Aluminum alloy has excellent characteristics such as low density, good corrosion resistance, high electrical conductivity and thermal conductivity. Replacing steel with aluminum alloy will greatly reduce the quality of welded structure. Steel has good weldability and mechanical properties, and aluminum-steel welded structures have been widely used in automobile and shipbuilding industries.
The melting point of aluminum is 660℃, which is 700-900℃ lower than that of steel. When welding, aluminum with low melting point melts first, but steel has not melted yet. Because of the great density difference between aluminum and steel, aluminum in the molten pool will float on the steel, which will cause uneven weld composition after cooling. There is a big difference between the linear expansion coefficients of aluminum and steel, which will lead to a large residual stress in the welded joint and produce welding cracks.
In order to realize the reliable connection between aluminum and steel, it is necessary to overcome the obstacle of oxide film on the surface of aluminum and aluminum alloy to prevent or reduce brittle intermetallic compounds on the interface between aluminum alloy and steel.
Extended data
Aluminum-steel composite structure has the advantages of high strength, corrosion resistance, light weight and so on, which has been widely concerned in aerospace, shipbuilding, automobile manufacturing and other fields and has a good application prospect.
The thermophysical properties of aluminum and steel are quite different, and the solid solubility between them is low. In the process of joining, the interaction between Al and Fe atoms is easy to produce brittle Al-Fe intermetallic compounds, which reduces the mechanical properties of the joint and greatly limits the application of Al/steel heterostructure in industrial production.
In order to increase the wettability and spreadability of molten aluminum on the steel surface and effectively control the growth of Al-Fe intermetallic compounds at the aluminum-steel interface, scholars at home and abroad mainly study adding alloying elements and coatings to obtain aluminum-steel joints with good shape and certain mechanical properties.
Depositing transition layer on the surface of aluminum alloy and steel can not only prevent the formation of Fe-Al brittle intermetallic compound, but also ensure the solder to be fully wetted on the surfaces of the two metals. In order to improve the welding rate of aluminum-steel dissimilar joints, reduce the welding temperature and reduce the number of brittle-hard composite phases, foils or coatings such as Ag, Cu and Ni are usually selected as barrier layers.
Intermetallic compounds formed at the interface of aluminum-steel welded joints have great influence on the mechanical properties of welded joints. Therefore, the phase composition, species, microstructure distribution and formation process of intermetallic compounds at the interface of aluminum-steel welded joints, as well as the thickness of intermetallic compound layer, are studied mainly from two aspects: adding alloying elements and coating, so as to obtain aluminum-steel welded joints with good shape and meeting certain mechanical properties requirements.
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