To comprehensively understand the microstructural evolution during friction stir welding (FSW), the plastic deformation stage and the post-annealing stage of the FSW were separated by a stop action technique associated with liquid CO2 cooling and a subsequent annealing treatment. During the plastic deformation stage, the initial large grains in the base metal were subdivided with the increasing strain and temperature, and the stir zone showed ultrafined grains with a large quantity of low angle boundaries and a symmetrical simple shear texture. During the subsequent annealing stage, static recrystallization occurred, which led to the selected grain growth, and disappearance of dislocations, that produced the simple shear texture change. The stir zone showed a recrystallized-like structure in appearance caused by the static continuous and discontinuous recrystallization. The static restoration caused by the post-annealing effect can significantly affect the microstructure, which is produced by the plastic deformation, and it should not be ignored when discussing the microstructure evolution of the FSW.
Xu, N., Ueji, R., & Fujii, H. (2016). Dynamic and static change of grain size and texture of copper during friction stir welding. Journal of Materials Processing Technology, 232, 90–99. https://doi.org/10.1016/j.jmatprotec.2016.01.021