Evolution of interfacial intermetallic layers in steel-aluminum joints: Thermodynamic and adhesion perspectives

The dissimilar welding of zinc-coated steel to aluminum has long posed significant challenges due to differences in melting points, and the limited solubility of iron in aluminum. Traditional fusion welding often results in the formation of thick, brittle intermetallic compounds (IMCs) at the interface, compromising joint integrity. Weld-brazing reduces these issues by utilizing lower heat inputs, thereby reducing IMC thickness and minimizing the risk of zinc coating evaporation. However, despite studies on the weld-brazed steel-to-aluminum joints, the exact role of interfacial IMC layers, along with the influence of alloying elements on the thermodynamic formation and adhesion properties of IMC layers, remains underexplored, contributing to the ongoing challenges in dissimilar weld-brazing of steel to aluminum. This study investigates the thermodynamic formation of interfacial layers at steel-aluminum interfaces, emphasizing their adhesion properties, morphology, and composition. The analysis highlights how these interfacial characteristics influence the failure behavior of joints in two zinc-coated steel systems: galvanized (GI) and ZnAlMg (ZAM) coatings. The study identified differences in the composition and morphology of IMC layers at horizontal and vertical interfaces, with the vertical interface considered as a critical failure region under shear-tensile loading. The results showed a higher peak load in GI-coated joints compared to those with ZnAlMg coating, which was attributed to differences in the morphology, thickness, and composition of the interfacial layers. This study underscores the critical influence of zinc coating composition on the mechanical performance of weld-brazed joints and provides insights that could improve the reliability of steel-aluminum joints in the automotive industry.