| Dissimilar metal joints between aluminum alloy and steel combine low density, high corrosion resistance and high conductivity of aluminum alloy and high strength of steel. Due to its advantages in weight and energy saving, aluminum alloy/steel dissimilar metal joints have experienced increasingly broad application in automotive, aerospace and transportation industry. But the large difference in thermal-physical properties between the two materials and the inevitable formation of brittle intermetallic compounds during welding process increase the challenge to build a sound joint.In this paper, dissimilar metal joining of pure aluminum to 304 stainless steel was conducted by tungsten inert gas welding with Al-12wt.%Si flux-cored filler metal, the influence of welding parameters on the microstructure and mechanical properties of pure aluminum/steel TIG fusion-brazed joints was studied. TIG fusion-brazing of 5052 aluminum alloy to 304 stainless steel in lap joints was carried out with Al-12wt.%Si filler wire and modified precoating flux with different composition. The effect of added ZnO powder on the microstructure characteristics and mechanical properties of the resultant joints was investigated. A mathematical model of aluminum alloy/steel lap joints made by TIG fusion-brazing process was developed to calculate the temperature field and fluid flow field by the computational fluid dynamics(CFD) software FLUENT.The results indicate that the tensile strength of pure aluminum/304 stainless steel welded joint was 92MPa,73% of that of pure aluminum base metal. The joints fractured in the heat-affected zone (HAZ) on pure aluminum side at welding current of 50A and welding velocity of 60mm/min. The interface layer showed two morphological characteristics under the influence of arc force:the root and middle section of the joints were flat, but the head section was serrated. Plate-like structure and Chinese script structure phases were detected in the weld. The interface layer, plate-like and Chinese script structure phases were identified as Ts-Al7.4Fe2Si.For the 5052 aluminum alloy/304 stainless steel welded joint in the same welding condition, the thickness of intermetallic layer had a critical value about 5-8μm. When the thickness of intermetallic layer exceeded this threshold, the intermetallic layer could be easily broken into pieces, then the residual intermetallic layer would keep on growing until the critical value. Thermit reaction occurred when ZnO powder was added into the flux, the ZnO powder reacted with liquid aluminum alloy leading to the formation of Al2O3, which influenced the growth of intermetallic layer and resulted in the detachment of interfacial layer in a large scale. The Al2O3 existing in the weld as slag inclusion deteriorated the resultant joints. The tensile strength of 5052 aluminum alloy/304 stainless steel dissimilar metal joint was 139MPa without addition of ZnO powder into the flux, however, that degraded to 91 MPa when 20wt.%ZnO powder was added. The intermetallic compounds formed between the liquid aluminum alloy and solid steel, the block-like phases and branch-like phases were [Fe, Cr, Si]2Al5.Numerical results indicate that asymmetric temperature field of aluminum alloy/steel lap joint was produced as a result of relatively lower heat conduction ability of the steel substrate, and the temperature gradient was larger on the steel side than aluminum alloy side. The maximum temperature in weld pool was up to 1218K. The heat exchange between the steel and aluminum alloy was stronger than that between steel and air on the top surface of steel, which led to wide high-temperature zone and low temperature gradient in overlap region but narrow high-temperature zone and high temperature gradient in other region. The maximum velocity was 0.449m/s in the welding pool. Vortices occurred due to the severe flow of the fluid in the welding pool. Some metallic elements with low boiling point evaporated from the welding pool under the influence of vortices, and thereafter pores formed. |
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