Interfacial microstructure and fracture behavior of laser welded-brazed Mg alloys to Zn-coated steel

Laser lap welding-brazing of AZ31B Mg alloy to Zn-coated dual phase 980 steel using Mg-based filler was performed. The microstructures at the brazed side were characterized. Fracture behavior was then investigated by in situ scanning electron microscopy observation. The results indicated that heterogeneous reaction layers formed along the interface between the seam and the steel, which had a significant influence on the fracture behavior. Thick and diverse morphologies of reaction products were observed at the seam head and the seam root, where the crack initiated during in situ tensile test. The crack from the seam head then propagated along the Mg-Zn reaction layer/the original Fe-Al phase smoothly due to their weak bonding. When the crack propagated to the direct laser irradiation zone, the newly formed Fe-Al phase at the interface played a positive role in inhibiting the crack propagation, making the crack deflect into the seam zone. Owing to this, the joint strength was ultimately enhanced to 200 N/mm. While the crack from the seam root just propagated into the seam and contributed little to the failure of joint.