Influence of drilling parameters by EDM on the HSLA steel microstructure

Abstract

Samples of SAE 945X steel were drilled by Electrical Discharge Machining (EDM). Statistical techniques were implemented to study the process parameters on surface quality and the material removal rate, but mainly how the material microstructure is affected. Scanning electron microscopy and energy dispersive X-ray spectroscopy show a Cu-rich recast layer formed at high discharge energy. The formation of the recast layer results in shrinkage stresses due to the difference of the thermal expansion coefficient between copper and steel. This phenomenon produces micro cracks in the recast layer and the propagation of them to the base metal. Likewise, the heat affected zone has a transformation of martensitic between 14 and 35µm in depth at low current level and at high current levels, respectively. By contrast, low energy levels show a thickening of cementite and ferrite recrystallization. In this context, this research is aimed at studying the effect of parameters by electrical discharge machining process on the microstructures due to thermal effects and copper diffusivity that are present during removal of material by the electrical discharge machining process at different energy discharge levels, with the purpose of evaluating the feasibility of this process for the machining of high-strength materials. In addition, it is postulated that at high conditions of machining, copper can be diffusive inside lattice of martensite and induce plastic strains greater than the yield stress of the steel, generating microcracks in areas with high cooling rates on the walls of the perforations.