Mechanism of formation and evolution of intermetallic in aluminum/steel FSW joint assisted by ultrasonic vibration

In order to explore the formation and evolution of intermetallic compounds layers (IMCLs), the butt welding of 3 mm thick Q235 carbon structural steel and 2024-T3 Al alloy was realized by friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW). The formation and evolution mechanism of IMCLs was systematically analyzed. The results showed that after applying the ultrasonic vibration, the grain diameter of Al alloy in the central region and interface region decreased by 20.57 % and 15.58 %, and the proportion of recrystallization grains increased from 0.99 % and 57.37 % to 13.27 % and 59.62 %, respectively. The average IMCLs thickness of FSW and UVeFSW at the forming zone was 2.71 μm and 1.22 μm, respectively. In the generation stage of IMCLs, the acoustic softening effect of ultrasonic promoted the mixing degree of materials, making its initial IMCLs thicker. In the evolution stage of IMCLs, the ultrasonic significantly reduced the welding thermal cycle, making the growth rate of IMCLs lower than the destruction rate of the interface caused by the enhancement of plastic flow, which caused the Al-rich phase IMCLs near the Al side were break and flow into the nugget zone, which eventually led to the gradual thinning of IMCLs in UVeFSW.