SOME STUDIES ON ELECTRIC DISCHARGE MACHINING OF HASTELLOY USING POWDER METALLURGY ELECTRODE

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1 Research Article SOME STUDIES ON ELECTRIC DISCHARGE MACHINING OF HASTELLOY USING POWDER METALLURGY ELECTRODE * 1 Vijay Kumar, 2 Naveen Beri, 2 Anil Kumar, 1 Paramjit Singh Address for Correspondence * 1 Deparment of Mechanical Engineering, Amritsar College of Engineering and Technology, Amritsar, Punjab, India. amoghvijay01@gmail.com 2 Department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab, India. ABSTRACT In this paper, an attempt has been made to study the process performance of electrical discharge machining with powder metallurgy tool electrode during the machining of hastelloy using positive polarity. Where current and voltage are taken as process input parameters and material removal rate, tool wear rate, percentage wear rate, surface roughness are taken as output parameters. A set of ten experiments were performed on electronica make smart ZNC electric discharge machine and relationships were developed between input and output parameters. The study indicates that, the maximum material removal rate is at the average value of current and above average value of voltage within selected range of process input parameters, the minimum tool wear rate is with the minimum value of current and voltage, the minimum Percentage wear rate for below average value of current and minimum voltage, the minimum average surface roughness for average value of current and voltage. KEYWORDS Electrical discharge machining (EDM), Material removal rate (MRR), Tool wear rate (TWR), Percentage wear rate (Percentage WR), Surface roughness (SR). INTRODUCTION It is clear that the past few years have seen an increasing interest in the novel applications of electrical discharge machining to see the potential of this technique for better process performances it is obvious that lot of work has been done to optimize the EDM process and the work related to improvement in surface quality is still on the anvil. Very little surface alloying occurs using conventional tool electrodes compared to partially sintered PM electrodes, because binding energy between grains is considerably lower. This powder metallurgy approach to surface alloying is relatively new and there is little information on aspects such as preferred particle size, sintering temperatures and pressures.despite the promising results, electric discharge machining process using powder metallurgy electrode is used in industry at very slow pace. One of the key reasons is that many

2 fundamental issues of this new development, including the machining mechanism are still not well understood. The complexity of this process deserves a thorough investigation the compacting and sintering conditions under which the powder metallurgy electrodes are produced greatly affect their performance. One of the challenges of the research is to produce surfaces that are highly alloyed and have high wear resistance, but meet the surface roughness and topography requirements of the mould making, die casting, rolling and aerospace industries. It is observed that EDM surface alloying and the application of PM electrodes for the same has a great potential for future research. However, very little research work on electric discharge machining has been reported on the applications of electrode formed by powder metallurgy route. From the above discussion a problem of studying, analyzing, and optimizing the machining characteristics using powder metallurgy tool electrode during electric discharge machining of hastelloy steel by varying different machining parameter on ZNC EDM is identified for the problem research work. The process input parameters (current and voltage) will be varied and the response obtained will be analyzed with a view to improve process performance PAST WORK Gangadhar A., et al., [1] they have studied metal transfer from the powder compact tool electrode in EDM by cross-sectional examination, electron spectroscopy for chemical analysis and X-ray diffraction analysis of the work surface. The associated changes in the surface topography were analyzed by scanning electron microscopy and the harmonic contents of the surface profiles. Mohri N., et al., [2] they presented a new method of surface modification by electrical discharge machining using composite structured electrode. Surface modifications on work pieces of carbon steel or aluminum were carried out in hydrocarbon oil using composite electrodes. Copper, aluminum, tungsten carbide and titanium were used as electrode material. It was revealed that there is electrode material in the work surface layer and these surfaces have fewer cracks, higher corrosion resistance, and wear resistance Yoshiki T., et al., [3] they carried out surface modification of aluminum using a new method named electrical discharge alloying. Deposition of titanium contained in electrodes and carbon decomposed from hydrocarbon working fluid occurs on

3 aluminum substrates. With an electrode of a Ti-36mass%Al premixed green compact, composite layers mainly consisting of Ti-C and Ti-Al are formed. The process makes it possible not only to coat the composite layers with a thickness of 100 µm within a few minutes but also to obtain different Ti-C volume ratios by changing the process parameters. Shunmugam M. S., et al., [4] they investigated the use of tungsten carbide powder in compact form. The wear resistance was evaluated by cutting tests and considerable improvement over the uncoated surfaces was observed. EDM with pulsing relaxation generator using a powder compact electrode and reverse polarity setting modified the surface properties of a machined surface greatly. Samuel M. P., et al., [5] the authors have discussed the performance of PM electrodes on various aspects of EDM process. Due to the ease of manufacturing and control over the properties of electrodes, the PM technique has an advantage over other methods of electrode fabrication. PM electrodes affect the micro- and macro variables in EDM and the properties of PM electrodes can be controlled over a wide range by adjusting the compacting and sintering conditions. Zaw H. M., et al., [6] they investigated the use of compounds of ZrB2 and Ti-Si with Cu at various compositions for EDM electrodes by either solid-state sintering or liquid phase sintering. The performance of the newly formed electrode material is compared with that of conventional electrode materials such as Cu, Graphite, and Cu-W etc. Katsushi F., et al., [7] in this paper the author has proposed a surface modification method by EDM with powder suspended in working fluid to avoid the production process of the green compact electrode. Titanium powder was suspended in working oil like kerosene. Ti-C layer grew to a thickness of 150 µm with a hardness of 1600 Hv on carbon steel with an electrode of 1 mm in diameter. Guu Y.H., et al., [8] they investigated the effect of Titanium Nitride (Ti-N) coating by physical vapor deposition (PVD) on the fatigue life of AISI D2 tool steel. The specimen coated by Ti-N exhibited considerably increased hardness, a better surface finish, and decreased super facial tensile residual stress on the surface, increasing their fatigue limit. Wang Z. L., et al., [9] they described a new method of surface modification by EDM. This revolutionary new method is called electrical discharge coating (EDC). The process of EDC begins with electrode wear during EDM, and a kind of hard carbide is created through the chemical reaction

4 between the worn electrode material and the carbon particles decomposed from kerosene fluid under high temperature. The carbide is piled up on a work piece quickly and becomes a hard layer of ceramic of about 20 µm thickness in several minutes. This paper studies the principle and process of EDC systemically by using a Ti powder green compact electrode. Experiments and analyses show that a compact Ti-C ceramic layer can be created on the surface of the metal work piece. The hardness of the ceramic layer is more than three times higher than that of the base body, and the hardness changes gradually from the surface to the base body. Tsai H. C., et al., [10] they proposed a new method of blending the copper powders contained resin with chromium powders to form tool electrodes. The results showed that using such electrodes facilitated the formation of a modified surface layer on the work piece after EDM, with remarkable corrosion resistant properties. Furthermore a higher MRR is obtained as compared to Cu metal electrodes, the recast layer was thinner, and fewer cracks were present on the machined surface. Simao J., et al., [11] they reviewed published work on deliberate surface alloying of various work piece materials using EDM and have given the details of operations involving PM tool electrodes and use of powders suspended in the dielectric fluid, typically aluminium, nickel, titanium, etc. Guu Y.H. et al., [12] in this paper the author has studied the surface characteristics and machining damage caused by EDM on AISI D2 tool steel work piece. This indicates that thickness of recast layer and surface roughness are proportional to the power input. Guu Y.H., et al., [15] the surface morphology, surface roughness, and micro-cracks of AISI D2 tool steel were studied by means of atomic force microscopy (AFM) technique reported that surface roughness and depth of micro-cracks were proportional to the power input. Beri N., et al., [17] correlated the usefulness of electrodes made through powder metallurgy (PM) in comparison with conventional copper electrode during electric discharge machining. Gloria P., et al., [19] saw surface alloying/modification during EDM of martensitic hot-work tool steel components (AISI H13) die-sinking operations using powder metallurgy (PM) tool electrodes as a means of achieving enhanced work piece wear resistance without resorting to a subsequent coating operation. Kumar S., et al., [20] reviewed Surface modification by electrical discharge machining using conventional electrode, powder metallurgy

5 electrode, powder suspended in dielectric. Garg R.K. et al., [21] they saw hard composite materials can be machined by many non-traditional methods like water jet and laser cutting but these processes are limited to linear cutting only. Electrical discharge machining (EDM) shows higher capability for cutting complex shapes with high precision for these materials. Lopez S., et al., [22] investigated that Electro conductive, homogeneous, and dense ceramic/semiconductor/metal nano composites were obtained by hot pressing Experimentation Experiments were conducted on a ZNC EDM machine manufactured by Electronica Machine Tools Ltd. (EMTL) India. It is energized by a 50-Ampere pulse generator and a controller to produce rectangularshaped current pulses. The existing dielectric circulation system of the EMTL EDM machine needs about 60 liters of dielectric fluid in circulation. The process input parameters viz current and voltage, are varied keeping duty cycle, pulse on time, flushing pressure as average values for the optimization of MRR, TWR, Percentage WR and SF. A set of ten experiments were performed using copper chromium (75% Cu and 25% Cr) powder metallurgy electrode. In the first five experiments the and subsequently machined by wire electrical discharge machining (EDM). The addition of semiconductor and metal phases to a high-performance ceramic material produces nano composites which preserve the excellent mechanical properties of the ceramic/metal material while possessing the electrical conductivity necessary for EDM. The role of the low concentration of metal has been investigated from a mechanical and an electrical point of view, and found to be critical to the final behavior. current was varied taking other four input parameters as average value, then another set of five experiments was done with varying voltage keeping other four parameters as average value. The necessary readings are taken after each experiment to calculate process output parameters. Measurement of MRR: MRR = Work piece weight loss (g) Machining Time (min) Measurement of TWR: TWR = Work piece weight loss (g) Machining Time (min) Measurement of Percentage WR: Percentage WR = TWR X 100 MMR Measurement of Surface Roughness Various methods are available for measuring of surface roughness of work piece. The arithmetic surface roughness value (R a ) is

6 used in present work to measure the surface finish. Various measurements of roughness are carried out at the bottom of holes by using Mitutoyo SJ-201P surface roughness tester. The parametric variation chart for each experiment is shown in following table: Table1: Polarity Positive (+) Machining time 20 min. Electrode lift time 0.2 sec. Table 2: Hastelloy Material Composition Element Ni Co Cr Mo Fe Si Mn C Ti % Table 3: Parametric Variation Chart Exp No. Current (A) Voltage (V) Pulse on time (µs) Duty cycle Flushing pressure(pa) Results and Discussion Total ten experiments were performed on hastelloy steel by using powder metallurgy MRR, TWR, Percentage WR, and SR. The final phase of experimental work is analysis and discussion of results. The variations of tool electrode. At the end of each all the four output parameters are plotted experiment; calculations were done for against the variable input parameters. Analysis of Material Removal Rate

7 Current Amount of power used in EDM, is measured in terms of its amperage, and is termed as most important machining parameter in MRR. It is observed that with an increase in current, MRR improves up to 18 amperes and then decreases (fig.1). This can be explained as initially with increase in current MRR increases because of higher erosion of work piece material with increase in current. Thereafter with further increase in current net deposition on work piece material from powder metallurgy tool electrode takes place. This explains the decrease in MRR with further increase in current. Voltage MRR increases, by increasing open circuit voltage, because electric field strength increases. It is observed that with an increase in voltage, MRR improves up to 60Volts and then starts to decrease continuously (fig. 2). This can be explained as initially with increase in voltage, MRR increases because of higher erosion of work piece material with increase in voltage. Thereafter with further increase in voltage net deposition on work piece material from powder metallurgy tool electrode takes place. This explains the decrease in MRR with further increase in voltage. Analysis of Tool Wear Rate

8 Current It is observed that with an increase in current, TWR increases (fig. 3) because as the current increases higher energy is available for the electrode wear. This causes more and more erosion of tool material with increase in current Voltage It is seen that higher the voltage more is the TWR (fig. 4) because electric field strength increases. This results in greater wear rate of electrode with voltage increase. Analysis of Percentage Wear Rate Current Percentage wear rate is the ratio TWR & MRR. It is observed that with an increase in current upto18ampers, Percentage WR decreases and then with further increase in current Percentage WR increases (fig. 5). These variations in trend can be explained with increase in current up to 18 amperes rate of tool wear is less as compared to material removal hence downward trend of the curve. After 18 amperes due to net deposition on work piece material from powder metallurgy electrode results in higher TWR as compared to MRR. Therefore curve has an upward trend. Voltage It is observed that with an increase in voltage, Percentage WR varies slightly up to 60 Volts and then increases with voltage rise (fig. 6). These changes can be explained as increase in voltage up to 60 Volts rate of tool wear is comparable to material removal hence small variation in thee curve. After 60 Volts due to net deposition on work piece material from powder metallurgy tool electrode results in higher tool wear as compared to material removal. Therefore curve shoots upwards.

9 Analysis of Surface Roughness Current It is seen that with an increase in current upto14ampers, surface roughness decreases i.e. surface finish improves due to net material removal and then with further increase in current surface roughness increases due to net material deposition (fig. 7).These variations in trends can be explained with increase in current up to 14amperes rate of tool wear is less as compared to material removal. This result in decreases in surface roughness hence curve moves downward. After 14 amperes due to net deposition on work piece material from powder metallurgy tool electrode results in higher surface roughness. Voltage Surface roughness decreases up to 60 Volts, due to net material removal and then with further increase in voltage surface roughness increases due to net material deposition (fig. 8).These can be explained as, with increase in voltage up to 60 Volts rate of tool wear is less as compared to material removal. After 60 Volts due to net deposition on work piece material from electrode results in higher surface roughness CONCLUSIONS The present experimental work identifies the EDM process performance and the machining conditions with powder metallurgy tool electrode and its effect on process output parameters viz. material removal rate, tool wear rate, percentage wear rate, average surface roughness etc. on hastelloy steel within the selected range of parameters. Within the experimental range of the selected parameters the following conclusions are drawn from this study:

10 1. The maximum material removal rate is obtained at current =18A; voltage =60V. 2. The minimum tool wear rate is obtained at current =6A; voltage = 40V. 3. The minimum percentage wear rate is obtained at current =10A; voltage = 40V. 4. The minimum average surface roughness is obtained at current =14A; voltage= 60V. 5. Material removal rate is less at low current of 6A and it increases up to value 18A and then decreases at higher value of current. Low value of MRR at low current is because of less energy available for erosion of work piece material whereas at high current in spite of higher energy MRR is less because of net material deposition of electrode material on work piece. 6. Similarly MRR is low for small voltage but rises till 60V thereafter decrease continuously. Lesser MRR at small voltage is due to low electric field strength, which results in little erosion of work piece material. At higher voltage no doubt electric field strength is large but due to material deposition on work piece MRR is less. electrode wear. Likewise as voltage rise electric field strength increases resulting in greater wear rate of electrode. 8. The percentage wear rate decreases initially upto18ampers as tool wear is lesser compared to material removal. Thereafter due to net deposition on work piece material from electrode because tools wear is more as compared to work material. Percentages wear rate increases. 9. The percentage WR varies slightly up to 60 Volts and then increases with voltage rise. Initially up to 60 Volts rate of tool wear is comparable to material removal hence little variation in the curve. After 60 Volts deposition of electrode material on work surface takes place leading to higher tool wear than material removal. This result in increased Percentage wear rate. 10. The surface roughness decreases with current of up to14ampers and voltage of up to 60 Volts because of net material removal from work piece. But afterwards as current and voltage rises beyond these values due to net material deposition surface roughness increases 7. TWR rises continuously with current and voltages rise because as the current increases more energy is available for the

11 ACKNOWLEDGEMENTS The authors would like to acknowledge the support of the 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 (F. No.: 8023/BOR/RID/RPS-129/ and F. No.: 8023/BOR/RID/RPS- 144/ ) in this direction. REFERENCES 1. Gangadhar A., Shunmugam M. S., Philip P. K. Surface modification in electro discharge processing with a powder compact tool electrode, Wear, Vol. 143, Issue 1, 1991, pp Mohri N., Saito N., Tsunekawa Y., Kinoshita N., Electrical discharge machining with composite electrode, Annals of CIRP, vol. 42, Issue 1, 1993, pp Yoshiki T., Masahiro O. and Mohri N., Surface modification of aluminum by electrical discharge alloying, Material Science and Engineering Annals, Vol. 174, Issue 2, 1994, pp Shunmugam M. S., Philip P. K., Gangadhar A., Improvement of wear resistance by EDM with tungsten carbide PM electrode, Wear, Volume 171, Issues 1-2, 1994, pp Samuel M. P., Philip P. K., Powder metallurgy tool electrodes for electrical discharge machining, International Journal of Machine Tool Manufacturing, Vol. 37, Issue 11, 1997, pp Zawa H. M., Fuhb Y. H., Neeb A. Y. C., Lub L., Formation of a new EDM electrode material using sintering techniques, Journal of Materials Processing Technology, Vol. 89, 1999, pp Katsushi F., Akinori S., Takezawa H., Mohri N.and Miyake H., Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid, Precision Engineering, Vol. 25, Issue 2, 2001, pp Guu Y. H., Hocheng H. Improvement of fatigue life of electrical discharge machined AISI D2 tool steel by TiN coating,material Science and Engineering Annals, Vol. 318, Issue 2, Nov 2001, pp Wang Z. L., Fang Y., Wu P. N., Zhao W. S., Cheng K., Surface modification process by electrical discharge machining with a Ti powder green compact electrode, Journal of Materials Processing Technology, Vol. 129, Issues 1-3, 2002, pp Tsai H. C., Yan B. H., Huang F. Y. EDM performance of Cr/Cu-based composite electrodes, International Journal of Machine Tool Manufacturing., Vol. 43, Issue 3, 2003, pp Simao J., Lee H. G., Aspinwall D. K., Dewes R. C., Aspinwall E. M., Work piece surface modification using electrical discharge machining, International Journal of Machine Tool Manufacturing Vol. 43, Issue 2, 2003, pp Guu Y. H., Hocheng H. Chou C. Y. and Deng C. S. Effect of electrical discharge machining on surface characteristics and machining damage of AISI D2 tool steel, Material Science and Engineering Annals,, Vol. 358, Issue 1-2, October 2003, pp Bernd M., Schumacher, After 60 years of EDM the discharge process remains still disputed, Journal of Materials Processing Technology, Vol. 149, Issues 1-3, 2004, pp Bulent E., Erden A. Remarks on surface integrity of electric discharge machined

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