Experimental and numerical studies of relationship between microstructures and mechanical properties in friction stir welding under water

To reveal the relationship between microstructures and mechanical properties in friction stir welding under water (FSWUW), a numerical model is proposed with full experimental validations. Monte Carlo model was established to simulate the recrystallization, and precipitate evolution model was developed to study the mechanical property changes. It was found that the main strengthening phase is changed from precipitates to Cu–Mg co-cluster in FSWUW due to the higher cooling rate. Results indicated that the maximum temperature is decreased by 20.2–44.7%, and the maximum cooling rate is increased by 92.4–344.5% in FSWUW compared with friction stir welding (FSW). The change of the temperature variations leads to the decrease in the average grain size by 6.6–44.8% in FSWUW. Due to the high cooling rate in FSWUW, the precipitate growth is limited, and Cu–Mg co-cluster consists of the main phase in the final microstructure, which leads to the change of the main contribution item to the yield strength in FSWUW. In FSWUW, the contribution from the Cu–Mg co-clusters is the main contribution to the yield strength and ranges 61.3–73.4% of the final yield strength. The average grain size and the maximum cooling rate decreases with the increase in the translational speed and the decrease in the rotating speed in the heat-affected zone of FSWUW.