Multiphase field simulation of dynamic recrystallization during friction stir welding of AZ31 magnesium alloy

During friction stir welding (FSW) of AZ31 magnesium alloy, dynamic recrystallization (DRX) takes place and enables fine equiaxed grains to be formed in the weld nugget zone (WNZ), which significantly affects the microstructure and properties of the weld joints. In this study, the multiphase field method was employed to analyze the DRX process in AZ31 FSW. The coupled Eulerian-Lagrangian model was first established to obtain the transient variation of temperature and strain rate at a checking point located in the WNZ center, which was then used as the input variables of DRX simulation. The multiphase field model including dislocation density was used to simulate the DRX behavior driven by both thermal and mechanical factors. It was found that the whole DRX process in AZ31 FSW consists of four stages, at each of which the dislocation density, grain boundary mobility and nucleation rate are varied at each stage, so that the DRX features are different. The calculated results of the final grain structure are in good agreement with the experimentally measured ones.