Inhibition of intermetallic compounds using pure Ni or Ni-Al2O3 nanocomposite coating during the ultrasonic-assisted soldering of Mg/Al dissimilar metals

The widespread utilization of lightweight Mg and Al alloys underscores the importance of establishing reliable connections between them. However, joining dissimilar Mg/Al alloys presents challenges, primarily due to uncontrolled formation of intermetallic compounds during the process, leading to weakened joint performance. To inhibit its formation, a Ni or Ni-Al2O3 nanocomposite coating served as a barrier layer during ultrasonic-assisted soldering AZ31 Mg/6061 Al using Sn-3.0Ag-0.5Cu solder. By investigating the impacts of Ni/Ni-Al2O3 composite coating on microstructure, intermetallic compounds formation, and joint performance, we found that the Ni coating partially inhibited Mg diffusion into the solder at 260 degrees C for 20 min, thus limiting excessive Mg2Sn compound formation and increasing shear strength by approximately 110% compared to uncoated joints. However, with an extension of the holding time to 30 min, block-shaped Mg2Sn compound reemerged as the primary cause of joint failure. Using Ni-Al2O3 composite coating further broadened the process window of Mg/Al soldering, eliminating block-shaped Mg2Sn compound in the joint at 260 degrees C for 40 min. The incorporation of Al2O3 further enhanced joint properties, with the maximum shear strength of the Mg(Ni-Al2O3)/SAC/Al joint reaching approximately 66.6 MPa, an improvement of 18.3% compared to the joint with Ni coating. This enhancement shifted the fracture position from the Ni/SAC interface to the SAC solder. Additionally, due to grain refinement in the Ni matrix, the microhardness of the Ni-Al2O3 composite coating surpassed that of the Ni coating. A model of Ni/Ni-Al2O3 composite coating preventing intermetallic compounds formation of was proposed. This nanocomposite coating-ultrasonic assisted soldering method effectively avoids the issue of excessive intermetallic compounds formation when joining dissimilar metals.