Microstructures and high temperature mechanical properties of friction stirred AZ31-Mg alloy

An annealed AZ31-Mg alloy was given a FSP (friction stir process) at various rotation speeds (region from 1500 to 2800 rpm) to explore the room temperature tensile properties first, and the FSP1500 specimen which possessed the lowest average grain size was selected to investigate the tensile deformation mechanism in the range 100-500 °C afterwards. The results indicate that increasing the rotation speed not only increased the friction stir heat, but also caused the average grain size of the stirred zones to increase. When the rotation speed of FSP was >2200 rpm, the intensity of (0 0 0 2) reduced, however, (1 0 1 1) and (1 0 1 2) increased. Regardless of the variation of rotation speed, the equiaxed grain structure of the stirred zones possessed a micro-texture. The recrystallization of FSP increased the elongation of the specimens at room temperature, but the refinement grain had no contribution to tensile strength to cause an unusual Hall-Petch effect. After high temperature tensile testing, the tensile strength of the AZ31-O specimen was higher than the FSP1500 specimen (<300 °C), and the FSP1500 specimen had better uniform elongation than the AZ31-O specimen (<200 °C). In addition, both dynamic recrystallization and grain growth occurred during the duration of tensile testing. The retained stress induced by FSP and the variation of grain boundary energy caused the following effects: (1) no deformed twin formed during tensile processes at <200 °C and (2) unusually large grains could be observed at >300 °C tensile testing. This result also caused the high temperature deformed resistance of the FSP1500 specimen to be lower than that of the AZ31-O specimen. When the tensile temperature was more than 350 °C, some of the grains of the stirred zone grew fast and stopped the recrystallization characteristics of FSP from being enhanced.