Available online at ScienceDirect. Procedia CIRP 42 (2016 )

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1 Available online at ScienceDirect Procedia CIRP 42 (2016 ) th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) High Speed EDM and Laser Drilling of Aerospace Alloys Mohammad Antar a,b*, Dimitrios Chantzis a, Sundar Marimuthu a,c, Philip Hayward a a The Manufacturing Technology Centre Ltd, Ansty Business Park, Coventry CV7 9JU, United Kingdom b University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom c Loughborough University, Epinal Way, Loughborough LE11 3TU, United Kingdom * Corresponding author. Tel.: ; address: Mohammad.Antar@the-mtc.org Abstract EDM and laser processing are extensively used for drilling cooling holes in various aero and land based turbine components, including combustion casing, vanes and blades. However, as it is envisaged that future generation of aero engines will typically have in excess of 150,000 cooling holes, which will result in enormous pressure on technology providers to meet some very challenging targets in relations to productivity and hole quality. This represents a significant R&D opportunity to improve the performance of current hole drilling process to keep up with the ever increasing customer demands. This paper investigates high speed hole drilling (0.8mm diameter) of nickel based aerospace alloy (5-10mm thick) with the state-of-the-art EDM and laser drilling machines. EDM trials were performed using GF Agie Charmilles, 7-axis Drill 300 unit and laser trials were performed using a DMG LT50 PowerDrill unit equipped with an IPG 20kW QCW fibre laser sources. A 2-level 3-factor full factorial design was used to identify the preferred operating parameters for each process. The main investigation concentrates on identifying suitable hole drilling regimes for EDM and laser drilling process on basis of drilling speed, recast layer thickness and hole taper. Results showed a step change in drilling speed (4-5 folds) compared to previous generations of EDM and laser machines (ND:YAG laser and standard EDM drill), with significant enhancement in hole quality and integrity. EDM showed significantly better results with regards to recast layer (10-15μm compared to ~80μm for laser) and geometric accuracy / taper particularly for thicker samples. Laser drilling, however, was far superior in terms of speed with <3s drilling time for 10mm thick samples compared to 48s best recorded EDM drilling time. Crown Copyright 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier B.V. ( Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review XVIII). under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) Keywords: Electrical discharge machining (EDM), Laser beam machining (LBM), Drilling 1. Introduction Modern gas turbines operate at temperatures well in excess of the 2000K which is higher than the melting point of nickel super-alloys widely used in the combustion chamber and turbine blades. One of the most emerging cooling technologies is effusive cooling, the effect behind which is the result of many subsequent rows of cooling holes [1]. Usually the number of holes varies from part to part in a gas turbine engine where, indicatively, there are around holes on the afterburner component of an engine, typically with a 0.4mm diameter [2]. Moreover a key trend in turbine design is using many different complex shape holes in order to maximise cooling and subsequently fuel efficiency [3]. Diesel fuel injection nozzles, on the other hand, is another application which demands high accuracy holes [4]. Electrical discharge drilling and laser drilling are, perhaps, two of the most well established methods for producing cooling holes in aero and land based turbine engine components, typically ranging between 0.3 and 0.9mm in diameter. Both processes have particular advantages, compared to conventional or other types of drilling, when it comes to processing hard alloys or a micro, complex shape holes or when high aspect ratios are required. Key applications for EDM drilling include machining cooling holes in land turbines (where workpiece thickness is typically above 5mm and larger hole diameter compared to aeroengines), micro-holes (typically less than 0.1mm) for medical components and diesel injection nozzles. The process Crown Copyright 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi: /j.procir

2 Mohammad Antar et al. / Procedia CIRP 42 ( 2016 ) is normally characterised by recast layer and necking effect [5] caused by the dielectric fluid escaping through the hole exit as the electrode approaches the end of the workpiece which reduces the amount of erosion particles suspended in the dielectric and hence reduces lateral secondary erosion [6]. However, one of the key limitations for the process, especially in turbine engines, is related to drilling through Thermal Barrier Coating (TBC) which is normally applied to blades and vanes at the hot sections of the engine [7]. Whereas on this front laser offers a major advantage with regards to flexibility as a number of studies demonstrated laser material processing from glass to nickel based alloys and ceramics. A number of recent studies highlighted the application of EDM drilling for high precision applications, in some instances sub-micron holes, which is mainly driven by the negligible cutting forces and high level of control associated with the process [8]. Other studies also reviewed the effects of electrode materials, polarity and other factors, concluding that the choice is directly related to the desired responses. For instance it was reported that positive polarity results in better surface roughness than negative polarity and Cu electrode provides better surface roughness but a WC electrode provides a higher removal rate [9-11]. Most industrial macro laser drilling is performed using percussion or trepan laser drilling process, in which a millisecond laser pulse with a power density of W/cm2 is used to produce holes by combination of melting and vaporisation. Trepanning drilling process is most often used in high value manufacturing industries, due to its ability to produce holes of high quality, and is accomplished by an initial percussion drilling (repeated pulse at single point), followed by a laser cut by moving the workpiece or moving the laser beam. The high quality in trepanning drilling process is attributed to the fact that the melt is mostly flushed downwards through the hole exit, however, trepanning drilling process is significantly slower than the percussion drilling; typically by a factor of 4 10 [12]. Generally, millisecond laser drilling process is associated with a number of undesirable defects including recast layer, heat affected zone, oxide layer and spatter. Recast layer formation is considered very critical for rotating components like turbine blades, as it can lead to initiation of fatigue cracks and ultimate failure of the laser drilled components. Bandyopadhyay et al. [13] drilled 4 and 8mm thick sections of IN718 and Ti-6Al-4V materials to investigate the influence of type of the material, thickness, pulse frequency, energy on hole quality. They found that for the same material thicknesses processed under identical conditions, the recast layer formed in case of Ti-6Al-4V material were always thicker than those in holes drilled in IN718. Sezer et al [12] also investigated the effect of drilling angle on recast layer and found that recast thickness increases with decreasing drilling angles to the surface and vice-versa. Geometric accuracy or hole taper is one of the inherent manufacturing problems associated with laser drilling. Typically, materials with higher thermal diffusivities tend to give a larger taper [14]. Various methods were developed and reported to control it, ranging from process parameters modelling and optimization [15-16] to ultrasonic vibration assisted laser drilling [17] and pulse modulation [18]. This paper aims at demonstrating the capabilities of the latest EDM and laser processing technologies / equipment and identifying preferred key process parameter based on processing time, recast layer and geometric accuracy (taper) of the holes. 2. Experiments 2.1. Design of Experiments A 2-level, 3-factor full factorial design was undertaken for the experimental trials in both EDM and laser processes. Lists of all parameters (fixed and variable) and levels are provided in Tables 1-3. For each experiment, 3 replications were performed and the average value was recorded. All experiments were conducted in a randomised order to provide protection against extraneous factors affecting the results. Additionally, the experimental design was further improved by adding three centre point runs (one for each factor) randomly within the test matrix. Minitab software was used to perform statistical analysis and produce main effect plots. Output measures were related to drilling time and recast layer thickness for the EDM trials and hole taper and recast layer thickness for the laser trials. The reason for variation is that for EDM a fixed set of parameters was employed for the hole entry and exit whereas the optimisation process was focusing on the main cut only. Therefore the change of parameter levels was not likely to affect the hole taper. On the other hand, for trepan laser drilling, speed is a process input (factor) rather than an output. Table 1. EDM Parameters and their respective levels. Parameters Units Level 1 Level 2 Compression um SBOX nf Current (I) A Table 2. Laser Parameters and their respective levels Parameters Units Level 1 Level 2 Peak Power kw Laser Speed mm/min Pulsing Frequency Hz Table 3. Constant process parameters Parameters Process Value Gas Laser Oxygen Gas Pressure Laser 4 bars Spot Diameter Laser 0.8mm Focus Length Laser 150mm Wavelength Laser 1070nm No. of Orbits Laser 3 Electrode Material EDM Tungsten Carbide Electrode Diameter EDM 0.8mm

3 528 Mohammad Antar et al. / Procedia CIRP 42 ( 2016 ) Experimental set-up Workpiece materials were 5 and 10mm thick Inconel 718 nickel based super-alloy, annealed and precipitation hardened to ~ 40RC. All experimental trials were conducted on A DMG LT50 Power Drill unit equipped with a 20kW Q-switched pulsed fiber laser and a GF Agie Charmilles 7-axis Drill300 EDM Drilling machine, see Figure 1. Tungsten Carbide electrodes (0.8mm diameter) were used for EDM trials due to their high stiffness, low wear properties and the brittle behaviour, which makes them a preferred option for applications requiring very small holes. Fig. 1: (a) DMG LT50 Power Drill; (b) GF Agie Charmilles Drill300 (c) machine setup for EDM trials (d) laser beam profile Selected workpiece samples were sectioned and subsequently set in bakelite using a Buehler Ltd. mounting press and then ground and polished. Samples preparation was done according to ASTM E3-11 Standard and Kalling s No. 2 etchant (40-80ml ethanol, 40ml HCl and 2g CuCl2) was used to reveal the recast layer. A Zeiss Imager M2m digital microscope was used for recast layer and hole taper analysis. In each test sample, five recast layer thickness measurements were taken across the hole depth on each hole side (i.e. 10 measurements in total) and the average value was recorded. The hole taper was calculated as per Equation 1: (1) 3. Results Tables 4 and 5 summarise the results for both the EDM and laser factorial experiments. For the EDM drilling, average recast layer thickness was relatively similar across the different test pieces and ranged between ~7µm and 15µm for the 5mm thick samples and 10µm and 15µm for the 10mm ones. This was further supported by the statistical analysis of the experimental design where none of the main operating parameters had significant effect on the recast thickness for the 10mm samples while SBOX was the only significant factor (marginally) in the 5mm DoE. On the other hand, process parameters appeared to have clear effect on the drilling time for both workpiece thicknesses as this varied between 28s to 58s and 48s to 154s for the 5mm and 10mm samples respectively. This was in line with findings by Diver et al [6] who reported 30s drilling time for a similar machining conditions. Here, current was the most significant factor followed by SBOX whereas compression did not seem to have statistically significant effect at 5% confidence level. For laser drilled samples, as somehow expected, recast layer was much thicker compared to the EDM tests and ranged between 14µm and 28µm for the 5mm samples was up to 80µm for the 10mm samples. With regards to hole taper, up to ~2.5% and 3.5% (equivalent to ~ 125µm and 350µm) were measured for the 5mm and 10mm samples respectively. This was also significantly higher than the taper for the EDM samples which varied, on the measured holes, between 20µm and 50µm. It should be noted, however, that drilling time was much lower using laser (1.5s-3s) compared to EDM. Table 4. Experiments and Results for EDM drilling Exp Parameter 5mm thickness 10mm thickness No. Recast Drill Recast Drill Time Time (um) (sec) (um) (sec) COMP SBOX Current (I) Table 5. Experiments and Results for Laser Drilling Exp. No. Parameters 5mm thickness 10mm thickness Recast Hole Recast Hole Taper Taper (um) (um) Peak Power Speed Frequency % % % % % % % % % % % % % % % % % % Figure 2 shows the main effect plots for all key parameters against the different output measures. The process parameters effect was more apparent on the EDM drilling time which (%) (%)

4 Mohammad Antar et al. / Procedia CIRP 42 ( 2016 ) Fig. 2. Main effect plots for all key parameters against different responses dropped as the compression (a parameter for controlling Servo movement, directly related to the gap voltage), SBOX (a machine-specific parameter for controlling the peak/pulse machine ignition current) and Current increased, with the latter being the most significant. This was somehow expected, however it was interesting that the increase in the parameters levels did not lead into significant adverse effect on the recast layer and the hole quality in general. Also, the process effects did not seem to change significantly when varying the workpiece thickness, where drilling speed was also similar (~0,2mm/s best recorded value) for both 5mm and 10mm samples. For laser drilling, on the other hand, recast layer thickness decreased with decreasing peak power and increasing trepanning speed and frequency (i.e. shorter pulse time) with the latter being the most significant. This is directly related to the lower thermal input when these variables are set at the lower levels. In the same vein, Bandyopadhyay et al [13] reported that shorter pulse duration minimized recast layer formation and reduced micro cracking at the side walls of the laser drilled hole, whereas the thickness of the recast layer increased with thickness of the material drilled. Even though none of the main laser process parameters was statistically significant in relation to hole taper, drilling speed and, to a less extent, peak power appeared to have some effect on the hole taper. For the 5mm samples less taper was measured at the higher speed levels, whereas it was the opposite for the thicker samples. This indicates that not enough average power was employed in the latter case, causing relatively high taper as the laser beam struggled to cut through at the bottom of the hole. All main process parameters with the associated statistical significance and percentage contributions are listed in Table 6. Fig. 3. Recast for EDM samples (a) 5mm and (b) 10mm Fig. 4. Recast for laser samples (a) 5mm and (b) 10mm

5 530 Mohammad Antar et al. / Procedia CIRP 42 ( 2016 ) Figures 3 and 4 show micrographs for both EDM and laser drilled samples for different test piece thicknesses. Generally, continuous, non-uniform recast layers were observed after laser drilling on both 5mm and 10mm samples (although much thicker at the later). Much thinner and mostly discontinuous recast layers were left on the EDM drilled surfaces and microcracks when evident, remained within the recast layer. For EDM, no thermal banding was obvious below the recast layer and the microstructure appeared unaffected, whereas in some cases up to ~50µm thick heat affected zones were observed on the laser drilled samples. Table 6. % Contribution of each parameter to the response EDM 5mm % Drill Time COMP SBOX I Error EDM 10mm % Drill Time COMP SBOX I Error Laser 5mm % Hole Taper Peak Power Frequency Speed Error Laser 10mm % Hole Taper Peak Power Frequency Speed Error Table 7. EDM optimum combination of parameters Response COMP SBOX Current Drill Time 5mm mm Recast 5mm mm The combinations of parameters resulting in the most desired output measures in both processes are listed in Tables 6 and 7. Table 8. Laser optimum combination of parameters Response Peak Power Frequency Speed Hole Taper 5mm mm Recast 5mm mm Conclusions Recast layer thickness on EDM drilled samples was marginally affected by the operating parameters and even the workpiece material thickness. This suggests the possibility of running the process at the higher parameters levels to achieve enhanced drilling speed without significant adverse effects on recast thickness. The uniform beam profile of the fibre laser helped to achieve significant increase in laser drilling productivity. For the same thickness, fibre laser trepanning drilling showed around four times enhancement in drilling speed compared to traditional lamp pumped Nd:YAG laser [19]. In contrast to EDM, workpiece thickness had significant effect on the average recast layer on laser drilled samples. This may be related to the fact that as the thickness increased it became more difficult to flush away the molten material, resulting in thicker recast layer. Relatively high hole taper was measured after laser drilling, particularly for the 10mm samples. Furthermore, for the 5mm samples taper decreased with increasing trepanning speed, whereas the opposite was noticed for the 10mm samples. This may be related to potential loss of beam power in the thicker samples where trepanning speed then became a significant factor in compensating for this loss (i.e. slower beam motion resulting in better hole geometry). On the other hand, laser beam power was high enough in the 5mm samples where speed did not have a significant effect (18% PCR compared to 65% for peak power. Multi-orbit trepan laser drilling could be a feasible option for improving geometric accuracy and potentially reducing recast layer, however speed maybe compromised. Overall, EDM is the better option if hole quality and precision are of higher importance than speed whereas laser provides a much better alternative otherwise. Also, with regards to quality, EDM appears to be less affected by material thickness or aspect ratio which gives another advantage over laser drilling. References [1] P. Renze, W. Schröder, and M. Meinke, Large-eddy simulation of film cooling flows at density gradients, Int. J. Heat Fluid Flow,2008, vol. 29, pp [2] van D. M.H.H, de V. D, and B. J.E., Laser Precision Hole Drilling in Aeroengine Components, 1989, in Proceedings of 6th Lasers in Manufacturing [3] P. Thompson and P. Busti, Laser Processing of Shaped Holes History and Future, IOP Conf. Ser. Mater. Sci. Eng., 2011, vol. 26, p , [4] L. Li, C. Diver, J. Atkinson, R. Giedl-Wagner, and H. J. Helml, Sequential laser and EDM micro-drilling for next generation fuel injection nozzle manufacture, CIRP Ann. - Manuf. Technol., 2006, vol. 55, no. 2, pp [5] Y. Guu and H. Hocheng, Effects of work piece rotation on machin- ability during electrical-discharge machining, Mater. Manuf. Process, 2001, vol. 16, no. 1, pp ,. [6] C. Diver, J. Atkinson, H. J. Helml, and L. Li, Micro-EDM drilling of tapered holes for industrial applications, J. Mater. Process. Technol., 2004,vol. 149, pp [7] F. J. Wos, Laser Hole-Shaping Improves Combustion Turbine Efficiency, [8] K. Egashira, Y. Morita, and Y. Hattori, Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes, Precis. Eng., 2010, vol. 34, pp [9] M. G. Her and F. T. Weng, Micro-hole machining of copper using the electro-discharge machining process with a Tungsten carbide electrode

6 Mohammad Antar et al. / Procedia CIRP 42 ( 2016 ) compared with a copper electrode, Int. J. Adv. Manuf. Technol., 2001, vol. 17, pp [10] Y. Li, M. Guo, Z. Zhou, and M. Hu, Micro electro discharge machine with an inchworm type of micro feed mechanism, Precis. Eng., 2002, vol. 26, pp [11] J. a. Sánchez, S. Plaza, R. Gil, J. M. Ramos, B. Izquierdo, N. Ortega, and I. Pombo, Electrode set-up for EDM-drilling of large aspect-ratio microholes, Procedia CIRP, vol. 6, pp , [12] H. K. Sezer, L. Li, M. Schmidt, a. J. Pinkerton, B. Anderson, and P. Williams, Effect of beam angle on HAZ, recast and oxide layer characteristics in laser drilling of TBC nickel superalloys, Int. J. Mach. Tools Manuf., 2006, vol. 46, pp , [13] S. Bandyopadhyay, H. Gokhale, J. K. S. Sundar, G. Sundararajan, and S. V. Joshi, A statistical approach to determine process parameter impact in Nd:YAG laser drilling of IN718 and Ti-6Al-4V sheets, Opt. Lasers Eng., 2005, vol. 43, pp , [14] W. K. Hamoudi and B. G. Rasheed, Parameters affecting Nd:Yag laser drilling of metals, Int. J. Join. Mater., 1995, vol. 7, no. 2 3, pp [15] R. Biswas, a. S. Kuar, S. Sarkar, and S. Mitra, A parametric study of pulsed Nd:YAG laser micro-drilling of gamma-titanium aluminide, Opt. Laser Technol.,2010, vol. 42, no. 1, pp [16] R. Biswas, A. S. Kuar, and S. Mitra, Process optimization in Nd:YAG laser microdrilling of alumina aluminium interpenetrating phase composite, J. Mater. Res. Technol., 2015, pp. 1 10, [17] M. Lau, W., Yue, T.-M., Wang, Ultrasonic-Aided Laser Drilling of Aluminium-Based Metal Matrix Composites, Ann. CIRP, vol. 43, no. I, pp , [18] L. Li, D. K. Y. Low, M. Ghoreshi, and J. R. Crookall, Hole Taper Characterisation and Control in Laser Percussion Drilling, CIRP Ann. - Manuf. Technol., 2002, vol. 51, pp , [19] Marimuthu, S, Antar, M, Chantzis, D, High Speed Quasi-CW Fibre Laser Drilling of Aerospace Alloys, Proceedings of Lasers in Manufacturing, 2015,pp , Munich