| Magnesium alloy as a structural material of high performance has a wideapplication in the fields of aerospace, automotive industry, electronic equipment. It hasbecome one of the greatest potential for the future development of new engineeringmaterials. However, the steel has been dominated in the manufacturing industry. Thedissimilar combination magnesium alloy and steel can make full use of their intrinsicproperties. It plays an important positive significance in energy efficiency of automotive,aerospace and other modern industries. However, because the physical and chemicalproperties was too much difference between magnesium and steel, So achieve thedissimilar combination magnesium alloy and steel is still very difficult. This shows thatthe dissimilar materials welding of magnesium and steel will become a serious problem.Mg alloy AZ31B and zinc-coated SPHC steel sheets with thicknesses of2mm and1mm are studied in spot welding process and mechanism of nugget in this paper. Theeffect of process parameters on mechanical properties of magnesium alloy and steeljoints are studied system, and the process parameters of spot welding are optimized. Thefracture characteristics of joints are analyzed. Characteristics of microstructuresstructure and mechanism of joints are studied. Some improvement measures for fourcommon welding defects of joints are discussedThe results show that: the shearing force increased at first and then decreased withthe increasing welding time (2~14cyc), welding current (20~37.5KA) and electrodeforce (5~8KN). The maximum shearing force of6.961kN is obtained at the parametersT=8cyc、I=32KA、P=7KN. The welded joints are fractured in the nugget pullout failuremode, So it can meet engineering application. The nugget pullout failure is a mixedfracture mainly on ductile fracture supplemented by the brittle fracture. The joint iscomposed of three parts molten magnesium alloy nuggets, steel heat affected zones andthe interface of magnesium alloy and steel. Morphology of joints is Bowl-shaped.Magnesium alloy nuggets is composed of equiaxial grains and columnar grains.Microstructure of steel heat affected zones is lath martensite and ferrite. Al5Fe2intermetallic compound are formed in the magnesium/steel interface. Mg-Zn eutecticstructure MgZn2are found at the edge of nugget, and complex compound about Al5Fe2were found at the center of nugget. Addition of zinc could significantly lower the reaction temperature of Al and Fe, and contribute to the formation of Al-Fe intermetalliccompounds. The eutectic reaction of Mg-Zn create a good condition of mechanical seal,lead to aluminum elements enrichment in the interface, thus aluminum elements reactswith the iron formation compounds, the high strength joints is formed at last. |
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