Interfacial characteristics and strengthening mechanism of Al-steel dissimilar metal half-lap joint laser welding-brazing

A butt joint of laser welding-brazing SUS316 steel and 6061-T6 aluminium alloy was successfully achieved by using the half-lap type bevel without filler metal. The designed bevel effectively transforms the conventional butt joint into a half-lap joint, improves the weld formation, enhances the uniformity of interfacial reaction, and significantly improves the mechanical properties of the joint. This investigation focused on the effects of welding speed and bevel shape on weld appearance, microstructure and mechanical properties, and the mechanism of interface reaction caused by the difference in bevel shape was revealed. The results demonstrate that as welding speed decreases, the interfacial reaction between solid steel and liquid aluminium intensifies, and the thickness and type of intermetallic compounds increase. In comparison with joint without bevel, the half-lap type bevel has been shown to suppress defects such as porosity and flux inclusions, balance the temperature field in welding pool, promote the homogeneity of interfacial reaction, and enhance the average thickness of the intermetallic compounds layer from 1.97 mu m to 4.59 mu m under the same heat input condition. In addition to the species of intermetallic compounds, Fe2Al5, FeAl3 and FeAl, there are also Fe-Cr-Al and Fe-Ni-Al phases consisting of Cr and Ni atoms diffused into the interface from the solid steel, which increase the joint toughness to some extent. The maximum tensile strength of the half-lap bevelled joint can reach 125.42 MPa, which is 53.6% higher than that of the joint without bevel.