Microstructure and mechanical properties of friction stir processed Al-Mg2Si alloys

The microstructure and mechanical properties of friction stir processed Al-Mg2Si alloys were studied by TEM and EBSD. The results showed that an increase in the tool rotation speed (300-700 r/min) led to a decrease in the defect area (from 10.5 mm(2) to zero), whereas the defect area demonstrated the opposite trend (increased to 1.5 mm(2) from zero) upon further increasing the rotation speed (700-1200 r/min). The types of defects were transformed from tunnel defects to fusion defects as the rotational speed increased. The coarse Mg2Si dendrites were broken and fine particles (smaller than 10 mu m) formed in the weld nugget (WN). The amount of low-angle grain boundaries increased significantly from 57.7% to 83.6%, which was caused by an increase in the content of the deformed structure (from 1.7% to 13.6%). The hardness, ultimate tensile strength (UTS) and elongation were all greatly improved for the weld nugget. The hardness values of the WNs had the following order: R300<R1200<R500<R900<R700. The UTS and elongation had the following order: BM (base material)<R300<R1200<R500<R900<R700. The UTS and the elongation for the WN were increased by one and three times, respectively.