Research on the mechanical properties and fracture mechanism of ultrasonic vibration enhanced friction stir welded aluminum/steel joint

Friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW) were used to connect 3 mm thick 2024-T3 Al alloy and Q235 carbon structural steel. The weld forming quality, mechanical properties, fracture location and interface behavior of the whole and layered joints were compared and analyzed, and the different mechanisms of the ultrasonic vibration to improve the tensile strength of the joint were summarized. As the results show, the ultrasonic vibration promoted the plastic flow ability of materials and improved the welding seam forming quality. Moreover, the ultrasonic vibration made the plastic deformation of the material in the nugget zone more uniform, which improved the strength of the nugget zone. In the middle layer of the joint, the ultrasonic vibration reduced the thickness of intermetallic compound layers (IMCLs), eliminated the laminated structure that harmed the connection quality of the joint, and formed a micromechanical interlocking structure in which IMCLs was inserted into the steel, which improved the interfacial bonding strength and improved the tensile strength by 11.1%. The ultrasonic vibration optimized the macro-mechanical interlocking structure in the joints of the lower layer, which enhanced its mechanical bonding ability with Al alloy.