Effect of UPT process on surface hardness and residual stress of parts cut

Wirecut electrical discharge machining (EDM) produces surface layers containing high tensile residual stresses, which reduce fatigue resistance and promote crack initiation. This study examines the effectiveness of ultrasonic peening treatment (UPT) in improving the surface integrity of Mo40 (DIN 1.7225) alloy steel specimens cut under finishing, semifinishing, and roughing conditions. UPT was applied using a lathe-mounted system, and the influence of feed rate, number of passes, and cutting mode was evaluated through a Taguchi L9 experimental design. Residual stresses were measured by X-ray diffraction, while microhardness and surface morphology were assessed using indentation testing and video microscopy. The results show that UPT consistently transformed the tensile residual stresses of wirecut surfaces into strong compressive stresses, reaching values up to 1655 MPa. Increasing the number of passes intensified the compressive stress field, whereas higher feed rates reduced it due to lower overlap between impacts. Surface hardness increased after the first pass for all cutting conditions, with rough-cut samples exhibiting the highest improvement. Additional passes further enhanced hardness through progressive surface densification. Overall, the findings demonstrate that UPT is an effective post-processing method for improving the mechanical condition of wirecut EDM surfaces by generating high compressive residual stresses and strengthening the near-surface layer.