Microstructure And Properties Of Brazed Interface In MIG Arc Brazed-fusion Welded Joint Of Aluminum Alloy To Steel Plate

As a novel method to join dissimilar metals with big difference in melting point, the brazing-fusion welding technology can make that the molten base metal with lower melting point and filler metal form the fusion welded joint, while the molten base metal and filler metal spread at the top surface of the unmelted base metal with higher melting point and form the brazed joint with it. This method effectively avoids the mass formation of the brittle and hard intermetallic compounds (IMCs). The brazing-fusion welding of aluminum alloy to steel is one of hot spots in recent years, and various welding processes were studied based on different welding heat sources. However, both the spreading behavior of liquid aluminum alloy on the top surface of steel plat and the intermetallic compounds formed at brazed interface are still of the crucial importance to the mechanical properties of the joint. So in present studies, the microstructural variation and mechanical properties of the brazed interface are studied in detail. Then the evolution mechanism of the brazed interface is also clarified which provides the data for revealing the joining mechanism of aluminum alloy to steel and promotes the actual application of this dissimilar metal welding process.The 5052 aluminum alloy plate with the thickness of 1 mm was lap joined on the galvanized steel plate with the thickness of 2 mm, and AlSi5 (ER4043) filler wire was used as the filler metal. The effects of initial temperature on the microstructure and mechanical properties of the joint were analyzed firstly. It is found out that there are three distinct regions exists at the brazed interface, which are Zn-rich zone at weld toe, central zone and Zn-rich zone at weld root, respectively. The relatively lower temperature at Zn-rich zones in welding process leading to the segregation of Zn along the grain boundary forms these two Zn-rich zones. There forms IMCs in the central zone of the brazed interface. On account of lower Gibbs free energy than that of the other Fe-Al intermetallic compounds, Fe2Al5 forms firstly at the brazed interface and grows along its c axis. At the top of Fe2Al5, the higher content of Al and Si atoms leads to the reaction between them and Fe2Al5, which forms Al8Fe2Si, Fe4Al13. At higher initial temperature, there appears a Fe-rich phase in nano-scale. The higher peak temperature and longer high temperature residence time caused by the increased initial temperature results in the inter-diffusion of Fe2Als and Fe, thus forms this new phase. In addition, the new phase can only form at high temperature and be suppressed because of high cooling rate. Although the thickness of IMCs layer increases with the increasing initial temperature, the spreadability of liquid aluminum alloy was also improved greatly which incrases the tensile strength of joint.The influences of heat input on the microstructure and shear strength of the brazed interface were also studied. When the heat input is relatively lower, the interface at steel side is quite straight. However, the interface becomes wavy when the heat input is higher. It is believed that heat input governs the formation and morphology of the wave through influencing the peak temperature and high temperature residence time. During the formation of the wave, IMCs also begin to grow towards the molen pool. Since the preferred growth of IMCs is along the vertical direction of the wave, pores defects can easily appear. While the shear strength of brazed interface increases first then decreases with the increasing heat input. At the lower heat input, the bonding strength of central zone is lower than that of Zn-rich zone at weld toe which leads to the fact that the cracks propagate along the brazed interface and then expand to the Zn-rich zone ar weld toe. When the heat input is higher, the rising bonding strength caused the crack runs through the whole brazed interface.The effects of alloying elements on the brazed interface were also clarified. The thickness of IMCs is greatly reduced by adding the Zn and Sn stripe, and there also forms Fe2Al5 while the growth of Al8Fe2Si and Fe4Al13 are suppressed obviously. When adding Si powder, Si is likely to occupy vacancies in c axis, so it is supposed to hinder the reaction of Fe and Al, which results in the thinner IMCs layer. Meanwhile, the phases of the IMCs are still Fe2Al5, Fe4Ali3 and Fe2Al8Si. When Ti powder is used, Ti avoids the liquid Al alloy directly reacting with Fe, Zn which also greatly reduces the thickness of IMCs layer and the morphology of intermetallic compounds becomes smoother. The studies indicate that it is possible to control the IMCs and properties of the joint by adding alloying elements in brazing-fusion welding process of aluminum alloy to steel.