INVESTIGATION OF WORK SURFACE INTEGRITY AND PROCESS OPTIMIZATION IN ELECTRICAL DISCHARGE MACHINING

Abstract

Surface integrity is generally defined by its topological, mechanical, and metallurgical state of subsurface layers. Since the defects produced from different machining processes can affect the performance of component, experiments are conducted to understand the effect of changing operating parameters before new machining strategies are accepted. The MDS (minimum data set) data includes the basic surface topography that is normal in assessing a machining surface and some of the metallurgical measurements. High capital cost of WEDM equipment demands the need for optimal utilization of the capacity of WEDM process. Cutting speed (CS) is a basic performance indicator to meet this goal. However, WEDM process parameters have great impact on CS and as such productivity and machining efficiency. It is therefore necessary to choose appropriate combination of parameters to achieve higher CS. In this research, surface integrity was characterized by recast layer, surface morphology, microstructure and microhardness. Experimental investigation was carried out to determine significant wire electrical discharge machining (WEDM) process parameters contributing to recast layer formation and its surface roughness (SR). Additionally, an empirical relationship between process parameters and cutting speed has been developed. Five cutting parameters selected to study their effects on surface integrity and cutting speed are wire speed, pulse (spark gap), pulse-on time, pulse ratio, and power. Machining conditions were varied systematically based on factorial design of experiments to explore influence of these process parameters on hardened high-strength low-alloy (HSLA) steel using molybdenum wire. Significant WEDM process parameters were identified through analysis of variance (ANOVA). Scanning electron microscope was used for recast layer, elemental analysis, morphology, and microstructures. To illustrate the effect of microstructure and compositional changes in the recast layer, micro-Vickers hardness testing of WEDMed surface was also performed. ANOVA results showed that pulse-on time and wire speed significantly affect recast layer. Topographical images showed presence of surface irregularities like micro-voids, craters, and globules which affect morphology of WEDMed surface. Scanning electron microscope (SEM) micrographs indicated the presence of three distinct layers namely recast layer (amorphous and columnar structure), heat-affected zone(HAZ) and bulk material. Energy dispersive spectroscopy ii (EDS) revealed that recast layer is free of alloying effects from molybdenum wire electrode. Surface layer was, however, found to be oxidized. A statistical model of CS was developed and validated through residual analysis and confirmatory experiments. Pulse ratio, power and pulse were the major process parameters significantly affecting CS as determined by ANOVA. Contour plots have also been developed to choose optimal process parameters. Stability of moly wire was studied employing SEM and EDS analysis. It was found that original surface quality and dimensions of moly wire are not affected by WEDM process. By virtue of this, dimensional accuracy of parts is ensured. Keywords: Cutting speed; HSLA steel; Moly wire; Process parameters; Surface integrity