Effect of Al₂O₃ nano sol content in Ni-Al₂O₃ composite coating on intermetallic compound formation and properties of Mg/Al soldered joints

Combining Mg and Al dissimilar metals further reduces structural weight, but the formation of intermetallic compounds (IMCs) affects Al/Mg joint properties. To prevent IMCs, a Ni-Al₂O₃ composite coating was pre-plated on the Mg alloy substrate, and then Sn3.0Ag0.5Cu (SAC 305) solder was utilized to facilitate the joining of AZ31 Mg/6061 Al through ultrasonic-assisted soldering. We investigated the impact of Al₂O₃ nano sol content in the coating on microstructure evolution, IMCs formation, and mechanical properties. Results indicated that the Ni-Al₂O₃ composite coating effectively suppressed the Mg-Sn reaction, thereby preventing the formation of Mg₂Sn IMC and significantly enhancing joint strength. In joints with a Ni-Al₂O₃ composite coating containing 50 mL/L Al₂O₃ nano sol, no Mg₂Sn IMC was detected after 50 min of holding at 260 °C, achieving a maximum shear strength of approximately 67.2 MPa. Increasing the Al₂O₃ concentration further expanded the soldering process window. For the joint with Ni-Al₂O₃ (100 mL/L Al₂O₃ nano sol) composite coating held at 260 °C for 70 min, the coating was dissolved to a thickness of about 5.8 µm, but no Mg₂Sn IMC was observed. The Ni-based solid solution formed near the coating/solder interface was strengthened, leading to fractures occurring within the SAC solder, and the maximum shear strength further increased to 73.9 MPa. The strengthening mechanism of the joints facilitated by using the Ni-Al₂O₃ composite coating was revealed by comparing with pure Ni-assisted joints. Therefore, employing a Ni-Al₂O₃ composite coating as a barrier layer represents a promising strategy for inhibiting IMC formation during the joining of dissimilar metals.