Characterization of Intermetallic Compounds in Dissimilar Material Resistance Spot Welded Joint of High Strength Steel and Aluminum Alloy

Dissimilar materials of H220 Zn-coated high strength steel and 6008 aluminum alloy were welded by median frequency resistance spot welding. Interfacial characteristics and kinetics of growth of intermetallic compound layer at steel/aluminum interface in the welded joint were investigated. The intermetallic compound layer was mainly made up of eta-Fe2Al5 and theta-FeAl3 phases, and its morphology and thickness varied with positions along the interface. The growth behavior of the intermetallic compound layer was dominated by eta-Fe2Al5, which exhibited parabolic characteristic. The growth coefficient of eta-Fe2Al5 could be expressed as k = k(0)exp(-Q/RT) with k(0) of 132 m(2)/s and 0 of 239 kJ/mol. The kinetics of growth of the intermetallic compound. layer indicated that its formation and growth were mainly driven by reactive diffusion between Fe and Al atoms, and hence the thickness and morphology of the layer were dependant on interaction time between liquid aluminum alloy and solid steel, and also interfacial temperature history during welding. The brittle intermetallic compound layer at the steel/aluminum interface was the weak zone where cracks inclined to derive and propagate during tensile shear testing. The fracture surfaces of the welded joint displayed mixed fracture morphology with both brittle and ductile features.