| Because of the universality and cross-cutting of magnesium, aluminum alloy used, as well as the universality of resistance spot welding, this paper is mainly studied two different materials’weldableness for magnesium alloy and aluminum alloy with thickness of2mm for each.The result shows that the tensile shear of the welding joint will increase first, then decrease along with the welding current, welding time and electrode pressure increase. When the welding current is too low and the time is too short, it easy to cause lack of penetration and or melting nuclear smaller to the welding joint; On the contrary, it would cause serious spatter when the welding current is large and the time is too long; When the electrode pressure is too small, two-sample will not contact well, then it will easily cause spatter under such conditions; On the other side, it will reduce the contact resistance and current density when the electrode pressure is too large, loss of heat resulting in the joint lack of heat, and even unfused. By using the orthogonal experimental design, the optimum welding parameters for the welding of magnesium alloy and aluminum alloy with thickness of2mm can be acquired:electrode pressure6.6KN, welding current43KA, welding time6periods, the tensile shear can reach3.91KN. By using the modern test equipments, such as scanning electron microscope(SEM), Microhardness testing, X-ray diffraction (XRD), energy spectrum analysis, can analyze the spot welding, and the results show that the heat on each side of the joint is unbalance because of the physical property for magnesium alloy and aluminum alloy is difference, and finally cause the nugget shape is like a hat. The entire welded joint is composed of three parts:the magnesium side, the transition layer and aluminum side. The magnesium side is formed by the base metal, the heat affected zone and the nugget, aluminum side is formed by the heat-affected zone and base metal. The microhardness values in the magnesium side are65HV,85HV and125HV, in aluminum side are values50HV and26.5HV, but the microhardness values in the transition layer reach up to251HV. The nugget is mainly composed of dendrites and columnar grain, and more closer to the center of the specimen, the grain is coarsening. The main constituent phases of Spot welding joint are Al, Mg, ε-Mg, Mg17Al12. The study also showed that, A1element in the aluminum side transited to the magnesium side during the Mg/Al dissimilar materials spot welding process, a transition layer emerged in the magnesium side, the Mg and A1elements mainly existed in the transition layer as the form of Mg17Al12which is one kind of inter-metallic compounds. Macroscopic fracture of spot welded joints are flush and bright, micrograph showing is river pattern, it was cleavage fracture, nor obvious dimples, it was typical brittle fracture.Study also showed that the sample surface cleaning state have a great impact on the performance of the spot welding joints. The surface without cleaning will contains the oil slick, the oxide film and any other impurities that would cause the joint overheating and result serious splash, it is also prone to form inclusions and other defects. During spot welding joints in the solid-liquid phase, when the liquid metal can not be timely to fill the small gaps, which are appeared due to the internal forces or contraction, will cause microcracks. |
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