| The joining of aluminum alloy and steel attracts more and more attention of researchers for its function of weight lightening. Hybrid structures of aluminum alloy and steel used in vehicles, shipbuildings, airplanes etc. can bring economical and environmental benefit. However, lots of hard and brittle intermetallic compounds (IMCs) will form due to their distinct differences on physical and chemical properties, making it difficult to weld aluminum alloy to steel based on fusion welding. As a new welding method, brazing-fusion welding has advantages such as higher efficiency and higher flexibility than other processes like solid state joining etc. Lap joining of aluminum alloy to steel had been studied a lot and it’s relatively mature compared with butt joining. As butt joining has much broader application in engineering, it is very necessary to research the butt joining of aluminum alloy to steel.By taking 5052 aluminum alloy and low carbon steel of 1.5 mm in thickness as base metals, AlSi5 welding wire of 1.2 mm in diameter as filler wire, the butt welding of Al alloy plate to steel plate were studied by experiments. The differences of weld ability between butt welding and lap welding were discussed. It was found that the wetting and spreading conditions can be improved by enhancing the temperature of steel surface, which is a critical point to realize the butt joining. Some methods were used to enhance preheat the steel palte. As a result, setting grooves was not beneficial for the welding appearance, however, setting different heat dissipation environment for aluminum alloy and steel can remarkably improve the weld appearance. But the tensile strength of weld is low that it can’t be used in engineering structures. All these results indicate that only the process measures cannot obtain the joint with good appearance and high strength, some metallurgical measures should be used to improve weld appearance and mechanical properties.A special flux was developed to improve the weld appearance and mechanical properties. KAIF4 and K3AIF6 were used as main composition of flux and a proportion of 80% KAIF4 and 20% K3AIF6 was proved the optimal proportion to obtain good weld appearance. By this flux the liquid filler metal spreads well both on the top surface and back surface of steel. The addition 8% K2SiF6 into the flux can make it easy to smear and the tensile strength of weld was increased by 7%.Effects of alloy elements of Zn, Sn and Cu on weld appearance of aluminum alloy and low carbon steel were studied. Alloy powders and alloy strip were used in the experiments which were certified that they cannot improve the weld appearance. However, flux mixed with alloy elements can improve the weld appearance. An orthogonal experiment was applied to find the optimal proportion of alloy elements, by which the tensile strength of weld can reach up to 142 MPa,65% of that of aluminum alloy.Microstructures of joints were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Consecutive IMCs layer was found on the surfaces of steel, which means that there is good metallurgical bonding between Al alloy and steel. The IMCs have different appearance at different positions of steel, which are mainly FeAl2, Fe4Al13, Al7.2Fe1.8Si. The fracture mode of the joints was characterized as cleavage fracture and the fracture occurred on intermetallic layer. |
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