Experimental study of the effects of the ultrasonic peening process on residual stress and surface hardness of parts cut with Wire EDM

Wire EDM (WireCut) is a machining process utilizing electric discharge to gradually cut a workpiece with a spark-generating thin wire. It offers benefits like non-contact cutting, versatility with materials, and precise dimensional accuracy. However, it is important to note that surfaces cut by WireCut process may exhibit tensile residual stresses with significant values in the surface layers. Ultrasonic Peening Treatment (UPT) can be employed as a new approach to modify the surface structure of wire EDMed workpieces. UPT, which induces compression in the surface layers of the workpiece, generates compressive residual stress and enhances mechanical properties including hardness. In this study, UPT was performed on wire EDMed workpieces made of alloy steel Mo40 (DIN 1.7225) using an ultrasonic vibrating tool on a lathe, at 3 levels: finishing, semi-finishing, and roughing. The impact of parameters such as the number of passes, feeding rate, and the type of wire EDM cut on the constant amplitude of vibration were studied to understand their effect on the residual stress generated. The experiments conducted using the Taguchi method revealed that in every case, this process resulted in the conversion of tensile residual stresses into substantial compressive residual stresses in wire EDMed workpieces. The compressive residual stress also reached 1654 MPa. More passes lead to higher compressive residual stress, while a higher feeding rate leads to a decrease. Surface hardness increased by 30%, 31%, and 48% for the semi-finishing, finishing, and roughing workpieces in the initial pass. This trend continued with each subsequent pass.Key words: Residual stress - Surface hardness -Wire Electric Discharge Machining (WEDM) - Ultrasonic Peening Treatment (UPT) - Mo40 alloy steel