EXPERIMENTAL STUDIES ON FIBRE LASER MICRO-MACHINING OF Ti-6Al-4V.

Transcription

1 EXPERIMENTAL STUDIES ON FIBRE LASER MICRO-MACHINING OF Ti-6Al-4V. A. SEN 1*, B. DOLOI 2, B.BHATTACHARYYA 3 1* PRODUCTION ENGINEERING DEPARTMENT, JADAVPUR UNIVERSITY, KOLKATA, INDIA, , abhishek.sen1986@gmail.com 2 PRODUCTION ENGINEERING DEPARTMENT, JADAVPUR UNIVERSITY, KOLKATA, INDIA, , bdoloionline@rediffmail.com 3 PRODUCTION ENGINEERING DEPARTMENT, JADAVPUR UNIVERSITY, KOLKATA, INDIA, , bb13@rediffmail.com Abstract Titanium alloy plays a significant role in the advancement of engineering in the area of advanced structures and technologies for aerospace and power industry, medicine, automatics and mechatronics, and measurement equipment, owing to its unique combination of physical, chemical and mechanical such as high strength and stiffness at elevated temperatures, high corrosion resistance, fatigue resistance, high strength to weight ratio and ability to withstand moderately high temperatures without creeping. The conventional cutting methods not only face difficulties for cutting these alloys due to their poor thermal conductivity, low elastic modulus and high chemical affinity at elevated temperatures but also undergo from higher cost associated with the machining of Ti-6Al-4V caused by lower cutting speeds and shorter tool life. The application of micro-grooves on Ti-6Al- 4V predominantly lies in the biomedical devices and the implantation into the bone which further needs to be integrated with the surround tissue for the bone healing. The aim of the present research work is to investigate the fibre laser ablated micro-grooves on Ti-6Al-4V of 1.1 mm thickness in atmospheric condition with different parametric combinations and also to find out the parametric effects on the groove geometry in terms of width, depth, and surface roughness. To understand the parametric effect of the geometry of the micro-grooves, various ranges of process parameters were considered for the experimental analysis such as (a) No of passes of 1 to 8; (b) scan speed of 40 to1000 mm/sec; (c) Pulse frequency of 50 to 100 khz; (d) average power of 2.5 to30w. Groove geometries were measured using optical microscope and the surface roughness was measured using AFM (atomic force microscope). Keywords: fibre laser, micro-groove, Ti-6Al-4V. 1 Introduction Ti-6Al-4V is used extensively in hip and dental implants for its low cytotoxicity and biocompatibility, corrosion resistance, wear resistance and fatigue resistance [1]. In case of aerospace industries, it is widely used in aircraft structural components, airframes etc. Both the aforementioned fields not only require complex micro features and components but also require stringent quality checks. While considerable amount of researches have been carried out of titanium alloys but still micro-machining of titanium alloy is considered in rudimentary stage. In most applications, Ti-6Al-4V implants have relied on surface roughening and porous coatings to improve osseointegration. The potential for micro-parts as medical tools operating at cellular level is increasing, research into the micro-machining of titanium alloys is gaining more interest now-a days. The challenge in machining titanium alloys is chiefly the high tool wear associated with the reactivity of titanium with tool materials and its low thermal conductivity. On the other hand laser cutting is a high-speed, repeatable, and reliable method for a wide variety of material types and thicknesses producing very narrow and clean-cut width. Laser beam achieve atomic motion when three conditions must, in general, be met: Firstly, threshold intensity is needed to initiate the process, generally a few MW.cm -2 ; Secondly, vibrational energy photon absorption or by multiphonon cascades following electronic excitation must be localized on a small group of atoms or a molecular size cluster in the laser-irradiated solid for longer than a few vibrational periods. Thirdly, the energy absorbed must be sufficient to initiate and sustain the breaking of bonds and the meso-scale motion of atoms, ions, molecules and clusters. Many researchers have studied the laser cutting performance of Ti alloys in different conditions. When these alloys are cut with oxygen assist gas, at even low pressure, the uncontrolled burning of cutting front starts that results wide kerf and poor surface quality. For air assisted laser cutting, the reaction of titanium with oxygen and 229 1

2 EXPERIMENTAL STUDIES ON FIBRE LASER MICRO-MACHINING OF Ti-6Al-4V nitrogen produces a thin layer of hard and brittle oxides and nitrides, and generates much thicker HAZ in comparison to that of nitrogen or argon. In order to overcome these problems titanium alloys may be cut by using inert gases such as argon and helium. Rao et al. have used nitrogen (N 2 ), argon (Ar) and helium (He) for the pulsed laser cutting of 1mm pure titanium sheet. Almeida et al. have performed factorial designed experimentation on Nd: YAG laser cutting of Ti and Ti alloy sheets. They found that use of nitrogen assist gas increases surface hardness from 2 to 3 times due to the formation of TiN while a mixture of He and Ar gases reduces the irregular edges and also eliminates the nitride formation. It has been reported that micro-milling of titanium alloy for medical applications especially for implants can create free-form surfaces, which can improve biocompatibility. Therefore, in order to produce functional micro-products, not only part features and tolerances have to be concerned but surface quality must be considered as well. Several approaches have been used in order to gain a control over the surface finish of micro-features; for example, process parameters optimization, surface generation modelling and simulation, effects of using lubrication, etc. Sun et al. [2] performed laser assisted machining trials on commercially pure (alpha) titanium alloy and found significant reductions in the cutting forces due to laser preheating and smoother surface finish due to the diminished fluctuations in the cutting forces. Similarly, Dandekar et al. [3] observed an increase in tool life up to a cutting speed of 107m/min during laser assisted turning of Ti 6Al 4V alpha beta titanium alloy. In this research work basic experimental studies have been carried out on fibre laser micro-machining of Ti-6Al-4V of 1.1 mm thickness in atmospheric conditions with different parametric ranges so as to identify the influence of the various process parameters such as pulse frequency, scan speed, no of pass, and average power during fibre laser micromachining of Ti-6Al-4V. 2 Experimental planning Laser micro-machining is based on the interaction of laser light with solid matter and it is very complex process. Laser beam energy is focused on the surface and the absorptivity not only depends on the material but also on the surface structure, the power density, and the wavelength. Lasers used for micromachining are characterized by short pulse lengths from the millisecond range for applications to the pico-second and even femtosecond area for ablation of metals. Laser beam micro-grooving process of Ti-6Al-4V can be characterized by pyrolithic process based on a rapid thermal cycle, heating, melting, and (partly) evaporation of the heated volume. In the recent years, fibre lasers have emerged as the most promising alternatives to the conventional solid state lasers and the merger between the most innovative and advanced technologies in the laser world active optical fibres and semiconductor diodes. In the present research, all the experiments were carried out on multi-diodes pumped Ytterbium doped fibre laser machining system of 50W, made by M/S Sahajanand laser Technology. The basic setup consists of an undoped cladding, a doped core of higher refraction (active medium) and 8 no of diodes for pumping. Ytterbium acts as the gain media. Light incidents on the boundary between core and cladding at angles greater than critical angle and undergoes a total internal reflection. Light rays are guided through the core without refraction into cladding. Mode of travel along the axis of the waveguide is constant with its characteristic propagation and group velocity. Due to its long interaction length i.e. 3 m and high surface to volume ratio it enforces nonlinear effects and avoids excessive heating. The laser head of the fibre laser is consisted of electric and galvo unit and from which the generated laser beam is focused on the work piece with the aid of F-theta lens of 100mm. The diode pointer assures the work piece is on the focal plane. The experimental setup of fibre laser is shown in the figure 1. The fibre laser system has a wavelength of 1064 nm and pulse duration of 120 ns. The mode of operation in which all the experimental works are carried out is pulsed mode. The description of the fibre laser machining setup is listed in table 1. The material used for this experimentation is titanium alloy of grade 5 i.e. Ti-6Al-4V of 1.1 mm thickness. The chemical compositions of Ti-6Al-4V are listed in the table 2. In the present study the aim was to investigate the influence of various process parameters on various responses such as width, depth and surface roughness. All the experiments were carried out keeping pulse width as 90% and in atmospheric conditions. Another important aspect was to reduce surface roughness and width as low as possible to use it successfully in various engineering applications. To reduce the detrimental effect of recyrstallizaion, longer pulsed widths have been used at a given pulse energy due to the lower peak power with longer pulsed width. The range of parameters was chosen from the previous research works and based on the results of series of pilot experiments. In this present experimentation, most of the experiments were conducted on 30W of average power. Total 49 of micro-cutting operations were performed in which total experimentation was divided into four groups, designed for four different parameters. Each group includes several series of micro-cutting parameters and the results are listed in table 3. For each series, a single type of parameter is changed with other parameters are kept constant so as to identify its effect on the width, depth and surface roughness of fibre laser ablated micro-grooves. The dimensions of the depth and width were measured using Leica optical microscope using 5X lens and surface roughness (R a ) of the micro-grooves was computed by AFM. After 229 2

3 the micro-machining the samples were cleaned with the ultrasonic cleaner for 15 minutes so as to reduce dust particles. characteristics to study the influences of parameters have been performed. 3.1 Influence of scan speed on width, depth, and surface roughness During pulsed fibre laser micro-grooving operation of titanium alloys, minimization of surface roughness, width and as well as increase in dimensions of depth of the micro-groove criterion are most important aspect for various fields of engineering applications. Therefore influences of the fibre laser micromachining parameters such as scan speed, pulse frequency, no of pass and average power on width, depth, and surface roughness phenomena during laser micro-grooving on Ti-6Al-4V have been analyzed. Figure 1 Photographic view of Fibre laser Machining System Table 1 Specification of the Fibre laser Specification Description 1.Laser type Pulsed 2.Wavelength 1064nm 3.Mode of operation Pulsed 4.Mode of Laser beam TEM 00 5.Beam Diameter 8.8-9mm 6.Laser beam spot 18-20µm(after focus) Diameter 7.Average Power 50W 8.Pulse width 120ns 9.Pulse Repetition Rate KHz 10.Output fibre length 3 meter Table 2 Chemical composition of Titanium alloy sheet (grade-5) Al Fe Sn V Ti 6.22% 0.187% 0.56% 3.35% 89.6% 3 Influence of laser process parameters on micro-grooving criteria Due to complexity of relationship between lasercutting parameters and the cutting quality, four parameters i.e. scanning speed, pulse frequency, average power, and no of pass were considered so as to find out the influence of fibre laser micro-grooving process parameters on various responses such as width, depth and surface roughness. The graphical software Origin was utilized to analyse and plot the three responses with respect to process parameters. The parametric analyses of the laser micro- grooving Fig 2 Effect of scan speed on width, depth and surface roughness Figure 2 shows the influence of scan speed on width, depth, and surface roughness on fibre laser ablated micro-groove surface while other process parameters were kept constant such as no of pass of 8, pulse frequency of 50 KHz and average power of 30 W. Increase in scan speed leads more material is melted per unit time during micro-grooving operation and higher mass flow rate is achieved at higher scan speed. Low scan speed produces high level of spot over lapping and continuous power density per unit length which gives complete micro-cutting with uniform smooth surface. But when the scan speed is increased at higher value of pulse frequencies, it produces low spot over lapping, discontinuous power density, and less time for melting the materials which results rough cutting. Thus surface roughness increases with the increase in scan speed. The decrease in the width and the depth show low power transmission to the work-piece through the fibre laser in terms of increase in the scan velocity. 3.2 Influence of pulse frequency on width, depth, and surface roughness: 229 3

4 EXPERIMENTAL STUDIES ON FIBRE LASER MICRO-MACHINING OF Ti-6Al-4V Figure 3 shows the influence of pulse frequency on width, depth, and surface roughness of fibre laser ablated micro-groove surface while other process parameters were kept constant as no of pass of 8, scan speed of 40mm/sec and average power of 30 W. Table 3 Results of the total 49 experiment Ex. No of passes Scan Speed (mm/sec) Frequency (KHz) Average Power (W) Width (µm) Depth (µm) R a (µm)

5 Pulsed fibre laser micro-machining operation provides a high instantaneous power for a period of pulse duration and followed with a period of power off. At high values of pulse frequency, peak power of the fibre laser beam is lower. So at lower value of pulse frequency causes excessive removal of material from the micro-grooved surface. Thus there will be considerable amount increase of width and depth as seen in figure 3. An increase in pulse frequency makes the laser closer to a continuous wave which impacts more power to the substrate. When the pulse frequency increases, changes in dimension of width and depth found to be smaller due to low heat generation. At higher values of pulse frequencies, the spot overlap increase which gives continuous power density and smooth cut surfaces. focal plane tends to increase depth, but it does not affect in the depth of the micro-groove as it would be. So after certain no of passes depth will increase but will also lead to decrease in the dimension of width. As the pulse width was kept constant for all the experiments at 90% and thus leads to low peak power and low rate of penetration. In the case of surface roughness it can be seen that it has a tendency to increase with no of passes due to over lapping phenomena. Fig 4 Effect of no of pass on width, depth and surface roughness Fig 3 Effect of pulse frequency on width, depth and surface roughness 3.3 Influence of no of pass on width, depth, and surface roughness Figure 4 shows the influence of no of pass on width, depth, and surface roughness of fibre laser ablated micro-groove surface while other process parameters were kept constant as scanning speed of 40mm/sec, pulse frequency of 50 KHz and average power of 30 W. It can be seen from fig 2 that width is increasing with the no of passes but after certain increment of no of pass it is gradually decreasing. As no of passes are increasing, the rate of penetration into the work piece will be more. Along with the increase in depth dimension, excess amount molten material will be removed from the surface. Thus depth and width will increase. But as all the experiments were carried out without any assist gas some amount of molten material will be resolidified on the wall of the microgroove. Adding a second pass without changing the 3.4 Influence of Average power on width, depth, and surface roughness Figure 5 shows the of average power on width, depth, surface roughness of fibre laser ablated micro-groove surface while other process parameters were kept constant as no of pass of 5, scanning speed of 100mm/sec, pulse frequency of 50 KHz. Laser power determines the direct energy input to the microcutting process. Both cutting quality and the cutting performance depend on the laser power. The amount of resolidified material on the cut surface increases with increase in laser power. Increase in laser power increases the width and the depth due to increase in more power density, power input per unit area. The laser energy affects both the width and depth of the micro-grooves, but it does not change both with the same functional dependence. Increase of average power with moderate setting of other parameter leads to linear increases of laser beam energy. This high amount of more energy melts and evaporates more material from the micro-groove zone creating more depth deviation. Thus it will increase width and depth simultaneously. As the average power is increasing, so it will lead to poor surface quality. Figure 6(a) & (b) exhibits the photographic view of the micro

6 EXPERIMENTAL STUDIES ON FIBRE LASER MICRO-MACHINING OF Ti-6Al-4V grooved surface of Ti-6Al-4V in which width and depths are shown respectively at parametric combinations of 30W of average power; scan speed of 40mm/sec; no of pass of 8 and pulse frequency of 50 KHz. Fig 5 Effect of Average power on width, depth and surface roughness 6(a) mm/sec; pulse frequency of KHz during the micro-grooving of Ti-6Al-4V using 50W fibre laser. Different process parametric settings lead to different width, depth and surface roughness. From the results of present experimentation, the following conclusions can be drawn. (a) Depth, and width tends to decrease with the higher scan speed but surface roughness has a tendency to gradually decrease owing to low spot over lapping, discontinuous power density. (b)with the increase of pulse frequency, peak power of the fibre laser tends to decrease thus causes reduction the dimensions of width and depth. At higher values of pulse frequencies, the spot overlap increase which gives continuous power density and smooth cut surfaces. (c)as the increment of no of pass, depth and width will increase. But after certain no of pass, width will tend to decrease owing to resolidification of the molten material on the grooved surface. Rough surface quality can be achieved also due to increase of no of pass. (d)as the average power increases; width, depth simultaneously increases due to high amount of more energy melts and evaporates more amount of material from the micro-groove zone. Poor surface quality can also be seen with increase of average power. Further research work can be carried out with parametric settings so as to achieve optimized values of better micro-machining quality along with low dimension of width and high dimension of depth. Acknowledgement The authors acknowledge the financial support and assistance provided by CAS Ph-IV program of Production Engineering Department of Jadavpur University under University Grants Commission, New Delhi & TEQIP phase II program of Jadavpur University also M/S Sahajanand laser Technology for extending the machining facilities. 6(b) Figure 6(a) & 6(b) showing microscopic view of the width & depth of the micro-groove of Ti-6Al- 4Vrespectively Conclusions The present research paper deals with the influence of four process parameters such as average power of W, No of passes of 1-8; scan speed of References 1. Fasasi A.Y. et al. (2009), Nano-second UV laser processed micro-grooves on Ti6Al4V for biomedical applications, Materials Science and Engineering: C, Volume 29, Issue 1, 1 January, pp Sun S.J., Harris J., Brandt M. (2008), Parametric investigation of laser-assisted machining of commercially pure titanium, Advanced Engineering Materials, Vol. 10, pp Dandekar C.R., Shin Y.C., Barnes J. (2010), Machinability improvement of titanium alloy (Ti 6Al 4V) via LAM and hybrid machining, International Journal of Machine Tools and Manufacture,Vol. 50,pp Kibria G, Doloi B, Bhattacharyya B (2013), Predictive model and process parameters optimization of Nd:YAG laser micro-turning of 229 6

7 ceramics, International Journal of Advanced Manufacturing Technology, Vol.65, pp Pandey A.K, Dubey A.K (2012), Simultaneous optimization of multiple quality characteristics in laser cutting of titanium alloy sheet, Optics & Laser Technology, Vol. 44,pp