Role of ultrasonic in synergistic strengthening of friction stir welded alloys by grain refinement and rapid nanoscale precipitation

The trade-off between welding and strength loss of metal materials has been a troublesome question for decades. Improving joint efficiency is of considerable importance for industries. In the present study, grain refinement and rapid nanoscale precipitation in Al–Li alloys welds are concurrently achieved by ultrasonic enhanced friction stir welding (UFSW), improving the mechanical properties of joints. The results indicated that microstructures in the stir zone (SZ) were refined by the ultrasonic due to the enhanced dynamic recrystallization (DRX). Moreover, prominent nanoscale precipitation was observed in the SZ during UFSW. Under the effect of the ultrasonic, T₁ (Al₂CuLi) phases can precipitate through heterogeneous nucleation mechanism on a pre-existing θ′ (Al₂Cu) phase, instead of starting from a supersaturated solid solution as that in traditional heat treatments. Therefore, both the precipitation velocity and nucleation rate were accelerated in UFSW. Based on the experimental discovery of orientation relationships between the T₁ and θ′ phases (0001)_T1//(312)_θ′ and (2-1-10)_T1//(-112)_θ′, a theoretical model for T₁ precipitation via heterogeneous nucleation was established, which coincided well with the observation by selected area electron diffraction. Excess vacancies introduced by the ultrasonic contributed to the enhanced DRX and nanoscale precipitation via the heterogeneous nucleation mechanism.