The Research On Method And Mechanism Of Dissimilar Metal Joining Of Aluminum To Steel By Pulsed DE-GMA Welding Brazing

Aiming at the problems of existing welding methods for joining aluminum to steel,such as low efficiency, high cost or having special requirements on the shape of thework piece, a novel welding method was proposed innovatively in this paper conduciveto control metal transfer and heat input of base metal precisely in welding process. Thewelding method has different mechanism from the existing precise-control-type arcwelding technology, and was proved to be conducive to dissimilar metal joining ofaluminum to steel from theory and practice.A rapid prototyping test system was established to sense, observe and control thewelding process according to the characteristics and control requirements of PulsedDE-GMAW, and the corresponding software control program was also developed.Control and coordination of current waveform of the main and bypass was also realizedby the developed control system.A mathematics model for Pulsed DE-GMAW was developed and a control schemethat stabilizing the whole coupling arc by controlling the main arc was proposed. Thensimulation for arc length stability in welding process and influence of pulse currentparameters to metal transfer were carried out. On this basis, control tests for arc lengthstability in Pulsed DE-GMAW for joining aluminum to steel were done adopting twodifferent feedback modes, average arc voltage feedback of main welding arc and visionfeedback. Both the control effect of the two control schemes was well.The influence of main and bypass arc parameters to weld formation was studied bythe flat surfacing technology tests for Pulsed DE-GMAW. The results show thatincreasing the main current makes for good spreading and wetting of aluminum droplet,but the effect of the bypass current to droplet wetting is relative small. Then theinfluence of pulse current parameters to metal transfer of Pulsed DE-GMAW processwas researched by high-speed camera tests, and the results show that the peak currentand peak time can be also increased to improve the metal transfer frequency andstabilize the welding process in addition to increasing the bypass current in order torealize the stable spray transfer, in the case of maintaining the same total current. At thesame time, the base current of main and bypass can be set small to decrease the totalwelding heat input due to the stability role of bypass arc in Pulsed DE-GMAW processfor joining aluminum to steel.The thermal cycle curves for Pulsed DE-GMAW were measured and influence lawof different bypass parameters to thermal cycle curve was studied. On the basis of builtheat source model and obtained thermal cycle curves, the temperature field distributionat the interface of aluminum-steel was simulated and analyzed. With the obtainedtemperature field distribution, the growth of the intermetallic compound(IMC) layerwas then simulated and the results was consistent with the tests, which proved thecorrectness of the assumed diffusion model. On this basis of simulation for temperaturefield distribution and growth of the IMC layer, the influence of the cooling time, peaktemperature and integral of thermal cycle on the thickness of IMC was identified anddiscussed. It was identified that cooling time has the biggest influence on the thicknessof the IMC and the effect of peak temperature and integral of thermal cycle is small.Mechanical properties tests were carried out for lap joint specimens of5052 aluminum alloy and galvanized steel. The results show that the average tensile strengthis144.85MPa and the maximum tensile strength is up to186.73MPa, about88.5%ofthe base metal strength. The influence of bypass arc parameters on mechanicalproperties of lap joint was studied. The results show that the strength of aluminum-steellap joints depends not only on the base metal heat input, but also by the influence ofweld geometry parameters. In Pulsed DE-GMAW process for joining aluminum to steel,the bypass arc parameters should be selected appropriately. Not only the lower basemetal heat input should be ensured to inhibit the growth of harmful IMC of aluminumand steel and reduce the softening of HAZ of aluminum side, but also a larger weldshape factor should be ensured to make the droplet wetting and spreading better ongalvanized steel.By observing and analyzing the microstructure of the aluminum-steel joint, it isfound that the interface zone of aluminum-steel joints made by Pulsed DE-GMAWmainly consisted of a uniform continuous Fe2Al5, or Fe2Al5ZnXnear the steel side andelongated FeAl3phase near the aluminum side. Zinc-rich zone is located at the edge ofweld bead, and the main component is Al-Zn solid solution. An IMC growth model wasproposed based on microstructure analysis. The IMC growth stages could be as follows:diffusion of solid iron and molten aluminum, the nucleation and growth of Fe2Al5at thealuminum-steel interface, formation of the continuous Fe2Al5phase layer and hinderingthe diffusion of iron atoms, aluminum atoms diffusing through Fe2Al5layer to continuereacting with the iron atom, FeAl3crystallization in the aluminum melting zone. Theroles of zinc on droplet spreading wetting and stabilizing coupling arc are played inPulsed DE-GMAW process for joining aluminum to steel.