Quasi-in-situ characterization of microstructure evolution in friction stir welding of aluminum alloy

The in-situ investigation of microstructure evolution during friction stir welding (FSW) can provide very important theoretical guidance to control the microstructure and mechanical properties of the FSW joint. In this study, a quasi-in-situ investigation method of the microstructure evolution in friction stir welding of AA1050 aluminum alloy was developed. The microstructure along the material flow path was characterized and analyzed by taking the strain, strain rate, and temperature into account simultaneously. The results show that the microstructure evolution during the FSW of aluminum alloy can be divided into four stages according to the material flow state and thermal history. Firstly, grain coarsening by static recovery and static recrystallization occurs at the static preparatory stage (or called the preheating stage). Secondly, grain refining by continuous dynamic recrystallization occurs at the acceleration stage, where the temperature gradually increases to nearly a constant. The grain size is mainly controlled by the accumulative total strain. The strain rate only affects the rate of dislocation generation and the type of β-fiber simple shear texture components. Thirdly, at the high-velocity flow stage, a dynamic equilibrium between plastic deformation and dynamic recrystallization is maintained, and the grain size only fluctuates within a small range. Finally, a second grain-coarsening process occurs through normal grain growth due to thermal activation at the deceleration stage. This process is inevitable because it occurs under the cover of the tool shoulder. The grain coarsening during the normal cooling stage was avoided in this study due to the rapid cooling used, which has been widely reported by published documents.