Post-Weld Cold Rolling and Annealing of Al/Cu Friction-Stir-Welded Joints: Interfacial Evolution, Tensile Behavior, and Local Cupping Response

Post-weld cold rolling and subsequent annealing were applied to friction-stir-welded Al/Cu joints to examine their interfacial evolution, tensile behavior, and local cupping response. Multi-pass cold rolling reduced the sheet thickness from 3.0 to 0.4 mm and transformed the initially inclined Al/Cu interface into a near-horizontal laminated structure with an average inclination of 3.97° relative to the sheet plane. Rolling fragmented and thinned the interfacial reaction layer (narrow nanoscale reaction-product layer) and increased the average tensile strength from 99 to 370 MPa, whereas the elongation remained low at approximately 0.6%. Annealing at 300 °C for 30 min promoted recovery and recrystallization, reduced the hardness difference across the joint, and increased the elongation to 8.8%, with an average tensile strength of 182 MPa. The macroscopic tensile fracture path changed from the Al/Cu interfacial region (broad transition zone) in the as-welded joint to the laminated region (rolled interfacial region characterized by alternating Al-rich/Cu-rich lamellae) after rolling and annealing. In contrast, interface-centered cupping specimens exhibited a lower load-carrying capacity and cracking along the interfacial region. These results show that post-weld rolling and annealing improve the uniaxial tensile response through interface flattening and microstructural homogenization, while the interfacial region remains susceptible to cracking during combined stretching and bending deformation.