Hybrid laser-MIG keyhole welding of thick steel/Al butt joints

Dissimilar joining of steel/Al joints has become more and more significant in industrial applications with particular weight saving interest. However, directly welding of steel/Al joints even in thin thickness is very difficult. In the present study, the hybrid laser-MIG source focused on the steel side in keyhole mode was introduced to weld steel/Al butt joints of 6 mm in thickness. Effects of the laser offsets on the weld shape, interface microstructures and ultimate tensile strength (UTS) of the steel/Al joints were investigated. Sound steel/Al butt joints were obtained by using this hybrid laser-MIG keyhole welding process. Increasing the laser offsets from 0.6 mm and 0.8 mm, the welds exhibited better shape with a smooth appearance attributable to the reduced heat input. The positions through the thickness of the steel/Al joints played an important role in the morphology and thickness of the intermetallic compound (IMC) layers at the Al/weld interface. The IMC layers had thick irregular morphology at the upper part and the lower part, while the layers at the middle part exhibited relatively thin and uniform morphology. With some certain welding conditions, the Al/weld interface at the lower part of the steel/Al joints transformed to a welding-brazing mode from a fusion one. The island-shape structures were formed at the Al/weld interface, and the IMC layers were composed of Fe₂Al₅ layer and needle-like Fe₄Al₁₃ phases. The maximum UTS of 87.0 MPa was obtained at a laser offset of 0.6 mm. Although the failure occurred in the IMC layers revealed a brittle fracture, the fracture morphology and locations were a mixed failure, which had a certain resistance to the crack propagation of the IMC layers.