Air atmosphere diffusion bonding of Al–Mg–Li alloy using Cu nano-coating interlayer: Microstructural characterization and formation mechanisms

• A new process of diffusion bonding in air for Al–Mg–Li alloy was proposed, which greatly reduced the welding cycle and equipment requirements. • The Cu coating nano-interlayer not only inhibited the formation of oxide film but also promoted atomic diffusion. • The shear strength of Al-Mg-Li alloy joint was 187.9 MPa reaching up to 90.6% of that in BM. • The width of Al-Al diffusion interface was only about 1 μm and the joint was enhanced by nano-MgO. The process of diffusion bonding in air was successfully used to join 5A90 Al–Mg–Li alloy, which can greatly reduce the welding cycle and equipment requirements. Before diffusion bonding, the Cu nano-coating prepared on the surface of Al–Mg–Li alloy by ultrasonic electrodeposition was used as interlayer to hinder the formation of oxide film and increase the atomic diffusion. The effects of coating thickness, diffusion temperature and interlayer types on the microstructure and performance of joints were investigated. When the diffusion temperature was 530 °C and the coating thickness was 5.12 μm, the optimal shear strength of the joint with 1 μm-thick diffusing interface was 187.9 MPa, which was 90.6% of that in base metal. As the decomposition of unstable Li 20 Mg 6 Cu 13 Al 42 , Cu atoms diffused into the Al matrix to precipitate Al 6 CuMg 4 , and enhanced nano-MgO phase was formed in situ. In addition, the element diffusion behavior and formation mechanisms of the joints were discussed in detail.