Microstructural evolution and mechanical properties of cooling medium assistant friction stir processed AZ31B Mg alloy

AZ31B magnesium alloy prepared by conventional friction stir processing (FSP) usually exhibits an intense basal texture, resulting in an unsatisfactory strength and ductility. In this work, cooling medium-assisted FSP was conducted on the AZ31B magnesium alloy. The effects of a liquid CO2 coolant on the thermal cycle, microstructure, and mechanical properties of the stir zone (SZ) were evaluated. The adoption of a liquid CO2 coolant resulted in markedly decreased peak temperature and increased cooling rate. The SZ exhibited a fine grain structure with abundant dislocations and (1012} twins. The grain refinement mechanism was attributed to the combinational effect of discontinuous dynamic recrystallization, continuous dynamic recrystallization, and twinning-induced geometric dynamic recrystallization. The SZ showed a best combination of ultimate tensile strength of 293 MPa and fracture elongation of 18.6%. The interaction of dislocations and (1012} twins rendered the plastic deformation more stable during tensile testing.