Effect of the processing parameters of friction stir processing on the microstructure and mechanical properties of 6063 aluminum alloy

After friction stir processing (FSP), fine equiaxed a-Al grains formed in the weld nugget (WN) of 6063 Al alloy, and their size increased with increasing tool rotation speed in the range of 300-700 rpm. The grain sizes in the bottom zones were smaller than those in the upper zones in the WN, and two-pass FSP led to a larger grain size than single-pass FSP. Tunnel defects can be observed in the thermomechanically affected zone (TMAZ) for a low tool rotation speed of 300 rpm. When the rotation speed exceeds 700 rpm, a good combination interface can form between the WN and the TMAZ. Electron backscatter diffraction (EBSD) results showed that the fraction of the high-angle grain boundary (HAB) was obviously increased after FSP in the WN. The alloys exhibited a more concentrated distribution and higher densities. Transmission electron microscopy (TEM) analysis results showed that the densities of needle-shaped precipitates were reduced in the WN. Some high-density dislocation pile-ups formed at some grains. The ultimate tensile strengths (UTSs) were lower than that of the base material (BM) at lower tool rotation speeds for both single- and two-pass FSP. With increasing speed, the UTSs increased gradually and then became higher than that of the BM. The strain values were all lower than that of the BM.