Influence of heterogeneous microstructures on the mechanical properties of low-temperature friction stir processed AZ91D Mg alloy

Abstract

Abstract A coarse homogeneous grain structure with a strong basal texture intensity is usually formed in AZ91D Mg alloy, which is produced by friction stir processing, and its strength and ductility are often unsatisfactory. In this work, the microstructure and mechanical properties of die-cast AZ91D were modified using low-temperature friction stir processing. A fine-grained structure was obtained in the stir zone, and abundant {10-12} twins, dislocations, and β-Mg17Al12 precipitates formed in the grains. The grain refinement mechanism was determined to be a mixture of continuous dynamic recrystallization, twinning-induced dynamic recrystallization, and particle-stimulated nucleation recrystallization. The appearance of a heterogeneous microstructure was attributed to the low heat input. The new grains originated from the {10-12} twins and randomized {0002} basal texture, and various strengthening mechanisms contributed to the increased yield strength. The {10-12} twin boundaries provided more locations for dislocation nucleation and slip, which increased the strain hardening capacity and ductility; therefore, a satisfactory combination of strength and ductility with a strength of 294 MPa and a total elongation of 18% was obtained in the stir zone. This study provides an efficient one-step strategy to produce heterogeneous grain structures that can balance the strength and ductility of AZ91D without using a cooling medium.