Investigation on microstructure and mechanical properties of cold source assistant friction stir processed AZ31B magnesium alloy

Abstract

Magnesium alloys subjected to friction stir processing usually exhibit undesirable mechanical properties due to their intense basal texture throughout the processed zone. In this work, cold source assistant friction stir processing was innovatively conducted on AZ31B magnesium alloy to modify the microstructure and mechanical properties in the plate thickness direction. The results showed that liquid CO2 has a high cooling efficiency, which can prevent grain coarsening and activate the {10–12} twinning behavior in the surface region, resulting in an ultra-refined grain structure with abundant twin boundaries. The {10–12} twins reduced basal texture intensity, and thus the contribution of grain boundary strengthening and second phase strengthening in the strength and hardness modification became more obvious. {10–12} twin boundaries can also enhance the dislocation storage capacity, resulting in the well-matching of the ultimate tensile strength and elongation. This work provides a simple and effective surface modification strategy to enhance the hardness and strength of a magnesium alloy without ductility loss. Additionally, it also gives an idea for manufacturing a gradient structure in magnesium alloy thick plate.