Indian Journal of Engineering & Materials Sciences Vol. 25, December 2018, pp. 437-444

Performance evaluation of mechanical micro-drilling, electrical discharge machining and laser beam machining on Nimonic 80A alloy

Sake Sudhakara,c, Praveen Kumara, G Srinivasa, S Ravishankarb, D Chakradharc & Harish C Barshiliaa* aNanomaterials Research Laboratory, Surface Engineering Division, CSIR – National Aerospace Laboratories,

Post Bag 1779, Bangalore 560 017, India bAircraft Prototype Manufacturing Facility, CSIR – National Aerospace Laboratories, Post Bag 1779, Bangalore 560 017, India

cDepartment of Mechanical Engineering, National Institute of Technology, Surathkal 575 025, India

Received 22 June 2017; accepted 14 September 2018

Micromachining techniques such as mechanical micro-drilling, electrical discharge machining (EDM) and laser beam machining (LBM) play an important role in the manufacturing of micro-devices used in mechanical, electronics, aerospace and medical applications. In this paper, an effort has been made to compare the performance of these micromachining techniques with regard to tool wear, burr formation and surface integrity. This is done by producing 20 micro-holes of approximately 800 μm diameter on a rectangular block (90×30×3 mm3) of Nimonic 80A superalloy. TiAlN coated WC micro-drills, Cu electrodes and CO2 laser beam are used to produce these holes in conventional micro-drilling, EDM and LBM, respectively. The quality of the drilled hole (diameter, surface roughness and micro-burr formation), tool diameter analysis, taper angle and material removal rate (MRR) are compared and reported. A comprehensive analysis is also carried out on overcut, which leads to hole inaccuracy. Results show that mechanical micro-drilling produces better results in the above mentioned characteristics in comparison to LBM and EDM techniques. The relatively better performance of mechanical micro-drilling is attributed to the usage of TiAlN coating on WC tool.

Keywords: Micro-drilling, Ni-based superalloys, Electrical discharge machining, Laser beam machining, Burr formation

Miniaturization of devices in various sectors is becoming a necessity due to their potential applications in mechanical, electronics and medical applications. This consequently led to the development of various machining processes such as micro-drilling, laser beam machining (LBM), electron beam machining (EBM), ultrasonic machining, electrical discharge machining (EDM), etc.1 This technique produced micro-holes with greater accuracy for engineering applications in aerospace jet engines, seal slots in compressors and micro-holes in fuel injectors. Most of these applications make use of Ni-based superalloys such as Inconel, Monel, Rene, Nimonic, etc due to their high strength, exceptional fatigue and corrosion resistance at elevated operating temperatures2,3. However, significant challenges have been observed in machining Ni-based superalloys due to their low thermal conductivity and high thermal strength4-7. This is further complicated by the presence of hard abrasives in the microstructure of nickel and the work hardening effect. As a result, modified micro-machining techniques such as micro-

drilling with a coated micro-drill (such as TiAlN, TiAlSiN, etc), micro-EDM and micro-LBM are popularly used to machine these superalloys.

Among the above mentioned techniques, conventional micro-drilling is widely employed owing to its independence on work-piece properties, lower thermal deformation and minimum post-processing. Additionally, mechanical micro-drilling produces deeper holes with better roundness and smoothness, higher surface integrity and improved tolerance8. However, the development of high temperatures in the heat affected zone limits the use of this technique. It has been reported that temperatures of 1000oC and above are generated in the machining zone with a cutting speed of 300 m/min in the micro-drilling of Ni-based superalloys9-11. This subsequently affects the tool life of the micro-drill. A higher depth-per-pass during machining leads to greater heat generation which results in burr formation and other surface defects. The literature reveals that conventional machining of Ni-based superalloys produces higher plastic deformations, variations in hardness12, residual stress13, microstructure changes, tears, lapse and phase

————— *Corresponding author (E-mail: harish@nal.res.in)

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transformations closer to the machined surface14-16. However, these issues can be minimized by using a nanostructured physical vapor deposited coating such as TiAlN on micro-cutting tools and by supplying a proper amount of coolant.

Electrical discharge machining (EDM) is another popular machining technique that causes material removal due to spark erosion between tool and work-piece material. This technique is usually applicable to conductive materials. The relatively small amounts of material removed in each step is used to produce micro-dies, micro-holes, micro-slots and micro-gears on complex contours such as curvatures and inclined surfaces17,18. Jahan et al.19 observed higher surface finish and circularity when using an EDM operation as compared to other machining techniques. However, slow machining time, shape inaccuracies and the hazards associated with EDM limit the application of this process. Moreover, as the size of the hole decreases, the debris accumulated in the gap leads to greater difficulty in machining.

Laser beam machining (LBM) makes use of a high pressure, gas assisted jet to clear the machining zone. LBM is reaction force free pyrolytic process in which melting and vaporization causes material removal. Nd:YAG and CO2 are the commonly used lasers. An advantage associated with LBM is that it can machine any material regardless of its hardness and electrical conductivity. Thus, it has widespread applications in several industries such as aerospace, automotive, biomedical and MEMS20,21. Through the use of LBM, it is possible to produce a micro-hole of a given dimension on complex profiles at different orientations in most materials. In 2014, a German gas turbine company (SIEMENS Energy, Fossil Power Generation) designed the world’s largest gas turbine SGT5-8000H by carving cooling holes of size 0.4 mm - 1.2 mm diameter at 15°- 90° on curved turbine blade surfaces using an LBM technique. Voisey et al.22 used Nd:YAG LBM with varied power densities and observed that the material removal rate increases with increasing power density. LBM is an advanced and rapid machining process in comparison to mechanical drilling and micro-EDM. However, it suffers from a few limitations with respect to surface integrity of the work-piece. It has been reported that while machining

LBM leads to heat affected zones, tapered holes, poor surface finish and formation of recast layer followed micro-cracks in the machining of Ni-based superalloys20,23,24. A recast layer is formed in LBM due to over flow and accumulation of molten material on to the walls of the hole. Metallurgical changes induced by LBM influence mechanical properties of the work-piece. The recast layer consists of micro-cracks that lead to fatigue cracking of the machined component25. This paper presents a comparative study on the machining characteristics of mechanical micro-drilling, micro-EDM and LBM with respect to surface integrity, burr formation and micro-hole quality.

Materials and Methods A homogeneous rectangular Nimonic 80A alloy of

dimensions 90×30×3 mm3 with composition as shown in Table 1 was used in this study. The machining conditions used for mechanical micro-drilling, EDM and LBM are described in Table 2. The optimized cutting parameters for micro-EDM were selected by performing a set of experiments by changing the peak current (A) and the pulse on time (B). The optimized parameters were selected based on the lowest tool wear. (A) and (B) were varied as per the data reported in the literature26. It can be seen from Table 3 that the optimized values of the current and time are 0.6 A and

Table 1 — Composition of the work-piece material used in this study (wt%).

Elements Cr Co Fe Ti Al P S C Mn Ni

wt% 20.3 1.3 1.5 2.68 1.48 0.005 0.006 0.04 0.06 73

Table 2 — Machining conditions for three micro-machining techniques

S. No. Machining process Machining parameter settings

1) Mechanical micro-drilling Machine used Victor Taichung Feed rate (mm/min) 5 Cutting speed (m/min) 12 Coolant Soluble oil Tool WC, TiAlN coated

2) Micro-EDM Machine used Tool craft Discharge current (A) 5 Pulse duration (ms) 6 Voltage (V) 145 Tool Cu electrode Dielectric fluid Synthetic fluid

3) LBM (CO2 laser with N2 as assisted gas)

Machine used Trumph laser Wavelength (μm) 10.6 Gas used N2 Pressure (bar) 2 Pulse duration (ms) 30 Power (W) 1000 Tool CO2 laser

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6 μs, respectively. The tool wear in this case is minimum at 1.02%. In addition, the lateral surface of the tool tip was smooth and contained lesser craters.

The optimized cutting parameters for micro-drilling were determined by performing a full factorial set of experiments at various cutting speeds (vc) and feed rates (fz) with a TiAlN coated tungsten carbide micro-drill (SGS Solid Carbide Tools). These parameters were chosen based on the recommendations from the tool manufacturer27. The results of the test are tabulated in Table 4. It was noted that the lowest tool wear was observed at a cutting speed (vc) of 12 m/min and a feed rate (fz) of 5 mm/min. These parameters were hence considered as optimized cutting parameters. Additionally, it was observed that the drill broke in two cases, i.e., at vc = 15 m/min, fz = 7 mm/min and when vc = 15 m/min, fz = 9 mm/min.

CO2 laser (single pulsed mode) with N2 gas was used for the laser beam machining (LBM). The process parameters for this case were selected from a previous study28. The optimized parameters were obtained by varying the power and gas pressure through a full factorial design of experiments. These results are shown in Table 5. It can be seen that minimum taper angle was produced at a power of 1000 W and a gas pressure of 2 bar. These parameters

were hence classified as optimum process parameters. After obtaining the optimized cutting parameters

for mechanical micro-drilling, EDM, and LBM, 20 holes of diameter 800 µm were machined using the above techniques. The hole diameter, tool diameter, burr formation and surface roughness were studied in each case and analyzed to observe the influence of the physics of the operation on the above mentioned parameters. Results and Discussion Hole quality analysis

Figure 1 shows the FESEM images of micro-holes (at the entrance and exit) made by EDM, mechanical micro-drilling and LBM. For all these cases, it was observed that the hole entrance diameter was higher than that of the exit diameter. The continuous arching of the electrode at the entrance during the machining process was thought to be the reason for the above

Table 5 — Optimized cutting parameters for LBM.

Exp. No. Factors Results

Parameter Power (W) Gas Pressure (bar) Entrance diameter (μm) Exit diameter (μm) Taper angle (deg.)

1 1000 O2 4 840 780 0.5729 2 1000 O2 2 845 789 0.5633 3 1000 N2 4 840 783 0.5442 4 1000 N2 2 835 781 0.5150 5 750 O2 4 824 750 0.7066 6 750 O2 2 820 760 0.6684 7 750 N2 4 827 744 0.7925 8 750 N2 2 830 740 0.8593

Table 3 — Optimized cutting parameters for EDM

Expt. no. A - Peak current (A)

B - Pulse on time (μs)

Tool wear (%)

1 0.3 6 1.52 2 0.3 12 1.90 3 0.3 30 2.79 4 0.6 6 1.02 5 0.6 12 2.66 6 0.6 30 3.78 7 0.9 6 2.53 8 0.9 12 3.54 9 0.9 30 5.69

Table 4 –— Optimized cutting parameters for mechanical micro-drilling

Expt. no. Cutting speed (m/min)

Feed rate (mm/min)

Tool flank wear (µm)

1 8 5 10.0 2 10 5 11.9 3 12 5 9.5 4 15 5 35.6 5 8 6 11.9 6 10 6 18.9 7 12 6 16.5 8 15 6 40.0 9 8 7 13.9 10 10 7 22.3 11 12 7 22.2 12 15 7 Drill break 13 8 9 18.8 14 10 9 28.6 15 12 9 28.9 16 15 9 Drill break

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observation in case of EDM. In the case of LBM, it is postulated that the hole at the entrance is subjected to continuous melting and vaporization, which results in larger entrance diameter than that of exit diameter. The difference between entry and exit diameters of micro-holes is low in mechanical micro-drilling than EDM and LBM. Figure 2 shows the hole diameter at the entry and exit with increasing number of holes. From the graph, it is clear that in EDM, the hole diameter decreases with machining time due to the increase in wear of the Cu electrode (at the tip and lateral surface). However, the reduction in the diameter of micro-holes made by LBM and mechanical micro-drilling is considerably low. The diameter of the micro-hole produced is compared to assess the dimensional accuracy in each process. Based on the results, micro-drilling shows the minimum deviation in hole diameter with increasing number of holes (from 1st to 20th) at the entrance and the exit. Since minimum deviation of diameter indicates better accuracy, micro-drilling is recommended for applications where the dimensions of the micro-hole are critical.

Surface roughness Surface roughness of the holes made on aerospace

alloys plays a critical role in the fatigue life of the

component29,30. Hence, it is necessary to create holes with minimum surface roughness. Figure 3(a) shows the FESEM images of the cross-sectional view of the micro-holes created in all three techniques. Micro-drilling (with TiAlN coated micro-drill) exhibits a better surface finish as compared to EDM and LBM. Figure 3(b) depicts the surface roughness variation across 20 holes for all the three micro-machining techniques. The average surface roughness measured for mechanical micro-drilling, LBM and EDM are 0.152, 2.608 and 3.326 μm, respectively. Surface roughness in EDM is higher because of the micro-craters formed by the continuous arching between

Fig. 1 — FESEM images of the holes at the entrance and exit for(a) EDM, (b) mechanical micro-drilling and (c) LBM

Fig. 2 — Variation of hole diameter with increasing number ofholes at entry and exit

Fig. 3 — (a) Cross-sectional view of the micro-holes and (b) surface roughness with increasing number of holes

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electrode and the work-piece. Even though LBM is an advanced non-conventional machining process, its surface roughness is higher than micro-drilling. This is because the molten metal flows into the cavity and gets solidified to form a recast layer. In case of mechanical micro-drilling, material removal takes place with ease due to the presence of high performance nanostructured TiAlN coating having high hardness, resulting in better surface roughness. Evaluation of hole taper

The angle between the entrance and exit diameter of the micro-hole is termed as taper angle (Fig. 4(a)). Due to this, secondary machining operations might be required which cause an increase in machining time. So, it is better to choose the micro-machining technique which causes the least taper angle for critical dimensional applications. The taper angle (α) of a micro-hole is calculated using formula:

𝛼 tan … (1)

where t is thickness of the work-piece, Dt is micro-hole entrance diameter and Db is the micro-hole exit diameter. The variation in the taper angle with increase in the number of holes for all three different micro-machining techniques is plotted in Fig. 4(b). The average values of the taper angles are 0.36o, 0.48o and 0.58o for mechanical micro-drilling, EDM and LBM, respectively. Among the three machining techniques, the taper angle is observed to be more in the case of LBM. Nevertheless, the variation in taper angle for all three techniques is not significant for number of holes drilled up to 20. The FESEM image of cross-section of the LBM drilled micro-hole is shown in Fig. 4(a). In LBM, the huge diameter variation is due to the convergent nature of the

incident beam, which leads to the continuous heating and vaporization at the entrance of the hole. In EDM and mechanical micro-drilling, as the number of holes increases, the taper angle varies due to tool wear. The taper angle in micro-drilling is minimum since the size of the micro-hole obtained by this process is very close to the tool diameter. Hence, mechanical micro-drilling is recommended for making holes with minimum taper angle.

Burr formation Burr is an extra protruded portion on the machined

surface, which can be removed by a deburring process. However, the complete elimination of burr is difficult. As a result, we have to adopt a process which produces minimum burr height. Figures 5(a)-(c) show the FESEM images of entry and exit burrs of micro-holes machined using EDM, LBM and mechanical micro-drilling techniques, respectively. The burr height was measured with increasing number of holes and is plotted in Fig. 5(d). The average burr height for EDM, micro-mechanical drilling and LBM are 10, 16, and 100 μm, respectively. In case of LBM, the molten metal solidifies and protrudes out at the entry and exit of the micro-hole. In mechanical micro-drilling, an exit burr is observed as the drill reaches the hole exit, since the material surface is pushed out without being cut by the thrust force31. As the tool wears, burr size also increases at the entrance and exit of the holes. In EDM, material removal rate takes place by spark erosion followed by flushing (dielectric fluid) and hence, the burr height is negligible in this technique. Even though, the burr height is negligible in EDM, the hole quality is poor because of the craters formed on the walls of the hole.

Material removal rate (MRR) In any machining process, MRR explains the rate

at which the operation progresses. MRR is a crucial parameter, which determines the performance of any machining process. MRR depends on cutting parameters such as speed, feed and machining time. In mechanical drilling, higher MRR is possible with enhanced speed, feed rate and depth-of-cut (DOC), thus reducing the machining time. In EDM, MRR depends on the erosion depth, pulse duration, discharge voltage, and thermal properties of both work-piece and electrode32. In case of LBM, the laser wavelength, frequency, pulse duration and power density are the factors which affect MRR. In EDM, the enhancement of discharge current leads to higher MRR but the electrode wear also increases due to the

Fig. 4 — (a) A schematic representation of taper angle and(b) variation of taper angle with respect to number of holes

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vigorous sparking action in the inter-electrode gap region. The MRR for a particular operation is calculated using:

Volume of material removedMRR

Machining time … (2)

The MRR for each machining operation is plotted in Fig. 6. The time taken to produce each micro-hole by EDM, mechanical micro-drilling and LBM techniques were 120, 36, and 10 s, respectively. In EDM, MRR (0.05024 mm3/s) is very low when compared with the other machining processes. This is due to the higher machining time in removing the same volume of material. MRR in case of LBM is the highest (0.6028 mm3/s) among all the three techniques. Therefore, LBM is being used as an advanced rapid micro-machining technique. Even though LBM exhibits high MRR, factors such as high taper angle and poor hole quality limit its applications. So, micro-drilling, which leads to moderate MRR and satisfactory hole quality is preferred for various industrial purposes. Tool diameter analysis

The diameter of the micro-tool was compared before and after the machining operation. During

machining, some amount of the tool erodes at the interface of the tool and the work-piece, which directly influences the tool diameter. In EDM, the tool wear depends upon the thermal conductivity of the work-piece, machining time, current density, etc. In mechanical micro-drilling, the tool wear depends upon the frictional forces in the machining zone, operating temperature, thermal conductivity and hardness of the

Fig. 5 — FESEM images showing burr height at the entrance and exit for (a) EDM, (b) laser drilling, (c) mechanical micro-drilling and (d) variation of burr height with respect to number of holes

Fig. 6 — Variation in material removal rate for the different machining techniques

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work-piece. In case of LBM there is no tool, so the wear is neglected. After making 20 holes using each process, the tool diameter was measured (Fig. 7(a)). It was observed that, in both EDM and mechanical micro-drilling, the tool gradually wears out as the number of holes increases. In micro-drilling using a TiAlN coated micro-drill, the tool diameter reduced from 790 μm to 787 μm after drilling of 20 holes. In EDM process, the diameter of the Cu electrode reduced to 750 μm from 790 μm after machining 20 holes. Figure 7(b) shows the FESEM images of the wear occurring on the TiAlN coated micro-drill and the Cu electrode, respectively. It is clear that a large diameter reduction in tool (~40 μm) was observed in EDM, whereas, the diameter reduction was low (~3 μm) in case of micro-drilling. This is because of the continuous spark erosion that occurs during machining in EDM. Moreover, it was also noticed that the Cu electrode gets eroded along with the work-piece material. However, in mechanical micro-drilling, tool diameter variation is lower because of less wear on the TiAlN coated tool. Minimum tool wear was attained in case of micro-drilling through the use of a proper coolant (soluble oil) and optimum coefficient of friction (0.6).

Over cut Over cut is defined as the extent of enlargement in

hole diameter beyond the tool diameter during machining. It is given by the equation:

2

D DOver cut

… (3)

where Da is average diameter of the micro-hole and D is the tool diameter. Da is given by:

2t bD D

D

… (4)

where Dt, Db are entrance and exit diameters of the micro-hole. Over cut values obtained from the three machining processes are calculated and shown in Fig. 8. In case of EDM and mechanical micro-drilling, over cut decreases with increasing the number of holes because of tool wear. However, in case of LBM, such as conclusive change is not observed. The average over cut values for EDM, LBM and mechanical micro-drilling were: 71.1, 6.2, and 5.7 μm, respectively. So, it is clear that the over cut is maximum in case of EDM and minimum in case of mechanical micro-drilling. From the above mentioned formula, it is seen that the over cut is directly proportional to the average diameter of the micro-hole. For EDM, the average diameter of the micro-hole is 942.3 μm, and hence, the over cut is the highest. In EDM, the hole gets enlarged (oversized) due to the continuous arching at the walls of the hole during the machining process. The diameter of the micro-hole produced in micro-drilling is very close to the required diameter (800 μm). Minimum overcut in micro-drilling is possible due to the optimum machining conditions attained by using TiAlN coating on the cutting tool.

Fig. 7 — (a) Variation in tool diameter with increasing number ofholes and (b) FESEM image of the tool tip (electrode, micro-drill)

Fig. 8 — Graph representing over cut with increasing number of holes

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Conclusions Micro-holes are made on Nimonic 80A by using

mechanical micro-drilling, EDM and LBM techniques. In all the three cases, typical characteristics such as hole quality, surface roughness, burr height, hole diameter analysis, taper angle, MRR, tool wear and over cut are studied and compared. The use of a mechanical micro-drilling process produced superior results as compared to other machining techniques with respect to surface roughness (0.1562 µm), taper angle (0.3636˚), burr height (16 µm), tool wear (3 µm) and over cut (5.7 µm). We predict that these results will have a significant impact in the micro-machining of Nimonic 80A.

Acknowledgements This research work was supported by

CSIR–Networked Projects (ESC 0112 and ESC 0101). The authors would like to thank the Director, CSIR-NAL for his support and Mr. Siju John and Mr. Jakeer Khan for their help in giving relevant inputs. References

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