Influence of ultrasonic peening treatment parameters on surface hardness, wear, and friction behavior of wire EDMed hardened Mo40 alloy steel

Wire Electrical Discharge Machining (WEDM) allows for the accurate machining of materials with high hardness and limited machinability but inherently generates tensile residual stresses and surface microcracks due to rapid thermal cycling. These defects can deteriorate the mechanical integrity and wear resistance of machined components.This research applied Ultrasonic Peening Treatment (UPT) as a cost-effective and practical post-processing approach aimed at improving the surface integrity, microhardness, and wear characteristics of WEDM-processed AISI 4140 (DIN 1.7225) steel. Specimens cut at discharge currents of 7, 9, and 15 A were treated to the UPT with feed rates ranging from 0.08 to 0.16 mm/rev and 1–5 passes. Surface roughness, microhardness, and wear behavior were characterized using profilometry, Vickers hardness testing, scanning electron microscopy (SEM), and pin-on-disc tribometry. A Taguchi L9 orthogonal array and ANOVA were applied to identify the dominant parameters influencing surface properties. The discharge current and peening feed rate were found to have the most significant effects (p < 0.05), with the optimal condition (15 A, 0.12 mm/rev, 3 passes) yielding a 48% increase in surface hardness, a hardened layer depth of approximately 200 μm, and a 65% reduction in wear rate relative to the untreated surface. SEM analyses revealed pronounced dislocation rearrangement and nanoscale grain refinement near the surface, confirming that the UPT effectively mitigates WEDM-induced tensile stresses and refines the microstructure. Overall, the combined WEDM–UPT process demonstrates a robust pathway for producing high-performance components with superior surface and mechanical properties.