Effects of Sn Interlayer on Microstructure and Properties of Mg/Steel Dissimilar Friction Stir Lap Welded Joints

This study employed friction stir lap welding (FSLW) with Sn foil as an interlayer to achieve high-quality welding of AZ31B magnesium alloy and Q235 steel. The influence of Sn interlayer thickness on the interfacial microstructure, mechanical properties, and bonding mechanism of the joints was systematically investigated. The results show that without Sn foil, only a small amount of Al-Fe intermetallic compounds (IMCs) formed at the joint interface, and the highest average tensile shear line load was 103.1 N/mm. With the addition of Sn, a composite structure consisting of Mg₂Sn, Sn solid solution, and FeSn formed at the interface. When the Sn foil thickness was 0.02 mm, the joint achieved the highest average tensile shear line load of 291.3 N/mm. As the Sn thickness increased, the IMCs layer gradually thickened, and the line load of the joint exhibited a trend of first increasing and then decreasing. The Sn addition transformed the interfacial bonding mechanism from “partial metallurgical + mechanical bonding” to “bidirectional metallurgical bonding” significantly enhancing joint strength. However, an excessively thick IMCs layer led to a decrease in joint strength.