Behavior of Crack Generation in EDM using Thermal Stress Analysis

In EDM processing of steel, a hard white layer, different from the parent metal, forms on the surfaces because of rapid heating and cooling. Cracks and micro-craters appear on the surface of the white layer depending on machining conditions used. Such heat-affected zones (HAZ) can greatly shorten the life of metal molds.In this study, it is considered that the generation of cracks is caused by tensile stress based on the phase change of the material. Thus, an analysis of thermal stress including the phasechanged process is suggested. Thermal stress distribution is calculated using FEM with consideration of complicated boundary conditions due to phase change. And, cracks generated by EDM of die steel for cold working SKD11 were studied experimentally under various machining conditions. The form of generated cracks were examined, and the following results were obtained. Tensile stresss above 850MPa arises in the solidified region (white layer) of the material, which exceeds the value of tensile strength of the parent material used. The behavior of generated cracks could be readily explained with consideration of both the change in time and spatial distribution of the thermal stress.