Parametric Optimization of KERF Width and Surface Roughness in Wire Electrical Discharge Machining (WEDM) of Hybrid Aluminium (Al6061/SIC/GRAPHITE) Composite using TAGUCHI-Based Gray Relational Analysis

In this present investigation, the effect and optimization of machining parameters on the kerf width and surface roughness in wire electrical discharge machining (WEDM) operations of Al 60661 hybrid composite was studied. The hybrid metal matrix composite was fabricated by stir casting process using particulates SiC and graphite each in Al6061 alloy. The experiments were designed with L27 orthogonal array. The experimental studies were conducted under varying pulse on time, pulse off time, peak current, gap set voltage, wire feed rate and wire tension. The effect of the machining parameters on the kerf width and surface roughness (SR) is determined by using analysis of variance (ANOVA). A multi-response optimization, Taguchi-based grey relational analysis was used to find the optimal process parameter setting for the best quality machined characteristics. Confirmation test was conducted at selected optimal parameter levels, which shows improvement in grey relational grade, thus confirming the strength of grey relational analysis. Index Term-WEDM, ANNOVA, surface roughness, kerf width, hybrid composite, Taguchi, stir casting INTRODUCTION Tosun et al. studied that metal matrix composite are finding increased applications in aerospace and automobile industries due to their enhanced properties such as high strength to weight ratio, high stiffness and good wear resistance [1]. Hung et al. reviewed that hybrid composite materials are fabricated with more than one reinforcement materials to achieve better mechanical properties [2]. Aluminium composites containing solid lubricants such as graphite, MOS2 showed better friction and wear behaviour. Machining of these HMMCs is difficult by conventional machines due to abrasive nature of reinforcement. Lauws et al. reviewed that among nonconventional machining methods, wire electrical discharge machining is most versatile and useful technological process to machine these materials [3]. WEDM process has capability to machine intricate shapes and profiles irrespective of hardness of materials. The most important response in WEDM is material removal rate (MRR), surface roughness (SR) and kerf width. The high value of MRR will result in reduced production cost, whereas low value of SR will improve the product quality. Good quality surfaces improve fatigue strength, corrosion and wear resistance of the work-piece. Kerf, that is, cutting width, determines the dimensional accuracy of the finished parts. LITERATURE REVIEW Patel et al. investigated effect of discharge current, pulse on time, duty cycle and gap voltage on metal removal rate and lower surface roughness of Al2O3-SiC-TiC ceramic composite during EDM [4]. Nilesh Ganpatro et al. investigated the electric discharge machining characteristics of silicon carbide particulate reinforced aluminium matrix composites. They found that decreased MRR due to an increased percentage of ceramic composite [5]. Velmurugan et al. investigated the experimental investigations on machining characteristics of Al 6061 hybrid metal matrix composites processed by electrical discharge machining [6]. They found that surface roughness of composite increases with increase in current, pulse on time, voltage and flushing pressure. Mahapatra et al. studied on parametric optimization of WEDM process on D2 tool steel using L27 orthogonal array (OA) Taguchi method [7]. The results of analysis of variance (ANOVA) showed that discharge current, pulse duration and discharge flow rate are the significant factors for maximizing the MRR and surface finish (SF). Genetic algorithm (GA) was used to obtain the optimum machining parameters for multiobjective outputs using several combinations of the weight. The study concluded that optimal machining performance with maximization of MRR and SF occurs under equal importance of weighting factors. Datta et al. studied the process behaviour of WEDM with process parameters on MRR, SF and kerf width for D2 tool steel using Taguchi L27 OA. GRA was also adopted to International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:01 96 164005-1701-4343-IJMME-IJENS © February 2017 IJENS I J E N S convert the multi-objective criteria into an equivalent singleobjective function [8]. Optimal setting was verified and it showed good agreement with practical values through confirmatory tests. Shyamlal et al. investigated multi – response optimization of wire electrical discharge machining process parameters for Al7075/Al2O3/SiC hybrid composite using Taguchi-based grey relational analysis [9]. The optimum parameters for multiple parameters optimization setting were found to be the pulse on time of 4 ms, pulse off time of 6ms,pulse current of 2 A and the wire drum speed of 4 m/min for optimum value of surface roughness and kerf width. Shah et al. investigated the effect of different WEDM process parameters on the machining responses, such as the MRR, kerf and surface roughness while machining tungsten carbide samples using the Taguchi method [10]. Based on the study, the authors concluded that the material thickness has little effect on MRR and kerf, but was a significant factor for SR. Results of ANOVA showed that discharge duration and wire tension were the most significant factors for the kerf. The percentage contribution by discharge duration and wire tension was 39.64 and 26.47 %, respectively. Kerf increased with increase in discharge duration, but decreased with wire tension. Open voltage and pulse interval time affected kerf to a lesser degree. Gupta et al. evaluated the effect of WEDM process parameters on kerf width using the response surface methodology (RSM) while machining high-strength low alloy steel (HSLA) [11]. It was revealed from the results that kerf width decreased with increase in process parameters such as discharge duration, pulse interval time, spark gap voltage and discharge peak current. The analysis of results indicated that spark gap voltage, discharge duration, discharge peak current and pulse interval time had a significant effect on the kerf width. Yan et al. examined the effect of WEDM machining parameter discharge duration on kerf (width of slit) while machining 10 and 20 vol % Al2O3/6061 Al composite [12]. The experimental results revealed that the width of slit increased with increasing the discharge duration. The wire electrode wear rate (EWR) increases with increase in the percentage of reinforcing Al2O3 particles (0, 10, and 20 vol % of Al2O3/6061 Al). Also, it increased gradually as the discharge duration increased. The effects of process parameters including wire tension, flushing rate and wire speed of WEDM on wire breakage were investigated. A very low wire tension, a high flushing rate and a high wire speed are required to prevent wire breakage. Amitesh et al. studied the investigation of surface integrity, material removal rate and wire wear ratio for WEDM of nimonic 80A using GRA and Taguchi methods [13]. The authors found pulse on time and pulse off time to be the most significant factors for MRR. The two factor interactions (Ton x Toff and Ton x IP) have been found to contribute most to the variation in WWR. MATERIALS AND METHODS PREPARATION OF HYBRID COMPOCITE In this study, the hybrid MMC has been fabricated by stir casting process. The hybrid composite consists of 10 wt% SIC and 5 wt% Graphite particulates in metal matrix Al6061 alloy. The Al alloy of 6xxx series is having great potential to be utilized in aerospace and automotive industries because of its high strength-to-weight ratio and good resistance to corrosion. The weight % composition of Al6061 alloy is shown in Table I. Reinforcements SiC and graphite in particulate form are used to fabricate the hybrid composite. These reinforcements have 10– 13 micron size particles of SiC& graphite. Table I Composition of Al6061 alloy Mg Si Fe Cu Cr Al 1.1 0.64 0.48 0.33 0.04 Remaining Machining parameters and response six input process parameters in WEDM, namely, pulse on time, pulse off time, pulse current, gap set voltage, the wire drum speed and wire tension were chosen to study their effects on surface roughness and kerf width while machining the hybrid composite. The ranges of these process parameters were selected on the basis of the pilot experiments. The levels of various parameters and their designations are presented in Table II Table II Process parameters and their levels Symbol Process parameter Level 1 Level 2 Level 3 A Pulse on time 108 117 126 B Pulse off time 40 50 60 C Pulse current 90 160 230 D Gap set Voltage 10 30 50 E Wire drum speed 3 4 5 F Wire tension 4 8 12 EXPERIMENTAL DESIGN USING TAGUCHI METHOD Taguchi technique is an efficient tool for the design of a high-quality manufacturing system. It is a method based on OA experiments, which provide much reduced variance for the experiment with optimum setting of process control parameters. The six control parameters, that is, pulse on time (A), pulse off time (B), peak current (C),Gap set voltage (D), wire drum speed (E) and wire tension(F) at three levels were selected in this study. The experiments were done according to Table III. This table only represents particular level of the various factors of the process at which the experiments would be conducted. Column 1 presents serial order of experiments. Columns A–F indicates various levels of the parameters according to OA. The Minitab 15 software was used to analyse the results. International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:01 97 164005-1701-4343-IJMME-IJENS © February 2017 IJENS I J E N S EXPERIMENTAL SET-UP FOR WEDM PROCESS Experiments were conducted on Electronica Sprint cut (Electra-Elplus 40A Dlx) CNC wire electrical discharge machine to study the surface roughness and cutting speed affected by the machining parameters at different levels. The sparks are generated between the work piece and the wire electrode. The dielectric fluid is continuously fed into the machining zone with required pressure. The material is getting removed by a series of discrete sparks taking place at the area to be machined through electro-thermal mechanism. Experimental set up of the wire electrical discharge machine is shown in Fig. 1. During machining process small gap maintained between the work and wire material. The machined particles were flushed away by the continuous flow of the dielectric fluid. Table III L27 Orthogonal array Experimental run Control factors and levels A B C D E F 1 1 1 1 1 1 1 2 1 1 1 1 2 2 3 1 1 1 1 3 3 4 1 2 2 2 1 1 5 1 2 2 2 2 2 6 1 2 2 2 3 3 7 1 3 3 3 1 1 8 1 3 3 3 2 2 9 1 3 3 3 3 3 10 2 1 2 3 1 2 11 2 1 2 3 2 3 12 2 1 2 3 3 1 13 2 2 3 1 1 2 14 2 2 3 1 2 3 15 2 2 3 1 3 1 16 2 3 1 2 1 2 17 2 3 1 2 2 3 18 2 3 1 2 3 1 19 3 1 3 2 1 3 20 3 1 3 2 2 1 21 3 1 3 2 3 2 22 3 2 1 3 1 3 23 3 2 1 3 2 1 24 3 2 1 3 3 2 25 3 3 2 1 1 3 26 3 3 2 1 2 1 27 3 3 2 1 3 2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:01 98 164005-1701-4343-IJMME-IJENS © February 2017 IJENS I J E N S Table IV L27 S/n ratios of kerf width and SR Experiment number SR(μm) SNRSR Kerf( μm) SNRCS 1 2.605 -8.316154553 302 -49.6001 2 2.82 -9.004982166 300 -49.5424 3 3.185 -10.06218873 292 -49.3077 4 2.555 -8.147818089 304 -49.6575 5 2.625 -8.382586155 300 -49.5424 6 2.985 -9.498886709 301 -49.5713 7 2.52 -8.028010816 302 -49.6001 8 2.965 -9.440493954 303 -49.6289 9 3.115 -9.86916102 297 -49.4551 10 3.625 -11.18616022 303 -49.6289 11 3.48 -10.83158488 309 -49.7992 12 3.475 -10.81909618 314 -49.9386 13 3.57 -11.05336432 317 -50.0212 14 3.59 -11.10188897 313 -49.9109 15 3.975 -11.98674266 322 -50.1571 16 2.7 -8.627275283 317 -50.0212 17 3.125 -9.897000434 313 -49.9109 18 2.57 -8.198662467 327 -50.2910 19 3.515 -10.91850659 302 -49.6001 20 3.725 -11.42252554 317 -50.0212 21 3.81 -11.61849951 302 -49.6001 22 3.67 -11.29332129 310 -49.8272 23 3.32 -10.42276167 324 -50.2109 24 3.47 -10.8065895 322 -50.1571 25 4.125 -12.30847906 316 -49.9937 26 4.256 -12.58003239 325 -50.2377 27 4.11 -12.27683644 318 -50.0485 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:01 99 164005-1701-4343-IJMME-IJENS © February 2017 IJENS I J E N S Table V Calculated Grey relational coefficients and overall Grey relational grade Experiment number SR(μm) Norm GRGSR Kerf(μm) Norm GRGCS GRG 1 2.61 0.048963134 0.3445812 302 0.28571429 0.411764706 0.378172944 2 2.82 0.17281106 0.3767361 300 0.22857143 0.393258427 0.384997269 3 3.19 0.383064516 0.4476534 292 0 0.333333333 0.390493381 4 2.56 0.02016129 0.3378747 304 0.34285714 0.432098765 0.384986712 5 2.63 0.060483871 0.3473389 300 0.22857143 0.393258427 0.370298681 6 2.99 0.267857143 0.4057971 301 0.25714286 0.402298851 0.404047976 7 2.52 0 0.3333333 302 0.28571429 0.411764706 0.37254902 8 2.97 0.256336406 0.402038 303 0.31428571 0.421686747 0.411862364 9 3.12 0.342741935 0.4320557 297 0.14285714 0.368421053 0.400238401 10 3.63 0.636520737 0.5790527 303 0.31428571 0.421686747 0.500369724 11 3.48 0.552995392 0.5279805 309 0.48571429 0.492957746 0.510469141 12 3.48 0.550115207 0.5263796 314 0.62857143 0.573770492 0.550075058 13 3.57 0.60483871 0.5585586 317 0.71428571 0.636363636 0.597461097 14 3.59 0.616359447 0.5658409 313 0.6 0.555555556 0.560698247 15 3.98 0.838133641 0.7554395 322 0.85714286 0.777777778 0.766608645 16 2.70 0.103686636 0.3580858 317 0.71428571 0.636363636 0.497224722 17 3.13 0.348502304 0.4342171 313 0.6 0.555555556 0.494886332 18 2.57 0.028801843 0.339859 327 1 1 0.669929522 19 3.52 0.573156682 0.5394655 302 0.28571429 0.411764706 0.475615106 20 3.73 0.694124424 0.6204432 317 0.71428571 0.636363636 0.628403405 21 3.81 0.743087558 0.6605784 302 0.28571429 0.411764706 0.536171546 22 3.67 0.662442396 0.5969739 310 0.51428571 0.507246377 0.552110121 23 3.32 0.460829493 0.481153 324 0.91428571 0.853658537 0.667405765 24 3.47 0.547235023 0.5247884 322 0.85714286 0.777777778 0.651283085 25 4.13 0.924539171 0.8688689 316 0.68571429 0.614035088 0.741451978 26 4.26 1 1 325 0.94285714 0.897435897 0.948717949 27 4.11 0.915898618 0.8560158 318 0.74285714 0.660377358 0.758196569 Average value of overall GRG = 0.5252 GRA: grey relational analysis; SR: surface roughness; CS: cutting speed GRG: grey relational grade; GRGSR: grey relational coefficient for surface roughness; GRGCS: grey relational coefficient for cutting speed Fig. 1. WEDM experimental setup (Electronica Sprint cut The wire is held by a pin guide at the upper and lower parts of the work piece. Since the wire is subjected to complex oscillations due to electrical discharge between wire and the work piece. So it is essential to hold the wire in its designed position against the work piece. The wire is not reused once used for a cut at set level of process parameters. A fresh length of wire is used for each cut in next experiment. The work specimen size used in this study is 95 x 80 x 8 cm rectangular plate. 10 x 5 x 8 mm size rectangular work was cut from the specimen. Zinc coated brass electrode wire of 0.25 mm diameter was used in this study. Deionised water was used as dielectric fluid at room temperature. After machining, the specimens were cleaned with acetone. SR (Ra) was measured in micrometer using Mitutoyo Surf test SV-2100. On each International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:01 10