| With problems of the energy crisis and environmental pollution becoming more and more urgent, the light weight, energy saving, environmental protection and safety of the automobile and the rail passenger cars have become the inevitable trend. It is a direct and effective way to realize the light weight of vechicles by means of using aluminum alloys in the design and manufacture. However, due to the formation of brittle intermetallic compounds(IMC) and great differences in physical and chemical properties between alumimum and steel, the welded joints are seriously deteriorated. This has become one of the key problems which restrict the light weight of automobile and passenger cars. Therefore, it is of great theoretical significance and practical value to carry out research on the resistance spot welding of aluminum alloy and steel.In this paper, the microstructure and fracture behavior of dissimilar materials resistance spot welded joints of 6061-T6 aluminum alloy and SUS301 L stainless steel were studied firstly. The results showed that, the welded joints were mainly composed of aluminum alloy nugget, aluminum/steel interface IMC layer and stainless steel nugget. The aluminum alloy nugget was composed of celluar crystal, cellular dendrite and dendritic crystal. The IMC layer of welded joint had a double layer structure. The IMC layer on aluminum alloy side which was mainly composed of Fe4Al13, was stripe-like along the interface or needle-like growing towards the inside of aluminum nugget, while the main phase of IMC layer attaching to the steel side was stripe-like Fe2Al5. The stainless steel nugget was mainly composed of γ-Fe solid solution. In the tensile shear testing, the welded joints fractured along the aluminum/steel interface. Cracks initiated and propagated in the IMC layer. Therefore, The IMC layer was the weak link of aluminum/steel welded joints.In this investigation, it was found that the electrode morphology(size) and welding parameters had obvious influence on microstructures and mechanical properties of aluminum alloy/stainless steel resistance spot welded joints. When the F type electrode was used, the indentation ratio, nugget diameter and tensile shear load of the joints increase with the increasing of welding current and welding time. The increasing of tensile shear load was mainly due to the enlargment of nugget diameter. Further increase of welding current and welding time led to the decrease of tensile shear load, which was mainly related to the increase of IMC layer thickness. The welding current of 7kA, welding time of 250 ms and electrode force of 2.5kN were considerd as proper welding parameters to improve the microsturcures and mechanical properties of welded joints, under which condition, the indentation, nugget diameter and tensile shear load were 27.7%, 4.89 mm and 2.918 kN respectively. When the optimized electrode(an electrode with a flat tip in 10 mm diameter against the steel specimen, while an electrode with a domed tip in 35 mm against aluminum alloy side) was used, the microstructures and mechanical properties of welded joints were improved when the welding current was 13 kA, welding time was 300 ms and electrode force was 3.5kN. The indentation ratio, nugget diameter and tensile shear load were 13.5%, 7.29 mm and 4.579 kN, respectively. Compared with using F type electrode, the indentaion ratio was decreased 51.3%, while welding nugget and tensile shear load were increased 49.1% and 57.8%, respectively. Therefore, using optimized electrode could improve the welding quality and mechanical properties of aluminum alloy /stainless steel resistance spot welded joints better.The effects of alloy elements(Cu, Si, Zn, Ti, Ni) on microstructures and mechanical properties of resistance spot welded joints were studied. The results showed that under certain conditions, the Cu and Si could inhibit the growth of IMC layer and reduce the brittleness of Fe-Al IMC. The Zn could improve the wetting and spreading of liquid aluminum alloy on the surface of steel. The aluminum/steel interface microstructure was obviously refined by adding Ti powder. The Ni promoted the formation of(Fe, Ni)2Al5 and(Fe, Ni)4Al13, and improved the properties of IMCs and the mechanical properties of welded joints. By adding Cu, Si, Zn, Ti, and Ni powder, the tensile shear loads of joints were increased by45.7%, 48.8%, 19.7%, 27.6% and 64.0% respectively compared with those of welded joints without adding powders. |
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