Effect of ultrasound on microstructure evolution of friction stir welded aluminum alloys

The dynamic recrystallization (DRX) in ultrasound affected friction stir welding (UFSW) was investigated using Al–Cu alloy and compared with another joint produced by the same setup but without applying the ultrasound. The effect of the ultrasound on the evolution of recrystallized grains was significantly dependent on the local thermal cycle. For the stir zone (SZ) exposed to high temperatures, both the nucleation rate and growth rate of equiaxed grains during DRX were significantly promoted by the ultrasound. Compared with the SZ without the effect of the ultrasound, the bulging frequency of the local high angle boundaries in the SZ of UFSW joint was increased to 2–4 times, producing a considerable number of fine nuclei. Excess vacancies introduced by the ultrasound were the intrinsic reason for the mentioned phenomena. The sub-grain mobility was radically accelerated due to the enhanced dislocation climb by the excess vacancies and thus the normalized nucleation criterion in the UFSW was reduced, contributing to a larger nucleation rate during DRX. Grain boundary sliding and migration were both facilitated by the ultrasound due to the enhanced movement of grain boundary dislocations and grain boundary self-diffusion, leading to a larger growth rate of equiaxed grains in the SZ of UFSW joint.