Interfacial Evolution Mechanisms in Vacuum Diffusion Bonding of 5A90 Al-Li Alloy through Surface Oxide Regulation Using Cu Coatings

To address the inhibition of diffusion bonding by the surface oxide film of 5A90 Al-Li alloy, Cu interlayers were fabricated by electroplating and vapor deposition. Combined with coarse and fine grained substrates, the effects of Cu deposition route and substrate grain size on interfacial evolution and joint properties were systematically investigated. The results show that the electroplated Cu interlayer more effectively mitigates the detrimental effect of residual interfacial oxides, thereby providing a more favorable condition for interfacial diffusion and metallurgical bonding. Meanwhile, the fine grained substrate, with its higher grain boundary density, provides additional fast diffusion paths for Cu and alloying elements, promoting elemental redistribution, disappearance of the original interface, and local microstructural reconstruction. The highest shear strength of 235 MPa was obtained for the electroplated-fine grained specimen. The interfacial region largely retained its fine grained characteristics, accompanied by recovery, orientation adjustment, and localized microstructural rearrangement. The relatively low GOS and KAM values indicate reduced intragranular orientation gradients and limited local strain accumulation, which are favorable for deformation compatibility during shear loading. After diffusion bonding, the interfacial region consisted predominantly of an α-Al matrix with localized Mg, O, and Cu rich regions. These chemically and structurally heterogeneous regions exhibited non-uniform strain partitioning under external loading and consequently acted as preferential sites for crack initiation and propagation during shear deformation.