Research On Friction Stir Welding-Brazing Hybrid Welding Process For Al/Mg Dissimilar Metals

Al/Mg composite structures are widely used in industry areas such as vehical, aerospace, aviation and ship manufacturing. However, aluminum and magnesium are very active in chemical properties. They are also have huge differences in grain structures and other properties. Welding these two metals with traditional melt welding methods are very difficult for the formation of porosity, cracks and inclusione. So it is hard to obtainte sound joints of these two metals by melt welding methods. The formation of intermetalic composites is inevitablely in friction stir welding of these two metals. Thus, decreasing the mechanical properties of the dissimilar joints.A novel hybrid welding process named Friction stir welding-brazing(FSW-B) is proposed in this paper. During the FSW-B, Al/Mg dissimilar metals are welded in lap configuration, friction stir welding and brazing are conducted at the same time. Al/Mg dissimilar metals joint with friction stir welding and brazing hybrid welding can be obtained. It have advantages that belongs to friction stir welding and brazing. Sound dissimilar joints can be achieved by using the new welding technology.In this paper, 2A12 aluminum alloy and AZ31 magnesium alloy were joined by FSW-B processing. Pure zinc was used as the brazing filler metal. Dissimilar joints’ microstructure, interface structure and elements distribution are studied by SEM、EDS and micro-XRD. Also, the mechanical properties of the dissimilar joints are analyzed.Results show that: aluminum/magnesium dissimilar metals joint with friction stir welding and brazing hybrid welds can be obtained by FSW-B process. The brazing weld are consist of lap area, solid state diffusion area and brazing area. The main components in the brazing weld are solid solutions consisted of Al-Zn and Mg-Zn, and Mg Zn2 intermetallic compounds(IMCs). There are no Al-Mg IMCs.The brazing filler metal has two different utilizations in the dissimilar joint.(1) Molten zinc in the weld nugget was a kind of liquid lubricant which can decrease the heat input during the welding processing. Furthermore, chemical reaction took place between Zn and Al、Mg, thus, forming Al-Zn-Mg intermetallic compounds then reducing the formation of Al-Mg IMC.(2) Part of molten Zn in the shoulder affected area was compressed out by the shoulder forging force. The compressed Zn can clean the original contact face of the dissimilar metals which promoting the metallurgy reaction then forming brazing weld. The two effects of the added zinc improve the hybrid joints’ mechanical properties.The tensile shear force of the dissimilar joints made by FSW-B hybrid welding process are increasing first and then decreasing as the welding speed increased. When the processing parameters are selected as the welding speed at 23.5mm/min and the rotation ratio at 375 rpm, the tensile shear force of the FSW-Bed joint reach the maximum level. The ultimate tensile shear force is 5.5k N that is 10% higher than that of the FSWed joint with the ultimate tensile shear force of 5.0k N. Joints welded by FSW-B are superior to the joints welded by resistance spot welding with a 40% higher in tensile shear force.Brazing weld in the joints made by FSW-B are failed first during the tensile shear testing, then the whole joints failed. The fractured mode of the dissimilar joint is brittle fracture. Fractured surface at the brazing weld are mainly on river pattern and cleavage step are the main structures in the friction stir weld. The micro hardness distribution of the dissimilar joint are not even. The IMCs and the solid solution are attributed to the unevenness. The hardness transition of the brazing area in the joint are very smooth.