Achievement of high-quality joints and regulation of intermetallic compounds in ultrasonic vibration enhanced friction stir lap welding of aluminum/steel

The lap joints of 6061-T6 aluminum alloy/Q235 steel were obtained by conventional friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW). The weld forming characteristics and microstructure differences of the joints with or without ultrasonic were compared and analyzed, and the effects of acoustic energy on the mechanical properties and intermetallic compounds (IMCs) layer of the joints were revealed. The results showed that ultrasonic vibrations could effectively improve the weld surface forming quality, increase the volume of plastic flow materials, significantly improve the welding speed of defect-free joints and expand the welding process window. The additional ultrasonic vibration can refine the grain structure of the weld nugget zone and the thermo-mechanically affected zone, improve the mechanical properties of the lap joints, and change the joint's fracture position and fracture mechanism. The assistance of acoustic can significantly enhance the mechanical interlocking ability of the center of the lap interface and change the type and thickness of the IMCs layer.