Electrical discharge machining of cast metal matrix composites

Abstract

Metal Matrix composites (MMCs) are well-known for their superior mechanical properties over those of unreinforced alloys. A full scale application of such advanced materials however is hindered by the high cost of machining. Unacceptable short tool life and the resulted sub-surface damage of MMCs are the major issues when the composites are machined by conventional machining processes. This paper investigates the feasibility of applying electrical discharge machining (EDM) process for cast aluminum MMCs reinforced with silicon carbide particles (SiC p). Statistical models of the process were developed to predict the effect of process parameters on metal removal rate, re-cast layer, and surface finish. It was found that the SiC particles shield and protect the aluminum matrix from being vaporized, thus reduce the metal removal rate. The un-melted SiC particles drop out from the MMC together with surrounding molten aluminum droplets. While some aluminum droplets are flushed away by the dielectric, others trap the loosened SiC particles then re-solidify onto the surface to form a re-cast layer (RCL). No crack was found in the RCL and the softened heat-affected zone (HAZ), which is below the RCL. The input power controls the metal removal rate and the RCL depth, but the current alone dominates the surface finish of an EDM'ed surface.