Effective joining of Mg/Ti dissimilar alloys by friction stir lap welding

Mg/Ti dissimilar alloys were lap welded by friction stir lap welding (FSLW) with the tool pin slightly penetrating into the bottom Ti alloy during the welding process. The joint microstructure, mechanical properties and fracture behavior were systematically investigated. The metallurgical bonding associated with an Al enrichment layer and the mechanical interlocking bonding relying on the pin marks on the bottom Ti alloy surface were formed at the lap interface. Decreasing the welding speed was beneficial to increasing the interfacial bonding strength due to the widened Al enrichment layer and the improved mechanical interlocking resulting from the denser pin marks. When the welding speed decreased to 80 mm/min from 120 mm/min, the fracture location transferred to the upper Mg alloy from the lap interface. The maximum joint tensile-shear strength was 424 N/mm and the joint efficiency reached 89.2 % with respect to the base Mg alloy. FSLW technique provides an effective method to acquire a high-quality joint of Mg/Ti dissimilar alloys.