Characterization of the 15-5 Stainless Steel Electric Discharge Machining Recast Layer

Electric discharge machining (EDM) is a non-conventional machining process that is used for tough, hard materials or materials that require no tool force. Wire EDM produces a recast layer and heat-affected zone as a result of rapid melting and quenching at the surface of the cut. A secondary machining operation has been required to remove this layer. Previous estimates of the depth of the recast layer are likely too conservative due to improvements in the technology; therefore, the goal of this study is to characterize and more accurately investigate the size of the layer. Two EDM machine parameters, voltage and pulse on-time, were varied for the machining of 15-5 PH stainless steel. Three levels of each factor were investigated: recommended machine settings, -25%, and -50%. Average recast layer thickness was evaluated using a scanning electron microscope (SEM). Microhardness testing was also performed on ground, polished, and etched samples. Compared to previous studies and industry specifications, the recast layer was significantly smaller or not observed at all. SEM imaging generally indicated an oxide layer with porosity and microcracking near the surface. Microhardness results showed no difference between the surface and base material at maximum voltage, but significant difference between recast layer and base material for all lower voltages. However, a secondary machining operation is still recommended to remove any recast or oxide layers, but less material must be removed than prior industry estimates