Softening issue in the thick-plate friction pull plug welding of AA2219-T87

The softening of aluminum alloy is an important factor affecting its service life. In the present work, the softening characteristics and mechanism in the friction pull plug welding (FPPW) joint of AA2219-T87 plate with a nominal thickness of 18 mm were studied, and an effective measure to mitigate the softening issue was proposed. The softening zone in the thick-plate FPPW joint is located in thermo-mechanically affect zone near the bonding interface and is 6–12 mm away from the top surface of the plate. The disappearance of the strengthening transition phase θ′ is the main cause for the softening; the degree of dynamic recrystallization, which influences the grain size and the work hardening, also affects the softening; but solid solution of Cu atoms in α-Al has little effect on the hardness change in the softening zone. After optimizing the geometric structure of joint and FPPW process parameters, the softening zone was still the weakest point of the joint. After solution and aging treatment on the above-mentioned optimized joint, the minimum hardness in the softening zone increased from 76 HV₁ to 95 HV₁, the tensile strength increased from 318 to 355 MPa, and the fracture position shifted from the softening zone to the plug.