The Effect Of The Transitional Layer On The Friction Stir Brazing For Aluminum And Steel

In this thesis, a recently novel friction stir brazing (FSB) technique was used to join commercial pure aluminum and low carbon steel, of which the process was energy-efficient and clean. The electrochemical plating was used to add transitional metal layer to the surface of the steel to control the formation of intermetallics in the joint interface. The mechanical property tests of joints were carried out, with the results compared with the joints of none transitional metal layer in the same conditions of welding parameters. The morphology and microstructure in the interface of joints were observed and analyzed by SEM and EDS to futher reveal the joining mechanism. The effect of the transitional metal layer on the mechanical performances and microstructure of joints was studied. The resuls of the experimental analyzation are shown as follows:For the friction stir brazing of aluminum and steel, the zinc-base filler metal can be used, which possesses good wettability during the joining process. The tensile shear test samples can fracture at the aluminum base material of which more than70%of tensile strength can achieved after welding. The peel tests showed that without transitional metal layer the joint can be easily peeled with very low failure load.The proper range of parameters was determined after the joining experiment and relevant tests and analysis, which were the tool insert depth of0.1-0.4mm, preheating time of10-20s, the tool rotation speed of1300-1500rpm, welding speed of about40mm/min.With the addition of Ni and composite Ni/Cu composite transitional layer, the relatively sound joints were achieved, where all samples of tensile shear test fractured at the base aluminum and the samples of longitude peel test failed on the upper surface of aluminum, and the samples with transitional layer of transverse peel test can also failed at the surface of aluminum, while the samples with none transitional metal layer failed in the interface of joints during peel test. The joints with transitional metal layer have higher peel resistance than the ones with none transitional layer.The Al-Fe intermetallics were barely generated in the interface of joint in the center zone when Ni was used as transitional layer, and instead the less brittle Al-Ni intermetallics with thin thickness formed. However, in the fringe region of joint the Al-Fe intermetallics were more easily generated in the interface. The thickness and diffusion distribution of Ni layer were consistent and homogenous in the center region which played a very good protection to the base metals, while with the heat input gradually increased from the transition zone to the fringe the Ni layer began to gradually decrease and then disappeared which leads to losing the effect of protection.When composite Ni/Cu were plated as transitional layer, the intimate contact between the base metals were completely achieved as well. In the center region of joint interface, the microstucture was very continuous and dense and its distribution was relatively uniform. A certain amount of the Cu layer dissolved into the molten filler metal of zinc and diffused into the interfacial seam, which result in the formation of zinc rich phase of which a very small amount remained and dispersed in the ceter region of joint interface and most was extruded with liquid Zn-Al alloy from the center region and gathered in the fringe region. A few Al-Cu intermetallics were genrerated in the joint interface which gathered in the form of continuous thin layer and distributed as dispersed particles. The Ni layer that distributed continuously and homogenously in the whole joint interface played a very good protection to the steel substrate. The brittle Al-Fe intermetallics were also barely generated in the interface region. The molten zinc filler metal in the center of joint played a good effect of protection on the joining interface, which prevented the heavy thickness Al-Cu intermetallics and simultaneously dissolved an amount of Al and Cu by protecting the Ni layer and effectively restricted the generation of extensive brittle intermetallics as well as the Ni layer.Under the proper parameter conditions, the molten zinc as filler metal played a significant role of wetting and protecting the base materials by emerging as Zn-Al alloy liquid in the FSB joining process and was immediately extruded from the weld center under the interfacial effect of friction and extrusion, where the oxidation of base metals can be inhibited and the intimate joning of aluminum and steel can be favorably achieved.