Forming behavior and its effects on interfacial microstructure and tensile properties of thick 2219 aluminum alloy in friction plug welding

Friction plug welding (FPW) is a promising solid-state repair method for large aluminum alloy structures in aerospace applications, improving structural reusability and cost-efficiency. However, the unclear forming behavior and interfacial bonding mechanism hindered the improvement of joint quality, leading to suboptimal mechanical performance and restricting broader application. In this study, forming behavior of 20 mm thick 2219 aluminum alloy FPW joint was systematically investigated via integrated experiments and numerical simulations. During the welding, the workpiece material exhibited bidirectional material flow, while the maximum interface temperature during welding reached 548 °C. Meanwhile, the stress evolved from a localized concentration to a near-linear distribution along the thickness, and plastic deformation was more pronounced in the lower region of the joint. These spatiotemporally non-uniform thermo-mechanical fields influenced the spatial distribution and progression of dynamic recrystallization (DRX) across the bonding interface. DRX initiated at the lower region of the joint and gradually extended upward as strain and temperature evolved, accompanied by the progressive formation of medium angle grain boundaries. Notably, sustained high temperature and intense deformation in the middle and lower region led to Cu enrichment at interface and triggered the formation of Al–Al₂Cu eutectic structure, thereby deteriorating local tensile properties. In contrast, the upper region exhibited refined equiaxed grains and superior strength and ductility. These findings elucidate the forming behavior and microstructural evolution in thick-plate FPW, and provide a scientific basis for welding process optimization and the rational design of joint geometry to achieve improved performance.