Numerical Analysis Of Fluid Flow During Pulse MIG Brazing-Fusion Welding Of Aluminum Alloy To Steel

The aluminum(Al) alloy/steel composite structure combines the advantages of Aluminum alloy and steel, such as low density and high corrosion resistance of Al alloys and high cost performance ratio of steel, and has broad application prospects. Due to great differences of the thermal physical properties between Al alloy and steel, brazing-fusion welding is the most effective welding technology to join Al alloy to steel. In brazing-fusion welding process, the molten Al alloy base metal and filler metal form the fusion welding joint, and they spreads on the top surface of the unmelted steel base metal and form the brazed joint. During the wetting and spreading process, the alloying elements diffuse between liquid aluminum alloy and solid steel, and results in the formation of Fe-Al intermetallic compounds, which determins the properties of the brazed-fusion welded joint. The thermal process on brazed interface and dynamic behavior of the molten pool metal have the crucial influence on weld appearance and properties of joint. Therefore, the numerical analysis on the temperature field and flow field of molten pool in brazing-fusion welding is of important significance in theory and engineering applications.Based on the acquired waveforms of welding current and arc voltage in pulsed MIG arc welding experiments, welding current and arc power were fitted by square wave. Except that the welding current peak value does not vary with the wire feeding rate, the frequency, base value and peak duty ratio of welding current and base value, peak value and peak ratio of arc power increase with the wire feeding rate. The arc characteristic and the metal transfer process were analyzed based on their images. Arc heating region on Al alloy is narrower than that on steel plate. The heating region ahead of arc is wider than that behind arc. The reason is the combined influences of properties differences between Al alloy and steel physical, lap joint and welding torch angle. Drop transfer time decreases with the increase of wire feeding rate.Based on characteristics of the pulsed MIG welding process and overlap joint, the quadruple elliptical arc model and drop model were founded to simulate MIG brazing-fusion welding process of aluminum alloy/steel. Considering the effects of arc, droplet, as well as surface tension, electromagnetic force, gravity and buoyancy, a model for pulsed MIG brazing-fusion welding was established. The simulated results of fusion line, brazed seamd width, eight of fusion weld and brazed seam spreading angle agree well with that of the experiments, which indicates that the model is reasonable and can be used to analyze thermal process of molten pool and spreading behavior of liquid aluminum alloy on the steel.The temperature field in Al alloy/steel brazing-fusion welding processis not symmetrical due to the differences in thermal physical properties of aluminum alloy and steel. The temperature gradient in aluminum alloy side is larger than that of steel. Corresponding to periodcal impact period in welding process, temperature field of weld pool has a thermal cycle of steady-slow rising-sharp rising and sharp falling. Weld pool is mainly consisted of molten filler metal, which is more than 60% of the metal in the molten pool. In the cooling process, weld pool solidifies from the fusion line and brazing interface edge gradually into internal weld pool.Corresponding to the base, peak and drop impact period of welding process, the flow field of both molten pool and brazing metal has a dynamic spreading cycle of steady- slow spreading- weld pool oscillation. During cooling process, the welding pool have a counterclockwise vortex, otherwise brazing pool has a clockwise vortex. As the liquid Al alloy solidifies, the range of vortex gradually narrowed down, and the flow speed gradually slows down. The flow of Al/steel fusion-brazing welding pool is a dynamic spreading process. If the spreading velocity on the surface of the molten pool is greater than the returning velocity on brazing interface, molten pool in the process of spreading. Otherwise molten pool is in the process of returning. The transition of molten Al alloy into weld pool is crucial to the spreading of weld pool.