Surface Integrity of Cemented Carbides Machined by Electrical Discharge Machining after Polishing

Heat-affected zones (HAZs), in which there are many cracks and microcraters, are generated by electrical discharge machining (EDM) of cemented carbides. These defects cause a substantial decrease in the strength of the materials. As one of the countermeasures, HAZs can be removed by polishing, which is dependent on hand finishing. However, it is difficult to remove HAZs completely through polishing. At present, a polishing operator subjectively evaluates whether defects such as cracks exist on a polished surface based on the degree of brilliance of the polished surface. In this study, the relationship between the reflectivity of the polished surface and the transverse rupture strength (IRS) was experimentally examined based on the assumption that polishing to a quasi-mirror-finish surface is not necessarily accompanied by recovery of the strength of the material. Our results showed that the reflectivity of the quasi-mirror-finish surface with surface roughness of 0.2μm Ry is 3-4 percent lower than that of a mirror-finish surface with surface roughness of less than 0.1μm Ry. The IRS ratio of a quasi-mirror-finish surface is 0.7, and that of an electrical discharge machined surface is 0.6 compared with 1.0 for a mirror-finish surface. Thus, we observed that the reflectivity of the quasi-mirror-finish surface is near that of the mirror-finish surface, and that the IRS of the quasi-mirror-finish surface is near that of the electrical discharge machined surface.