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A comparative experimental study on fast hole EDM of Inconel 718 and Ti-6Al-4V A.T. Bozdana, O. Yilmaz, M.A. Okka, İ.H. Filiz Dept. of Mechanical Engineering, The University of Gaziantep, 27310, Gaziantep, Turkey Abstract This paper presents a comparative experimental study on machining and surface characteristics of through and blind holes (Ø1 mm) produced on aerospace alloys of Ti-6Al-4V and Inconel 718 by fast hole rotary EDM process using tubular hollow copper and brass electrodes. Several holes were produced using the identical process parameters, and the corresponding values of Material Removal Rate (MRR) and Electrode Wear (EW) were compared. Surface characteristics of machined hole surfaces were also evaluated based on micrographs obtained by Scanning Electron Microscopy (SEM). The results reveal that the achievement of desirable MRR and EW values and acceptable topography of machined surfaces were dependent upon the appropriate selection of tool electrode material and the choice of making through/blind hole. Keywords: Electrical discharge machining (EDM), Titanium, Nickel, Scanning electron microscope (SEM) 1 INTRODUCTION Conventional drilling techniques cannot be employed to produce small-size holes on difficult-to-cut materials as tool wear/breakage and slow machining rates lead to inaccurate hole dimensions and unacceptable surface quality. Electrical Discharge Machining (EDM) process has recently been used for drilling holes of varying sizes on various materials. During EDM process, a small gap is maintained between the workpiece and an electrode and the machining process takes place due to high-voltage sparks causing the removal of small particles away from the workpiece being machined. Fast hole EDM drilling process has also become popular due to use of rotary tubular electrodes through which dielectric fluid is continuously flowing in order to provide better flushing effect during the process, resulting in an improved surface quality of machined surfaces. Wang and Yan  used rotary EDM concept to make blind holes on Al2O3/6061 Al composite. Mohan et al. [2,3] studied drilling of holes on Al-SiC metal matrix composite using rotary tubular electrodes made of different materials. Drilling of through and blind holes on plastic mould steel using tungsten carbide electrode was reported in Opoz et al. . The characteristics of microholes produced by EDM on carbide using copper tool electrode were examined by Yan et al. . Nakaokua et al.  investigated the hole drilling on sintered diamond by rotary micro-EDM using tungsten electrode. Diver et al.  produced micro-EDM tapered holes on case hardened steel (18CrNi8) using rotary WC electrode. Various components in aerospace industry such as turbine blades and disks, compressor blades and other engine components are made of special titanium and nickel-based alloys (namely Ti-6Al-4V and Inconel 718). These components have small-size cooling holes as they are working in a hostile environment (i.e. at high speeds and elevated temperatures). Consequently, such holes are required to have good surface finish and excellent dimensional accuracy. In spite of many studies on fast EDM hole drilling, the research on making of small-size holes on aerospace alloys is limited. Hascalik and Caydas  investigated the influence of sinking EDM parameters on machining of Ti6Al-4V using graphite, copper and aluminium electrodes. Asokan et al.  analysed the effect of EDM parameters on deep hole drilling of titanium alloy using copper electrode. Pradhan et al.  studied the optimisation of EDM hole drilling parameters on Ti-6Al-4V using brass electrode. The influence of EDM parameters in deep hole drilling of Inconel 718 using pure electrolytic copper was examined by Kuppan et al. . This paper presents a comparative experimental study on fast hole drilling of two aerospace alloys: Inconel 718 and Ti-6Al-4V. Two types of rotary tubular electrodes (brass and copper) were employed to produce through and blind holes (Ø1 mm) using identical process parameters. MRR and EW values calculated after the experiments were compared. The topography of machined surfaces was also compared based on SEM micrographs. 2 EXPERIMENTAL PROCEDURE 2.1 Test piece and electrode materials The test piece materials used in this study were common aerospace super alloys: Inconel 718 (IN718) and α+β type Ti-6Al-4V (Ti64). In spite of poor machinability and low mechanical properties, these materials are preferred in aerospace applications due to their specific thermal and physical properties. The chemical compositions of IN718 and Ti64 are given in Table 1 and 2, respectively. Table 1: Chemical composition of IN718 (wt. %). Ni Cr Fe Nb (+Ta) Mo Co Ti 50-55 17-21 Balanced 4.75-5.50 2.80-3.30 1.00 max. 0.65-1.15 Al Si Mn Cu C B 0.20-0.80 0.35 max. 0.35 max. 0.30 max. 0.08 max. 0.06 max. Table 2: Chemical composition of Ti64 (wt. %). Ti Al V Fe 89.464 6.08 4.02 0.22 O C N H 0.18 0.02 0.01 0.0053 5th International Conference and Exhibition on Design and Production of MACHINES and DIES/MOLDS 18-21 JUNE 2009 Pine Bay Hotel - Kusadasi, Aydin, TURKEY Brass and copper electrodes with tubular hollow shape (single-channel) were employed during the experiments. These are two commonly used tool electrode materials in fast hole drilling applications because of their desirable thermal and electrical properties. Table 3 shows melting points and thermal conductivities of base and electrode materials used in this study. Table 3: Properties of base and electrode materials. IN718 Ti64 Cu Br Melting point (ºC) 1336 1649 1084.62 900 - 940 Thermal conductivity (W/m-ºK) 11.4 7.2 391 159 Electrode Wear (EW) was determined according to depth of drilled hole and the amount of electrode consumption (i.e. the variation in electrode length): EW (%) = consumed electrode in length x 100 (2) machined hole depth SEM micrographs of machined hole surfaces were taken on JEOL JSM-6390LV SEM at x330 magnification. 3 EXPERIMENTAL RESULTS AND DISCUSSION Electrode Material _ Hole Type 2.2 Experimental setup The experiments were performed on test pieces of IN718 and Ti64 with dimensions of 6 mm x 11 mm x 35 mm using JS EDM AD-20 hole drilling machine. The surfaces of test pieces were ground prior to experiments. The flat surfaces of two specimens were aligned in order to ensure that the mating surfaces could be secured accurately using a specially designed and manufactured fixture. The holes were drilled on the line of intersection of these mating surfaces to enable implementation of standardised experiments, easier handling of test pieces after experiments, and performing reliable measurements on the machined hole surfaces. Table 4 presents the machining conditions. Several holes of Ø1 mm were produced at identical process parameters in order to make a back-to-back comparison. The through holes were 11 mm deep while the depth of blind holes varied from 5 mm to 8 mm. Figure 1 shows a pair of test pieces with drilled through and blind holes. Table 4: Machining conditions. Discharge current (A) Pulse duration (µs) Pulse interval (µs) Dielectric Dielectric flushing pressure (bar) Electrode rotation (rpm) Polarity of tool electrode ≈ 20 38 23 deionised water 100 200 negative 3.1 Material removal rate Figures 2 and 3 show MRR values for through and blind holes produced on IN718 and Ti64 test pieces using brass and copper tool electrodes, respectively. The results reveal that brass electrode exhibited higher MRR values than copper electrode during production of through and blind holes on both IN718 and Ti64. This is due to the fact that the brass electrode with a lower thermal conductivity transmits the heat occurring during machining to the base material, leading to a higher rate of erosion as compared with copper electrode. Material removal rate (mg/min) 0 10 20 30 40 50 60 70 80 90 Br_through Br_blind Cu_through Cu_blind IN718 Figure 2: MRR for through/blind holes produced on IN718 using Cu/Br electrodes. Material removal rate (mg/min) 0 Electrode Material _ Hole Type 3 6 9 12 15 18 21 24 27 Br_through 35 5 11 Br_blind All dimensions are in mm Cu_through Figure 1: The test pieces with through and blind holes. 2.3 Measurement procedure The drilling time for each hole was recorded using an electronic timer. The test pieces were weighed before and after drilling using a digital precision scale. Based on these measurements, Material Removal Rate (MRR) for each experiment was calculated by the following formula: Cu_blind Ti64 Figure 3: MRR for through/blind holes produced on Ti64 using Cu/Br electrodes. It is also evident that MRR values for machining IN718 were considerably higher than those for Ti64. This can be explained due to the effect of melting point. The melting point of IN718 is lower than that of Ti64 which causes a higher erosion rate during machining of IN718. MRR values for drilling through holes were also higher than those for blind holes, except for the case of drilling MRR (mg/min) = initial weight − final weight machining time (1) Ti64 using copper electrode where MRR for through and blind holes was equal. The duration of machining is longer in case of drilling through holes due to the greater hole depth. This results in generation of higher amount of heat that is absorbed by the base material, and thus the MRR increases in case of drilling deeper holes. 3.2 Electrode wear EW for through/blind holes produced on IN718 and Ti64 test pieces using both electrode materials are shown in Figures 4 and 5, respectively. Similar to MRR, brass electrode exhibited greater EW than copper electrode for drilling of through and blind holes on both base materials. During machining, melting of brass was easier because of its lower melting point, which leads to a greater amount of EW than copper. The only exception was the case of drilling through holes on IN718 though there was a slight difference of 0.5% in magnitudes of EW of brass and copper electrodes. It can also be seen from the results that EW values of both electrode materials for drilling through and blind holes on IN718 were much higher as compared with Ti64. There is a direct relationship between MRR and EW. Hence, IN718 exhibited higher EW due to the fact that greater MRR was obtained in machining of IN718. Electrode wear (%) 18 24 30 36 through holes, which causes higher amount of heat to be absorbed by the tool electrode. Therefore, production of deeper holes results in higher tool wear. The other reason could be the shape of blind hole. A typical blind hole after drilling is shown in Figure 6. As seen from the figure, a portion at the bottom of hole cannot be machined due to the flow of dielectric fluid through the electrode. During machining process, the tool electrode is trying to erode this portion, which increases the machining time and the electrode wear. drilling direction Figure 6: The shape of a typical blind hole. 3.3 Surface characteristics SEM micrographs of drilled hole surfaces are presented in Figure 7. Due to high EW, damages such as globules of debris, pockmarks and melted drops are observed on the surfaces of through holes produced on IN718 using copper electrode and also on Ti64 using brass electrode. During EDM, some particles eroding from the tool electrode stick on the machined hole surface, leading to formation of a rough surface with irregularities. IN718_through_Br Ti64_through_Br 0 Electrode Material _ Hole Type 6 12 42 48 54 Br_through Br_blind Cu_through Cu_blind IN718 IN718_through_Cu Ti64_through_Cu Figure 4: EW for through/blind holes produced on IN718 using Cu/Br electrodes. Electrode wear (%) 6 8 10 0 Electrode Material _ Hole Type 2 4 12 14 16 Br_through IN718_blind_Br Ti64_blind_Br Br_blind Cu_through Cu_blind Ti64 IN718_blind_Cu Ti64_blind_Cu Figure 5: EW for through/blind holes produced on Ti64 using Cu/Br electrodes. Moreover, production of through holes caused higher EW as compared with blind holes, except for the case of drilling through and blind holes using copper electrode on Ti64 where the same values of EW were obtained. Akin to MRR, this is due to the reason of machining deeper holes. The machining duration is longer in case of drilling Figure 7: SEM micrographs of EDM hole surfaces. Such damages are even more intensively dispersed on the corresponding surfaces of blind holes. This is due to the reason that the particles eroding from the surface being machined cannot be flushed away sufficiently. Thereby, even rougher surfaces are obtained during the production of blind holes upon solidification of debris and melted drops on the surface. Furthermore, craters and cracks of varying size were observed on the surfaces of through holes produced on IN718 and Ti64 test pieces using brass and copper electrodes, respectively. These cracks occur due to high pulse current and high pulse duration during machining. Melted drops and pockmarks are also more pronounced on the corresponding blind hole surfaces. It should be noted that these damages occurring during EDM process can be minimised by establishing the optimised process parameters to achieve desirable surface quality. 4 CONCLUSIONS Various through and blind holes have been produced at identical EDM process parameters on IN718 and Ti64 test pieces using brass and copper tool electrodes. The changes in magnitudes of MRR and EW have been compared. Surface characteristics of drilled hole surfaces have also been examined based on SEM micrographs. The following conclusions have been reached: 1. In general, brass electrode has provided a superior MRR for the production of through and blind holes on IN718 and Ti64 test pieces as compared with copper electrode. However, EW of brass electrode was only 0.5% less than that of copper electrode when through holes were drilled on IN718. 2. EW of brass electrode was considerably higher when through and blind holes were made on Ti64. However, both electrode materials exhibited similar magnitude of EW in case of producing holes on IN718, except for the case of drilling blind hole which caused lower EW. 3. The magnitudes of MRR and EW were influenced by the hole type. In general, MRR and EW were reduced in drilling of blind holes. The only exception was the case of drilling holes on Ti64 using copper electrode where both through and blind holes exhibited identical MRR and EW values. 4. SEM micrographs of machined hole surfaces reveal the effects of electrode material and hole type on their surface characteristics. Globules of debris, pockmarks and melted drops were observed on the surfaces of through holes produced on IN718 and Ti64 by copper and brass electrodes, respectively. Such damages on the surfaces of blind holes were more dispersed and pronounced on the corresponding test piece materials. 5. A comparative evaluation of effects of EDM process parameters on the characteristics of drilled surfaces as well as material removal rate and electrode wear is under way in order to achieve an improved surface quality and the optimum machining conditions. 5 ACKNOWLEDGMENTS The authors would like to thank The Scientific and Technological Research Council of Turkey (TÜBİTAK) for financially supporting this research work under 1001 Scientific and Technological Project Scheme, Grant No. 108M022. 6 REFERENCES  Wang, C.C., Yan, B.H., 2000, Blind-hole drilling of Al2O3/6061 Al composite using rotary electrodischarge machining, Journal of Materials Processing Technology, 102:90-102.  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