Ultrasonic-Assisted Dissimilar Friction Stir Lap Welding of AA2024 and AZ31: A Comparative Study of Cold and Hot Welding Parameters

In this study, ultrasonic-assisted friction stir lap welding (UaFSLW) successfully joined AA2024 and AZ31, and the weld performance was systematically investigated. The beneficial effect of ultrasonic assistance was pronounced under cold welding parameters (using a low rotational speed, 400 rpm); compared with conventional FSLW, the material flow during UaFSLW was significantly enhanced. This led to grain refinement (~20%, 0.4 μm) in the nugget zone and an elongated Al/Mg bonding interface (3.3 mm). Additionally, the inter-diffusion of Al and Mg atoms was accelerated, resulting in a continuous intermetallic compound (IMC) layer. As a result, the UaFSLW joint exhibited a tensile shear load 210 N (±5 N) higher than that of a conventional FSLW joint, attributable to enhanced metallurgical and mechanical bonding. By contrast, employing hot welding parameters (using a high rotational speed, 1200 rpm) resulted in grain growth and a thickened IMC layer in the nugget zone, while simultaneously eliminating the hook structure. The coarsening of microstructures, coupled with diminished mechanical bonding, collectively caused the tensile shear load of an FSLW joint to drop to 80% of the value obtained with cold welding parameters. Furthermore, under hot welding parameters, the ultrasonic assistance had a limited impact on microstructures, with the only notable improvement being an elongated Al/Mg bonding interface, which partially compensated for the loss in tensile shear load.