Solid-liquid composite diffusion in Al/Mg bimetallic joints with the Ni/Cu interlayer

Al/Mg bimetallic materials combine the complementary advantages of aluminum and magnesium alloys, demonstrating significant potential for lightweight structural and aerospace applications. However, the formation of brittle intermetallic compounds at the Al/Mg interface during diffusion bonding severely weakens interfacial bonding strength. To address this issue, the Ni/Cu composite interlayer was introduced to simultaneously achieve solid-state diffusion with the Al matrix and liquid-phase-assisted diffusion with the Mg matrix, thereby tailoring the interfacial structure and enhancing mechanical performance. The results reveal that as the bonding temperature increases from 460°C to 500°C, the Ni/Al interface consistently remains in a solid-state diffusion regime, while the Cu foil at the Mg side undergoes a transition from solid-state to liquid-phase diffusion, forming a Mg-Cu eutectic liquid. This transient liquid phase improves interfacial wettability, facilitates atomic transport, and is subsequently extruded under applied pressure. As a result, a three-dimensional wavy interface morphology develops on the Mg side, accompanied by the formation of Mg 3 AlNi 2 and Mg 2 Ni intermetallic layers. At 500°C, the Al/Mg interface evolves into a multilayered structure comprising: Al / Al 3 Ni 2 + Al 3 Ni / Ni / Mg 2 Ni / Mg 3 AlNi 2 /Mg, achieving a maximum interfacial shear strength of 53.78 MPa. Fracture analysis indicates that failure occurs between the Mg 3 AlNi 2 and Mg 2 Ni phase layers. These findings demonstrate that the Ni/Cu composite interlayer effectively suppresses the direct reaction between Al and Mg, achieving synergistic enhancement through solid-state diffusion on the Al side and liquid-phase diffusion on the Mg side.