A double molten pool laser welding process for joining magnesium alloys and aluminum alloys: The process, microstructure and mechanical properties of joints

Abstract

In this study, AZ91/6061 dissimilar metal joints were successfully manufactured using double molten pool laser welding, and the process, microstructure, and mechanical properties of these joints were investigated. During welding, two laser beams irradiate the base metal on two sides, forming two independent molten pools with a Ni interlayer; thus, the heat input of the weld on both sides can be individually optimized according to the thermophysical properties of Mg and Al alloys. Owing to the physical isolation of the Ni interlayer, the generation of extremely hard and brittle Mg–Al intermetallic compounds is completely inhibited. The microstructure of the Mg side weld metal comprised Mg2Ni, α-Mg, (α-Mg+Mg2Ni) eutectic structures, and dispersed Mg3AlNi2. The microstructure of the Al side weld metal comprised AlNi3, Al3Ni2, Al3Ni, α-Al, and (α-Al+ Al3Ni) eutectic structures, which were the weakest zone of the joints. The optimal welding parameters were 1.0 m/min and 1300 W on the Mg side and 0.6 m/min and 1200 W on the Al side. Under these parameters, the mean microhardness of the Al side weld was significantly higher than that of the Mg side weld, and the average tensile strength of the AZ91/6061 joint was 83.58 MPa. Fractures occurred in the Al side weld metal by cleavage fracture.