STUDY OF OVERCUT DURING ELECTRIC DISCHARGE MACHINING OF HASTELLOY STEEL WITH DIFFERENT ELECTRODES USING THE TAGUCHI METHOD

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1 Research Article STUDY OF OVERCUT DURING ELECTRIC DISCHARGE MACHINING OF HASTELLOY STEEL WITH DIFFERENT ELECTRODES USING THE TAGUCHI METHOD 1 Dinesh Kumar, 2 Naveen Beri, 2 Anil Kumar Address for Correspondence 1 Dept. of Mechanical Engineering, ARNI University, Kathgarh, Indora (HP), India. 2 Dept. of Mechanical Engineering, Beant College of Engineering & Technology, Gurdaspur, (Pb.) India. dinesh123badhan@yahoo.co.in ABSTRACT Electric discharge machining (EDM) has been used mostly in the tool and die industry and the material normally used as tool electrode are copper, tungsten, graphite, brass, silver, copper tungsten and copper chromium alloys. In the present work, hastelloy steel is used as a work piece for investigating using sintered tool electrode of copper tungsten (CuW) with straight and reverse polarity setup in standard EDM oil. The dimensional accuracy is greatly influenced by the overcut resulting from discharge gap and electrode wear. In this paper the effects of input parameters i.e. polarity, tool electrode material, peak current, pulse on time, duty cycle and gap voltage on the overcut was analyzed using analysis of variance and scattered graph. It was found that powder metallurgy tool electrode with reverse polarity gives minimum overcut and Best parametric setting for minimum overcut was found at ve polarity, CuW (Cu 40% W 60%) tool electrode, 4 amp current, 150 µsec pulse on time, 0.83 duty cycle and 60 volts gap voltage i.e A 2 B 2 C 1 D 3 E 2 F 2. KEYWORDS: Electrical discharge machining, powder metallurgy, overcut, analysis of variance. INTRODUCTION Since its development in the mid 1940 s, electrical discharge machining (EDM) provide an effective manufacturing technique that enables production of parts made of hard material with complicated profile that are difficult to produce by Conventional Machining Processes. EDM has widely used for the manufacture of components such as moulds and dies. The EDM process is thermal in nature and the material removal rate occurring via the high frequency spark of energy between the electrodes (a tool and a work piece). Tool and work piece from a pair of electrode separated by a gap of 10 to 200 µm and immersed in a dielectric, generally hydrocarbon oil or deionized water. A pulse generator (D.C. type) is used to initiate discrete spark which have duration in the region of µs and at the same time deionization of the dielectric occurs and the gap is flushed of debris. [1] The exact mechanism of the EDM process is not clearly understood. Experimental studies using a single-spark set up lead to results which are of limited value, because these studies do not faithfully reflect the actual EDM process where spark occurs at a frequency of several khz. [2]. A number of researchers have investigated the EDM process and tried to relate the input parameters to the output parameters experimentally and theoretically [3]. The new developments in the research of EDM have progressed due to the growing application of EDM process and the challenges being faced by the modern manufacturing industries, from the development of new materials that are hard and difficult-to-machine such as tool steels, super alloys, hastalloy, composites, ceramics, heat resistant steel, carbides etc. Typically, the major cost and time components in die and mould maching by EDM are in the tool electrode fabrication, which can account for over 50% of the total machining cost [5]. Basic materials used as EDM tool electrodes include copper, steel, tungsten, chromium, brass, and copper chromium alloys [6, 7, 8]. Conventional methods of fabricating the tool electrodes include stamping, coining, grinding, extrusion, drawing, and more commonly, turning, milling, incurring long processing time and material wastage especially if a complex geometry or profile is required [4]. EDM research has concentrated on achieving improved surface characteristics and overcut [9, 10, 11]. The most of work has been done using mechanically formed tool electrodes and the present EDM user is compelled to search for alternative tooling such as powder metallurgy (PM) method of electrode fabrication which is more economic and faster to manufacture. The tool electrodes made by using powder metallurgy technology from special powders have been used to modify EDM surfaces in recent years, to improve wear and corrosion resistance. Ferreira [12] observed that copper-tungsten tool

2 electrodes with negative polarity are suitable for the planetary EDM surface micro finishing of die steel (AISI H13) with good geometry accuracy and sharp details. Lee [13] investigated the small area EDM process using a copper tungsten tool electrode on AISI 1045 carbon steel and has reported that the values of the MRR, SR increase for higher values of pulse current. Gangadhar [14] reported surface deposition by EDM in a liquid dielectric using powder metallurgy compact tool electrode. Deposition of tungsten carbide on flank and rake of a HSS tool using powder metallurgy electrode and kerosene as dielectric resulted in low variation in cutting forces. Soni and Chakraverti [15] found that appreciable amount of elements has migrated from the tool electrode to work piece and vice versa and got alloyed in the resolidified layer causing a change in chemical composition and significant increase in surface hardness of the work piece during electrodischarge machining of high carbon high chromium die steel (hardened) with rotating copper-tungsten tool electrode. EDM of hardened steel (BS M40, 53Rc) work material of Cu electrode made through powder metallurgy and paraffin as dielectric has been reported by Samual [16]. Wang [17] described a new method of surface modification by EDM. From the above reviewed literature it is observed that powder metallurgy tool electrodes have a significant role in metal removal process in addition to their contribution in surface treatment/modification applications and a need is felt to correlate the usefulness of CuW tool electrode made through powder metallurgy (PM) with a view to optimize the process parameters [18, 19, 20, 21, 22, 23, 24]. The present experimental work is focused on the electrical discharge machining of hastalloy steel with different CuW1 (40% Cu and 60% W) and CuW2 (25% Cu and 75% W) tool electrode made through powder metallurgy technique in comparison with convention Cu tool electrode and an attempt has been made to obtain optimal setting of the process input parameters for minimum overcut with EDM oil as dielectric fluid. Taguchi methodology has been applied to plan and analyze the experiments. EXPERIMENTAL PROCEDURE AND PARAMETERS Experiments were conducted on a smart ZNC EDM machine manufactured by Electronica Machine Tools Ltd. (EMTL) India. Hastalloy Steel (chemical composition as shown in Table 1) was used as work piece material with standard EDM oil as the dielectric fluid. Cylindrical Cu tool electrode and CuW tool electrodes made through powder metallurgy i.e. CuW1 (Cu40%, W60%) and CuW2 (Cu25%, W75%) were used for the experimentation. The parameters selected for the study are polarity, tool electrode material, current, pulse on time, duty cycle and gap voltage Diameter of cavity was measured after each experiment with standard profile projector. Overcut is calculated by subtracting the diameter of electrode from the diameter of cavity and dividing by two. Two sets of 36 experiments (Table 2) were performed as per L36 ( ) Taguchi design and average value of each output parameter were statistically analyzed using Minitab 14.1 software. Table 1: Chemical composition of Hastalloy steel Element Ni Co Cr Mo Fe Si Mn C Ti % Table 2:Experimental Layout Using Mixed Orthogonal Array L36 (2 1 x 3 6 ) Exp. No Polarity (A) Tool Electrode Material (B) Current (C)amp. Ton Time (D) µsec Duty Cycle (E) 1 Positive Copper Positive CuW Positive CuW Positive Copper Positive CuW Positive CuW Positive Copper Gap Voltage (F)volts

3 8 Positive CuW Positive CuW Positive Copper Positive CuW Positive CuW Positive Copper Positive CuW Positive CuW Positive Copper Positive CuW Positive CuW Negative Copper Negative CuW Negative CuW Negative Copper Negative CuW Negative CuW Negative Copper Negative CuW Negative CuW Negative Copper Negative CuW Negative CuW Negative Copper Negative CuW Negative CuW Negative Copper Negative CuW Negative CuW RESULT AND DISCUSSIONS: The mean effect plots of the S/N ratios for the overcut were obtained using Minitab 14.1 software. Plots with the steeper slope and longer lines shows that the factor has significant impact on overcut. The average values of S/N ratios for overcut at different levels are plotted in Fig. 1 keeping the objective smaller is better. For this study the significance of the parameters in effecting the quality characteristic of interest i.e. overcut analysis of variances (ANOVA) was performed. The S/N ANOVA for overcut is given in Table 3. The result of ANOVA indicates that polarity, tool electrode material, current, Ton time, duty cycle, gap voltage effects the overcut.

4 Table- 3 Analysis of Overcut: Source DF Seq SS Adj SS Adj MS F P Polarity Tool Electrode Material Current Ton Time Duty Cycle Gap Voltage Current*Duty Cycle Current*Gap Voltage Duty Cycle*Gap Voltage Residual Error Total Fig. 1 Mean effect plot for S/N ratios for Overcut It is clear from fig.1 that overcut is minimum at the 2 nd level of parameter A (polarity), 2 nd level of parameter B (tool electrode material), 1 st level of parameter C(current), 3 rd level of parameter D (ton time), 2 nd level of parameter E (duty cycle) and 2 nd level of parameter F (gap voltage). The S/N ratio analysis suggest the same levels of parameters (A 2 B 2 C 1 D 3 E 2 F 2 ) as the best levels for minimum Overcut.

5 Scatter plot of overcut with different input parameters are drawn in Fig. 2 and it is clear from these graphs that overcut decreases continuously with increase of voltage on negative polarity and it increases slightly with increase of the voltage on positive polarity. Overcut continuously increases with increase of current value upto 9A and then decreases slightly upto 12A in negative polarity and overcut continuously increases with increase of current value on positive polarity. Overcut slightly increases upto average value of ton time and decreases with further up to final value on reverse and straight polarity. Overcut decreases very slowly up to average value and further it is increases upto final value of negative polarity and on positive polarity it is continuously decreases with increase of duty cycle value. CONFIRMATION EXPERIMENT Three set of confirmation experiments were performed at A 2, B 2, C 1, D 3, E 2 and F 2 levels of the parameters for minimum overcut as suggested by the mean effect plot of overcut and overcut obtained was 0.02mm, 0.01mm and 0.02mm. CONCLUSIONS During EDMing of hastelloy steel using the Taguchi approach following conclusions can be drawn:

6 Best parameteric setting for minimum overcut is with ve polarity, CuW (Cu 40% W 60%) tool electrode, 4 amp current, 150 µsec pulse on time, 0.83 duty cycle and 60 volts gap voltage i.e A 2 B 2 C 1 D 3 E 2 F 2 Reverse polarity with powder metallurgy tool electrode gives the better result in case of overcut study. Minimun value of current gives better results for overcut. It is approx. same as that of diameter of tool electrode. The maximum value of ton time and the average value of duty cycle and gap voltage gives the better result for overcut in this present work. ACKNOWLEDGEMENTS The authors would like to acknowledge the support of department of mechanical engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab, India and All India Council for Technical Education New Delhi, India for supporting and funding the research work under research promotion scheme in this direction vide F.No.: 8023/BOR/RID/RPS-129/ , F.No.:8023/ BOR/RID/RPS-144/ and F.No.:8023/BOR /RID/RPS-86/ REFRENCES: 1. J. A. McGeough., Advanced Methods of Machining, Chapman and Hall, P. Sadagopal and V. Radakrishnan., Development of and studies on a single spark EDM, Proc. X AIMTDR Conf., pp (1982). 3. R. F. Cooke and J. R. Crookall., An investigation of some statistical aspects of electro-discharge machining, Int. J. Mach. Tool Des. Res. 13, (1973). 4. K. Takase, S. Komeigi and I. Uchishiba., Electrical discharge machining with an "Arcless" circuit, Ann. CIRP 22, (1973). 5. G. Semon., A practical guide to electro-discharge machining, 2nd edition Geneva: Ateliers des charmilles, A. Arthur., Using rapid prototyping to produce electrical discharge machining electrodes, Journal of Rapid Prototyping, 2 (1) (1996) S. Singh, S. Maheshwari, P.C. Pandey, Experimental Investigations into Die- sinking Electric Discharge Machining of Hardened AISI 6150 Tool Steel using different electrode Materials, Strojnicky Casopis- Journal of Mechanical Engineering (Slovak), 56 (4)(2005) S. Singh, S. Maheshwari, P.C. Pandey, Some investigations in to the electric discharge machining of hardened tool steel using different electrode materials,journal of Materials Processing Technology, 149(2004) B. Bayramoglu and Duffil, A. W., Manufacturing linear and circular contours using CNC EDM and fame type tools, International Journal of Machine Tool and Manufacture, 35 (8) (1995) I. Ishida, Y. Takeuchi., L-shaped curved hole creation by means of electrical discharge machining and an electrode curved motion generator, International Journal of Advance Manufacturing Technology, 19 (4) (2002) P. Rajurkar and S. M Pandit., Formation and ejection of EDM debris, Transactions ASME, Journal of Engineering for Industry, 108 (1986) C. Ferreira., A study of die helical thread cavity surface finish made by Cu-W electrodes with planetary EDM, International Journal of Advance Manufacturing Technology, 34 (2007) H. Teng Lee, Fu-Chuan Hsu, Tzu-Yao Tai., Study of surface integrity using the small area EDM process with a copper tungsten electrode, Materials Science and Engineering A, 364 (2004) A. Gangadhar, M. S. Shunmugam and P. K. Philip., Surface modification in electro discharge processing with a powder compact tool electrode, Wear, 143 (1) (1991) J. S. Soni and G. Chakraverti., Experimental investigation on migration of material during edm of die steel (T215 Cr12), Journal of Materials Processing Technology, 56 (1996) M. P. Samuel and P. K. Philip., Power metallurgy tool electrodes for electrical discharge machining, International Journal of Machine Tool and Manufacture, 37 (11) (1997) Z. L. Wang, Y. Fang, P. N. Wu, W. S. Zhao and K. Cheng., Surface modification process by electrical discharge machining with a Ti powder green compact electrode, Journal of Material Processing Technology, 129 (1-3) (2002) M. S. Shunmugam, P. K. Philip and A. Gangadhar., Tribological behaviour of an electrodischarge machined surface with a powder metallurgy bronze electrode, Tribology International, 26 (2) (1993) Mohri, N., Saito, N. and Tsunekawa, Y., Metal surface modification by EDM with composite electrode, Annals of CIRP, 42 (1) (1993) P. K. Philip, M. S. Shunmugam, and A. Gangadhar,, Surface modification using electric discharge machining, Journal of Institute of Engineers (India), 1993, 4, N. Beri, S. Maheshwari, C. Sharma and A. Kumar, Parameteric optimization of electric discharge machining of EN-19 using response surface methodology, Strojnicky Casopis- Journal of Mechanical Engineering (Slovak), Vol. 60, no. 4, pp , N. Beri, S. Maheshwari, C. Sharma and A. Kumar, Performance evaluation of powder metallurgy electrode in electrical discharge

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