Some Investigations on Electrical Discharge Machining of OHNS...

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1 Some Investigations on Electrical Discharge Machining of OHNS Steel with a Copper Tungsten Powder Metallurgy Electrode Using Response Surface Methodology Dalbir Singh* 1, Naveen Beri 2 and Anil Kumar 3 1,2,3 Department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab, India Abstract: In the present work, a study has been made to optimize the process parameters of electrical discharge machining (EDM) of oil hardened non-shrinking (OHNS) steel with copper tungsten (30% Cu and 70% W) powder metallurgy (PM) electrode. Response surface methodology (RSM) has been used to plan and analyze the experiments. The experimental plan adopted is a standard RSM design called face centered central composite design (CCD). Pulse on time, gap voltage, input current of EDM were chosen as input variables to study the process performance in terms of material removal rate and surface roughness. The results identify the most important parameters to maximize material removal rate and minimize surface roughness. The validity of response surface equations as derived from quadratic regression fit was verified by conducting confirmation experiments. Keywords: Powder metallurgy (PM) electrode; response surface methodology; surface roughness; material removal rate. 1. INTRODUCTION Electrical discharge machining (EDM) is a common nonconventional material removal process. This technique has been *Corresponding Author: er.dalbirsingh@gmail.com Journal of Metallurgical Engineering, 1(1-2) January-December

2 Dalbir Singh, Naveen Beri and Anil Kumar widely used in modern metal working industry for producing complex cavities in dies and moulds, which are otherwise difficult to create by conventional machining [1]. However it is observed that considerable research work has been done on various aspects of electrical discharge machining of low carbon steels, carbides and a few die steels but sufficient data is not available on oil hardened non-shrinking (OHNS) steel even though it is widely used in die and mould making industries. Surface properties of OHNS steel can also be improved by using powder metallurgy electrode rather than using copper electrode. As a result the economical machining of OHNS steel still poses difficulty to manufacturers. There is a need to investigate the machining of this material with copper tungsten (CuW) electrode made through powder metallurgy technique by varying different machining parameters i.e. discharge current, gap voltage and pulse on time. The effect of using powder metallurgy electrode on the properties of work-piece was studied by Gangadhar, et al. [2] they 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 et al. [3] 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 the characteristics of the surface of work material remarkably changed. These surfaces have fewer cracks, higher corrosion resistance and wear resistance. Shunmugam, et al. [4] investigated that metal transfer from tool electrode to machined surface is appreciable in electric discharge machining when a powder metallurgy compact tool electrode is used under reverse polarity condition. Resultant surface shows reduction 90 Journal of Metallurgical Engineering, 1(1-2) January-December 2011

3 in friction coefficient and reduction in wear later they also investigated the use of tungsten carbide powder in compact form. It was found that reverse polarity setting modified the surface properties of a machined surface greatly. Tsunekawa, et al. [5] 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 aluminum substrates. With an electrode of a Ti-36 mass% Al premixed green compact, composite layers mainly consisting of TiC and TiAl was formed on the workpiece. Samuel and Philip [6] 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. Zawa, et al. [7] investigated the use of compounds of ZrB 2 and TiSi 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 CuW etc. Marafona and Wykes [8] performed an investigation into the optimization of EDM which uses the effect of carbon which has migrated from the dielectric to tungsten-copper electrodes. This work has led to the development of a two-stage EDM machining process where different EDM settings are used for the two stages of the process giving a significantly improved material removal rate for a given tool wear ratio. Li, et al. [9] investigated the effect of titanium carbide (TiC) in sintered copper tungsten (Cu W) electrodes on EDM performance whereas normally used electrodes materials are various types of copper, graphite, tungsten, brass and silver. The tool material with added TiC showed good performance. The surface roughness decreased with the increase of relative density and vice versa. They concluded that the highest relative density, Journal of Metallurgical Engineering, 1(1-2) January-December

4 Dalbir Singh, Naveen Beri and Anil Kumar lowest electrical resistively, and best EDM could be obtained by 15% TiC addition, i.e. the lowest tool wear rate, highest material removal rate and best surface finish. Wang, et al. [10] described a new method of surface modification by EDM. By using an ordinary EDM machine tool and kerosene fluid, a hard ceramic layer was created on the work piece surface with a Ti or other compressed powder electrode in a certain condition. This revolutionary new method is called electrical discharge coating (EDC) this method can find wide application in the fields of surface modification, such as the surface repairing and strengthening of cutting tools and molds. Simao, et al. [11] performed surface modification/alloying and combined electrical discharge texturing (EDT) of hardened AISI D2 sendzimir rolls using powder metallurgy green compact and sintered electrodes. The results indicate that Ti and/or W contained in the PM electrode together with C decomposed from the dielectric medium during sparking were transferred to the work piece surface. Hardness of rolls is more when textured with PM electrode as compared with conventionally processed EDT rolls. Tsai, et al. [12] 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. urthermore a higher MRR was obtained as compared to Cu metal electrodes, the recast layer was thinner and fewer cracks were present on the machined surface. Simao, et al. [13] 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. Mohri, et al. [14] investigated that during surface modification by EDM with semisintered electrodes, worn substances in the gap region form the material source of the layer generated on the work piece surface. A crystallize carbon layer or carbide layer from the working oil cover the surface of the insulated ceramic during EDM. Guu [15] analyzed the surface morphology, surface roughness and micro crack of AISI 92 Journal of Metallurgical Engineering, 1(1-2) January-December 2011

5 D2 tool steel by means of atomic force microscopy (AM) technique. An excellent machined finish was obtained at low pulse energy. The surface roughness and depth of micro-cracks were proportional to power input. Kansal, et al. [16] optimizes the process parameters of powder mixed electrical discharge machining using response surface methodology. The results identify the most important parameters to maximize material removal rate and minimize surface roughness. Khanra, et al. [17] have developed ZrB2-Cu composite by adding different amount of Cu and tested as a tool material for maching of mild steel. The result showed that ZrB2-40 wt% Cu composite has more MRR, with less tool removed rate than commonly used Cu tool. But diameteral over cut and average surface roughness are found to be lesser in case of Cu tool. Beri, et al. [18] performed experimentation on electric discharge machining of AISI D2 steel in kerosene with copper tungsten (30% Cu and 70% W) electrode (made through PM technique) and conventional Cu electrode. An L18 orthogonal array of Taguchi methodology was used to identify the effect of process input factors (viz. current, duty cycle and flushing pressure) on the output factors (viz. material removal rate and surface roughness). It was recommended to use conventional Cu electrode for higher material removal rate and CuW electrode made through PM for higher surface finish. All these input parameters are interrelated and it appears that there has been no attempt to find out their combined effect on various output parameters with CuW electrode made through powder metallurgy. Hence there is a need to investigate the effect of these parameters on material removal rate (MRR) and surface finish using CuW tool electrodes for efficient machining of OHNS steel. All these parameters are important for an effective study of the EDM process as a production operation. Material removal rate justifies the economy of the process and its selection in comparison to the conventional methods of machining. It also affects the surface finish and dimensional accuracy of the cavity produced. The rate of material removal depends upon various factors such as amount of current (energy) in each discharge, duty cycle, frequency of Journal of Metallurgical Engineering, 1(1-2) January-December

6 Dalbir Singh, Naveen Beri and Anil Kumar discharge, electrode material, electrode size, work-piece material, polarity and dielectric flushing condition. Surface finish produced on the machined surface plays an important role in production. It becomes more desirable when hardened materials are machined which require no subsequent polishing or grinding. The surface generated by EDM is actually a multitude of spherical craters produced by the energy contained in the sparks and this again depends upon the amperage, frequency, flushing pressure, material of electrode, polarity and finish on the electrode. These craters help in retaining lubrication during press working and are desirable in such applications. rom the reviewed literature it is observed that powder metallurgy tool electrodes have a significant role in metal removal process in addition to their contribution on surface treatment/ modification applications. Therefore a need is felt to correlate the usefulness of CuW electrode made through PM with a view to optimize the process parameters. The present experimental work is focused on electrical discharge machining of AISI D2 steel with CuW (25% Cu and 75% W) electrode made through powder metallurgy technique. Response surface methodology (RSM) has been used to plan and analyze the experiments and the experimental plan adopted is face centered central composite design (CCD). An attempt has been made to obtain optimal setting of the process input parameters, which may yield optimum material removal rate and surface roughness with kerosene as dielectric fluid. 2. EXPERIMENTAL WORK 2.1 Response Surface Methodology (RSM) Response Surface Methodology (RSM) is a sequential experimentation strategy for empirical model building and optimization. It is a collection of mathematical and statistical techniques that are useful for the modeling and analysis of problems in which a response of interest is influenced by several variables and the objective is to optimize this response. By conducting experiments and applying 94 Journal of Metallurgical Engineering, 1(1-2) January-December 2011

7 regression analysis, a model of response to some independent input variables can be obtained. RSM is often applied in the characterization and optimization of the process [16, 19, 20]. In RSM, it is possible to represent independent process parameters in quantitative form as: Y = f (X 1, X 2, X 3... X n ) ± ε (1) Where, Y is the response (yield), f is the response function, ε is the experimental error, and X 1, X 2, X 3... X n are independent parameters. By plotting the expected response of Y, a surface, known as the response surface is obtained. The form of f is unknown and may be complicated. Thus, RSM aims at approximating f by a suitable lower order polynomial in some region of the independent process variables. If the response can be well modeled by a linear function of the independent variables, the function (Equation (1)) can be written as: Y = C 0 + C 1 X 1 + C 2 X C n X n ± ε (2) However, if a curvature appears in the system, then a higher order polynomial such as the quadratic model (Equation (3)) may be used: n n 2 0 i= 1 i i i= 1 i i Y = C + C X + d X ± ε (3) The objective of using RSM is not only to investigate the response over the entire factor space, but also to locate the region of interest where the response reaches its optimum or near optimal value. By studying carefully the response surface model, the combination of factors, which gives the best response, can then be established. 2.2 Experimental detail Experiments were conducted on E-ZNC logic EDM machine manufactured by Electronica Machine Tools Ltd., India. OHNS steel (specimens 60mm 40mm 4mm) hardened to HRC was used as work piece material with EDM oil as dielectric fluid. Journal of Metallurgical Engineering, 1(1-2) January-December

8 Dalbir Singh, Naveen Beri and Anil Kumar Cylindrical CuW electrodes made through powder metallurgy (Cu25%, W75%, diameter 8mm) were used for the experimentation. Then erosion was switched on for a depth of cut of 1mm and time taken to complete the operation was noted. MRR was obtained using weight loss method. The surface roughness (Ra value in microns) was measured on a Surfcoder (model SE 1200, make Kosaka Laboratory Ltd., Japan). The following four independent parameters were chosen for the study keeping all other parameters constant: 1. Peak current, A; 2. Pulse on time, B; 3. Gap voltage, C; The ranges of these parameters were selected on the basis of preliminary experiments conducted by using one variable at a time approach. The coded and real levels of independent parameters for the three input variables are presented in Table 1. Table 1 The Coded and Real Levels of Independent Parameters actor symbol Parameter Levels A Current, I (Amp.) B Pulse on Time, Ton (µs) C Gap Voltage (Volts) Coded values for each parameter -1(low) 0 1 (High) or the present experimental work response surface methodology (RSM) was adopted to plan and analyze the experiments. The experimental plan adopted was face centered central composite design (CCD) and involved a total of 20 experimental observations (as depicted in Table 2). Each combination of experiments was carried out two times under the same conditions and their average was taken for experimental analysis. The Design Expert 7.1 software was used for regression and graphical analysis of the data obtained. 96 Journal of Metallurgical Engineering, 1(1-2) January-December 2011

9 The optimum values of the selected variables were obtained by solving the regression equation and by analyzing the response surface contour plots. Table 2 The Experimental Trial Conditions (X Matrix) Exp. No. Current (I) Pulse on Time Gap Voltage (amp) (A) (Ton) (Sec) (B) (Volts) (C) RESULTS AND DISCUSSIONS The EDM of OHNS steel was studied by conducting experimental work using standard RSM design. or analysis the data, the checking of goodness of fit of the model is very much required. The model adequacy checking includes test for significance of the regression model, test for significance on model coefficients and test for lack of fit. or this purpose, analysis of variance (ANOVA) was performed. Journal of Metallurgical Engineering, 1(1-2) January-December

10 Dalbir Singh, Naveen Beri and Anil Kumar 3.1 Analysis of Material Removal Rate The fit summary recommended that the quadratic model is statistically significant for analysis of MRR. The results of the quadratic model for MRR in the form of ANOVA are given in Table 3. After eliminating the non-significant terms, the final response equation for MRR is given as follows: MRR = * A * B * C * B * C * C 2 (In coded form) MRR = * Current E 003 * Pulse On Time * Gap Voltage E 003 * Pulse On Time * Gap Voltage E 003 * Gap Voltage 2 (In actual factors) Table 3 ANOVA Table for MRR Source Sum of Degree of Mean -Value Prob.> Squares reedom Square Model < significant A-Current < B- Pulse < On Time C- Gap Voltage AB 7.462E E AC BC A B C Residual Lack of it insignificant Pure Error Cor Total Journal of Metallurgical Engineering, 1(1-2) January-December 2011

11 igure 1 shows the estimated response surface for MRR in relation to the design parameters of peak current and pulse on time. As can be seen from this figure, the MRR tends to increase, considerably with increase in peak current for any value of pulse on time. Hence, maximum MRR is obtained at high peak current (10 A) and high pulse on time (150 µs). This is due to their dominant control over the input energy. ig. 1: (Gap voltage = 50V) Effect of Current & Pulse on Time on Metal Removal Rate ig. 2: (Pulse on time = 100 microsec) Effect of Current & Gap Voltage on Metal Removal Rate Journal of Metallurgical Engineering, 1(1-2) January-December

12 Dalbir Singh, Naveen Beri and Anil Kumar The effect of peak current and gap voltage on MRR is shown in ig. 2. This figure displays that the value of MRR increases with increase in peak current. The reason is same as stated earlier. The effect of gap voltage on MRR can also be seen from this figure. It also tends to increase with increase in gap voltage but at a lower rate. 3.2 Analysis of Surface Roughness or surface roughness, the fit summary recommended that the quadratic model is statistically significant for analysis. Table 4 presents the ANOVA table for a quadratic model for SR. After eliminating the non-significant terms, the final response equation for SR is given as follows: SR = * A+0.38 * B * A (In coded form) Table 4 ANOVA Table for SR Source Sum of Degree of Mean -Value Prob.> Squares reedom Square Model < significant A-Current < B- Pulse On Time C- Gap Voltage AB AC BC A B C E E Residual Lack of it insignificant Pure Error Cor Total Journal of Metallurgical Engineering, 1(1-2) January-December 2011

13 igure 3 shows that surface roughness tends to increase with the increase of current. It also increases with increase in pulse on time but at a lower rate. The best surface roughness is obtainable at the lower level of current (4.0 A) and pulse on time (50). igure 4 shows that surface roughness tends to increase with the increase of current. At 4.0 A current, the surface roughness increases negligibly with increase in gap voltage. At 10.0 A current surface roughness increases with increase in duty cycle. ig. 3: (Gap Voltage = 50V) Effect of Pulse on Time and Current on Surface Roughness ig. 4: (Pulse on Time = 100 microsec) Effect of Current & Gap Voltage on Surface Roughness Journal of Metallurgical Engineering, 1(1-2) January-December

14 Dalbir Singh, Naveen Beri and Anil Kumar 4. CONIRMATION EXPERIMENTS In the present study response surface equations are derived from quadratic regression fit, so to verify their validity confirmation tests are performed by taking the independent variable values within the ranges for which the formulas were derived. The three conformation experiments were performed for MRR and SR. Table 5 shows the results of the confirmation runs and their comparisons with the predicted designed for MRR and SR. It is observed that the calculated error is small (with in 10 %). This confirms the reproducibility of experimental conclusions. Table 5 Confirmation Test Result and Comparison with Predicted Result as Per Model Sl. Current Pulse On Gap Material Removal Rate Surface Roughness (SR) No. (I) (amp) Time (Ton) Voltage mg/min µm (µsec) (V) (Volts) Exp. Predicted Error% Exp. Predicted Error% CONCLUSIONS On the basis of the present experimental study, following conclusions can be drawn with in the experimental range regarding the effect of input parameters (current, pulse on time, gap voltage) on the response parameters viz MRR and SR during electric discharge machining of OHNS steel. 1. Electric discharge machining of OHNS steel is feasible with a CuW powder metallurgy processes electrode. 2. Material removal rate increases considerably with increase in peak current for any value of pulse on time and maximum MRR is obtained at high peak current (10 A) and high pulse on time (150 µs) for a gap voltage of 50V. 102 Journal of Metallurgical Engineering, 1(1-2) January-December 2011

15 3. Material removal rate also tends to increase with increase in gap voltage but at a lower rate. 4. Surface roughness increases with the increase of current. It also increases with increase in pulse on time but at a lower rate. The best surface roughness is obtainable at the lower level of current (4.0 A) and pulse on time (50) for a gap voltage of 50V. 5. The response surface equations as derived from quadratic regression fit were verified through confirmation tests by taking the independent variable values within the ranges for which the formulas were derived. Small value (with in 10%) of the calculated error confirms the reproducibility of experimental conclusions. 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 of Technical Education New Delhi, India for supporting and funding the research work under research promotion scheme (. No: 8023/ BOR/RID/RPS-129/ ) in this direction. REERENCES [1] Ho, K.H. and Newman, S.T. (2003). State of the Art Electrical Discharge Machining (EDM), International Journal of Machine Tool and Manufacture, 43, [2] Gangadhar A., Shunmugam M.S., and Philip P.K., (1991). Surface Modification in Electro Discharge Processing with a Powder Compact Tool Electrode, Wear, 143(1), [3] Gangadhar A., Shunmugam M.S., and Philip P.K. (1991). Surface Modification in Electro Discharge Processing with a Powder Compact Tool Electrode, Wear, 143(1), [4] Shunmugam M.S., Philip P.K., and Gangadhar A., (1993). Tribological Behaviour of an Electrodischarge Machined Surface with a Powder Metallurgy Bronze Electrode, Tribology International, 26(2), Journal of Metallurgical Engineering, 1(1-2) January-December

16 Dalbir Singh, Naveen Beri and Anil Kumar [5] Tsunekawa Y., Okumiya M., and Mohri N., (1994). Surface Modification of Aluminum by Electrical Discharge Alloying, Materials Science and Engineering A, 174(2), [6] Samuel M.P., and Philip P.K., (1997). Power Metallurgy Tool Electrodes for Electrical Discharge Machining, International Journal of Machine Tools and Manufacture, 37(11), [7] Zaw H.M., uh J.Y.H., Nee A.Y.C., and Lu L., (1999). ormation of a New EDM Electrode Material using Sintering Techniques, Journal of Materials Processing Technology, 89, [8] Marafona J., and Wykes C., (1999). A New Method of Optimizing Material Removal Rate using EDM with Copper-tungsten Electrodes, International Journal of Machine Tools and Manufacture, 40(2), [9] Li L., Wong Y.S., uh J.Y.H., and Lu L., (2001). Effect of TiC in Coppertungsten Electrodes on EDM Performance, Journal of Materials Processing Technology, 113(1-3), [10] Wang Z.L., ang Y., Wu P.N., Zhao W.S., and Cheng K., (2002). Surface Modification Process by Electrical Discharge Machining with a Ti Powder Green Compact Electrode, Journal of Materials Processing Technology, 129(1-3), [11] Simao J., Aspinwall D., Menshawy.E., and Meadows K., (2002). Surface Alloying using PM Composite Electrode Materials when Electrical Discharge Texturing Hardened AISI D2, Journal of Materials Processing Technology, 127(2), [12] Tsai H.C., Yan B.H., and Huang.Y., (2003). EDM Performance of Cr/Cubased Composite Electrodes, International Journal of Machine Tools and Manufacture, 43(3), [13] Simao J., Lee H.G., Aspinwall D.K., Dewes R.C., and Aspinwall E.M., (2003). Work Piece Surface Modification using Electrical Discharge Machining, International Journal of Machine Tools and Manufacture, 43(2), [14] Mohri N., ukusima Y., ukuzawa Y., Tani T., and Saito N., (2003). Layer Generation Process on Work-piece in Electrical Discharge Machining, Annals of CIRP, 52(1), [15] Guu Y.H., (2005). AM Surface Imaging of AISI D2 Tool Steel Machined by the EDM Process, Applied Surface Science, 242(3-4), [16] Kansal H.K., Singh S., and Kumar P., (2005). Parametric Optimization of Powder Mixed Electrical Discharge Machining by Response Surface Methodology, Journal of Material Processing Technology, 169, [17] Khanra A.K., Sarkar B.R., Bhattacharya B., Pathak I.C., and Godkhindi M.M., (2007). Performance of ZrB 2 -Cu Composite as an EDM Electrode, Journal of Material Processing Technology, 183(1), Journal of Metallurgical Engineering, 1(1-2) January-December 2011

17 [18] Beri N., Maheshwari S., Sharma C., Kumar A., (2008). Performance Evaluation of Powder Metallurgy Electrode in Electrical Discharge Machining of AISI D2 Steel using Taguchi Method, International Journal of Mechanical, Industrial and Aerospace Engineering, 2(3), [19] D.C. Montgomery, Design and Analysis of Experiments, (New York, Wiley 2007). [20] Beri N., Maheshwari S., Sharma C., Kumar A., (2009). Parametric Optimization of Electrical Discharge Machining of EN 19 Using Response Surface Methodology, Journal of Mechanical Engineering, 60(4). Journal of Metallurgical Engineering, 1(1-2) January-December