Study On The Connection Mechanism Of Dual Phase Steel To Magnesium Alloys With The Addition Of Sn Foil

Application of lightweight materials to achieve weight reduction of automotive, is necessarily related to the connection between dual phase steel and magnesium alloy. Because of the large difference of the melting point and boiling point between steel and magnesium, they can not achieve the simultaneous melting.In addition, they can not generate solid solution or intermetallic compound. Therefore, it is difficult to achieve the welding between dual phase steel and magnesium alloy.To explore an effective method of realizing the connection of dissimilar metal between steel and magnesium alloy, the 1.4mm thick DP600 and 1.8mm thick AZ31 plate are welded by fiber laser beam with the Sn foil addition in the method of laser heat-conduction welding of the steel plate on the upper and magnesium lower. By using the horizontal microscope, scanning electron microscope with EDS and EBSD, X-ray diffraction(XRD) and micro hardness tester, microcomputer control electronic universal testing machine, the microstructure, interface element distribution, phase distribution and compositions, grain size and mechanical properties of the welding joint are studied. At the same time, by using ANSYS finite element numerical simulation and the first-principle calculation based on density fuction theory, the connection mechanism of steel to magnesium welded joint with the Sn foil addition is studied from the angle of energy transfer and metallurgy. Through the research, the project results can provide theoretical foundation for the connection technology between Fe and Mg, and provide experimental basis for the application of laser welding of steel and magnesium.In order to obtain the optimal weld surface forming performance, the fiber laser lap welding process of dual phase steel to magnesium alloy with the Sn foils addition is optimized. When the welding laser power is 1400W, welding speed is 30 millmeter per second, and the defocusing amount is+2 millimeter, welding seam is formed well. By analyzing the microstructure of the welded joints, it is found that there are no softening organization in heated affected zone of dual-phase steel and no oxides and pores in the transition region of steel/magnesium. Besides, FeSn, Fe1.3Sn and Fe3Sn phases in steel side of the transition zone, and Mg2Sn phases in magnesium side of the transition region, which realized the "bidirectional" metallurgical combination between the steel/Mg dissimilar metals.Based on the optimized laser welding process parameters and weld pool shape, Gauss body heat source are selected as the heat source model of laser welding to dissimilar metal between steel andmagnesium. Considering the temperature dependence of material properties, initial conditions, boundary conditions and other factors, the nonlinear 3-D conduction finite element model of the steel/magnesium dissimilar metal laser welding is done by using ANSYS finite element software. The loading and moving of the Gaussian eat source are realized by using the APDL programming languages. Compared the weld pool shape under the same welding parameters with or without Sn foil, and the difference in depth and width of welding pool under different welding parameters, from the angle of energy transfer the fuction of Sn foil are explained to realize the connection of dual phase steel and magnesium alloy. To a certain extent, the effect of heat insulation of Sn foil is beneficial to the steel and magnesium plates melt at the same time.Based on the phase detection results in the interface region of steel and Mg welded joint, the thermodynamic properties and the intrinsic mechanical properties of ductility and brittleness of FeSn, Fe3Sn and Mg2Sn compounds are calculated by using first-principles method. Combined with the welding sample fracture morphology and EDS results in micro areas, it is also discussed for the relationship of the interfacial intermetallic compounds and the mechanical properties of the joints. It is found that the brittleness Mg2Sn intermetallic compound limits the improvement of the shear strength of the steel/magnesium joint.