| Summary: | The migration behavior of intermetallic compound (IMC) layer with some distance to the interface was observed in twin-spot laser welding-brazing of AA7075-T5 aluminum to DP590 galvanized steel. The microstructure characterization of different types of IMCs was systematically discussed using scanning electron microscopy, focused ion beam and transmission electron microscopy methods. It was found that the dense plate-like IMC layer formed at the interface was η-Fe2Al5 phase and the acicular IMC was θ-FeAl3 phase, while the sparse plate-like IMC layer that migrated into the weld metal was the mixture of the θ-FeAl3 phase and molten Al. A 3D thermo-fluid numerical model was developed to predict the melt flow behavior and analyze the mechanism of the migration behavior of the IMC layer. The simulation results revealed that the downward violent flow under the twin-spot center impacted the sparse IMC layer and entered into the gap between the steel and IMC layer, forming a broken migrated IMC layer. The joint strength increased with maximum migration distance and the maximum joint strength of 133.14 MPa was obtained. Thicker and sparse IMC was formed at the interface when the migration distance increased, which was adverse to the joint strength. Keywords: IMC layer macro-migration, Twin-spot laser, Al/steel welding-brazing, Melt flow behavior
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