Experimental study of the effects of the ultrasonic peening treatment on residual stress, surface hardness, and hardness depth of parts cut with wire EDM

The Wire EDM process is widely utilized in diverse industries due to its numerous advantages. Residual stresses in wire EDMed workpieces, mainly due to non-uniformity in heat flux, metallurgical phase change, and inhomogeneous plastic deformation, are tensile and have large values ​​in surface layers. These residual stresses lead to the formation of surface cracks and a decrease in superficial mechanical strength. The Ultrasonic Peening Treatment (UPT) is an innovative approach used in this study to generate compressive residual stress and modify the surface structure of wire EDMed workpieces, as a cost-effective alternative to traditional methods. The study highlights UPT's potential as an effective way to modify the mechanical properties of wire EDMed workpieces. The impact of parameters such as the number of peening passes, feeding rate, and the type of wire EDM cut on the residual stress generated in MO40 (1.7225) alloy steel was thoroughly investigated. The experiments performed with the Taguchi method demonstrated that the UPT caused a shift in residual stress from tensile to compressive, with a maximum residual compressive stress of 1654 MPa. In the initial peening pass, an enhancement of 30%, 31%, and 48% in surface hardness values was noted for the semi-finishing, finishing, and roughing workpieces. The depth of hardness distribution reached 200 μm, while surface hardness decreased with an increased feeding rate. SEM images showed improved dislocation rearrangement due to ultrasonic vibrations, resulting in smaller surface layers, even at the nanoscale.