Revealing the mechanism of superimposing ultrasonic vibration on microstructure evolution in friction stir welding by multi-physical multi-scale simulation

Abstract

Ultrasonic vibration-enhanced friction stir welding (UVeFSW) is a promising joining method for aluminum‑lithium (Al-Li) alloy. However, directly observing the microstructural evolution in UVeFSW only by experimental methods is still a challenge. The mechanism of ultrasonic vibration (UV) on the microstructure evolution during FSW of Al-Li alloy remains unrevealed. In this study, a multi-physical multi-scale model was developed by coupling the finite element (FE) method and the Monte Carlo (MC) method to quantitatively study the impact of superimposing UV on the microstructure evolution in FSW. The preheating effect, acoustoplastic effect and residual hardening effect of UV were all considered in the model. Welding thermal processes, microstructure evolution of the material on the advancing side (AS) and in the center of the weld were quantitatively analyzed. And dislocation density and grain size in UVeFSW and FSW of Al-Li alloy were compared. Temperature field simulation results show that superimposing UV on FSW has a certain preheating effect in front of the tool. Microstructure simulation results show that the main effect of UV on microstructure evolution is the dynamic recrystallisation (DRX) process, where the maximum dislocation density in the weld nugget zone (WNZ) increases from 2.49 × 1013 m−2 to 3.49 × 1013 m−2 after superimposing UV, leading to an increase in the plastic deformation stored energy. Thereby, the driving force for nucleation and DRX increases, resulting in more nuclei in the WNZ, together with an increase in the width of the WNZ. Superimposing ultrasonic vibration in FSW can significantly refine the grain size by enhancing the activity and multiplication of dislocation in the UVeFSW of Al-Li alloy. The model is validated by the experimental results.