Failure mechanisms in PWHT AA5052–AZ31 friction stir welded joints: Role of solid solution loss and intermetallic compounds evolution

Post-weld heat treatment (PWHT) is used to improve Al-Mg joints, but under certain thermal conditions, it may prove detrimental, causing unexpected embrittlement. This study examines the premature mechanical failure of friction stir-welded (FSW) AA5052-AZ31 dissimilar joints subjected to PWHT at 400 °C for 1 h. The as-welded joints exhibited better tensile strength (140 ± 5 MPa, 189 ± 5 MPa, and 112 ± 2 MPa for S-800, S-1000, and S-1200, respectively), attributed to the presence of fine, dynamically recrystallized grains and the retention of the Al-Mg solid solution. In contrast, PWHT resulted in a severe reduction in strength, with tensile values dropping drastically to 5–19 MPa. The hardness of as-welded samples in the stir zone varied from 120 to 140 HV, while for PWHT samples dropped down to 59–70 HV in the stir zone. Multiscale characterization techniques, including SEM, EDS, EBSD, and XRD, were employed to elucidate the failure mechanism. XRD and crystallite size analysis confirmed the decomposition of the Al-Mg solid solution and the growth of brittle intermetallic compounds (IMCs), primarily γ-Al 12 Mg 17 and β-Al 3 Mg 2 . These IMCs formed continuous layers along grains and cellular boundaries, weakening cohesion and promoting early crack initiation. EBSD results revealed that PWHT caused abnormal grain coarsening and a sharp rise in high-angle grain boundaries (HAGBs > 90 %), accompanied by a reduction in dislocation density. Kernel Average Misorientation (KAM) maps revealed localized strain along grain boundaries, which served as sites for the initiation of cracks. The collective solid solution loss, IMC layer growth, abnormal grain coarsening, and strain localization contributed to unstable crack propagation. This study made a novel contribution to the understanding of how PWHT adversely affects the microstructure and mechanical performance of Al-Mg dissimilar joints.