Improved microstructure and mechanical properties of friction stir-welded AZ61 Mg alloy joint

Abstract

Friction stir welding (FSW) is a common and effective technology used to join metals or alloys of low melting point. However, the FSW Mg alloy joint usually exhibits an unsatisfactory combination of strength and ductility due to an intense basal texture formed in the welded joint. In this study, large-load and low-speed FSW was conducted on an AZ61 Mg alloy to improve the thermal cycling during the welding process. The welded joint exhibited an ultrafine grain structure with high dislocation density and large {10-12} twins. This grain refinement was attributed to continuous dynamic recrystallization and twinning-induced geometric dynamic recrystallization. The appearance of {10-12} twins played a critical role on the basal texture randomization of the welded joint. During transverse tensile testing, the welded joint fractured at the base metal, indicating that a welding efficiency of 100% was successfully achieved. Compared with the conventional FSW, the enhanced strength of the stir zone of large-load and low-speed FSW is attributed to grain boundary strengthening, dislocation strengthening, and precipitation strengthening. The enhanced elongation is due to the increased Schmid factor and interaction between dislocations and {10-12} twins. This study provides a facile strategy for enhance the welding efficiency of the FSW Mg alloy joint and gives new insights for improved strength-ductility synergy of the Mg alloy.