Unveiling the Enhanced Mechanical Properties of Friction Stir Welded AZ31 Joint by Pre-Ultrasonic Surface Rolling

The hot extruded AZ31 Mg alloy joints were obtained by friction stir welding at a high rotation rate (FSW-H). To improve mechanical properties of the joints, pre-ultrasonic rolling on docking surface (PURS) was also carried out before FSW-H. The microstructure, texture distribution and tensile properties of the two joints were investigated. Compared with the FSW-H joint, the tensile results showed that the ultimate tensile strength and elongation of the PURS joint were improved by about 7% and 43.1%. The slightly increased strength of the PURS joint was mainly due to the weaker texture near the interface of the joints, which led to lower Schmid factor (SF) values for basal slip and extension twinning with hard orientation. The higher elongation of the PURS joint was attributed to the decreasing localized strain, which mainly caused by uniform SF values for the two deformation mechanisms and higher interface angle of transition boundary in the advancing side. During transverse tension test, the reduced possibility of premature cracks of the PURS joint was mainly due to repaired defects, decreased residual dislocation and suppressed {10-11} twins, which made the mechanical properties more stable.