Coupling Effect of Ultrasonic Assistance and Zn Interlayer on the Performance of Dissimilar Al/Mg Friction Stir Lap Welds

In this work, conventional friction stir lap welding (FSLW), FSLW with a Zn interlayer (FSLW-Zn) and ultrasonic-assisted FSLW with a Zn interlayer (UaFSLW-Zn) were used for joining dissimilar AA2024 and AZ31 alloys. Compared to the FSLW joint, the FSLW-Zn joint exhibited finer grains in the nugget zone, with Al-Mg-Zn intermetallic compounds (IMCs) replacing the brittle Al-Mg IMCs. However, the Zn interlayer failed to break up completely, resulting in blocky Al-Mg-Zn IMCs at the Al/Mg interface in the nugget zone. The application of ultrasonic assistance produced a wavy Al/Mg interface, further refined both the grains and the Al-Mg-Zn IMCs due to the enhanced material flow, the increased stored energy and the ultrasonic cavitation. The microstructural refinement in the UaFSLW-Zn joint consequently increased the hardness of nugget zone by approximately 15/10 HV compared to the FSLW/FSLW-Zn joints. The tensile shear load of FSLW joint was 1.33 kN, and this value increased to approximately 1.69 kN for the FSLW-Zn joint and 1.84 kN for the UaFSLW-Zn joint. Although all joints exhibited brittle failure, the wavy Al/Mg interface and the refined microstructures increased the elongation of UaFSLW-Zn joint by factors of 1.9 and 1.3 relative to the FSLW and FSLW-Zn joints, respectively.