Study on processing and strengthening mechanisms of mild steel subjected to laser cavitation peening

Abstract

A method of laser cavitation peening (LCP) was employed to strengthen the Q235 steel. The plastic deformation, residual stress, microhardness, phase, and microstructure of Q235 steel subjected to laser cavitation peening were investigated through a combination of experiments and simulations. The processing and strengthening mechanisms (LCP impact, plastic strain, grain refinement) were analyzed. Laser-induced cavitation bubble experiences three periodic pulsations during the process of LCP. The strengthening of Q235 steel by LCP can be attributed to the impact of the laser shock wave, bubble collapse shock wave, and water-jet. LCP causes plastic deformation in the surface layer of material and thereby introducing compressive residual stress and enhancing the microhardness. Dislocation structures including dislocation tangles, dislocation walls, and dislocation cells generate within the grains and near grain boundaries after LCP impact. Residual stress, microhardness, and dislocation density increase significantly with the increase of impact times. The high-density dislocation tangles and the sharing of dislocation cells refine the original coarse grains into equiaxed fine grains. The process of grain refinement is accompanied by the dissolution of cementite.