Preventing formation of intermetallic compounds in ultrasonic-assisted Sn soldering of Mg/Al alloys through pre-plating a Ni coating layer on the Mg substrate

Magnesium and aluminum alloys are widely used in various industries because of their excellent properties, and their reliable connection may increase application of materials. Intermetallic compounds (IMCs) affect the joint performance of Mg/Al. In this study, AZ31 Mg alloy with/without a nickel (Ni) coating layer and 6061 Al alloy were joined by ultrasonic-assisted soldering with Sn-3.0Ag-0.5Cu (SAC) filler. The effects of the Ni coating layer on the microstructure and mechanical properties of Mg/Al joints were systematically investigated. The Ni coating layer had a significant effect on formation of the Mg2Sn IMC and the mechanical properties of Mg/Al joints. The blocky Mg2Sn IMC formed in the Mg/SAC/Al joints without a Ni coating layer. The content of the Mg2Sn IMC increased with increasing soldering temperature, but the joint strength decreased. The joint without a Ni coating layer fractured at the blocky Mg2Sn IMC in the solder, and the maximum shear strength was 32.2 MPa. By pre-plating Ni on the Mg substrate, formation of the blocky Mg2Sn IMC was inhibited in the soldering temperature range 240–280 °C and the joint strength increased. However, when the soldering temperature increased to 310 °C, the blocky Mg2Sn IMC precipitated again in the solder. Transmission electron microscopy showed that some nano-sized Mg2Sn IMC and the (Cu, Ni)6Sn5 phase formed in the Mg(Ni)/SAC/Al joint soldered at 280 °C, indicating that the Ni coating layer could no longer prevent diffusion of Mg into the solder when the soldering temperature was higher than 280 °C. The maximum shear strength of the Mg(Ni)/SAC/Al joint was 58.2 MPa for a soldering temperature of 280 °C, which was 80.7% higher than that of the Mg/SAC/Al joint, and the joint was broken at the Mg(Ni)/SAC interface. Pre-plating Ni is a feasible way to inhibit formation of IMCs when joining dissimilar metals.