EXPARIMENTAL INVESTIGATION AND OPTIMIZATION OF EDM PROCESS PARAMETERS ON MACHING OF ALUMINUM BORON CARBIDE (AL-B4C) COMPOSITE

Abstract

Productivity and quality are two important parameters which have great concerns in today’s competitive global market. All the production/manufacturing units mainly concentrate on these two issues. In last decade manufacturing capabilities of machine tool industry has grown exponentially but still they are not utilized at their full potential. This limitation is due to failure to use these machine tools at their optimum operating conditions. In non-conventional machining processes like EDM, the most important aspect is the selection of optimum level of process parameters to obtain best output. The materials which can be easily machined by EDM process are composites, carbides, ceramics, heat treated tool steels, super alloys, heat resistant steels etc. The Al metal matrix composites (AMMCs) is extensively used in aerospace, automotive and military industries due to their high strength to wear ratio, good wear resistance, light weight, high specific stiffness, and low coefficient of thermal expansion. The reinforcing particles used in the composite material are carbides (SiC, TaC, WC, B4C), nitrides (TaN, ZrN, Si3N4, TiN), borides (TaB2, ZrB2, TiB2, WB) and oxides (ZrO2, Al2O3, ThO2). Boron carbide (B4C) is a good substitute to Al2O3 and SiC due to its high hardness (the third hardest material after diamond and boron nitride), high strength, low density (2.52 g/cm3), good wear resistance and good chemical stability. In the present work, Al 6061 alloy is used as a base metal which is reinforced with boron carbide particles (5% by wt.) to fabricate Al-B4C metal matrix composite using stir casting method. After fabrication each experimental specimen was cut as a rectangular block of dimension 51x51x12.7 mm3. The Al-B4C composite is mostly used as structural neutron absorber, armor plate materials and as a substrate material for computer hard disks. Due to possession of higher hardness and reinforcement strength Al-B4C composite materials are difficult to be machined by traditional techniques. There can be many non-conventional machining processes which can be used to machine these kinds of composite materials but Electric Discharge Machining (EDM) is most suitable method to machine Al-B4C composite in terms of power consumption cost and better material removal rate. Experiments were conducted using the SPARKONIK SN 35 (die sinking type) EDM, with servo control system. The polarity of the electrode was set as negative while that of workpiece was positive. Copper, graphite and EN-19 (tool steel) were used as electrode material and EDM oil (grade LL21) as dielectric fluid. The shape of the electrodes are cylindrical and each having a diameter of 30 mm. iv The comprehensive study of the literature survey has revealed that very little research has been done to obtain the optimal levels of EDM process parameters for best outcome in terms of MRR, EWR and SR during machining of Al-B4C composite. During a process, material removal rate (MRR) is considered as productivity estimate and our aim is to maximize it while simultaneously minimizing electrode wear rate (EWR) and surface roughness (SR). Maximizing MRR and simultaneously minimizing EWR and SR are the objectives which are opposite in nature. These opposite in nature objectives can be optimized simultaneously by selecting an optimal process parameters. The effect of input parameters of EDM process i.e. current, pulse on time, pulse off time and different electrode material (copper, EN-19 and graphite) on response parameters like MRR, EWR and SR were studied and investigated. Taguchi analysis was carried out to get the optimum levels of input process parameters i.e. current, pulse on time, pulse off time and electrode material to maximize MRR and minimize EWR and SR. For four parameters with three levels, L9 orthogonal array was used to conduct the experiments. The experimental results were also analyzed using analysis of variance (ANOVA), for identifying the significant input parameters. Based on the results of ANOVA analysis it was found that current and pulse on time are most significant factors affecting material removal rate and surface roughness respectively. It was also found that electrode material and current are most significant parameter for electrode wear rate. To obtain maximum MRR during machining of Al-B4C composite optimized level of input parameters is A3B1C3D3, i.e. 6 amps current, 6μs pulse on time, 6μs pulse off time with graphite electrode. To obtain minimum EWR during machining optimized level of input parameters is A1B2C2D1 i.e. 2 amps current, 8μs pulse on time, 4μs pulse off time with copper electrode. To obtain lowest SR during machining optimized level of input parameters is A1B3C3D3 i.e. 2 amps current, 10μs pulse on time, 6μs pulse off time with graphite electrode.