Optimization of Process Parameters for Electric Discharge Machining of Tungsten Carbide Tooling

Abstract

Tungsten carbide-cobalt composite (WC-Co) is widely used for manufacturing of tools, dies, and molds due to its high hardness, high strength, and excellent wear resistance. Applications of WC-Co in tooling require complex shapes to be machined including curved and tapered features. It is generally known that WC-Co is a difficult-to-machine material due to its very high hardness and non-homogeneous microstructure. Present research work is carried out to investigate the effect of workpiece geometry, wire-electrode properties, and wire electric discharge machining (WEDM) parameters on machining of complex profiles (curved profiles in taper mode) in WC-Co and to optimize the process for getting better machining performance. This research work is accomplished in two phases. Separate sets of experiments are conducted for the two phases. Phase-I is aimed at optimizing the dimensional accuracy for machining of concave and convex curved profiles in WC-Co through control of WEDM process parameters. Taguchi L27 orthogonal array is employed for design of experiments. Analysis of variance (ANOVA) and main effects plots are used to identify the significant factors affecting the two response variables namely, radius of concave profile and radius of convex profile. Regression optimization is carried out to determine the settings of WEDM parameters in order to obtain optimal values of response variables. Phase-II is carried out to compare the machining performance of uncoated brass wire and zinc-coated brass wire in terms of taper angle error, cutting speed, and workpiece surface roughness, for cutting of complex profile in WC-Co. The influence of workpiece taper and WEDM parameters on angular error, cutting speed, and workpiece surface roughness is also investigated and optimization is carried out. Taguchi orthogonal array L18 is used for design of experiments. ANOVA, main effects plots, and signal-to-noise (S/N) ratio analysis are used to identify significant factors and their optimal levels in order to get better machining performance. Heat-affected zone of selected workpieces is analyzed with the help of scanning electron microscope (SEM) and recast layer, micro-cracks, and micro-chipping are observed in the closed vicinity of machined profiles. Micro-indentation hardness of selected samples is also measured. Experimental results reveal that zinc-coated brass wire offers higher cutting speed but poor surface finish as compared to uncoated brass wire. At optimal values of process parameters, there is an increase of 31% in cutting speed and a reduction of 31% in workpiece surface roughness for machining using uncoated wire. The corresponding values for zinc-coated wire are 26% and 5%.