Investigation on material flow and microstructural evolution mechanism in non-thinning and penetrating friction stir welded Al–Cu aluminum alloy

Abstract

Non-thinning and penetrating friction stir welding (NTPFSW) was proposed through an innovative welding system. The traditional backing plate was replaced by a small-size stationary shoulder with a blind hole to accommodate the stirring pin. AA2219-T6 aluminum alloy joints were produced via NTPFSW and the examinations on material flow, dynamic recrystallization (DRX), precipitate evolution, and microhardness were carried out. The results revealed that the material flow around the root of the NTPFSW joint was unique owing to a stationary shoulder. Due to the bending moment M between the two sides of the NTPFSW tool being enormous, the inclination angle of the tool was large, and the long pin made this phenomenon even more pronounced. Analyses of DRX behavior indicated that discontinuous dynamic recrystallization (DDRX) was the primary microstructures evolution mechanism in the stir zone (SZ), resulting in the refined grains. Additionally, the dissolution of primary precipitates (θ′ phase) entirely occurred. By comparison, continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) were dominant in thermo-mechanically affected zone (TMAZ), and more θ′ phases were transformed into θ phases. Owing to the different microstructure evolution mechanisms and precipitates evolution behavior, the TMAZ was characterized by lower microhardness than SZ. These findings provided a basis for tailoring the microstructures and mechanical properties of NTPFSW joints.