Microstructure and High-Strain-Rate Superplasticity of Friction Stir Processed Al-Mg-Sc-Zr Alloy with Low Magnesium Content

Abstract:

The effect of speed ratio factor of friction stir processing on microstructure, microhardness and superplasticity of Al-3Mg-0.1Sc-0.1Zr alloy was investigated. The results show that with the increase in speed ratio factor and heat input, the area of stir zone and the grain size are increased, the dynamic recrystallization is more complete, while the peak hardness in stir zone is decreased. All alloys processed at different speed ratio factors show high-strain-rate superplasticity when they are tensile-tested at 475 ℃ with strain rate of 10^–2 s^–1. Three types of true stress-true strain curves are observed during tensile tests. The optimal elongation of 2500% is achieved in the alloy processed with a speed ratio factor of 4, and significant strain hardening occurs before tensile fracture, which improves the common softening loss of stress at the later stage of superplastic forming, implying high engineering application value. The outstanding superplasticity is mainly attributed to equiaxed fine grains with excellent thermal stability and a high proportion of high angle grain boundaries. Based on the analysis of grain aspect ratio, cavity evolution, and morphology of fracture profile, the dominant mechanism of superplastic deformation under all speed ratio factors is grain boundary sliding.