Microstructure And Properties Of Fe-Al Lap Joint Made With AC Double Pulse MIG Welding

In nowadays China, energy shortages and environmental degradation become more serious. Reducing the own weight of conveyances and their consumption of fossil energy, limiting their negative impact on the environment, improving its carrying capacity have become the focus in automotive and aerospace industries. Al alloys, which possess high specific strength, good corrosion resistance and low density, can partially substitute steel to manufacture structural components. However, components serving in serious conditions, such as in higher or lower temperatures or bearing heavier dynamic loads, have to be made by steel to guarantee safety and reliability. Therefore, investigation on aluminum and steel dissimilar materials joining is hot spot nowadays. Unfortunately, solubility of steel in aluminum is nearly zero, and brittle Fe-Al intermetallic compounds layers easily generate at the interface between aluminum and steel. Moreover, there are huge gap between the thermal conductivity, linear expansion coefficient and molten point of steel and aluminum, inducing residual stress and hot crack in the aluminum-steel joint. Porosity also tends to generate at the aluminum-steel joint. These factors weaken the mechanical properties of aluminum-steel joint.In this research, AC double pulse MIG welding has been introduced to make aluminum-steel joint. Due to the cyclic polarity changing and imposing of low frequency current pulse, molten pool temperature in AC double pulse MIG welding is comparatively lower than that in traditional DC pulse MIG welding. The thickness of Fe-Al intermetallic compound layer(IMC) and residual stress at the aluminum-steel interface is decreased and the UTS of the Fe-Al lap joint is improved. In AC double pulse MIG welding, cooling rate at different parts of molten pool is different, the root part of IMC layer is thinner than the intermediate part, and the thickness and morphology of the root part of IMC layer play an important role in inhibiting cracks generation, and controlling the mechanical properties of the joint.In the joint made with AC double pulse MIG welding, mean welding voltage, mean welding current and welding speed govern the linear heat input. The IMC layer thickness increases linearly with the increasing linear heat input. When the linear heat input is the same, higher welding voltage,welding current and welding speed increase the maximum temperature and high temperature residence time of the molten pool, then slightly increase the thickness of IMC layer. The IMC layer thickness is the major factor controls the mechanical property of the joint. The UTS of the joint increases with the decreasing thickness of IMC layer. Si element Added in filler wire can appear in intermetallic compounds layer by substituting Al atoms, and hinder the diffusion of Al atoms from molten pool to reaction front, inhibit the growth of IMC layer. The thickness of intermetallic compounds layer decreases and the UTSs of the Fe-Al joint increases with the increasing amount of Si atoms in IMC layer.AC double pulse MIG welding induces turbulent molten flow and comparatively low temperature in molten pool, interferes the escape of gas bubbles, and leads to more pores trapped in the middle of the joint, increases the porosity in Fe-Al lap joint. In the joint made with DC pulse MIG welding, the amount of pores is comparatively lower but the size of pores is larger, and tends to distribute near the up weld surface.Due to the weld seam of Fe-Al lap joint is as-cast structure with precipitated phases, serious corrosion occurred in this area. The corrosion behavior of Fe-Al lap joint made with AC double pulse MIG welding has been investigated by measuring open circuit potential, polarization curve and AC impedance spectroscopy in 3.5 wt. % NaCl and 3.5 wt. % Na2SO4 solutions. It is found that the corrosion current density of Fe-Al lap joint in 3.5 wt. % NaCl solution is higher than that in 3.5 wt. % Na2SO4 solution, but the surface independence, ionic conduction rate and electronic conduction rate of Fe-Al joint in 3.5 wt. % NaCl solution is lower than that in 3.5 wt. % Na2SO4 solution.