Microstructure Evolution and Fracture Mode of Laser Welding–Brazing DP780 Steel-5754 Aluminum Alloy Joints with Various Laser Spot Positions

Joining steel and Al alloys can fully utilize their advantages for both base metals (BMs) and optimize automobile structures. In this study, the laser welding-brazing technique was utilized to join DP780 steel and aluminum alloy 5754 (AA5754). The mechanical properties, microstructure, and fracture locations of steel-Al joints prepared using different laser spot positions were comparatively investigated. As the proportion of the laser spot on the steel BM increased from 50% to 90%, the tensile-shear strength of the steel-Al welded joint rose from 169 MPa to 241 MPa. Meanwhile, the fracture location of the joint shifted from the interface to the BM of the aluminum alloy. The change in the laser spot position could dramatically affect the interfacial microstructure and fracture mode of the steel-Al joint. When the proportion of the laser spot on the steel BM was relatively small (50%), the growth of intermetallic compounds (IMCs) was inhibited. The metallurgical bonding effect at the steel-Al interface was poor. In this case, the interfacial zone became the primary path for the crack propagation. Thus, interface failure became the dominant failure mode of the steel-Al joint. On the contrary, metallurgical bonding at the interface was remarkably improved as the proportion of the laser spot on the BM of the steel increased (to 90%). It was determined that the IMCs could effectively hinder the propagation of cracks along the interface. Eventually, the joint fractured in the Al alloy's BM, resulting in a qualified steel-Al joint.