Nano-scale characterization of intermetallic compounds in dissimilar aluminum-steel resistance element welded joints

To achieve the goal of lightweight vehicle and energy-saving, the joining of dissimilar materials, specifically aluminum (Al) alloy and steel, is gaining increasing attention in car body structure. In the present study, 6061 Al alloy and DP780 steel sheet were joined using the resistance element welding (REW) technique with a flat Q235 rivet. The intermetallic compounds (IMCs) in the welded joints were thoroughly analyzed using various characterization techniques such as scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), electron backscatter diffraction (EBSD), focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results showed that the IMC in the REWed joints consists of a tongue-shaped Fe2Al5 towards the steel side and a finger-shaped Fe4Al13 towards the Al side. Dislocations were observed at the grain boundaries of Fe2Al5 grains for the first time. Nanotwins and stacking faults inside the Fe4Al13 grains were observed indicating that the formation of planar defects was mainly by rapid cooling. The (100) twining was considered to be the main twining type during the solid/liquid interfacial reaction in dissimilar REW welding between Al alloys and steel.