| Comprehensive advantages of welding structure between steel and aluminum alloy in many aspects such as lightweight and high strength make it more popular in automobile manufacturing field, and this is conducive to the development of lightweight of automobile. However, joining of aluminum alloy and steel has great difficulty since brittle intermetallic compounds are easily formed in the joint because of the big difference of two materials in physical and chemical properties.5052 aluminum alloy and the third-generation steel used in car body are chose in this paper. Experiments were carried out in two aspects, including welding parameter control and the role of alloying elements. The method of laser welding-brazing of steel and aluminum alloy using butt joint with coaxial powder filling was first put forward to study welding characteristics and interface behavior. Such means as optical microscopy, scanning electron microscopy, X-ray diffraction, tensile testing machine, nano-indentation has been applied to systematically analyzed the micro structure, interface elements distribution, the main phase interface, fracture morphology and mechanical properties of joints, providing technical support for the realization of laser welding-brazing of steel and aluminum with high-quality.Firstly, welding characteristics has been analyzed, the influence on surface morphology, microscopic structure of the interface layer has been studied and mechanical properties of joint has been assessed with AlSi12, AlSi5, AlCu5, ZnAl15 alloy powder adding. Under the optimal process parameters, reliable and sound welding-brazing joints have been obtained. With Si element adding, the interface layer is mainly composed of Fe4Ali3, FeAl3Si and Fe0.905Si0.0905 compounds etc., and Fe-Si phase and Fe-Al-Si phase reduce joints brittleness. Cu element does not participate in Al-Cu and Fe-Al-Cu phases in the interface layer, but dissolves in Fe4Al13 compound precipitated, the reduction of Fe, Al atomic diffusion rate probably caused by Cu adding influence the interface reaction. Zn element solid-solute in Fe4Al13 compound, improving the performance of Fe-Al phase without changing the crystal structure of Fe-Al compound. With AlSi12 alloy powder filling, the highest tensile strength,128.3 MPa, was obtained, about 68% of the aluminum alloy base metal. And the increase of the Si element is beneficial to reduction of the hardness of interfacial layer. Take weld formability, micro structure of the interface layer and mechanical properties of the joint into consideration, AlSi12 powder has the best effect.Secondly, in welding process, the correlation of the thickness of interfacial layer, process parameters and joint strength has been researched to seek the control mechanism of intermetallic compound. There is a certain relationship between the interfacial layer thickness and the process parameters, and welding parameters are needed a reasonable match to control interfacial layer thickness within 10μm range. Joint tensile strength and the interface layer thickness does not exhibit perfect linear and functional correlation, but the effective thickness of interface layer are mostly concentrated on this range. Joint strength increases at first and then decreased within the effective range of laser power, substantially increases with the increase of welding speed, decreases with the increase of butt gap within the effective range of the gap.Results in this thesis have important theoretical and practical value on the development of new technology to connect dissimilar metals and applications of lightweight structure. |
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