R. S. Pawade a, Suhas S. Joshi #a, P. K. Brahmankar b & J. Ramkumar c

R. S. Pawade a, Suhas S. Joshi #a, P. K. Brahmankar b & J. Ramkumar c Department of Mechanical Engineering a Indian Institute of Technology, Bombay, Mumbai 400 076, India b Dr. Babasaheb Ambedkar Technological University, Lonere 402 103, India c Indian Institute of Technology, Kanpur 208 016, India ABSTRACT This paper reports the results of an investigation on the effect of electrode shape, electrode rotation, pulse current and duty cycle on material removal rate (MRR), electrode wear rate (EWR) and surface roughness in electric discharge machining of Inconel 718. Taguchi orthogonal array L 18 was used for the experiments. The results show that the pulse current and the electrode shape have the most dominating influence on MRR and surface roughness. In the case of EWR, besides pulse current, duty cycle also has significant influence. The MRR is higher when the electrode is rotated at 120 rpm. Compared to other shapes, cylindrical electrode gives better results in terms of MRR, EWR and surface roughness. SEM examination revealed that the surfaces produced using rotating electrode have uniformly distributed and regular shaped craters and spherical globules whereas the surfaces produced using stationary EDM have irregular and non-uniform appearance. Keywords: EDM, Electrode rotation, MRR, EWR, Surface damage 1. INTRODUCTION Electro-discharge machining (EDM) is a nontraditional material removal process with high frequency electrical discharges between the electrode and the electrically conductive workpiece. Besides die and mould manufacturing, it has emerged as an alternative for machining complex shaped components from superalloys in aerospace, missile and nuclear applications [1]. The superalloy such as Inconel 718 has extensive applications in aerospace, missile and nuclear components. It possesses very high specific strength, creep resistance, extreme corrosion and wear resistance. However, it is a difficult-to-cut material in conventional machining due to several problems such as rapid tool wear and extensive damages on the machined surfaces. In view of this, non-conventional process like EDM has a potential to machine such a material and has opened up an opportunity to machine components like turbine blades and other complex parts. However, it may give rise to problems related to surface quality. * Corresponding author: ssjoshi@iitb.ac.in It may result in metallurgical, mechanical and geometrical alterations that hamper the integrity of the machined workpiece and thereby the service life. The literature on electric discharge machining is vast. Several researchers have conducted experimental as well as theoretical investigations using various materials. EDM experiments have been carried out on hard materials like ceramics [2], cermets [3], tool steels [4], shape memory alloys [5], hardened steel [6], MMCs [7-9], and carbon-carbon composite [10]. However, Inconel 718 has been relatively unexplored. Further, very few investigations have been conducted using rotary electrodes of different shapes. The present work is, therefore, an attempt in that direction. In this investigation, effect of electrode shape, electrode rotation, pulse current, and duty cycle on material removal rate (MRR), Electrode wear rate (EWR) and surface roughness (SR) has been investigated. 2. EXPERIMENTAL WORK 2.1 Experimental Design A statistical experimental design based on Taguchi s orthogonal arrays was used [11]. It involves selection of quality characteristics, the factors that may influence the quality characteristics, levels of the factors, their interactions, the appropriate orthogonal array (OA), conducting the experiments, analysis of variance (ANOVA), analysis of means (AOM) and confirmation experiment. Following control factors were selected: duty cycle at 2 levels (8-12%), pulse current at 3 levels (8-14-22 amp), electrode shape at 3 levels (cylindrical-reverse tee-dovetail) and electrode rotation at 3 levels (60-120-0 rpm). In this experiment, a mixed orthogonal array L 18 (2 1 3 7 ) was selected. The control factors were assigned to columns of the orthogonal array as shown in Table 1. As mentioned above, MRR, EWR and surface roughness were chosen as the response variables in this study. It is necessary to produce EDM components having smaller surface roughness in a

56 I J M E M S: 5(1) 2013 short duration but with low electrode wear. Hence the smaller-the-better S/N ratio for surface roughness and EWR and the the larger-the-better S/N ratio for the MRR was chosen. Table 1 Factors Assigned to Orthogonal Array L 18 (2 1 x3 7 ) Factors Test run A Duty B Pulse C Electrode D Electrode cycle current shape rotation 1 8 8 C 60 2 8 8 T 120 3 8 8 D 0 4 8 14 C 60 5 8 14 T 120 6 8 14 D 0 7 8 22 C 120 8 8 22 T 0 9 8 22 D 60 10 12 8 C 0 11 12 8 T 60 12 12 8 D 120 13 12 14 C 120 14 12 14 T 0 15 12 14 D 60 16 12 22 C 0 17 12 22 T 60 18 12 22 D 120 2.2 Work Material and Electrode Workpiece in the form of a rolled sheet of Inconel 718 was used. The chemical composition of the work material is: Ni 52.8; Cr 17.1; Fe 16.9; Co 0.5; Ti 0.6; Mn 0.1; Si 0.05; C 0.04; Nb+Ta 5.2. Electrodes were fabricated from commercially available electrolytic copper rod of 8 mm diameter. Electrodes of the following shapes were used so as to assess the effect of electrode geometry on EDM performance: cylindrical (C), reverse tee (T), dovetail (D); see Fig. 1a. 2.3 Experimental Procedure All the 18 experiments (Table 1) were conducted in a random order. To ensure reproducibility of experimental data, each combination of experiments was replicated once. Die sinking EDM machine (Model - ZNC, Make - Electronica) with a pulse generator was used. The workpiece and electrode were maintained at negative and positive polarity respectively. A dielectric fluid (IPOL oil) was supplied through side jet flushing nozzle. As mentioned above, three configurations of tool electrodes as shown in Fig. 1a were used. A depth of 6 mm was set for machining the workpieces. A schematic of the EDM set-up is shown in Fig. 1b. Before and after each experiment, the machined plates were cleaned with acetone. A high precision electronic weighing balance was used to measure the weight loss of copper electrode and the work material after each experiment. The surface finish at three different locations on the hole surface was measured using surface roughness tester (Perthometer MAHR II make). Scanning electron microscope (FEI QUANTA 2000 HV SEM), equipped with energy dispersive X-ray analysis (EDX), was used to examine the alterations in surface topography of work material and tool electrode. 3. RESULTS AND DISCUSSION 3.1 Material Removal Rate Analysis of variance for MRR is shown in Table 2. It is observed that pulse current has the most significant effect on MRR as evident from ANOVA. It has the largest contribution on the variability of the MRR among the selected EDM parameters. The other parameters, electrode rotation and electrode shape have moderate influence on MRR (Table 2). However, duty cycle has insignificant effect on MRR. The influence of EDM parameters on the MRR is explained using means plots as shown in Fig. 2. It is found from the main effects plots that at 8 amp, the MRR is lowest. As the pulse current increases to 14 amp, a steep increase in the MRR is observed. Table 2 Analysis of Variance (ANOVA) for MRR Duty cycle 1 0.218 0.218 0.20 0.664 Pulse current 2 233.41 116.70 106.97 0.000 Electrode rotation 2 7.471 3.736 3.42 0.074 Electrode shape 2 8.443 4.222 3.87 0.057 Residual error 10 10.910 1.091 Total 17 260.45 Figure 1: Schematic of (a) Electrodes and (b) Experimental Setup Further, increase in the pulse current to 22 amp, shows further increase in the MRR. It is seen from the analysis of means (AOM) plots that the MRR is high when the electrode is stationary. But as the electrode

Effect of Electrode Shape and Rotation on EDM Performance of Inconel 718 57 24 23 22 21 20 19 18 17 16 15 Figure 2: 8 12 Main Effects Plots for MRR is rotated at 60 rpm, the rate of material removal decreases. Further, an increase in the electrode speed to 120 rpm again causes a decrease in the MRR. It seems that the rotation of electrode does not favour the increase in MRR alone, but in combination with other factors, it influences the material removal rate. It can be noticed from the AOM that the cylindrical electrode gives lower MRR. However, other electrodes, reverse tee and dovetail, do not have much influence on MRR. These electrodes, moreover, give equal material removal rate. 3.2 Electrode Wear Rate Signal-to-noise: Larger is better for MRR 8 14 22 0 60 120 C T D The effect of EDM parameters on electrode wear rate can be seen from the AOM plots presented in Fig. 3. It is observed that the pulse current has the most significant influence on the variability in EWR. In addition, duty cycle is also significant. However, other parameters have less influence on EWR. Table 3 Analysis of Variance (ANOVA) for EWR Duty cycle 1 96.85 96.85 11.55 0.007 Pulse current 2 2725.7 1362.8 162.49 0.000 Electrode rotation 2 2.18 1.09 0.13 0.880 Electrode shape 2 43.17 21.58 2.57 0.125 Residual error 10 83.87 8.39 Total 17 2951.8 30 It is observed from the main effects plots that higher duty cycle (12%) produces low EWR, whereas higher electrode wear occurred at lower duty cycle of 8%. Pulse current has similar effect on EWR as that of MRR. The electrode wear is lower at 8 amp. As the pulse current increases to 14 amp, the electrode wear increases substantially. Similar trend is observed with an increase in pulse current till 22 amp. It is seen from the AOM that EWR is higher when the dovetail electrode was used. However, EWR is less when the cylindrical electrode was used. The reverse tee electrode shows moderate amount of wear during EDM. 3.3 Surface Roughness The chips produced in EDM are in the form of spherical debris due to melting of work material by high frequency electrical sparks. The surface roughness of EDMed hole depends on the size of the craters. Fig. 4 shows the AOM for the surface roughness produced by EDM. It is observed that pulse current and electrode shape have more significant influence on surface roughness. These factors are significant as is evident from ANOVA (Table 4). However, duty cycle and electrode rotation have less effect on surface roughness. Table 4 Analysis of Variance (ANOVA) for Surface Roughness Duty cycle 1 0.3409 0.3409 0.24 0.633 Pulse current 2 45.445 22.722 16.15 0.001 Electrode rotation 2 3.1631 1.5815 1.12 0.363 Electrode shape 2 14.495 7.2476 5.15 0.029 Residual error 10 14.072 1.4072 Total 17 77.516 The best surface finish was observed when the pulse current was low (8 amp). An increase in pulse current to 14 amp resulted in moderate increase in the surface roughness. However, there is a large increase in surface roughness when the pulse current changes to 22 amp. -12 25 20 15 10 5-13 -14-15 0 Signal-to-noise: Smaller is better for EWR 8 12 8 14 22 0 60 120 C T D Figure 3: Main Effects Plots for EWR -16 Figure 4: Signal-to-noise: Smaller is better for Ra 8 12 8 14 22 0 60 120 C T D Main Effects Plots for Surface Roughness

58 I J M E M S: 5(1) 2013 Further, at low discharge energy, the craters are shallow and the surface irregularities are smooth, shallow and less frequent. Therefore, more uniformly spread spherical globules are found on the surface and hence the surface shows lower roughness values. As the current increases, high discharge energy produces deeper craters and therefore the surface has larger roughness. Electrode shape is found to have significant effect on surface roughness. It is seen from the AOM that the cylindrical electrode produces surface with lower roughness value. Higher value of surface roughness was observed for dovetail electrode. However, moderate surface roughness resulted when the reverse tee electrode was used. 4. SURFACE TOPOGRAPHY ANALYSIS Scanning Electron Microscopy (SEM) was used for examining topography of the machined surface and the electrode. The machined surfaces generated by EDM show variety of features. These include scattered debris, shallow craters, irregular size craters, globule appendices, clusters of spherical globules and pockmarks; see Fig. 5a-b. Shape of each crater is shallow and mostly symmetric around vertical axis, where the discharge channel is developed and collapsed. Figure 5: (a) Best (c) Best (b) Worst (d) Worst SEM Images of (a-b) Machined Workpiece and (c-d) Electrodes Some of the debris are hollow spherical shells, which is an indication of solidification from vapour state, and others have irregular solid shapes, an evidence of solidification from liquid state. During EDM, the gases from the melting zone escape continuously. Some of the gases finds their way through the layer of resolidifying metal and result in pockmarks at these locations. A release of large energy for small discharge duration causes the formation of large craters on the surfaces (see Fig. 5a-b). The surface texture produced at higher pulse current was rough. A matte type finish was seen on the EDMed surface, which is formed as a result of irregular crater formation. It was seen that at low pulse current, the craters were shallow and the density of globule appendages and pockmarks was less. On the other hand, at higher pulse current, deep craters and more globule appendages were observed on the machined surface. The resolidified layer of material may also include decomposition products of the dielectric liquid as alloying agent. Surface also contains superposition of craters over the solidified molten metal. Topography of the tool electrode consists of features such as adhered oxide layer, debris of molten material, electrode micro-particles and macro- as well as micro-cracks (see Fig. 5c-d). These deep cracks get formed due to presence of large temperature gradient on the electrode surface. In addition, cracks and foreign particles were also observed on the electrode surface. It was observed from the surface roughness results that the specimen # 3, 10 and 1 have relatively better surface finish as compared to the specimen # 7, 18 and 8. 5. CONCLUSIONS An experimental study was carried out to determine effect of various process parameters along with electrode shape and rotation on EDM performance while machining Inconel 718. The conclusions of this study are as follows: Pulse current is the most significant factor influencing material removal rate. Besides, electrode rotation and electrode shape also have significant influence on MRR. Highest MRR was found when the current was 22 amp, duty cycle was 8% and EDM was performed in stationary mode with dovetail electrode. Pulse current and duty cycle have significant influence on EWR. Cylindrical electrode and 12% duty cycle are found to cause less electrode wear. Electrode rotation does not show significant influence on EWR. Pulse current and electrode shape show significant effect on surface roughness. The optimum conditions that give better surface are: duty cycle 8%, pulse current at 8 amp, 60 rpm electrode and cylindrical electrode. Surface topography of EDMed surfaces shows defects such as crater, clusters of globules, globule

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