Microstructure And Mechanical Properties Of Fusion-brazing Joints Between Aluminum Alloy And Uncoated Steel By Tungste Inert Gas Welding

Dissimilar metal joint of aluminum to steel is widely used in automotive, aerospace and shipbuilding because it can combine the advantages of aluminum and steel, reduce the weight of structures and improve the fuel efficiency. However, the direct fusion welding of aluminum to steel is still a challenge due to the extremely low solubility of Fe in Al at room temperature and the large differences in physical-thermal properties between the two materials.In this paper, TIG fusion-brazing of5A02aluminum alloy to304stainless steel and5A02aluminum alloy to Q235low carbon steel was conducted with Zn-15wt.%Al and Al-12wt.%Si flux-cored filler wires. The effects of the temperature and duration time during post-weld heat treatment (PWHT) on the micro structure and tensile strength of lap joints were investigated. Moreover, the influences of presetting gap width and groove in steel side on the appearance, microstructure and mechanical properties of butt joints were also researched.The lap experimental results show that the concentration and size of Zn-rich phases in the weld changed with different PWHT temperatures and duration times, and consequently affected the joint strength. The fine Zn-rich phases uniformly distributed in the weld could enhance the joint strength; however, the coarse Zn-rich phases along the interfacial layer would degrade the bonding strength of the interfacial layer. The highest tensile strength of the joint with the optimal PWHT temperature and duration time was180MPa, which was about double of that of as-welded joint.The butt experimental results reveal that, for the joint made with Omm-wide gap and without groove in steel side, the wetting and spreading of molten filler metal and Al base metal on steel were poor and severe incomplete brazing zones occurred along the side and bottom surfaces of steel. However, presetting1.5mm-wide gap or groove in steel side could promote the wetting and spreading of molten metal on steel, and then significantly enhanced the joint strength. The corresponding phases in the interfacial layers of the joints made with Al-12wt.%Si and Zn-15wt.%Al flux-cored filler wires were Al7.4Fe2Si compound and [FeAl3]Znx compound, respectively. The thickness of interfacial layers was less than10μm when using Al-12wt.%Si flux-cored filler wire. When usina Zn-15wt.%Al flux-cored filler wire, the thickness of interfacial layers from5A02aluminum alloy/304stainless steel joints was less than10μm but that from5A02aluminum alloy/Q235low carbon steel joints even reached60-70μm. During tensile testing, the specimens from the joints made with Al-12wt.%Si flux-cored filler wire fractured through the weld or interfacial layer, but those from the joints made with Zn-15wt.%Al flux-cored filler wire even fractured in the Al alloy base metal. The microhardness testing results display that the Al-Si eutectic and Zn-rich particles dispersively distributed in the weld increased the microhardness of the weld. Si and Zn from the flux-cored filler wires diffused into the heat-affected zone and resulted in hardening in this area. Meanwhile, the hardening in the heat-affected zone from the joint made with Zn-15wt.%Al flux-cored filler wire was more obvious than that made with Al-12wt.%Si flux-cored filler wire.