Experimental and numerical assessment of interfacial microstructure evolution in dissimilar Al/steel joint by diode laser welding-brazing

Diode laser welding-brazing was used to the lap joining of 1.0 mm-thick DP980 steel to 1.5 mm-thick AA 5754-O aluminum alloy with a Zn-22Al filler metal. The results show that a composite-like reaction layer was generated at the fusion zone/steel interface. The reaction layer mainly consisted of dark Fe₂Al_5−xZn_x matrix and bright dispersion FeZn₁₀. The morphology and thickness of the reaction layer varied significantly along the weld toe to weld root. At the weld toe and weld root, the reaction layer had laminal characteristic with a thickness of 5 µm; while its thickness increased to 38 µm at the intermediate region. At the root-intermediate and intermediate-toe regions, two approximately symmetrical hook-like features were formed at the interface. To study the formation mechanism of the interfacial reaction layer, finite element (FE) analysis was conducted. According to the newly established heat source model, the heat transfer and fluid flow in the fusion zone was simulated. Under violent Marangoni and Buoyancy effects, a clockwise vortex and an anti-clockwise vortex were generated near the root-intermediate region and intermediate-toe region, respectively. These vortexes had a significant influence during the interfacial diffusion and dissolution, which changed the formation behavior of the interfacial reaction products.