Microstructure and mechanical properties of a double-pass friction stir processed AZ31B magnesium alloy

Double-pass friction stir processing (2P-FSP) was performed on a 6-mm thick AZ31B magnesium alloy plate. Compared to single-pass FSP, the 2P-FSP led to more severe thermal deformation, causing the average grain size in the stir zone to increase, but the dynamic recrystallisation process was more complete. Tensile test results revealed that the yield strength of the 2P-FSP stir zone was improved. Schmid factor analysis showed that the basal slip of 2P-FSP was more difficult to activate. The extension twins formed during the tensile process interacted with the original dislocations, resulting in a better match of strength and ductility. In addition, the physical and mathematical models were established based on the dynamic recrystallisation mechanisms and mechanical property enhancement mechanisms.