Microstructure Evolution and Strengthening Mechanisms of Mg–Steel Welds Subjected to Multiple Microshot Peening Treatment

Abstract

A surface modification through multiple microshot peening (MSP) was performed on Mg–steel weldment. Application of MSP was found beneficial to the elimination of surface microdefects owing to severe plastic deformation induced by MSP. Moreover, MSP treatment transformed the residual tensile stress of the weld surface into residual compressive stress, which was beneficial to inhibit the initiation and propagation of surface microdefects. Strain strengthening and grain refining were introduced into the shot peened joint, resulting in the notable increase in surface hardness and tensile strength. Compared with an untreated joint, the tensile strength of optimized Mg/steel weldment was markedly enhanced and raised 28% to 244 MPa, and fracture ultimately occurred in the Mg alloy base material. Moreover, the refinement of weld grain induced by MSP treatment was beneficial to strengthen the stress corrosion sensitivity of Mg/steel joints, while also promoting the formation of a denser Mg(OH)2 passivation film on the weld surface and enhancing the corrosion resistance of the joints.