Is the heat-affected zone truly the weakest link in welded joints?

Whether the heat-affected zone (HAZ) is truly the “weakest link” in welded joints remains debated. In this study, a multimodal characterization framework—integrating mechanical testing, 2D and 3D morphological analyses, and crystallographic EBSD mapping—was employed to evaluate a laser-welded high-strength aluminum alloy joint. The results show that the weld zone (WZ) exhibits the lowest strength and the highest plasticity, the base metal (BM) the highest strength and the lowest plasticity, while the HAZ consistently displays an intermediate response. EBSD further reveals that the HAZ retains a microstructure comparable to the BM, whereas the WZ exhibits elongated grains with pronounced orientation alignment. Local EBSD mapping around indents demonstrates extensive orientation distortion and strain localization in the HAZ and WZ, in contrast to the more continuous and uniformly distributed orientation deviation bands observed in the BM, consistent with stress transfer across grains. These findings indicate that the preferential cracking of the HAZ does not result from intrinsically inferior properties but from stress–strain mismatch and mechanical incompatibility between adjacent zones. This provides a new perspective for understanding crack initiation in welded joints and highlights the need to optimize transition-zone compatibility rather than focusing solely on mechanical properties or microstructural features.