beam profiler was used to aid the qualitative representation of the focused position. Other method (mentioned in section ) was being used to measure the actual beam spot size with respect to the material used for processing i.e. Ti6Al4V Laser power meter A Gentec Model UP55G-500F-H12-D0 laser power meter (Figure 3-6) together with a Gentec Electro-Optics DUO monitor was used to measure the average power. This enabled the calculation of the beam pulse energy based on the pulse repetition rate. Figure 3-6: Gentec power meter, Model UP55G-500F-H12-D0 and Gentec Electro-Optics DUO The resolution of the monitor was 15 µw on a 30 mw scale with 0.06 mv/w sensitivity. The accuracy of the monitor was up to ± 0.5%. During the measurement, the laser was fired at the power meter probe at an out of focus z position, a minimum distance of 200 mm away from the beam focus as shown in Figure 3-7. The power meter can measure up to 500W at a maximum of 55 mm aperture. Upon each measurement, the readings will take around 10 seconds to stabilise and the power meter will read out the average power. 65

66 Galvo Scanner Focal length Laser beam path Extra 200 mm z - axis Power meter placement Figure 3-7: Gentec power meter placement schematics This power meter can be used for measurement of power at wavelengths of 355 nm, 532 nm and 1064 nm with max power of 500 W. Furthermore, the power meter comes with a heat sink and powered cooling fan to reduce the effect of heat build-up during power measurement especially at high powers Optical spectrometer A model Specord 250 spectrometer shown in Figure 3-8 from Analytikjena is a monochromatic spectrophotometer used to measure the reflectance and the absorbance of the material (substrate) at various optical wavelengths. Figure 3-8:The Analytikjena Specord 250 spectrometer. 66

67 The spectrometer had a 1 nm slit spectral resolution. Full spectral wavelength ranges from nm with two lamps Halogen and Deuterium selectable between 300 and 450 nm. The spectrum scanning speed can be up to 6000 nm/min. The minimum width for the inspected area is 12.5 mm. The sample compartment size is 193 x 111 x 400 mm. The device measures energy, absorption, transmissivity, and reflectivity. All readings and analyses from the device are done at room temperature. The data collected are interpreted and presented to Windows PC based interface Win ASPECT software. Table 3-2 : Technical Data of Specord 250 [164] Technical Data SPECORD 250 Optical principle Optics Wavelength range Scanning speed Double-beam spectrophotometer with two large-area photodiodes Premonochromator and monochromator with holographic concave grating Light split into two beams by a beam-splitting plate nm up to 6000 nm/min The absorption measured is a function of reflectivity where, Absorptivity = (1- R) and R is reflectivity measured by the amount of light reflected back to the direction of the light source. Figure 3-9 shows the corresponding reflectance graph of a Ti6Al4V sample. 43.6(% Reflectance) 1064 nm Figure 3-9: Sample reflectance measurement on a as received' Ti6Al4V sample. 67

68 3.2.5 White-light optical profiling system A Wyko NT1100 surface profiler shown in Figure 3-10 based on white light interferometer is used to measure the surface profiles of the materials processed by the lasers. Depending on the settings, the resolution for the depth of measurements can be up to 0.1 nm. Figure 3-10: Wyko surface profiler NT1100. The measurements are contactless, and the full range is 1mm. The measurable values include average surface roughness (Ra), the Root Mean Square (Rq) average between height deviations and the maximum height (Rt) of the highest point (Rp) and lowest point (Rv). Table 3-3: Specification Data of Wyko NT1100 [165]. Specifications Measurement Techniques measurement capability Measurement array Light Source Video display Software Vertical Measurement Range Field-of-View Description optical phase-shifting and white light vertical scanning interferometer three-dimensional, non- contact, surface profile measurements user-selectable, maximum array 736 x 480 pixels tungsten halogen lamp ( user- replaceable); manual filter selection 127 mm (5in.) monochrome monitor Wyko Vision software 0.1 nm to 1mm 8.24 mm to 0.05 mm 68

69 The NT1100 uses white light interferometer for the production of high-resolution measurements of 3D surfaces. Roughness of sub-nanometre to millimetre high steps can be detected. Below are some images on how ablation depths are measured in Figure (a) 2d Top Profile y Bar x Bar (b) Cross sectional Y bar profile (c) Cross sectional X bar profile Figure 3-11: (a)2 D top profile (b) cross sectional Y bar profile (c) cross sectional X bar profile of ablation depth measurement. 69

70 3.2.6 Sample preparation equipment Extensive sample preparation devices including cutters, grinders and polishing machines are available in the Laser Processing Research Centre laboratory enabling the preparation of laser processed samples for characterisation. Figure 3-12 shows some of the devices for sample preparation. (a) (b) (c) (d) Figure 3-12: (a)struerslabotom 5 (b) StruersAccutom 5 (c) OmegaPol Twin Metallurgical Polisher and (d) Struers LaboPol- 35 Polishing. For observing microstructure of Ti alloys, best procedure would be to apply water-cooled grinding on 1200 grit SiC papers. After grinding, the sample was polished using synthetic cotton cloth with synthetic diamonds of grit up to 3 µm. A Kroll s reagent was used for the microstructure observation. The reagent has 5 ml HNO3 (65% conc.) and 3 ml HF (40% conc.) in 100 ml of H20[166]. The polished sample may be swabbed with the solution allowing it to process for a few seconds before observation. Extra care has to be taken when using this reagent. Alpha region dominates the microstructure with lighter appearance and the beta region with the darker appearance. 70

71 3.2.7 Keyence VHX-500F Microscope The microscope is a high-magnification digital optical microscope as shown in Figure The microscope has two types of optical magnification lenses. These lenses would enable the use of optical magnification of x10 to x200 and even up to x1000. Figure 3-13: Keyence 3D optical microscope. Images of the top surface profiles, cut edges, cross-sectional profiles, striations, taper, roundness and microstructure changes can be conveniently captured using this microscope to analyse details, for example, measurements of diameters, width and heataffected zones. Furthermore, 3D profile and depth is measured with the aid of an automated motorised z stage of resolution 1 µm and through method of depth by defocus with the accuracy of up to 1 µm. The microscope was also used to characterise material microstructures, cut edges characteristics, striations formations, taper, roundness, ablation depth, recast layer thickness, melts and dross. 71

72 3-14 below. A sample of how the width and depth measurement was made is shown in Figure 44 W 1MHz,Scan Rate:500 mm/s Z+15 mm above sample surface and 1000 over scans Width Depth Ablated Line cross sectional profile Figure 3-14: Sample micrograph Quanta 200 Scanning electron microscope A Quanta 200 High resolution scanning electron microscope shown in Figure 3-15 was used for high definition imaging of laser processed samples. SEM is a nondestructive analytical technique. One of their outstanding capabilities is the SEM can take images of up to 20,000 times magnification of finer details. Figure 3-15: Quanta 200 Scanning electron microscope. 72

73 The high resolution is made by projecting a beam of electrons and measuring the sample topography from the beam interaction. The sensitivity and resolution of the images captured will depend on the electrical conductivity of the samples. The resolution of SEMs are better than microscopes because beam of electrons has a shorter wavelength, therefore, lower diffraction limit when compared to white light interferometry and optical microscopes hence better maximum resolution. Furthermore, backscatter images from SEM equipped with EDX system can produce images with phase variations when present. Ps laser scan parameters: 44 W, 500 mm/s 100 over scans Z +16 mm above sample surface Width Depth Ablated Line Figure 3-16: Sample SEM image for analysing the cross-sectional depth and width of an ablated line. 3.3 Materials Titanium Alloy Ti6Al4V A sheet of 1 mm thick Ti6Al4V (Grade 5) AMS4911 from Aerocom Metals was cut into smaller pieces of 20 mm x 50 mm by guillotine when machining using the lasers and ease of characterisation under microscopes and concerned analytical equipment. The average surface roughness was Ra= 504 nm measured using a Wyko White light surface interferometer profiler. The average hardness of the untreated surface of the Ti alloy measured using Buehler Micromet 5100 series micro indenter was 395 HV. The overall reflectance measured using a Specord 250 from Analytikjena was measured to be 48.4% 73

74 at 1064 nm wavelength. The thermo-physical properties of the Ti alloy are shown in Table 3-4. Table 3-4: Ti6Al4V thermo-physical properties[167, 168]. Item Ti6Al4V Melting point(ti) ( K) 1667 Boiling Point(Ti) ( K) 3285 Latent Heat of Melting(Ti)(kJ.kg -1 ) 436(Ti) Latent Heat of Vaporisation (Ti) (kj.kg -1 ) 8960(Ti) Specific Heat Capacity (J.kg -1.K -1 ) 610 Density(kg.m -3 ) 4420 Reflectivity(Ti) Thermal Conductivity(W.m -1.K -1 ) 5.8 Thermal diffusivity (m 2 /s) at 20 C 2.7 x 10-6 Modulus of Elasticity (GPa) 110 Ultimate Tensile Strength (MPa) 965 Yield Strength (MPa) 875 Elongation (%) Hardness (Hv) 395(measured) Reflectivity (1064nm) 0.48 (measured) Titanium alloy was selected as material of choice due to its thermo physical properties and emerging applications in aerospace, motor vehicle parts, motorsports, and exotic personal equipment such as watches, eyewear and jewellery. 3.4 Experimental methods Machining procedure Literature suggests that the best method for material removal when using pulsed laser processing was the scanning method. By right, this method would give the best material removal rate and furthermore, when optimised may results in high quality finish. However, to study the characteristics of the ps laser processing for Ti6Al4V, a line ablation method was chosen. The line ablation was done in air to study the effects of pulsed laser processing at room temperature and pressure without the aid of expensive inert gasses or non-practical vacuum environments Line ablation When high pulse repetition frequencies are used, galvo scanning method was favourable. With the ability for high scanning speeds, pulses can be made to separate from one another and thus reducing the accumulation of pulses at one spot. In the light of 74

75 this, a line scanning method was employed for studying the interaction of varying scanning speeds, laser power and pulse repetition frequencies on the Ti6Al4V alloy as shown in Figure Galvo Scanner Focal scan path laser beam Ablated line 120 mm 120 mm FOV Figure 3-17: Line ablation. Prior to the line scanning procedure, the beam focus position was verified using the same procedure stated in section Beam diameter measurement procedure. Only when the focus position and beam diameter was determined beforehand the line scanning experiments can be executed. For the line ablation procedure, the laser was scanned along the surface of the sample, forming a line. After each line of specified length was scanned, the laser switches off; returns to the starting position, and then repeated the process until the required number of repetition was completed. The line scanning was repeated for specific units say 100 times. The numbers of repeats (or overscans) were determined beforehand by doing tests runs. The groove resulted from the scans were then measured and will be detailed in the following section. The laser operational parameters used include laser power, repetition rate, scanning speed and number of passes. 75

76 3.4.2 Experimental sample measurement procedure Machining groove width and depth measurement Characterizing the experimental samples was crucial to the development and optimization of a process regime. The laser machined samples were cleaned with distilled water and wiped clean before the measurement process. After sectioning, grinding, polishing and etching, the machined cross-sectional area characteristics of the laser machined samples were determined using the Keyence optical microscope for depth from defocusing (as mentioned earlier) and the SEM. Figure 3-18 illustrates the parameters measured, and the typical laser machinedcross-sectional profiles. (a) Laser machined profile top view Cross Sectional Area (b) Laser machine side view Figure 3-18: (a) laser machine profile, top view and (b) laser machined side profile view for measurement of cross-sectional area. By examining the samples crossectionally, the width, depth and cross-sectional area (refer Figure 3-14 for exemplar features) will give sufficiently accurate measurements for calculating the rate of material removal. Furthermore, the timing of machining can be noted down together with the length of scan to calculate a volume 76

77 removal rate based on the cross-sectional area. Overall, a comparison can be made by analysing the change in removal rates with a change in laser parameters Beam diameter measurement procedure Analysing the position of focus and beam diameter is critical for any laser processing and machining. Beam diameter measurements was done by firing one shot at five different sites for a specified z position above and below the sample surface as shown in Figure The power was set at 24 W to reduce effect of surface melting and to measure the size of impression by the pulse solely. The pulses were fired at room temperature and pressure without an aid or processing gas or vacuum environment. Prior to the experimental runs, the Ti6Al4V alloy was covered with a layer of graphite. The graphite layer enhances the effect of the pulses for ease of measurement of the diameters at different z positions. As a precaution, the pulses should only affect the layer of graphite on the sample surface. The pulses were fired in air. Figure 3-19: One pulse beam spot measurement for determining focus position. At focus, the beam diameter is the smallest. The smaller beam diameter allows for higher energy density deposited onto a smaller area at the surface of the sample. Figure 3-20 illustrates the technique for the measurement of the beam diameter. This was done with four measurements of the beam width. This method is similar to rotating the beam through a number of angles in this case 90 degrees and taking the diameter measurements and then averaging them. 77

78 (a) (b) No. Measurement (µm) Av 117 Figure 3-20: Measurement of diameter (a) method of measurement and (b) results of example measurement on sample. 3.5 Summary of Chapter 3 This chapter introduces the equipment, material and procedures that were used for the purpose of achieving the aims and objectives of this thesis. The variations of equipment used are to support the findings of this literature and the findings of the current experimental results and procedures. The material preparation was crucial to lay out the foundation for both microscopy and SEM analysis. State of the art equipment such as the scanning electron microscope and white light interferometer give cutting edge analysis in terms of resolution and accuracy especially in the cross-sectional area evaluation and the impression of the pulsed ablated sites which were to be presented in the next few chapters. The aim was to be able to characterise the processed laser machined sites to better understand and improving removal rate while maintaining machining quality by varying power, p.r.f and the machining conditions such as pulse per spot, scanning speed and in the later chapters, effects of varying z position with respect to sample surface position (focused position). 78

79 Chapter 4 The Characteristics of High Pulse Repetition Frequency 300W Picosecond Laser Ablation of Ti6Al4V 4.1 Introduction Titanium is the ninth [33] most abundant element on the earth s surface and the fourth most abundant structurally suitable metal after aluminium, iron and magnesium. The attractiveness of titanium and its alloys is that they have low density, high strength, high heat and corrosion resistances compared to similar engineering metals such as steel and aluminium alloys [33, 169, 170] and they are biocompatible. Due to these properties, many industries such as aerospace, automotive, medical, sports, personal wearable items including jewellery are using these alloys. Machining of Ti6Al4V is, however, challenging due to high tool wear and costs. On the other hand, industrial lasers have the advantage of con-contacting in cutting and drilling materials that have led to the reduction of tooling cost that consequently leads to a substantial reduction in the overall manufacturing cost [37]. The main challenges for laser machining would involve limiting the damages resulting from laser machining including heat affected zones, recast, striation, taper, and undesirable surface finishes [35]. Although high power Nd:YAG, CO2, fibre and disc lasers are commonly used in machining (e.g. cutting) of Ti alloys, but due to long beam interactions times, excessive heat affected zones (HAZs) are experienced (typically in hundreds of micrometres). On the other hand, very short-pulsed lasers such as femtosecond and picosecond lasers have been applied to micro machining of Ti alloys with very high edge quality leaving very small HAZs. However, material removal rates are too low to allow these techniques to be considered for macro-machining applications. These ultrafast lasers are typically at very low average powers, from a few Watts to tens of watts. Only recently (over the last 5 years), high average powered (> 100 W) ultra-fast lasers are becoming available. However, the characteristics of high-average powered ultrafast laser interactions with materials are scarcely known. Fan et al. studied the interactions between a higher repetition rates (more than 2 MHz), high average powered ultra-short laser and Cu [121, 122]. Most investigations report single point, static laser beam ablation of materials [107, 118, ]. The average powers of the ultra-short pulsed lasers in these studies are typical below 20 W. 79

80 Next, the ablation threshold can be interpreted from plots of depth per pulse against the log natural of laser fluence. It is assumed that every pulse removes material at the same depth acting per unit area taken from the beam spot diameter. Hence, to satisfy the Beer Lambert's equation, the optical penetration is directly proportional to the depth of ablation. The ablation characteristics is analysed using Beer Lambert s equation[49]: x = 1/α ln (F/FT) (4.1) Where α = the effective absorption coefficient (cm -1 ), FT = threshold fluence (J/cm 2 ), F = parameter fluence for acquired depth (J/cm 2 ), x = depth for ablation per pulse, 1/α = optical penetration (cm) and finally the thermal loading can be calculated using [174]: γ = F T. α (4.2) Thermal loading γ, is the amount of energy needed to raise a unit temperature for a unit volume of the material. A comparison can be made with the calculated results for absorption coefficient to theoretical equation of absorption coefficients [37]; the equation 4.3 is used for the calculation of the theoretical absorption coefficient for each element of the alloy; Where λ is the corresponding laser wavelength, k is the extinction coefficient. The optical penetration on the other hand is inversely proportional to the absorption coefficient and has its units in nm. Thermal loading α = 4πk λ A comparison of thermal loading for each of the elements is made for Ti, Al and V with the calculated thermal loading from the results of the ablation threshold. The thermal loading can be calculated from thermal loading capacity: (4.3) ρ = ρc p (T m T 0 ) + H m + H v (1 R) (4.4) where ρ is the density, C p is the specific heat capacity, T m is the melting temperature, T 0 the room temperature, H m latent heat of melting, H v latent heat of vaporisation and R is the reflectivity. 80

81 The work investigated here is for the processing regime and characteristic of a high average powered (up to 300 W), high repetition rate (up to 19 MHz) 10 picosecond pulsed laser interactions with Ti6Al4V at high scanning speeds for macro-machining and drilling applications. From this method, the ablation threshold, absorption coefficients, thermal loading coefficients and corresponding incubation coefficients by multiple pulses can be determined. 4.2 Experimental method The laser used for this investigation was an Edgewave 300 W ps laser (see Chapter 3, section 3.2.1). In this investigation, the pulse repetition frequency ranged from 500 khz to 19.2 MHz. The focal length of the F-theta lens was 330 mm resulting in a focused Gaussian beam of the diameter of 125 µm (as stated by manufacturer). The manufacturer specified the beam quality factor was M 2 = 1.2. The effects of processing parameters such as pulsed repetition frequency (p.r.f), average laser power and scan speeds on the characteristics of Ti6Al4V were to be studied. A series of experiments were carried out to observe the material responses including ablation depth, ablation kerf width, and heat affected zones from crosssectional analyses. The experimental layout is shown in Figure 4-1. The properties of the Ti6Al4V alloy used for this study can be found in Chapter 3, Section The samples were prepared in rectangular sections of 100 mm x 50 mm with a 1 mm thickness. The surface was wiped clean with acetone, and then wiped again with distilled water and finally dried before commencing the process of laser ablation. Also, samples were laser machined as received condition, in air and at room temperature. That is to say the processing is done onto samples without polishing being done or any special inert gas applied. This was to simulate the actual machining conditions if the process were to be adapted for industrial applications. The overall reflectance measured using a Specord 250 from Analytic Jena was 48.4% at 1064 nm wavelength. The measured reflectance is lower compared to the reported 51% as the samples were not polished [167]. 81

82 Laser Controller Chiller Aerotech Controller PC Laser Source Galvo Laser path Sample Galvo Fume Extractor Work piece Holder Figure 4-1: Edgewave 1064nm 300 W Ps laser configuration The laser parameters used for the ablation threshold study are as follows: the laser power of 26 W, 55 W, 62 W, 64 W, 90 W and 132 W. This power range allows for the variation of fluences thus varying the responses. Then the pulse repetition frequencies (p.r.f) 2.4 MHz, 4.8 MHz, 9.6 MHz and 19.2 MHz. The resulting range of fluence ranged from 0.01 to 0.45 J/cm 2. Scanning speeds chosen for this processing regime were 2400 mm/s, 4800 mm/s and 9500 mm/s. At this point, the scanning rates are to cover the low, medium and fast speeds. The pulse repetition frequencies and three speed settings allows for the comparison of varying the applied number of pulses per unit area. One hundred overpasses were done to be able to observe the resultant depth achieved, as trials with one pass does not acquire sufficient depth for measurement. The experimental runs were done once for each parameter. Therefore, the errors implemented for the results are measurement errors of ± 0.5 µm. Below Table 4-1 and Table 4-2 are the tabulated parameters for the experimental runs for the measurement of the width and depth of the laser processing. 82

83 Table 4-1: Table of parameters for 26 W, 55 W and 62 W Power(W) Frequency(MHz) Speed(mm/s)

84 Table 4-2: Table of parameters for 64 W, 90 W and 132 W Power(W) Frequency(MHz) Speed(mm/s)

85 The processed surfaces were first analysed using a Keyence VHX-500F digital optical microscope for the measurement of the kerf width. The depth was initially measured using a Leica DM 2500 M confocal optical microscope. Further verification was done for the kerf width and depth by measuring cross sections features using an optical microscope. These cross-sectional samples were mounted onto metallurgical plastic resin mounts and were ground using SiC papers up to 1200 grit. The samples were then polished using a synthetic polishing cloth with diamond pastes lower than 3 µm. When required, the samples were etched using Kroll s reagent of 2% HF, 6% HNO3 and 92% H2O for microstructure examinations. To characterise the laser parameters, the number of pulses per spot, pulse energy and fluence were calculated. The number of pulse per spot were calculated using the following equation [34] NOP = p.r.f d v (4.5) Where p.r.f is the repetition rate of Hz, d is the beam spot size in mm and the v the scanning speed in mm/s. The pulse energy was calculated from the measured average power, Pav, and the set pulse repetition rate, p.r.f, using: E = P av P.r.f The laser fluence was calculated by dividing the pulse energy in J with the beam spot area in cm 2. Hence: (4.6) Fluence = Energy / Beam area (4.7) Throughout this chapter, the error used for the measurements of depth and width are measurement errors taken to be half the unit of the measurement of the optical microscope that was ±0.5 µm. 85

86 4.3 Results Kerf width and ablation depth After the experimental runs mentioned in 4.2, the samples were placed under the optical microscopes for the measurement of the kerf width on the surface of the samples. The measurement process and details are as illustrated in the following Figure 4-2. (a) (b) zones Heat affected scan path of laser kerf width Process: Line ablation Parameters: 12.9 µj, 100 passes, 4810 mm/s Figure 4-2: Kerf width measurement, laser machined profile top view, (a) 2d representative and (b) measurement of kerf width and heat affected zone 86

87 Kerf Width (µm) Kerf Width (µm) A comparison of kerf widths at 2.4 MHz and 19.2 MHz pulse repetition rates at different average powers is shown infigure 4-3. It can be seen that the kerf width increases up to a maximum of 151 µm at the 2.4 MHz p.r.f with an average power of 132 W. The best fit curve for the data on the kerf width results were logarithmic trends. The kerf width increases with increasing number of pulses. At higher pulse repetition frequency (p.r.f) referring to Figure 4-3(a), the width at 19.2 MHz the general trend shows increasing width with increasing number of pulses especially for high power of 132 W. On the other hand, the widths at 90 W are wider than that at 132 W. In Figure 4-3(b) it was observed that there was no measureable width for 62 W and 90 W for about pulses for 19.2 MHz p.r.f (a) Number of Pulses 55W 62W 64W 90W 132W Log. (55W) Log. (62W) Log. (64W) Log. (90W) Log. (132W) (b) 62W 90W 132W Log. (62W) Log. (90W) Log. (132W) Number of Pulses Figure 4-3: Titanium Alloy kerf width vs. No. of Pulses (a) 2.4 MHz and (b) 19.2 MHz 87

88 Kerf Width (µm) Kerf Width (µm) The effect of fluence with a number of pulses per spot per pass was presented in Figure 4-4. for pulse repetition rates of 2.4 MHz and 19.2 MHz respectively. The number of pulses per spot was differentiated here by different scanning rates (a) 32 pps/p 62 pps/p 125 pps/p Linear (32 pps/p) Linear (62 pps/p) Linear (125 pps/p) Fluence (J/cm 2 ) (b) 250 pps/p 500 pps/p 1000 pps/p Linear (500 pps/p) Linear (1000 pps/p) Fluence (J/cm 2 ) Figure 4-4: Titanium alloy kerf width with different number of pulses per spot per pass vs. Fluence (a) for 2.4 MHz and (b) for 19.2 MHz Referring to Figure 4-4 (a) at 2.41 MHz, the variation of width with increasing fluence can be represented by an linear trend line for number of pulses from 32, 62 and 125 pulses per spot per pass. This variation indicates that as number of pulses increases, 88

89 width increases in a linear manner. Predictable material removal in the form of width can be made possible in the form of linear trend (up to 125 pulses per spot). For the case of 250 pulses per spot at 19.2 MHz shown in Figure 4-4 (b), no data for fluences under 0.05 J/cm 2 indicates that the material removal mechanism at a lower number of pulses requires fluence higher than 0.05 J/cm 2. The width increase to a max at 0.04 J/cm 2 achieving µm for 500 pps/p and µm for 1000 pps/p then reduce at 0.06 J/cm 2 fluence achieving only µm and µm kerf widths for 500 pps/p and 1000 pps/p respectively. The reduction in recorded width as recorded for higher p.r.f may be due to the deepening of the ablated site. This will be analysed in the following section for ablation depth measurements Ablation depth The depth of the ablation was measured initially using a confocal microscope with a measurement error of ±0.5 µm. The depth was further verified by cutting cross sections of the samples and measuring the depth of the grooves by the 100 line passes. The schematic and micrograph of cross sectional shapes of machined grooves is shown in Figure 4-5. (a) (b) Cross Sectional Area Depth Cross Sectional Area Process: Line ablation Parameters: 12.9 µj, 100 passes, 4810 mm/s Figure 4-5: (a) Laser machine side view and (b) Laser machine micrograph view 89

90 Depth (µm) Depth (µm) The result of varying pulse repetition frequency on the total ablation depth is plotted in Figure 4-6. These figures show the depth against a total number of pulses for different power settings. The plots are for two p.r.f settings, 2.4 MHz and 19.2 MHz. Both were done in air and r.t.p. conditions. The results show that with an increasing number of pulses, the depth increases linearly. This holds particularly true especially for results at 2.4 MHz with the depth of up to 254 µm achieved at 132 W and pulses. On the other hand, at the 19.2 MHz p.r.f, with a 132 W of power, as the number of pulses increases, the depth achieves a maximum of about 249 µm for pulses but maintains the depth to 246 µm and is better represented by a polynomial line. The ablation depths with the lower p.r.f of 2.4 MHz offer higher ablation rates at a muchreduced total number of pulses compared to 19.1 MHz. This was due to the increase in fluence at lower p.r.f compared to a high p.r.f (a) 55W 62W 64W 90W 132W Linear (55W) Linear (62W) Linear (64W) Linear (90W) Linear (132W) Number of Pulses (b) 62W 90W 132W Linear (62W) Linear (90W) 50 Poly. (132W) Number of Pulses Figure 4-6: Final ablation depth vs. total no. of pulses for p.r.f (a) 2.4 MHz and (b) 19.2 MHz 90

91 Depth (µm) Depth (µm) Figure 4-7 shows the depth measured for the depth acquired by 100 over scans against varying fluence used for different number of pulses per spot (p.p.s.). The results show that generally as fluence increases, depth increases and were better represented by a logarithmic trend line. For one case shown in Figure 4-7 (a), at 125 p.p.s., there was a steady increase indepth of up to about 255 µm that equates to 2.55 µm per pass with fluence of 0.45 J/cm 2. On the other hand, at low p.p.s. there were only minimal changes to the maximum depth achieved for 32 pps/p and 62 pps/p of 23 µm and 85 µm respectively. All experimental runs were done in air and at r.t.p. (a) pps/p 62 pps/p 125 pps/p Log. (32 pps/p) Log. (62 pps/p) Log. (125 pps/p) (b) Fluence (J/cm 2 ) 250 pps/p 500 pps/p 1000 pps/p Log. (500 pps/p) Log. (1000 pps/p) Fluence (J/cm 2 ) 0.1 Figure 4-7: Titanium Alloy depth per pass vs. fluence (a) for 2.4 MHz and (b) for 19.2 MHz 91

92 Depth can be logarithmic predicted at higher number of pulses. At pps of 250, a minimum fluence of 0.06 J/cm 2 was required to initiate ablation. However, at much lower fluence, depth of up to 0.11 µm per pass was achieved with a fluence of only 0.03 J/cm 2 but at a higher pps/p of This lower fluence initiation of material removal hints a laser material reaction of the nonlinear type. The measured experimental data shows that for higher number of pulses, interaction with the ablated particles, plume dynamics and plasma should be taken into consideration[175]. However, at higher p.r.f, high energy, interaction may leads to plume transparency due to the cascading effect of incoming laser pulses [104]. Furthermore, results from Figure 4-7 (b) shows that lower ablation depth was achieved for higher p.r.f compared to lower p.r.f at Figure 4-7(a). From these results, it can be seen that higher fluence and higher p.r.f were desirable for achieving high ablation depth. This was true for pulse per spot number at lower values because at higher p.p.s. values, other interaction components ensues. Another particular observation was the influence of laser fluence, where only fluences above a certain critical value allow noticeable ablation depths. Do note the error bars used for this data are of the measurement error of ± 0.5 µm Cross-sectional Evaluation Effect of scanning speed After analysing the width and the depth with varying number of pulses and fluences, another important laser parameter to be analysed is the laser scanning speed for scanned laser processing. Table 4-3 shows the cross-sectional profiles of the laser-ablated samples in air with varying scan speeds. The samples were laser machined in air to better simulate processing regimes and results for in air laser processing. The following cross sectional profiles were produced at a p.r.f of 4.8MHz, 62 W resulting in a laser pulse energy of 12.9 µj per pulse, resulting in 105 mj/cm 2 for three scan speeds of low, moderate and high values at 2400 mm/s, 4800 mm/s and 9500 mm/s respectively. It is worth noting that the approximate laser off time at this p.r.f was 208 ns. 92

93 Table 4-3: Depth and HAZ variation for different scan speeds and 12.9 µj pulses for Ti6Al4V alloy Low Speed (2400 mm/s) Moderate Speed (4800 mm/s) High Speed (9500 mm/s) Pulse Energy 12.9 (µj) 50 µm 50 µm 50 µm Number of pulse per pass Total Number of Pulses Width (µm) Depth (µm) HAZ (µm) Ratio of Depth to HAZ Depth per pulse (nm) The trend shows that the number of pulses influences the depth linearly for the material removal. As number of pulses increases the depth and width increases due to absorption of the laser pulses. At 105 mj/cm 2, the fluence is low enough not to initiate nonlinear effects at 4.8 MHz that incur only 208 ns of off time after every pulses. Figure 4-8 shows the top surface optical microscopic views of laser scanned Ti6Al4V at three different speeds. Observe the reduction in the width of the kerf as the scanning speed increases. 93

94 (a) (b) (c) 100 µm 100 µm 100 µm Figure 4-8: Titanium alloy top view variation for different scan speeds and 12.9 µj pulses for Ti6AL4V alloy (a) 2400 mm/s, (b) 4800 mm/s and (c) 9500 mm/s Effect of p.r.f. In this section, two top views of the maximum depth recorded for this study was shown in Figure 4-9. In one case as shown in Figure 4-9 (a), it can be noted that at 9.2 MHz and 132 W, with only about 109 ns of off time, some spatter was seen and possible surface oxidation and/or nitriding was observed from the bluish black discoloration as the experiment was performed in air and at r.t.p.. spatter 100 µm melt 100 µm Figure 4-9: Top view for parameter 132 W at (a) 9.2 MHz and (b) 2.4 MHz and scanner speed of 2400 mm/s. In a separate case as shown in Figure 4-9 (b), with 2.4 MHz p.r.f and 132 W, with more off time of 406 ns the discoloration, the bluish black colour was extended further at the edges of the kerf. This hints on two factors, first being the fluence was increased compared to Figure 4-9 (a). Secondly, this was experimental evidence that high energy pulses and high powered lasers with high p.r.f encounter plasma. Furthermore, high p.r.f. also results in prolonged plasma lifetime and will induce incubation effects. 94

95 (a) (b) 50 µm Figure 4-10: Cross-sectional view with laser power 132W at (a) 9.2 MHz and (b) 2.4 MHz and scanner speed of 2400 mm/s. Deepest depth was achieved in Figure 4-10 (a) at the following parameters; µj pulse energy with approximately 104 ns cooling time before next pulse resulting in pulses. The total measured depth was 315 µm. The next deep groove Figure 4-10 (b) was about a quarter of the number of pulses with pulses but nearly four times the pulse energy at µj and about four times the length of cooling time at 415 ns. The depth achieved for these parameters was µm. The difference in depth was 19% lower for low p.r.f. The results show that more material removal can be achieved at high p.r.f albeit lower pulse energy. This can be due to heat accumulation together with plasma aided machining as has been reported by Breitling et al. and Bulgakova et al. [138, 139]. Plasma interactions usually involves high temperatures, this explains the spatter observed at the entrance of the scan shown in Figure 4-9(a) at 9.6 MHz p.r.f. 95

96 Total Depth µm 4.4 Laser-Material Interaction Ablation Threshold, Optical Penetration, Absorption Coefficient and Thermal Loading In this section, the ablation threshold analysis from plots of depth per pulse against the laser fluence will be presented. Figure 4-11 shows the effect of fluence on total depth. The Figure 4-11 shows that as number of pulses increases, the minimal increase in fluence causes high increases of depth PPS 62 PPS 125 PPS 250 PPS 500 PPS 1000 PPS Log. (32 PPS) Log. (62 PPS) Log. (125 PPS) Log. (250 PPS) Log. (500 PPS) Log. (1000 PPS) Fluence J/cm 2 Figure 4-11: Effect of fluence on overall ablation depth for different pulse per spot per pulse. In Figure 4-11, the as number of pulses increases, the logarithmic trend line shows a steeper gradient and a lower x axis intercept. This shows that as number of pulses increases, there was a possibility of reduction in the fluence threshold. This can be due to several factors such as plasma coupling, pulse cascading and incubation effect. 96

97 Depth nm/pulse Depth per pulse Log. (Depth per pulse) y = 4.85ln(x) R² = Fluence (J/cm 2) Figure 4-12:Ablation depth per pulse vs Fluence for Ti6Al4V machining using 132 W PS laser The results of the plot are presented in log natural x axis for fluence in Figure above thus showing the other derived components of the Beer-Lambert s equation as shown in equations 4.1 mentioned earlier. The x-intercept from figure 4-12 defines the threshold fluence. The linear line equation shown below will be used: d = f(x) = m. ln x + c (4.8) Where d is the depth per pulse, m is the resulting gradient of the line and c is the y-intercept. The equation is then related to Beer Lamberts equation in 4.1. The process is shown below: (4.9) The gradient is: (4.10) 97

98 When d = 0, referring back to 4.9 and 4.10, the threshold fluence, FT is calculated from the following equation: c = 1 α ln F T (4.11) Both m and c values can be calculated from the resulting linear equation 4.8 from the plot. From the equation, the values of optical penetration 1/α and FT can be derived. Table 4-4: Summary of Ti6Al4V ablation study with 1064 nm Ps laser Ti6Al4V 1064 nm ps Ablation Study Unit Fluence Threshold (mj/cm 2 ) 45 Optical penetration (nm) 4.85 Thermal loading coeff.(kj/cm 3 ) Absorption Coefficient (cm -1 ) 2.06 x 10 6 The ablation threshold for the high pulse repetition rate, high power 1064 nm wavelength ps laser is 45 mj/cm 2. This value is lower than as reported for metals elsewhere [95, 117, 155]. Mannion et al.[176] reported the ablation threshold for Ti was J/cm 2 for 150 fs laser pulses at a wavelength of 775nm. Siegel et al. reported the threshold fluence to be 61 mj/cm 2 [34] for Ti when a 12 ps laser with a maximum operating power of 6 W operating at a 50 khz repetition rate was used. The ablation threshold reported in this study is lower compared with nanosecond pulsed laser ablation, for example, an ablation threshold of 700mJ/cm 2 was reported byyue et al.[118, 173] using a 248 nm wavelength Excimer laser with a 15 ns pulse width on Ti6Al4V. However, it is to be noted that the other studies reported in depth for a point spot of their respective samples. Furthermore, the lower threshold fluence may be due to the high p.r.f and the higher power of the laser system used. 98

99 4.5 Discussion Width and Depth The effects of laser fluence on width may be influenced by several factors; one of which was the interaction between the laser beam with vapour plume and or plasma [139, 144] especially at high powers such as 132 W and from having higher repetition rate. Furthermore, heat accumulation enhances heat conduction to bulk material and increases temperature [ ] hence aiding the process of vaporisation of the ablated sites. On the other hand, at higher pulse repetition rates, reduced ablation was observed. This can be due to several factors, first, being the fluence which was not sufficient to initiate material removal. Another, at high pulse repetition rates, plasma can initiate shielding and blocks the oncoming laser pulses from reaching the target [180]. Apart from plasma shielding, elevated temperature due to high repetition rate machining may lead to the formation of spatter and recasts from resolidified melt that was harder to laser machine as melts have higher reflectivity. Another would be due to presence of oxide layers after consecutive pulses that are harder to remove. Even at 90 W, heat accumulation initiates material melting. This effect was similar to incubation effect found in previous studies [117, 178, 181, 182]. The analysis of depth shows that the depth increases with increase in fluence. The depth variation does not represent linear effect but satisfies the condition for Beer Lambert s ablation. The condition to satisfy the Beer Lambert's equation was that the depth of penetration is directly proportional to the absorbance of the laser pulses Further analysis for ablation threshold and absorption coefficients for different p.p.s. A comparison of the ablation threshold with different pulses per spot (p.p.s.) was made to compare the results of the ablation threshold, optical penetration, thermal loading and the coefficient of absorption shown in Table 4-5. The results show a reduction in the ablation threshold fluence as the p.p.s increases from 62 to The threshold fluence reduces from 77 to 32 mj/cm 2. This was similar to other studies that reports on incubation of pulses lead to deepening of the profile and reducing the threshold fluence [178, 182, 183]. 99

100 mj/cm2 Table 4-5: Comparison of ablation thresholds and optical penetration for varying pulse per spot Pulse per spot per pass Fluence Threshold(mJ/cm 2 ) Optical penetration (nm) The effect of incubation can be characterised by the following equation[155]: F th (N) = F th,n. N S 1 (4.12) Where Fth is the threshold fluence, N is the number of pulses and S is the incubation coefficient where factor values in the range of 0 to 1 were considered softening and above 1 was considered hardening. A limitation of this model was that when a number of pulses approach infinity, the fluence threshold goes to zero, and that was practically impossible. When equation (4.12) was used, the value of S was calculated to be 0.6. Next, effect of increasing number of pulses on ablation threshold was shown in Figure 4-13 below. From the equation, when a number of pulses approach minimum, say 1, the threshold fluence is 384.7mJ/cm 2. Conversely, threshold fluence approaches 0 at pps y = x-0.4 R² = 0.93 Fluence Threshold Power (Fluence Threshold) Number of pulses per pass Figure 4-13: Effect of increasing number of pulses on fluence threshold for Ti6Al4V machining using 132 W PS laser system 100

101 Plasma distinguishing times as reported by Zeng et al. [184] could be a factor in influencing the material removal rates. Plasma shockwave can be detected up to 30 ns and was dependent on the energy of the pulse and the pulse width. After initial shockwaves, the process was followed by an atmospheric breakdown and then mixed with material ejection in the form of vapour. However, the plasma interaction subsides exponentially after 200 ns[58]. In these sets of experiments, for p.r.f of 2.41 MHz, the pulses were spaced at 415 ns compared to p.r.f. of 19.2 MHz, where the pulses were only spaced at 52 ns that was only 20 ns less than the reported plasma shockwave detection. Furthermore, the separations between the ps laser pulses were 208 ns and 104 ns for 4.81 MHz and 9.62 MHz respectively. At higher p.r.f, the interactions of plasma with the oncoming pulses were unavoidable. However, material removal was still significantly increased at higher repetition rates although heat accumulation was a major factor for the mechanism as shown in section For optical penetration, the penetration length decreases as number of pulses per spot increases. This was probably due to the effect of plasma absorption where at large number of pulses, penetration depth suffers hence reduces in length significantly. The longest penetration length was at 125 pulses per spot at 9.59 nm Application: 6 mm cutting of holes on 1 mm thick Ti6Al4V and 3.5 mm thick Al7075 Cutting of holes of 6 mm using the ps laser in air as means to eliminate the need and the effect of processing gas. Due to the small beam diameter of 125 µm (as stated by manufacturer), the drilling was made using multiple concentric rings to aid the material removal. All the machining experiments were made in air and using a galvo scanner as described earlier Multipath machining Trepanning is a common practice for drilling high-quality holes using a laser beam [66, 67]. Another strategy is the use of multipath[185]. A combination of multipath and trepanning drilling was used for the 6 mm drilling study in this research. A total of 10 rings equally spaced at 50 µm were used. An experimental investigation on the effect of p.r.f, the number of passes and scanning speed was made and results analysed and 101

102 reported. Figure 4-14 and Figure 4-15 shows the effects of operating parameters on a cpti alloy and Al alloy (Al7075) respectively. Figure 4-14: 1mm thick cpti Alloy ps laser 6 mm drilling Figure 4-15: 3.5 mm Al Alloy (Al7075) ps laser 6 mm drilling Effect of p.r.f In Figure 4-16, the plot of depth versus p.r.f shows that the optimal processing regime was towards the low p.r.f of 0.5 MHz and 1.1 MHz. After trial runs, the chosen laser parameters were 21 W for Titanium and 40 W for Aluminium with a speed of 1500 mm/s and 100 overscans. The cooling time for 0.5 MHz and 1 MHz are 2 µs and 0.9 µs respectively. This pulse separation allows significant time for cooling of vapour and plasma plume before the arrival of the next pulse.this procedure was again done in air and at r.t.p. Furthermore, at low p.r.f, the pulse energies were higher. The equivalent pulse energies were 42 µj for Titanium whereas it was 80 µj for Aluminium. Higher 102

103 Depth µm pulse energy was used for Aluminium, in this case, to anticipate the effect of twice the reflectivity of Aluminium alloys at about 0.9 for Aluminium and 0.45 for Titanium[167] Processing parameters: 21 W for Ti (below) and 40W for Aluminium alloy ( above) Scanning speed 1500 mm/s 100 over scan Frequency Mhz MHz Ti 21W Al 40W Figure 4-16: Effect of pulse repetition rates on depth MHz The result shows that increase in p.r.f; the ablation depth decreases. The decrease was because, at a higher p.r.f, fluence and pulse energy reduces. Furthermore, the increase in p.r.f reduces the cooling off times between the pulses and may lead to vapour and perhaps plasma interaction as mentioned earlier. However, incubation of pulses was reported to have a softening effect but was not observable at low powers of 21 W and 40 W. The maximum removal depth for scan speeds 1500 mm/s is observed at lower repetition frequencies for both at 40 µm and 140 µm for cpti alloy and Al alloy respectively. 103

104 Ablation µm Effect of number of passes Next, in Figure 4-17, the effect of depth acquired with increasing number of pulses was presented Processing parameters: 21 W for Ti (below) and 21W for Aluminium alloy (above) Scanning speed 1500 mm/s number of passes 21 W Titanium 21W Alu Number of Passes Figure 4-17: Effect of number of passes on depth number of passes From this graph, it can be estimated that to drill a 1 mm thick Ti alloy, it would take 2500 repetitions just to get through. The set parameters are 21 W of power, 500 khz p.r.f and scanning speed of 1500 mm/s. Aluminium alloy at 21W nearly 70 µm depth is achieved for 100 passes. Supposedly, for a through drill on a 3.5 mm thick Aluminium alloy, the number of passes is calculated to be 5000 passes. 104

105 Ablation µm Effect of Scanning Speed Figure 4-18 shows the reduction in achievable depth when the scanning speed is increased. The power used is 21 W and p.r.f of 500 khz scanning speed Processing parameters: 21 W for Ti (below) and 21W for Aluminium alloy (above) p.r.f : 500 khz 100 over scans Speed mm/min 21W Ti 21W Alu scanning speed Figure 4-18: Effect of scanning speed on depth The reason for this reduction in depth with increasing scanning speed was the reduction in a number of pulses being deposited per unit area, and this determines the accumulated energy per area. Therefore the energy deposited will be reduced hence reducing the ablation depth acquired Comparison of cutting 1 mm thick CpTi and 3.5 mm thick Al7075 It should be noted that the fluence used in the experiment was well above the fluence thresholds reported in the ablation study earlier for Ti alloy. For these experiments, fluence used for the cutting was 0.36 J/cm 2 for cpti cutting and 0.43 J/cm 2 105

106 for Al7075. The value of fluence was close to the optimal fluence of 0.4 J/cm 2 reported by Lopez et al.[179] for machining Aluminium, Copper and Molybdenum with a laser of 10 ps pulse width. Hole Entrance and Exit The entrance and exit of the laser-cut hole are shown in Figure 4-19 and Figure The machining of these holes with the 10ps 1064 nm laser at a repetition rate of 500 khz allow for enough cooling time between pulses to reduce the effects of plasma shielding and avoid plasma induced material removal that were thermal in nature. The result produces desirable entrance and exit qualities with no observable discoloration for cpti. (a) Entrance (b) Exit Processing parameters: Fluence: 0.36 J/cm 2 ) p.r.f : 500 khz scan speed: 1500 mm/s 2500 passes Figure 4-19: Titanium alloy cutting(a) at entrance and (b)and exit side 1mm thick (a) Entrance (b) Exit Processing parameters: Fluence: 0.43 J/cm 2 ) p.r.f : 500 khz scan speed: 1000 mm/s 6200 passes Figure 4-20: Al7075 alloy cutting(a) at entrance and (b) exit side Al7075 alloy 3.5 mm thick Figure 4.21shows that there was a formation of taper with the ps laser cutting. It was suspected that the Gaussian beam interaction contributes to the formation of the taper. The absorbed laser energy at the walls were at an angle that was depicted by the equation Fθ = F sin θ where F was the acting fluence and θ was the angle of taper. The taper angle formed was 73 Ο, for pulse energy of 44 µj. To reduce taper, one method was 106

107 to increase the pulse energy by using higher powers and a combination of high power and low repetition rates. However higher fluences may lead to undesirable effects such as HAZ formation, hardening, melt formation and rough surfaces[124]. For Ti and Al alloy, hardening to Ti may be caused by a formation of alpha case Ti,whereas, for Al alloy, the tendency for formation of oxides of aluminium would lead to substantial reduction in the effectiveness of laser machining due to their properties that were tougher for laser machining. Processing parameters: Fluence: 0.36 J/cm 2 ) p.r.f : 500 khz scan speed: 1500 mm/s 2500 passes 73 o Figure 4-21: SEM graph of cross-sectional view of the cpti 1mm thick sample cut with the ps laser parameters: Cutting of 1 mm depth for a 6 mm hole can be made possible with the following 2500 passes (Approximately 45 minutes) for 1 mm thick cpti alloy Depending in the depth achieved at 1250 passes, mm focusing in Z direction Scanning speed 1500 mm/s Dwell 1 second 22W, 44μJ 0.36 J/cm 2 with 41 Pulse per spot The operating parameters recommended for the cutting of Aluminium alloy are at laser fluences in the range of J/cm 2 at 0.5 MHz. 107

108 Processing parameters: Fluence: 0.43 J/cm 2 ) p.r.f : 500 khz scan speed: 1000 mm/s 6200 passes 76 o parameters: Figure 4-22: AL7075Alloy cutting 3.5 mm thick SEM graph of cross-sectional view 3.5 mm thick cutting of the 6 mm hole can be made possible with the following 6200 passes (Approximately 2 Hour) for 3.5 mm thick Al7075 alloy Depending on the depth achieved at 1500 passes,- 0.5 mm focusing in Z direction Scanning speed 1000 mm/s Dwell 1 second 26W,53 μj Max 0.43 J/cm 2 with 62 Pulse per spot Table 4-6: Properties of CpTi and Al7075[167] CpTi Al7075 Density (g/cm 3 ) Thermal Conductivity (W/mK o ) at 20 o degrees Melting Point ( o C) Vaporisation Point ( o C) (Al) Absorptivity The cpti holes showed better quality than the Al alloy. The Al alloy is harder to drill partly because the cpti alloy has a higher absorptivity compared to Aluminium alloy. Also, the Al alloy has a higher thermal conductivity. Also noted was the lower melting point of Aluminium at 635 o C that helps with the ease of material removal compared to more energy needed for cpti alloy at 1655 o C. The optimum laser fluence for the Ti alloy cutting was 0.36 J/cm 2 and for Al alloy was 0.43 J/cm 2 with a satisfactory 108

109 edge, circularity and overall quality but a presence of taper. The taper could be reduced with reducing pulse repetition frequency resulting in increasing the pulse energy. The fluence threshold as mentioned in section of 45 mj/cm 2 was the minimum required fluence for Ti alloy and this could be the bench mark for laser processing. Multipath Spacing From the results of numerical and graphical solution validation experiments as mentioned in section 4.4.1, to maximise material removal rate, the average power should be high, the scanning speed should be low and pulse repetition frequency should be kept low. To improve material removal rate, multiple parallel track (ring) approach was investigated. In this study, the scan speed was set at 1000 mm/s, with pulse energy of 80.4 µj resulting in fluence of 0.66 J/cm 2. The fluence is calculated with the following formula: Fluence = Pulse energy (J) Beam Area (cm 2 ) (4.13) The beam spot size is 125 µm as specified by manufacturer. A 6 mm hole was to be drilled with spacing between the centres of the two adjacent rings in the multi rings of the experiment which were 25 µm, 50 µm, 100 µm, 150 µm and 200 µm. The number of passes was 2000 times. There is total of 10 multi rings for this study. The material used for this investigation was a 1 mm thick CpTi alloy cut into smaller pieces for ease of mounting to the microscope with a width of 20 mm and length of 50 mm. 109

110 Figure 4-23: Depth for different spacing in 10 x multipath trepanning of cpti alloy As shown in Figure 4-23, the maximum depth achieved was at the 25-micron spacing. The depth reduces, as the spacing of multi rings increased larger. At 150 µm and 200 µm, a separation of the multi rings can be seen. Micrographs of the top profile are presented in Figure 4-24, showing the edges of the multi-ring machining. Separations of the multi rings were observed as corrugated features at 100 µm and more pronounced as the spacing increases to 200 µm. 110

111 Processing parameters: Fluence: 0.66 J/cm 2 ) scan speed: 1000 mm/s 2000 passes Figure 4-24: Different spacings in 6 mm multipath trepanning of cpti alloy (a) 25 µm, (b) 50 µm, (c) 100 µm, (d) 150 µm and (e) 200 µm The effect of increasing width of multi rings shows that there was more material being removed in the width direction. However with closer spacing, the depth acquired was deeper than the larger spaced rings. Trade off was needed when justifying the spacing as larger spaced rings shows discontinuity of the machined trench forming 111

112 corrugated channels as shown in Figure 4-24 (c), (d) and (e) with 100, 150 and 200 µm spacing respectively. Optimal spacing of rings should be more than 25 µm but less than 100 µm for a manufacturer specified beam diameter of 125 µm. The number of rings would increase the overall width of the multi-ring machining. Surface properties of ps laser machining of CpTi To understand the effects of multipath processing regime, a separate pocket machining process detailed below was made to analyse the post processing surface properties using EDX, to show if there were any oxides or nitrides present for high pulse energy, low scanning speed, and only 1 pass execution. Here we present the results of pocket milling using the ps laser on cpti alloy. The parameters used for these experiments are as follows: Scan Speed: mm/s Repetition Frequency: 102 khz Power: 9 W 40 µm spacing 1 Pass figure below: The size of the pocket machined and machining path directions are shown in the St Fi Figure 4-25: Pocket milling strategy to study the surface modifications As shown in Figure 4-25, the square pockets was made in a way that the laser switches off at the end of one path and goes back to the start line with a displacement vertically (y-axis) of 40 µm. The final box dimensions were 5 mm x 5 mm. Elementally, the contents of a cpti is approximately 99% Titanium[167]. Below are the results of Quanta 200 Electron Discharge X-ray (EDX) microscopy for the quantitative surface analysis. The results are arranged accordingly to their respective laser parameters and are compared with an untreated surface: 112

113 Table4-7: Processing parameters for analysing surface properties effect of scanning speed. Run Repeat(Over scan) Scan speed (mm/s) P.r.f (MHz) Table 4-8: Results for effect of scanning speed on surface properties Cpti Run Element Weight (%) Atomic (%) 1 Untreated Ti K Ti K Ti K Ti K Ti K (a) Untreated (b) Processing parameters: Power 9 W p.r.f : 102 khz scan speed: 100 mm/s 1 pass Figure 4-26: (a) EDX result, untreated, Pocket milling strategy to study the surface modifications and (b) EDX result, with 9 W, 102 khz p.r.f and scan speed of 100 mm/s, 113

114 Figure 4-27: Pocket milling strategy to study the surface modifications (a) untreated, (b)100 mm/s, (c) 250 mm/s, (d) 500 mm/s and (e) 750 mm/s while other laser parameters are kept constant with 9 W, 102 khz p.r.f and 1 pass only. It is worth to note that the electrical conductivity of CpTi is 1.82 x 10 6 Siemens /m [186]. The images of the SEM are affected by this low electrical conductivity. 114

115 From the results shown above in both Table 4-8 and Figure 4-27, there were no observable changes in the composition of the surface of the cpti in both untreated and upon exposure to pulse energies of up to 88 µj. This results supports the previous studies where laser parameters are optimised, ps laser machining was possible with negligible heat affected zones[54, 187]. In this particular case, the processing did not oxidise the cpti surface due to couple of factors. First, the pulse width too was short for oxidation to take place. Furthermore, processing done here was shown to be vaporisation and/or ablation dominated, using high pulse energy of 88 µj. This can be shown by the SEM pictures in Figure 4-27 showing no evidence of melts. Another reason is, there is insufficient oxidation taking place. This was due to the fact that the machining was only done with one pass. Therefore, insufficient energy deposited to initiate oxidation. While it was done in air, cpti being heat resistant hence does not gain enough temperature to react with the atmosphere. Adding to that, the consecutive pulse only happens at about 9.8 µs. This allows enough time for the plume to disappear before the next pulse hits. For oxidation to take place, interaction with plasma, the sample surface and atmosphere can generate elevated temperatures that may cause oxidation in laser machining. The pocket machining processing here was done in the air and at r.t.p. without the assistance of any pressurized inert gas or vacuum type setup while still producing undetectable oxidation levels. 4.6 Conclusion The present work demonstrated the characteristics of using high power, high pulse repetition rate picosecond pulsed laser on Ti6Al4V. The ablation threshold was found to be 45 mj/cm 2 and the absorption coefficient was 2.06 x 10-6 cm -1. The ablation threshold was further reduced to 32 mj/ cm 2 when multiple pulses of 1000 p.p.s. were used and were suspected to be attributed to the incubation coefficient. The incubation factor should be taken seriously as a surplus number of pulses would lead to heat accumulation and thus reducing the quality of the machining. It was desirable to use high power ps laser at lower repetition rates and high pulse energies. The high fluence, pulse energy enables the material removal through the vaporisation mechanism. However careful care has to be taken with high p.r.f as can result in nonlinear plasma interaction. The timing at which the consequent pulses hits was crucial for the coupling of the energy after the earlier pulses. The lower p.r.f of

116 khz results in 2 µs of off time offering ample time for the ablation plume, plasma lifetime and incubation effect to be minimised. Machining of titanium in air can be made with multiple path method. For cutting 6 mm, multiple rings of up to 50 µm spacing for beam diameter (specified by manufacturer of 125 µm). Successful cutting of 1mm sheet of Ti alloy is made with the following parameters: 2500 passes, Scanning speed 1500 mm/s, Dwell 1 second22w, 44μJ, 0.36 J/cm 2 and 41 pulses per spot. This results in a through hole with 73 o taper. The taper could be reduced with higher pulse energies. Furthermore, there are no observable elemental changes when the following parameters are used; Scan Speed: mm/s, p.r.f: 102 khz, Power: 9 W, 40 µm spacing with 1 Pass. No oxides are detected for the laser parameters. This was owed to relatively low pulse energies and low p.r.f. 4.7 Summary of Chapter 4 In this chapter, a study on characterising laser interaction of the 132 W picosecond pulsed laser on Ti6Al4V in normal atmospheric and r.t.p. conditions. Their consequent experiments was conducted, analysed and presented. This led to the contribution of the ablation threshold of 45 mj/cm 2 and other properties such as optical penetration, thermal loading and absorption coefficient values for Ti6Al4V for 10 ps 132 W operated at 1064 nm wavelength, high pulse repetition frequency laser was reported. The effect of increasing number of pulses was also reported for the incubation analysis. To further understand the interaction of the ps laser on the Ti alloy, further work on ps laser is to be conducted in the following chapter (Chapter 5), by the method of beam focus positions, with the aim of studying the effect of laser parameters on improving material removal with ps laser. 116

117 Chapter 5 Focus shift in high power, high repetition frequency picosecond laser materials processing 5.1 Introduction In picosecond laser materials processing, it was commonly assumed that the laser beam focal position does not change. This may not be the case with high average powered and high repetition rate picosecond lasers. Understanding this phenomenon and identification of conditions under which the laser beam focus shift occurs are very important in high power picosecond laser machining of materials. This chapter presents, for the first time, the basic phenomena of the laser beam focal shift to up to 15 mm from the normal focal position during high averaged power picosecond laser machining of Ti6Al4V alloy. 5.2 Experimental Methods In this investigation, an Edgewave 1064 nm, Innoslab Nd:YVO4 laser (Model: SN476) with the10 picosecond pulse width as mentioned in Chapter 3 section 3.2.1was used. The focused laser beam diameter was measured using two methods, first is the mentioned referred to in Chapter 3 section , including utilising a commercial Spiricon LBS 100 beam profiler referred in Chapter 3 section The maximum power for this sensor was 1 W. The profile of the beam was then presented by LBA-FW beam profiler software. Resolution of the camera was 1600 x 1200 pixels at 4.4 μm per pixel. Due to maximum power cap limitation of the sensor, a method of measuring the diameter by analysing an ablated site from laser pulses at varying z positions was employed. The details of this method were explained in Chapter 3 section Prior to firing the pulses, the Ti alloy sample was sprayed with a layer of graphite enough to cover the surface. Then the impressions as a result of the pulses were measured against the z position from the sample surface. 117

118 Sample Sample Sample Figure 5-1: Illustration of focal position where Z = 0 was the standard laser beam focal plane on the workpiece surface, Z = -ve was when the focal lens moved towards the workpiece surface, and Z = +ve was moving the focal lens away from the workpiece surface. Ti6Al4V sheets of 1 mm thick were used and their properties can be referred to in The titanium alloy was prepared in small pieces of 20 mm x 50 mm for ease of placement under the scanner for processing. The width and depth of the machined grooves were measured by imaging the cross sections of the samples. The process was shown in Figure 5-2. Each sample was cut into 4 pieces. Three pieces were used for cross-sectional measurements and one for surface characterisation. The material preparation includes grinding using SiC paper with up to 1200 grit and then polished using a synthetic polishing cloth with a diamond paste of up to 3 μm. Figure 5-2: Sample processing for measurement of width, depth and cross-sectional area 118

119 Measurements of width, depth and cross-sectional area were made using the Keyence VHX 500 optical microscope. Figure 5-3 illustrates the measurements taken of the kerf width, the depth and the cross sectional area. The measurements are repeated three times for three sites and the results tabulated. (a) Laser machined profile top view Cross Sectional Area (b) Laser machine side view Figure 5-3: (a) Laser machine top profile and (b) Laser machine cross-sectional profile The laser parameters used were 24 W, 500 khz p.r.f and a scanning speed of 1000 mm/s. The groove scanning was of 100 mm in length and was repeated 100 times. The parameters gave 48 μj pulse energy, resulting in a fluence of 0.44 J/cm 2. The fluence used was 10 times more than the threshold fluence while maintaining a high p.r.f but below the incubation effect mentioned in The scanning speed results in 58 pulses per spot.these parameters were ideal to measure the effect of material removal rates with varying z positions. Initial trials at negative ( -ve ) z positions shows no observable removal rates beyond -10 mm below focus hence these laser scans were performed from z = -10 mm into focus to z = +39 mm above the sample surface. This suggests that the material interaction were dominant when the focus was placed above the sample surface. Results of width, depth and material removal rates were then plotted against the z positions. 119

120 5.3 Results Beam profiling and diameter The qualitative 2D and 3D beam profile graphs from the beam profiler are shown in Figure 5-4 and 5-5. When referring to section 5.2 Experimental methods, the resolution of the camera was 1600 x 1200 pixels at a rate of 4.4 μm per pixel. However, too few pixels were being employed at the focused position. The reason for having too few pixels was the use of multiple power attenuation filters used to attenuate the power of the beam power so as not to exceed 1W threshold of the sensor. Do note that the author does realise the tool used was not the right one for the job. However, results of the beam profiling were taken as qualitative representations of the beam profiles to find out the region of beam focus position. Validation and final beam diameter measurements at focus position ensue with a different method elaborated later in the following section. From the profiler experiments, the focused distance from the objective lens to the sensor of the beam profiler was measured to be 203 mm. This coincides with the rated effective working distance for the galvo scanner from Hurryscan Labs at 204 mm. Figure 5-4 below represents the variation of the beam diameters and 2d profile with different z positions. The focus position was at z = 0. In figure 5-4, higher intensity peak and smaller beam diameter are observed at z = 0. The beam profiles gradually reduce in their peak intensity and enlarge in their width as shown at -20 and +20 from z- focus plane position. 120

121 low peak z = +20 mm larger area (above surface) (above surface) highest peak smallest area z = 0 mm (on surface) (on surface) low peak larger area z = -20 mm (below surface) (below surface) Figure 5-4:2D Beam profiles for different z positions from z = +20 mm above to z = 0 mm and to z = -20 mm below-focusedposition (all units in mm) 121

122 low peak z = +20 mm (above surface) (above surface) highest peak z = 0 mm (on surface) (on surface) low peak z = -20 mm (below surface) (below surface) Figure 5-5: 3D Beam profiles with different Z positions from z = -20mm below, to z = 0 mm and to z = +20 mm with respect to the focused position Figure 5-5 above shows the 3D profiles of the beam at different z positions concerning the beam profiler focused position. As a general trend, the peak reduces in height and then increases in diameter as z moves away from 0 mm the focused position 122

123 Diameter µm in both the positive and negative directions. Again this was only taken as a qualitative presentation of the beam intensity and diameter profiles with varying z positions in both the positive and negative range. The smallest beam diameter and highest peak profile and its respective position with respect to z axis aids in validating the focused position measurements by pulsed ablation method. Then, a method of measuring of three ablated spots on a graphite sprayed alumina was done with varying z positions as mentioned in Chapter 3 section The three ablated spots were measured using an optical microscope for their width. Their measurements were averaged and the averaged values of the measured width were then plotted against the corresponding z positions as shown in Figure Theoretical Measured Z Position from Surface (mm) Figure 5-6: The comparison between the theoretical beam diameter and experimentally measured diameter The comparison of the actual measured diameter and the theoretical diameter calculated along z axis was also shown in Figure 5-6. The measured diameter at z = 0 mm was measured to be 118 µm and the beam diameter grows to about 260 µm at z = +15 mm above the sample surface. On the other hand, at -ve z positions, the measured diameter was smaller than the one measured for positive z, only up to 217 µm at z = -20 mm below the sample surface. On the other hand, the minimum theoretical beam diameter[37] for a 1064 nm wavelength laser, passing through a focal lens f of 330 mm and the collimated beam diameter before focusing d = 3 mm (as stated by manufacturer) was given by the following equation as mentioned in 2.8: 123

124 d min = 4fλ πd = mm (5.1) The theoeretical beam diameter was calculated to be 149 µm. A deviation of 21%, due to the efficiency of the beam delivery system. Delivery system as there may be losses from the reflecting mirrors and the lens hence affecting the beam throughput power. Furthermore there may be losses in absorption of the pulses interaction with the graphite layer. Where, λ is wavelength, M 2 is the beam quality and mode. M 2 for this laser was 1.2. Then, theoretical propagating beam diameters with respect to the focused beam diameter (w0 for this case is dmin) with a focusing lens f of 330 mm can be determined geometrically with respect to the z position using the following equation 5.2: w(z) = w λz z is the distance from the focal plane position or the sample surface (-ve towards (5.2) the lens and the convergent side or +ve away from the lens and the divergent side) or in another word, the position of the focused beam waist. The graph of measured beam diameter and the calculated theoretical beam diameter were presented in Figure 5-6. Another important beam optical characteristic was the Rayleigh range and can be deduced from the equation 2.9. Furthermore, the depth of focus is the range of the beam propagating axis, where the intensity of the beam maintains within a range of 5% of its focused value given by the following equation referred to 2.7. Therefore beam specification summary can be tabulated in Table5-1 as follows: Table 5-1: Summary of beam properties Item Value M λ 1064 nm z range for experimental study (mm) -20 to +20 Focused beam diameter from measurement µm Theoretical Beam diameter Dmin 149 µm Rayleigh Range, ZR 11.4 mm Depth of Focus, DOF 3.64 mm πw /2 Next, graphical presentations of width, depth and volume removal rates with respect to the z positions are presented in Figure 5-7 below. It was observed that there was an increase in the width and depth of the ablation lines up to a maximum of

125 μm for the kerf width and μm for the depth, at z = +18 mm above the sample surface. However, classical laser material interaction suggests that maximum removal rates were achieved at the beam focus with a reduction in width and depth when beam was fired away from the focal plane position. From figure 5-7 it indicates that, at z positions of -10 to 0, no material removal was observable under cross-sectional observation. The reason for this may underlie the actual cause of the shift in machining efficiency at z = +18 mm above the sample surface. A few suspects were plasma, ablation plume interaction, thermal lensing causing shift in focus position, and optical effect of plasma with the beam causing self-focusing and filamentation. However these effects have their own particular minimum requirements for happening and this will be studied further through this chapter. Another observable conditions was that there was no recorded measurements of width and depth at z = 0 mm while this was actually the focused position apart from there were no observable and recorded measurements of width and depth for z < 0 positions. One of the reason could be the lack of depth and width observable through the method of cross sectional measurement using the cross sectional optical microscopic method. An example of the cross-sectional measurement was shown below in Figure 5-8. The maximum material removal depth and rate occurred at a laser beam position at 15 and 16 mm above the sample surface with a maximum material removal of 0.73 mm 3 /min. This suggests that effective material removal regime was shifted away from the laser focused position during processing. This point will be further verified later in this chapter and the proceeding chapter through varying laser parameters and processing regimes and analysing methods. 125

126 Removal rate mm 3 /min Depth µm Width µm (a) Width Position from surface of sample Z(mm) (b) Depth Position from surface of sample Z(mm) (c) Removal Rate Position from surface of sample Z(mm) Figure 5-7: Results of (a) width, (b) depth and (c) removal rate in mm 3 /min measurements for Ti6Al4V againstz-axis position in mm on the surface of the sample. (Laser parameters are 24 W, 500 khz p.r.f and a scanning speed of 1000 mm/ repeated 100 times) 126

127 (a) (b) Figure 5-8: (a) The cross-sectional micrograph for z = +5 mm and (b)the cross-sectional micrograph for z = +15 mm In Figure 5-8, a comparison of the cross-sectional views of the laser ablated samples between the scanned lines at z = +5 mm above the sample surface and z = +15 mm above the sample surface was presented. From the micrographs it was evident that scanning at z = +15 mm above the sample surface has a wider and deeper groove than that when scanning at z = +5 mm above the sample surface. The values of the width and depth at these positions are 76 µm and 18 µm for z = +5 mm and 107 µm and 31 µm at z = +15 mm. 5.4 Discussion The results show significant increase in removal rates as machining position z was moved further above the sample surface. The maximum removal rate occurs when z was 15 mm above the surface. Then, a drop in material removal rate then occurs when z position was more than about 18 mm. The difference in material removal rates when compared at z = 0 and z = +15 mm may be due to several possibilities. First, as the focusing lens moved away and above the sample from the focus plane, the diameter of the beam increases. This leads to a reduction in fluence and the peak power intensity as pulse energy stays the same at 48 µj. Due to the widening of the beam diameter whilst maintaining the pulse energy, more surface area produces better area for material removal of the material during ablation. These results in better material removal rate, as shown by the increase of removal rate from only 0.29 mm 3 /min at z = +5 mm above the sample surface to 0.72 mm 3 /min at z = +15 mm above the sample surface. There was an increase of 2.48 folds in removal rate 127

128 for a z position change of a factor of 3 in the positive direction (away from the focal plane). In terms of the increase in area for particulate removability, the increase in width from 181 µm at z = +5 mm above the sample surface to 259 µm at z = +15 mm above the sample surface resulting in fluence of 187 mj/cm 2 for z = +5 and fluence of 91 mj/cm 2 for z = +15 respectively are still acquiring fluences exceeding the ablation threshold fluence of 45 mj/cm 2 mentioned in Chapter 4 section This further supports Shille et al.'s findings that ablation can still take place outside the focal plane position provided high p.r.f is used and fluence is above the threshold fluence. Such defocused processing regimes have also been reported by Chang et al. [125, 126] for fs lasers showing 10 µm defocusing offers better material removal rates. However, for these experiments, at ve z position, the material removal rate and depth does not show any variation as the distance between the focusing lens and the focus plane decreases. This suggests that the material removal mechanism only occurs after the focused position, below the plane of focus, hence suggesting that it may be cause by several factors such as self-focusing or filamentation, thermal lensing or plasma effects. Therefore, to understand the effect of varying removal rate with z positions, further experimentation on the effects of power and scanning speed is to be made Effect of z on material removal with varying powers Further investigation is done for the effect of power on material removal rates for different z positions from z = -15 mm to z = +15 mm from the sample surface. The laser parameter ranging from 5 W to 40 W are chosen to investigate the effect of power to material removal rate while the p.r.f, number of passes and scanning speed are kept constant at 1 MHz, 1000 over scans and 1000 mm/s. In figure 5-9, it was shown that the maximum material removal rate occurs at different positions of z for different powers. For example, for 10.5 W, the maximum removal rate occurs when the focus was at z = +5 mm above sample surface. At 19.1W, maximum material removal rate then shifts to z = +10 mm above sample surface. For 19.1 W and 40 W the maximum material removal rate occurs at z = +15 mm. 128

129 Z Position (mm) Volume Removal rate (mm 3 /min) W 10.5 W 19.1 W 31.3 W 40 W Z Position (mm) Figure 5-9: Effect of power on material removal rate for different positions of Z positions above the sample surface At low powers, maximum material removal rate occurred near the beam focused position as shown in 5W and z = 0 mm above sample surface. This effect of maximum material removal rate shows the influence of pulse power was to be factored in when designing a ps laser processing system Corresponding z with max removal (mm) Poly. (Corresponding z with max removal (mm)) y = 0.71x x - 1 R² = Power (W) Figure 5-10: Effect of z position with maximum removal rate with power. The effect of maximum material removal rate for different z positions are shown above in Figure The maximum removal z position shifts as power increases. A 129

130 Volume removal rate (mm 3 /min) polynomial trend line best represents the position of z in mm above the sample surface with varying powers Effect of z on material removal rate when varying scanning speed The effect of scanning rate was further studied when power, p.r.f and number of over scans are kept constant at 40 W, 1MHz p.r.f and 1000 respectively. The scanning speed was within the range of 1000 to mm/s mm/s 3000 mm/s 4000 mm/s 5000 mm/s 6000 mm/s mm/s Z from sample surface (mm) Figure 5-11: Effect of scanning speed and z position on the material removal rate Figure 5-11 shows the optimal material removal rate also depends on the scanning speed and the z-position from the surface of the sample apart from being influenced by varying power. The trend shows that at 1000 mm/s the maximum material removal rate occurs at z = +15 mm above the sample surface and 3000 mm/s the maximum removal rate position in z reduces to z = +10mm above the sample surface and at higher speeds, the z position reduces down to the z = 0mm above the sample surface at 10000mm/s. By varying the scanning speed, the number of pulses acting onto an area changes. The effect of maximum material removal rate with scanning speed may be due to the interaction of the beam pulses with the ablated plume and perhaps plasma. Better material removal rate will be observed when minimal ablated plume and/or plasma interaction takes place. This can be done by moving the beam away from the recently ablated area spot quickly (such as the case of high scanning speeds) avoiding ablation 130

131 plume, plasma and shockwave interaction with oncoming pulses. At mm/s, the effective number of pulses per spot is 12 pulses per spot with 1 µs of cooling time between pulses Effect of fast scanning speed on material removal rate with varying z position Focusing on the effect of scanning speed, the above phenomenon of having maximum material removal rate at z positions further than the focus on sample surface however did not occur when the scanning speed was higher. This was demonstrated with 44 W, 1MHz p.r.f scanned at 10 m/s at different z positions with 1000 overscan. Table 5-2 shows the effect of better machining efficiency when the laser beam was focused near the target surface (i.e. z = 0) compared with when z = +15 mm above the sample surface when scanning speeds were very high, in this case 10 m/s. The consequent reason for this may be the number of pulses per spot area was lower at the focus position than when the beam was at out of focus position. Referring to Figure 5-6, the beam diameter was 259 µm at z = +15 mm above the sample surface, resulting in number of pulses per spot area of about 26 pulses for the scanning speed of 10 m/s. This number reduces further at z = 0 mm above the sample surface where the 118 µm beam size results in only 12 pulses per spot. A factor of half the pulses per unit area improves the material removal rate shown in the cross sectional area by 10 times. These results further suggest the influence of the plasma interaction and ablated particles play a role in the efficiency of material removal [142, 188]. 131

132 Table 5-2: Effect of z position at high-speed scanning of Ti6AL4V Position z = 0 mm above sample surface z = +15 mm above sample surface Energy(µJ) Scanning Speed (mm/s) Diameter (µm) NOPPSPP Number of overpasses Top View Angle view Cross Section Width(µm) Depth(µm) CSA(µm 2 )

133 5.4.4 Effect of slow scanning speed on material removal rate with varying z Effect of scanning speed can influence the number of pulses acting on a specific area along the scanning line. On the other hand, at a lower scanning speed of 500 mm/s, the ablation depth was higher when the laser beam was focused at z = +15 mm above sample surface compared with that at z = 0 as shown in Table 5-3. The increase in depth by about five folds from 21 µm to 98 µm at z = 0 mm above sample surface and z = +15 mm above sample surface respectively shows that at slow scanning speed of 500 mm/s the depth much improved at z = +15 mm above sample surface. When taking into account the beam diameter change from 118 µm to 258 µm at z = 0 mm above sample surface and z = +15 mm above sample surface respectively the number of pulses per unit area was calculated to be 234 and 518 pulses per spot. It was apparent that the particulate ejection, ablated particles plasma, shockwave interaction plays a significant role in controlling the ablation efficiency. The secondary effects from the interaction of the ablated plume, plasma were the optical focusing and filamentation of air[189]. The improvement at out of focus machining of depth and removal volume differences was 5 times and 6 times respectively. This was unusual as the depth and volume removal rate should be higher at near focus position than that further away from the focus. However, another area of investigation would be in the subject thermal lensing through slab heating and distortion of the optical properties [190]. This could be discussed further in the following sections. 133

134 Table 5-3: Effect of Z position on low-speed scanning of Ti6AL4V Position z = 0 mm above sample surface Z = +15 mm above sample surface Energy(µJ) Scanning Speed (mm/s) Diameter (µm) NOPPSPP Number of overpass Top View Angle view Cross Section Width(µm) Depth(µm) CSA(µm 2 )

135 5.4.5 SEM backscatter results for groove machining at different z positions The use of SEM imaging was done with high magnification and backscattering mode to observe different micro structural changes from machining with varying z positions. The SEM used here was a Quanta 200 equipped with EDX and backscatter mode. The laser pulses used were at 44 W fired at pulse repetition rate of 1MHz, and the scanning was done to a 1 mm thick Ti6Al4V with 500 mm/s scanning speed and repeated 100 times. The resulting pulse energy was 44µJ and 250 pulses per spot. Figure 5-12 shows a cross section and top features of the ablated area. The lighter diagram in the inset of each cross-sectional picture as well as the high magnification top topography shows the backscatter images There were no observable variation from the backscatter images as shown in Figure 5-12 from the cross sectional observation. Furthermore, when using high magnifications of the top profile observation, micro ripples are observed. These micro ripples are only present at z positions above the focused position at z = 0 mm above sample surface. The micro ripples formation are also called as laser induced periodical surface structures as reported by Schille et al [191] can be observed from Figure 5-12 at z position + 4 mm from the surface of the sample. 135

136 Figure 5.12: The SEM and backscatter images for cross section and high magnification for different z. Laser parameters: 44 W, p.r.f. of 1MHz, 500 mm/s scanning speed and repeated 100 times 136

137 The Figure 5-13 below shows backscatter images from SEM to show the difference in depth between z = +4 and z = +16 mm above sample surface. Figure 5-13:Backscatter SEM comparison for z = + 4 and z = + 16 mm above sample surface Figure 5-14: Volume removal rate for different z positions at 44W, 1MHz repetition rate and 500 mm/s with 100 repetitions. Figure 5-14 shows the increase in material removal rate as z was moved away from the focus with 44 µj pulse energy when scanning speed was 500 mm/s resulting in NOP. The improvement between the material removal at the focus and Z = +16 mm above sample surface was 500% improvement. 137

138 5.4.6 Study the effect of 15 seconds ps laser exposure of 1MHz p.r.f at 44 W with ps laser on ceramic with varying z. Figure 5-15 below are micrographs of ablation sites resulting from a static beam fired onto a ceramic tile for 15 seconds. Previously, experimental data were for line ablation method where the scanning speed determines the number of pulse deposited to a specific area along the line. In this case, the stationary beam was fired for a period of 15 seconds to observe the effect of width and depth of ablation sites for different focus positions. The powers used are 44 W with 1 MHz p.r.f where cycle duty before each pulse was1µs. Figure 5-15: Micrographs of 15 seconds exposure with static ps laser pulses on ceramic tile. The micrograph in Figure 5-15 shows that the impression of the diameter reduces and gets narrower as the z position was away from the focused position. When beam diameter broadens, the effective area increases. This happens provided that fluence was still above the threshold limit; the effective area of width should increase with increasing z. 138

139 Width (µm) Depth (µm) Width Depth Z Position (mm) Z Position (mm) Figure 5-16: Measurements of width and depth for 15 seconds 1 MHz p.r.f 44 W pulses exposure. When referring to Figure 5-16, the results show that as z gets further away from the focus (z = 0) the width decreases and depth increases. This should not be the case. Ideally, as z moves away from the focus position, the width should increase due to broadening of the beam, and depth should be decrease due to reduced intensity per unit area. The procedure to determine focus position at z = 0 mm above sample surface were done prior and this experiment shows that the exposure time plays a role in the resulting width and depth at different z positions away from the focused position. Therefore, it was not just the effect of laser beam intensity on the target surface; rather it was shifting of focus position that results in the change in effective power at different z. However, in support of this findings of off focus material removal efficiency, Shille et al. [192] demonstrated that there was an increase in ablation depth when focusing up to 1 mm away from the focused position as long as there was sufficient repetition of laser pulses and the laser fluence is kept well above the threshold fluence. In contrast to the current findings, Shille's findings shows better efficiency when the beam area increases whilst maintaining minimum fluence and p.r.f to trigger ablation hence resulting in better depth pointed to thermal coupling or incubation effect. Rather, the current case may point out to the interaction of the plume, plasma and shockwave whilst also supporting the incubation and thermal effect. Further plasma diagnostics could determine the underlying effect of increased material removal at different z positions. 139

140 5.4.7 Shifting maximum material removal rate: Thermal lensing? Thermal lensing is a thermal effect that can cause the change in the thickness of an optical medium by thermal expansion resulting in refractive index change [ ].This effect can happen at any lens or optical mediums such as the laser cavity, mirrors and focusing lens. For this particular case, the effect of thermal lensing of the cavity and mirrors setup of this ps laser was not considered as these were both cooled. However, thermal lensing were suspected for the focusing lens in this case the F-theta focusing lens that was located after the mirrors of the galvo system and was placed between the galvo and the processing site. Incoming laser Incoming laser Focusing Lens Focusing Lens Focusing length Focusing length after thermal lensing Shift in focus position Before thermal lensing After thermal lensing effect Figure 5-17: Illustration of thermal lensing of the lens resulting in the focus position effect. Refractive index can be separated into temperature and stress dependent variation as shown in equation (5.3)[193, 196]: n(r) = n0 + n(r)t + n(r)ɛ (5.3) 140

141 Where n(r) is the resultant refractive change, n0 is the initial refractive index, nt the refractive change due to change in temperature and nɛ is the refractive change due to distortions. The distortion is in a form of conduction, heating up the area and volume under the influence of the beam, hence inducing a 'bump' that leads to increase in thickness of this part of the lens and thus creating lens face distortion. The distortion increases the hyperbolic curvature of the focusing lens therefore increasing the focusing length. Suppose we take a lens face distortion of: b l(x, z) = σ y [T(x, y, z) T 0 ] dy (5.4) 0 Where σy = dn/dt and T is the temperature rise due to the absorption of the laser pulse in the room temperature and b can be the thickness of the lens undergoing distortion, supposing the temperature is evenly distributed for the length of b. Suppose the lens used are fused silica, with the properties as shown in the table below: Table 5-4: Properties of fused silica[197] Fused Silica Properties Value Absorption (ppm/cm) 2 dn/dt (10-6 K) 10 K (W/mK) 1.38 C (J/kg.K) 746 ρ (kg/m 3 ) 2202 α (10-6 K) 0.55 The resulting rise in temperature at surface (z = 0) by one pulse can be calculated by the following equation[198] when : T(0, t < τ) = 2αI 0(1 R) (Dt) k π 1 2 (5.5) Where α is the absorption coefficient, I0 is the laser pulse intensity, R is the reflectivity, k is the thermal conductivity, D is the thermal diffusivity and t is the pulse duration. If laser pulse intensity was to be calculated with the following laser parameters, 44 W,1 MHz p.r.f and 117 µm beam spot size and 10 ps laser pulse duration resulting in 41 GW/ cm 2. The resultant gain at the centre of focus temperature was 10 degrees at the peak of a picosecond pulse. When this temperature was calculated with the values in 141

142 Table 5-4 and using equation 5.5, the resultant l is calculated to be 0.5 µm shift in the distortion of the lens as shown in Figure When referring to the ceramic tile results shown in Figure 5-15, the time of the pulses acting was 15 seconds whereby the resultant number of pulses for the 1 MHz repetition rate results in 15 million pulses of 44 µj each resulting in a total accumulated energy of 660 J. With this, it was speculated that there would be enough temperature gain that can induce thermal lensing and was supported by the temperature rise calculated using equation 5.5 and change in length calculated using equation 5.4. However, due to the infinitesimal changes of the temperature and the small resultant change in refractive index of the focusing lens as used by the ps laser processing system, it was unlikely that thermal lensing can affect the focus position by as much as 15 mm. Due to this reason, the underlying problem may lie in the plasma interaction of the ablated plume with the oncoming laser pulses. 5.5 Conclusion There is sufficient evidence to show that there was an increased material removal rate when machining at a distance of z away from the focus position. An increase of 2.48 folds in removal rate for a z position change of a factor of 3 in the positive direction (away from the focal plane). Results show that at low scanning speeds such as 500 mm/s, maximum removal rate attained was at z = +15 mm above the target surface. However at high scanning speeds such as mm/s, the maximum attainable removal rate was at z = 0. The maximum removal rate is at a rate of 500% better at z = +15 mm above sample surface than z = 0 mm above sample surface for 44 µj pulses. At mm/s, the effective number of pulses per spot is 12 pulses per spot with 1 µs of cooling time between pulses. Another, the laser average power also influences the z position for achieving the maximum material removal rate. For 10.5 W, the maximum removal rate occurs when the sample is at z = +5 mm above sample surface then at 40 W, the maximum material removal rate occurs at z = +15 mm above sample surface. The gradual change in the z positions dictates that power influences the maximum removal rate z positions. 142

143 On the other hand, effect of self focusing and thermal lensing can be ruled out as the laser output conditions and parameters does not meet the minimum condition for them to be the main cause to deviate the focus position by as much as 15 mm. Then, material plasma interaction may also play a role in the beam focal position shift. Plasma interaction was not to be overlooked. The interaction of plasma with the oncoming pulse was now expected to be the underlying cause of the shift in the maximum material removal focus positions. To understand further the interaction, the of plasma[144] can be studied. In the next chapter, plasma diagnostics of high power, high p.r.f ps laser will be studied for pulse to pulse interaction together with burst and multiple pulsed modes. This was to evaluate plasma behaviour and associate the plasma properties with the move in z positions for maximum material removal rates. 5.6 Summary of Chapter 5 The effect of changing effective focus positions when machining with high power, high p.r.f ps laser systems have been demonstrated. For laser parameters of 40 W and 1 MHz p.r.f. the optimum z position was found to be 15 mm above the focused position/ surface of the sample. The causes were discussed and further actions have been planned. Further understanding of the plasma behavior and diagnostics could be investigated to evaluate the cause of the z position shift. The following chapter will deal with plasma characterisation to conclude the findings on the effect of z position for maximum material removal. 143

144 Chapter 6 Understanding the Transient Behaviour of Picosecond Laser Ablation of Ti6Al4V Using High-Speed Holographic Imaging 6.1 Introduction In previous chapters, the diminishing rate of material removal upon increasing pulses was reported. However, the efficiency of material removal can be improved by changing of the z focus position and increasing scanning speed resulting in reduced number of pulses per spot while maintaining high pulse energies. It was suspected that the diminishing returns were due to the interaction of multiphase ablated particles, plasma shockwave, plasma plume and the atmosphere with the oncoming pulses. In the past, study of material removal mechanism by ultrafast lasers has been investigated by several authors [199, 200]. Understanding the pulse to pulse study strategy helps to shed light on what happens between the pulses hence their resultant depth and morphology of ablated sites. On the other hand, application of high speed controllers allows for the use of tailored pulses [103] thus creating pulse to pulse strategies that were applicable to increase material removal efficiency. One of the pulse to pulse strategies was burst mode. This strategy was reported by several authors[102, 191] with promising outcomes for improving material removal efficiency. For this strategy to work, pulse timings were to be separated long enough to avoid plasma shielding but close enough for the purpose of preheating. Another study by Emmelmann et al. [201] reported that with the use of correct pulse time spacing, the effect of induced plasma coupling could increase material removal rate while still reducing the effect of HAZ. Bruening et al [92]have also reported the pre heating effect leading to higher penetration depth hence leading to better removal rate. On another note, studies on the effect of shockwaves, material ejecta and plume [59, 202]on metal substrates could prove valuable for ultra short laser pulsed processing with high p.r.f.. Apart from focusing on burst and tailored pulses, the effects of plume and plasma have to be taken into serious consideration to achieve improved material removal rates. 144

145 Laser absorption and strong heating Vaporisation stage Ablated particle expands Plume interaction with surroundings Ablated depth and melts Figure 6-1: Ablated particles and plasma stages in a high p.r.f ultrafast lasers interaction. Here is an important study in identifying the underlying causes of diminishing returns of high pulse repetition frequency picosecond laser was ensued. A state of the art high-speed holographic imaging was utilised in collaboration with Dr Krste Pangovski from the Centre for Industrial Photonics, Institute for Manufacturing, University of Cambridge. With this study, the importance of laser material interaction could be dissected into different key phases as such shown in Figure Experimental Methods Picosecond laser system A picosecond pulsed laser that could be referred to in Chapter 3 section 3.2 was used. The z-axis terminology used in this study was zero at focus plane, and this was placed at the top of the sample once the focus position was known as shown in Figure 6-2 below. The focusing at ve values of z would entail the focus position below the sample whereas the focusing values of z +ve would imply the focusing position above the sample surface (getting further away from the focusing lens). The method was adapted from the one detailed in Chapter 5. Laser shots were fired at one spot to study the effects of pulse bursts and effect of varying z positions. 145

146 galvo z galvo galvo z Focus plane Focus plane sample Focus plane on sample above sample Below sample Below sample z -ve Focus position On sample z = 0 Above sample z +ve Figure 6-2: Beam focal position and position of z with respect to the sample surface Ti6Al4V (Grade 5) sheets specifications can be referred to in Chapter 3 section 3.3. The pulse energy used for this experiment is 44 µj. This was used in combination of one, two and three shots with nominal ON time of 2 ms resulting in 2000 pulses per shot as shown in Figure 6-3. V Burst of pulses ms ramp 2 On 2 On ms time "one shot" ms ramp 2 Off Figure 6-3:Schematics of the burst mode The shots of pulses were to understand and evaluate the effect of multiple pulses on material removal rates with varying z positions. This out of focus study of multiple pulses has not been done before. The average laser power was 44 W, and the p.r.f. was 1MHz. In post processing, the depths of the ablated profiles were measured using a Keyence VHX-5000 equipped with Z-stage for 3D depth imaging as practiced and referred to Chapter 3 section The 3D depth measurement was using a high-speed 146

147 automatic depth from defocus method at the accuracy of up to 0.5 µm. The depth results were further verified by a Wyko white light interferometer for accuracy as mentioned in Chapter 3 section Surface morphology was examined using a Quanta 200 SEM of specification that can be referred to in Chapter 3 section Results and Discussion Micrographs of ablated sites Comparison of micrograph of 2000 pulses at different z position was presented in Figure 6-4. These shots were done in air to study the effects and processing regime in air and at room temperature and pressure (r.t.p) without the aid of expensive processing gas and vacuum or inert environment. (a) (b) (c) Figure 6-4:Microscopic images of sites with 2000 pulses at 44 µj per pulseand one shot at (a) z = 0 mm above sample surface, (b) z = +7.5mm above sample surface and (c) z = mm above sample surface. Laser parameters are 44 W and 1 MHz p.r.f and 2000 pulse per burst ps pulses shot at a rate of 1 million pulses per second with pulse energy of 44 µj on a Ti6A4Vsurface shows that when beam was focused of surface, the impression of ablated area was oval in shape. The oval shape was due to manufacturer's setup where the laser was found to be linearly polarised. The processing was done in air as to study the implications of using the high powered ps laser without any expensive processing gas. As z moves away from focus, the width increases showing divergence of the beam profile hence making the diameter larger as shown in Figure 6-4 (b) and (c) at z = +7.5 mm above sample surface and z = mm above sample surface. A shiny morphology at the centre shows melt regime and surrounded by dark discoloration of black, blue and purple for z = 0 mm above sample surface and z = +7.5 mm above sample surface. The melt from the centre settles around the edges of the crater whereas material discoloration was observed at the outer edges. Again, these pulses were fired in air and at r.t.p. hence 147

148 the interaction of the Ti alloy with the nitrogen in air through nitriding was not avoidable thus giving a slightly yellowish colour. Furthermore, the melt shows intensity I applied was I>Iv>Im (Intensity for vapourisation and intensity for melting). The morphology changes at z = +15 mm above sample surface where the centre and surrounding was bluish and blackish colour owing to possible heating, oxidation and the yellowish colour was due to nitriding. However the process was insufficient to cause material removal through melt expulsion resulting in a small bump. The small bump was measured to be 1.2 microns in height and was darker colour than the surrounding that was bluish in colour Determining the effect of multiple pulses with one, two and three shots by varying z positions. Figure 6-5 below shows the morphology of shots each having 2000 pulses with 44 µj subjected on Ti alloy surface to observe the effect of number of shots at varying z positions. The numbers of pulses for one shot were 2000 pulses, and for two and three shots are 4000 and 6000 pulses respectively. The pulses in a shot come at a repetition rate of 1 MHz. The shots were done manually and have the space of about 3 to 5 seconds between each shots interval. 148

149 2000 pulse per shot z position : 0 mm focus position z position : +7.5 mm above focus position z position : +15 mm above focus position one shot two shots three shots Figure 6-5:Microscopic images of sites with 2000 pulses at 44 µj and one, two and three shots at the corresponding focus position, +7.5 mm above focus position and mm above focus position. Laser parameters were 44 W and 1 MHz p.r.f. The second shot turns the centre of the crater from shiny to purplish blue after the second burst and turns to darker bluish tone after the third burst at sites where the shots are fired at focus position and was suspected to be due to interaction of the metal with air and hence oxidation. This suggested that at multiple burst, further development of oxides were observed. On the other hand, when observing the craters formed by laser pulses fired at +15 mm above focused position, the feature at the centre of ablated area turns from blackish to bluish colour after second and third burst with micro cracks visible. This can be due to resolidification of melt and melt flow dynamics[203] and a small material removal by melt expulsion taking place hence the formation of the micro cracks. Similar results were observed with SEM images in Figure 6-6 below. 149

150 z = 0.0 mm above sample surface z = +7.5mm above sample surface z = +15.0mm above sample surface Figure 6-6: SEM images of the craters of 2000 pulses per shot, one, two and three bursts at z = 0.0 mm, z = +7.5 mm and z = mm above the sample surface. Laser parameters are 44W and 1 MHz p.r.f. The results show the first shot was having significant depth compared to consecutive shots at z = 0 mm above sample surface. However, not much can be observed from the features of z = +7.5 mm above sample surface and z = +15 mm above sample surface even with multiple shots. A study with specified intervals of up to 100 of the shots mechanism should ensure to compare the difference between manually firing the shots and programmed shots execution. 150

151 Comparison for burst to burst interaction on ablation site For comparison purpose, another set of experiments was done to compare the pulse burst interaction of the ps laser system. This time it was done with one burst at a time in the form of a dwell mode in a program resulting in a hardware reaction time of 6 milliseconds between start of two executions. The reaction time was confirmed through measurement by measuring the signal going into and coming out of the laser system with a LeCroy WaveRunner 6100A1GHz Oscilloscope. The reaction time was 2 ms during execution with a separate rise and down time of 2 ms each as shown in Figure 6-7. This allows for an artificial modulation of the laser burst pulses. (a) shot 2 shot 3 (b) shot 2 shot 3 Figure 6-7: Illustration for comparison of time between executions for (a) un specified off time (long, about 5 seconds) and (b) 4ms offtime. Effect of high number of repeats of burst with 4 ms off time Once the modulation time was known, 2 ms ON ramp, 2 ms EXECUTION and 2 ms of OFF ramp altogether assembles a 4 ms off time between firing the bursts. With this modulation method, an experiment was again done with 44 W of power and 1 MHz p.r.f. The resulting pulse energy was 44 µj. The aim was to get controlled number of pulses with the limitation of the control systems resulting in shortest laser on time of 2ms and these results in total of 2000 pulses per shot. Below, the resulted SEM images for the 4 ms off time with 1, 5, 10, 20 and 100 shot repeats with one, two and three bursts with varying z focus at z = 0 mm above sample surface, z = +7.5 mm above sample surface and z = mm above sample surface are shown in Figure 6-8 below. This was to understand the effect of shot repeats and multiple burst at different z positions. This has not been done before with different z (mm) above the sample. 151

152 Figure 6-8: SEM images of 1,5, 10, 20 and 100 shot repeats and one, two and three bursts and varying z positions z 0 mm above surface, z +7.5 mm above surface and z mm above surface. Laser parameters were 44 W and 1 MHz p.r.f and 2000 pulses per shot 152

153 Depth (microns) In contrast with pulses coming in less shots (Figure 6-6), here Figure 6-8 shows morphology of many shot repeats. Each shot delivers 2000 pulses. The resultant time from start of first pulse to start of next pulse for this method was about 167 Hz with 4 ms cooling time between shot repeats. The crater here were more pronounced in depth toward more shot repeats and gets deeper with increasing number of burst as shown when comparing one burst and three bursts. The cooling time of 4 ms allows for the plume to dissipate and disappear before the next 2000 pulses come in. But within the shot, during the delivery of the 2000 pulses, the pulses are only having less than 1 µs of cooling time. Due to the artificial regulation of the shot repeats, deeper ablation crater can be achieved. Furthermore significant depth and change in morphology can be observed with multiple bursts of the shot repeats as shown across horizontally for all z axes Results for depth Comparisons of the depth measured for small number of shot repeat were presented in the figure below. These were depths done with 2000 pulses each shot at 44 µj each pulse energy with 1 MHz p.r.f. as shown previously in Figure 6-5. The delay time between shot are estimated to be up to 5 seconds. The delay time between pulses in a shot on the other hand was 1 µs and was controlled by the p.r.f z=0 z=7.5 z=+15 Linear (z=0) Linear (z=7.5) Linear (z=+15) Repeats of 2000 pulse Figure 6-9: The depth with different number of shot each having 2000 pulses at z = 0 mm above sample surface, z = +7.5 mm above sample surface and z = mm above sample surface. Laser parameters 44 W, 1 MHz p.r.f. and each shot delivers 2000 pulses 153

154 Depth per pulse (nm) Depth (microns) With increasing number of shots, there was significant gain in depth especially at the focus where z = 0 mm above sample surface attaining a maximum of about 20 µm depth after the third shot. Reduced ablation depth measured was observed for z = +7.5 mm above sample surface when compared with z = 0 mm above sample surface only attained 6 µm depth after the third shot. Furthermore, at z = mm above sample surface, no depth was recorded. Upon observation, features of resolidified melt showing a bump feature of height 1.3 µm after third shot was recorded. For multiple shots, below are the characteristics of the depth achieved for multiple shots of 2000 pulses. Below in Figure 6-10 (a) an increase in total depth measured for number of repeats and multiple bursts of up to 98.5 µm for 3 bursts of 100 shot repeats of 2000 pulses bursts compared to only 18.6 µm for 3 bursts of one shot repeat of 200 pulse per burst. (a) Burst 2 Burst 3 Burst Log. (1 Burst) Log. (2 Burst) Log. (3 Burst) (b) Shot repeats of 2000 pulses Shot repeats of 2000 pulses 1 Burst 2 Burst 3 Burst Log. (1 Burst) Log. (2 Burst) Log. (3 Burst) Figure 6-10: The (a) depth and (b) depth per pulse with different number of shot repeats and bursts for z = 0 mm above sample surface with laser paramaters of 44 W and 1 MHz p.r.f. and 2000 pulses per burst 154

155 These depth results in Figure 6-10 were the measurements of craters shown in SEM pictures in Figure 6-8.There was a significant decrease in depth per pulse as number of pulse increases as shown in Figure 6-10 (b).the diminishing returns were also applicable for burst mode shots of pulses. The difference between one, two and three burst have minimal effect in depth per pulse. The limitation of these results only shows depth and morphology of the ablated sites subjected to multiple pulses at once and repeated many times over with varying bursts. A method for pulse to pulse analysis could help understand further the dynamics of pulse to pulse interaction between the oncoming pulses, ablated particles and plasma formation for their effects in the material removal mechanism as mentioned in previous chapters Effect of using artificial shot repeat and bursts with varying z positions The increase in total depth measured for shot repeats and multiple bursts of up to 98.5 µm for 3 bursts of 100 shot repeats of each shot having 2000 pulses per shot compared to only 18.6 µm for 3 bursts of 1 shot repeat of 2000 pulse per shot shows that there was significant material removal with this method. However, the rate of returns with increasing number of total pulses shows significant presence of limitations for material removal. This could be related to findings from previous two chapters where significant reduction in gains as number or pulses increases was observed. To highlight this, a significant reduction of depth per pulse was calculated from 3.3 nm per pulse to 0.16 nm per pulse for 1 shot repeat with 3 burst and 100 shot repeat with 3 burst respectively.the reduced gain in ablation efficiency after the first shot indicates plasma plume interaction [140, 141, 204]. The reason for this was insufficient time interval between pulses inside the burst for the plasma development and lifetime to dissipate before the next pulses comes by. Although the pulse to pulse separation was 1 µs, the accumulated energies from the pulses could result in prolonging of the plasma and ablated plume life span that could initiate incubation effect or plasma coupling. To tackle this diminishing gains in material removal mechanism, findings by studies suggests that by reducing the number of pulses, the ablation efficiency increases[124] due to sufficient cooling, eliminating plasma interaction and avoiding incubation of pulses[178, 181, 182]. 155

156 Further study on pulse to pulse interaction between the pulses and the ablation plume and plasma was to be carried out to understand the pulse to pulse dynamics. The following section and results were done in collaboration with Dr Krste Pangovski, Centre for Industrial Photonics, Institute for Manufacturing, University of Cambridge Pulsed digital holography Holography imaging system was used to aid the study of the plume dynamics. This study was in collaboration with Dr Krste Pangovski, Centre for Industrial Photonics, Institute for Manufacturing, University of Cambridge for the application of the holography method. With this method, the underlying plasma and plume effects for pulse to pulse operation can be studied. The plasma effect as speculated in previous chapters and section could be further understood. Figure 6-11: Holography setup for studying ps laser material interaction of Ti6Al4V A similar setup of holographic imaging equipment as used by Pangovski et al [205] to analyse the laser material interaction and plume/ plasma dynamics was shown in Figure A reference carrier beam was split between a plain wave carrier O(x,y) and the reference carrier R(x,y). The beam recombined at the holography unit with the CCD sensor. With these setup interferences, fringes can be observed corresponding to the phase changes detected from the observation area. The interferences can be manipulated to analyse the phases and other measurable quantities such as refractive index. The 156

157 phases that cause refractive index change can be given by the following equation [206, 207]: φ(x, y) = 2π d [n(x, y, z) n λ 0 ]dz 0 (6.1) λ is the wavelength, n0 is the refractive index of the reference and n(x, y, z) n0 is the change in refractive index. To analyse this, the data of the phase must be put through an unwrapping procedures[ ]. The results of the unwrapping can be now used with (6.1) to determine the changes in refractive index and other measurement possibilities. The reference beam used for the imaging is a 532 nm laser with a minimum pulse width of 400 ps. The field of view (FOV) is dependent on the observational area. To suppress plasma emission and luminescence, a narrow band pass filter at 532 nm and FWHM 1nm is used. A pulse generator from Stanford Research Systems DG35 was used for triggering the processing and imaging systems sequentially. This allows for variation in the times of imaging to cater for the different pulse to pulse repetition frequencies. Minimum separation time can be adjusted from as small as 1 ms to a maximum of 60 s. Thorlabs DET10A photodiode measures the exact time between the peaks of the process pulse to the peak of the imaging pulse. With this setup, a sequence of holograms is acquired to produce the visualisation of the laser material interaction Holography technique Holography studies are based on changes in the electron density. This is by assuming that the change in refractive index was by the contribution of electrons. The electron density can be estimated by the following equation [211]: Ne(x,y) ~ m -1 λ -2 (1-n(x,y)) (6.2) Where the electron density is in cubic centimetres (cm 3 ) and the wavelength is in micrometres. The temperature distribution can be determined by Lorentz-Lorentz[212]: (6.3) T0 is the reference temperature where the refractive index of air is n0, P0(atmospheric pressure) = kpa (kg/ m. s 2 ) and Rc = J/K.mol is the molar gas constant and A is the molar refractivity of air. 157

158 Furthermore, the effect of ablation and material removal follows the Inverse Bremsstrahlung (IB) mechanism. The absorption length of a pulse through the plasma can be defined as [213]: I IB (x, y) = a c T e 3 2 ( N c N e (x, y)) 2 (6.4) Where Te is the electron temperature in electron volts (ev), Ne is the electron density, from (6.2), and critical plasma density Nc = cm 3 (λ=1064 nm). For the estimation of the absorption length, the plume temperature Tp calculated from (6.3) is assumed to be in equilibrium with the electron temperature. With this assumption, the residual pulse energy after propagating through distance z through the plume can be estimated by: z l I(z) = I 0 e IB (6.5) I0 is the incident pulse intensity and I(z) is the residual pulse intensity after propagating through a distance z through the plume. The IB characteristic absorption length ac is defined as ac = ZlnΛ in cm. Ze is an electric charge of metallic ions where e is the electron charge, Z is the charge number and lnλ being the Coulomb logarithm where Λ 15 is the plasma parameter [214]for many lasers produced plasmas. The study is for 1, 5, 10, 20, 100 pulses with one, two and three burst and varying z positions z = 0, z = and z = The pulses are initiated separately (pulse to pulse).this is done using a fast function generator Stanford Research Systems DG35[205].The pulses are at 44 W of power, and 1MHz p.r.f. resulting in 44 µj energy. Plume Results Plume results were compiled by the collaborator Dr Krste Pangovski and the corresponding images in figure 6-12 shows the plasma plume results for different number of pulses at different times corresponding to the number of pulses. The legend for coloration blue to green and to consequently yellow and red refers to the refractive index of less than 1. Furthermore the Field of View (FOV) is identified as of x = 1.2, y = The results (shown in Figure 6-12) of plume diagnostics shows the plume formation increasing in height as number of pulses increases from 1 to 100. Pulses below 100 at z = 0 shows complete plume disconnection from the sample surface and this may 158

159 be the cause for reduced efficiency in the depth measured. The disconnection of the plume can be observed for all burst numbers. The reduction in efficacy was because the incoming laser pulses were absorbed away from the sample surface by the plume. The plume then connects to the surface at 100 pulses resulting in better depth and material removal rate due to the suspected incubation, plasma coupling effect and the reheating effect. The event disappears under one microsecond to as sudden as 200 ns as been reported by König et al. [58]. Reduced effect of plasma was observed at all pulses when fired position was z = +15 mm above sample surface. This helps explains the results in chapter 5 where material removal was reported to be improved at out of focus position and were shown here due to minimal plasma interaction. The results here associates itself with the type of material removal such as the vaporisation only type of material removal as observed at z = +15 mm above sample surface and the melt and heat expulsion type of material removal as observed at z = 0 mm above sample surface. This observation may suggest that vaporisation material removal type yields better removal rates when using the scanning method as reported in previous chapters. On the other hand, the particle ejecta images presented in the following section can better associate between the plasma plume diagrams and the particle ejection. 159

160 t 0 s t 5 µs t 10 µs t 20 µs t 100 µs Figure 6-12: Plume results for Z = 0 mm, Z = +7.5 and Z = +15 (mm above sample surface) for 1, 5, 10, 20 and 100 pulses with one, two and three bursts. Each images are having field of view (FOV) of approximately x = 1.2 mm by y = 3.65 mm with sample surface and the focused laser plane placed at the bottom of the FOV. Laser parameters are 44 W and 1 MHz p.r.f. (Results courtesy from Dr Krste Pangovski) 160

161 Particle Ejecta Results The following Figure 6-13 shows the particle ejecta conditions for different number of shot burst with varying z positions at z = 0, z = +7.5 and z = +15(mm above sample surface). The figure shows the presence of particle ejecta for specific time corresponding to the number of pulses (shots). The Field of View of approximately x = 1.2, y = The focused plane and sample surface at the bottom of the FOV. t 0 s t 5 µs t 10 µs t 20 µs t 100 µs Figure 6-13: Particle ejecta results at z = 0, z = +7.5 and z = +15(mm above sample surface) for 1, 10, 20 and 100 shots with one, two and three bursts. Each images are having field of view (FOV) of approximately x = 1.2 mm by y = 3.65 mm with sample surface and the focused laser plane placed at the bottom of the FOV. Laser parameters are 44 W and 1 MHz p.r.f..(results courtesy from Dr Krste Pangovski) 161