FACTORS AFFECTING THE MACHINABILITY OF AN AL/SIC METAL-MATRIX COMPOSITE

Because of the structure of metal-matrix composites (MMCs), the inclusion of a hard abrasive ceramic reinforcing phase makes these materials difficult to form and machine. This paper presents results from an ongoing investigation into the factors affecting the finish machining of an Al/SiC MMC. The influence of cutting tool coatings on flank wear and surface finish was investigated and it was found that a triple-coated carbide, having a top layer of TiN, performed best in terms of flank wear but gave the poorest surface finish. Overall, the worst results were obtained when machining with an uncoated carbide. The micromechanism of the cutting process was investigated with the aid of a ''quick-stop'' device used to produce a cut chip attached to the workpiece surface. The chip/workpiece specimens were examined subsequently using a scanning electron microscope, the SEM results indicating widespread failure due to tensile fracture rather than to shearing. The underlying cutting mechanism appeared to be quite different from that of an aluminium alloy similar in composition and mechanical properties to that of the aluminium matrix material of the MMC.