Relationship of axial force–torque responses with joint formation behavior and interfacial bonding quality in friction plug welding of thick aluminum alloy

Friction plug welding (FPW) is a solid-state welding technique that has shown great potential for repairing high-performance aluminum alloy structures. However, its application remains limited by the difficulty in controlling joint formation and identifying interfacial bonding quality. In this study, 20 mm thick 2219 aluminum alloy FPW joints were investigated by welding experiments and numerical simulations to establish a process–response framework linking axial force–torque responses, joint formation, and interfacial bonding quality. The results showed that different macroscopic defects corresponded to distinct process response patterns: upper lack-of-bonding defects showed a continuously increasing axial force response, whereas lower lack of bonding defects exhibited a rise–stabilization–decrease pattern. For macroscopically defect-free joints, the steady-stage process variables were closely related to joint geometry and showed clear correlations with layer-dependent ultimate tensile strength (UTS). The upper-layer UTS increased with the average axial force F steady-avg , which increased the normal stress and strain at the upper interface, eliminated kissing bond defects and improved interfacial bonding quality. In contrast, the lower-layer UTS deteriorated with increasing effective heat input E steady , as excessive E steady promoted eutectic formation at the lower interface and premature crack initiation during tensile loading. Based on these findings, a synergistic optimization strategy combining joint geometry design with axial feed regulation was proposed to eliminate macroscopic defects while regulating interfacial microstructure. This study provides a theoretical basis for using axial force–torque response as indicators for defect evaluation, bonding-quality assessment, and process optimization in FPW and other solid-state processing techniques.