| Magnesium alloys as the lightest structural materials have been attracting people's attention increasingly with their excellent properties. Thus, they have a great potential in application in industry. Currently, steel and Al alloys are still dominant materials in industry; therefore, welding of Mg alloys to those materials is inevitable. Dissimilar material welding could improve the flexibility, adaptability, etc., which was desired in industry increasingly. Thus, the welding of Mg alloys to other dissimilar materials is attracted to people such as the welding of Mg to Ti alloys, to A1 alloys and steel. Melting and boiling temperatures, crystal structures are in great difference between Mg and Fe elements; meanwhile, both of them does not interact with each------no solid solution or reaction. Therefore, the combination and weldability are going to be faced for the joining of Mg alloys to steel through fusion welding. Despite the closeness between Mg and A1 elements in physical properties, massive detrimental phases that degraded the joining would produce in the fusion welding of Mg, Al alloys. Hence, how to inhibit the production of such phases effectively would be encountered. Hybrid laser-TIG welding technique which is a type of fusion welding, integrated orderly the welding techniques of laser and TIG. It could implement the welding speed equivalent to that of single laser welding and surpass considerably the penetrating depth created by laser or TIG welding alone. In view of the problems mentioned above between Mg and Fe or Al elements. The present work attempted to investigate the joining process, bonding mechanisms and interlayer selection principles between Mg alloys and steel with the addition of another metal interlayers by the hybrid welding technique. Besides, based on the above method, Fe element was used to suppress the reaction between Mg and A1 elements. Accordingly, the joining process and mechanisms of Fe-added Mg/Al joints were also examined by the hybrid welding technique. The main content of the present work is as follows:The effect of welding parameters of the hybrid heating source was investigated first on shear strength and microstructures of the direct welding of AZ31B Mg alloy to Q235 steel. The results showed that the penetration depth inside the steel was a critical factor that affected the joint strength. The maximum shear strength of direct joint was 115 MPa, and optimum welding parameters were obtained. Microstructures of the joints showed that lots of micro-gaps appeared at the FZ/steel interfaces. The fracture surfaces presented dual features of ductility and brittleness. The fracture location was along the FZ/steel interface. The elementary analysis showed that no inter-diffusion occurred between the FZ and steel, suggesting that the joining of Mg/steel direct joints was mechanical.The influences of the addition of Cu, Ni, Sn and Cu-Zn alloy interlayers were examined on the joint microstructures and performances. The results showed that the strength of Cu-added joints could reach as high as 170 MPa, and that of Ni-, Cu-Zn alloy-added joints could achieve 166,161 and 159 MPa, respectively. While the maximum strength of Sn-added joint was 117 MPa, which was actually tensile strength, as the non-uniform composition in the joint fusion zone (FZ) deteriorated the joint shear strength. Two joining modes were proposed, which were "semi-metallurgical" and mechanical modes. The joining modes of Cu- and Ni-added joints are "semi-metallurgical", while that of Sn- and Cu-Zn alloy-added joints are mechanical. Compared with Ni-added joints, Cu interlayer strengthened the microstructures in the fusion zone; and Cu-Zn alloy interlayer strengthened the joint in contrast with direct joints, however, Cu interlayer was more suitable for the interlayer of Mg alloy/steel joints. Besides, some common features of interlayer-added joints were as follows:Micro-gaps between the FZ and steel disappeared, suggesting that compact bonding was attained. Fractures presented characterization of quasi-cleavage and rupture location traversed the FZ. Nonetheless, the fracture surface of Sn-added joints displayed dual characteristics, and the fracture path was along the location where the composition was non-uniform.The joining mechanisms for hybrid welding of AZ31B Mg alloy to Q235 steel are as follows, the formation of penetration depth inside steel is a determinant factor of the strength for the same type of Mg alloy/steel joints, but an imperative precondition for the dissimilar types of the joints. The strengthening effect is a determinant factor of the strength for dissimilar types of the joints. "Semi-metallurgical" bonding contributed to the improvement of joint strength, but not a determinant one. The wettability of weld metal on steel was enhanced with the addition of various interlayers, comparing with that of AZ31B Mg alloy on steel. The improvement of wettability contributed to a compact bonding, leading to the disappearance of the micro-gaps, which favors the enhancement of joint strength. In addition, a quantitative estimation model for Cu, Ni and H62 interlayer selection was established, which provided a reference for the welding of various thicknesses of Mg alloys to steel.AZ61 Mg alloy was also lap joined to Q235 steel by the identical welding and inspecting methods as aforementioned. The results showed that the ultimate shear strength of Cu and Ni-added joints increased highly; Microstructure variation, fracture characterization, bonding modes and micro-hardness profile were similar to the previous results, suggesting that the adaptability of the above joining mechanisms is well for joining of dissimilar Mg alloys to steel. The effect of laser defocusing amount (LDA) on the penetration depth was examined mainly during the experiment procedure of the hybrid lap welding of AZ31B Mg alloy to 6061 Al alloy. The results showed that when the LDA was -2mm, the maximum penetration depth 1.3 mm inside Al alloy could be acquired. The influence of Fe-interlayer thickness on the joint properties and microstructures was also investigated. The results showed that when the thickness was 0.13 mm, higher shear strength 100 MPa could be reached. Microstructures showed that massive production of Mg-Al intermetallics was inhibited in the FZ, whilst Al-Fe intermetallics were generated as well, which degraded the joint strength significantly. The addition of excessively thick Fe interlayer was adverse for the improvement of Mg/Al joint shear strength. Threshold strength for Fe-added joints was discussed finally.
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