Study On Improving Properties Of Steel-aluminum Laser Welded Joints By Rare Earth Alloying Elements Powder

In response to the national policy of energy saving,emission reduction and sustainable development,automobile lightweight technology has become the research focus of modern scientists in the new era.Among them,the use of steel-aluminum welded structure instead of all-steel structure in the car body is one of the important ways to realize the lightweight of the car.Because steel and aluminum are prone to generate brittle Fe-Al intermetallic compounds during the welding process,the strength of the welded joint is reduced.Therefore,restraining the formation of Fe-Al intermetallic compounds and reducing the thickness of Fe-Al intermetallic compounds are the key to improving the joint strength.In this paper,the laser deep fusion welding of 1mm thick DP600 double-phase steel and6061 aluminum alloy was carried out.Study the effects of optimizing welding process parameters and adding intermediate layer alloying element powder on welded joint forming morphology,mechanical properties,fracture morphology,and the types and distribution of intermetallic compounds at the weld interface;A two-dimensional numerical model of laser deep penetration welding of steel and aluminum was established to simulate the dynamic formation process of the keyhole in laser deep penetration welding of steel and aluminum.In terms of numerical simulation,a two-dimensional numerical model of steel-aluminum laser deep penetration welding was established,and the dynamic formation process of the keyhole of steel-aluminum laser deep penetration welding was simulated.The Level-Set method is used to track the gas-liquid interface of the keyhole,and the solid-liquid interface of the keyhole is processed by a hybrid continuous model.The simulation results successfully simulated the dynamic formation process of the steel-aluminum laser deep penetration welding keyhole and the distribution of the internal temperature field in the molten pool at different times,and the accuracy of the model was verified through experimental comparison.In terms of optimizing welding process parameters,a single factor experiment was used to analyze the influence of laser power,welding speed,and defocusing amount on the weld surface formation,and to determine the process parameter conditions for this experiment.The orthogonal experiment was used to optimize the welding process parameters,and the best process parameters were obtained:the laser power was 420W,the welding speed was10mm/s,the defocus was+0.6mm,and the shielding gas flow rate was 15L/min.The mechanical properties of the welded joint obtained under the optimal process parameters were 75.6N/mm,and the fracture morphology of the welded joint was analyzed to be brittle fracture.EDS point scanning of the weld interface shows that the IMC at the weld interface is mainly sheet-like,rod-like Fe₂Al₅ phase and needle-like Fe₄Al₁₃ phase.In terms of adding intermediate layer alloying elements,based on the above optimized process parameters,four kinds of rare earth alloying elements powders of Al-0.3%Ce,Al-0.5%Ce,Al Si12-0.3%Ce and Al Si12-0.5%Ce were preset on the surface of aluminum alloy to conduct steel-aluminum laser welding experiments.Compared with the case when no alloying elements are added,the influence of adding the intermediate layer powder on the mechanical properties of the welded joint and the shape of the weld interface is analyzed,and the morphology of the side fracture of the tensile specimen steel and the type and distribution of intermetallic compounds at the joint interface are analyzed.The results show that when Al-based Ce powder is added,the weld cross-section formability obtained by adding Al-0.3%Ce intermediate layer is better than adding Al-0.5%Ce intermediate layer,and there are almost no defects such as holes and cracks.When the Al-0.3%Ce intermediate layer is added,the joint line load is 83.8N/mm,which is an increase of 10.8%,and the fracture morphology is mixed fracture.When the Al-0.3%Ce intermediate layer is added,Fe Al_0.3iron-rich phase Fe₃Al phase,Fe Al₂ phase and Fe Al phase are formed inside the weld interface.The IMC at the boundary of the weld interface is mainly flake Fe₂Al₅ phase and zigzag Fe₄Al₁₃ phase,and compared with no powder added,the thickness is significantly reduced.Ce atoms segregate at the grain boundary to effectively hinder the formation of Fe-Al compounds.At the same time,the oxides and sulfides that can be formed by Ce and the internal O elements and S elements in the molten pool during the welding process are used as Heterogeneous nucleating agent refines the crystal grains and improves the mechanical properties of the joint;When Al Si12-based Ce powder is added,the fracture cross-section formability when Al Si12-0.3%Ce intermediate layer is added is better than that when Al-0.5%Ce intermediate layer is added,and there are no defects such as cracks and holes.When the Al Si12-0.3%Ce intermediate layer is added,the joint line load is 86.7N/mm,which is an increase of 14.7%,and the fracture morphology is mixed fracture.When the Al Si12-0.3%Ce intermediate layer is added,Fe Al_0.8Si_0.1 phase,Fe Al phase and Fe Al₂Si_0.1 phase are formed inside the weld interface.The IMC at the boundary of the weld interface is mainly Fe₂Al₅phase,Fe₃Al phase,Fe₄Al₁₃ phase,and the thickness of the IMC reduce.In addition to the effect of Ce,the ductile Fe Al_0.8Si_0.1 and Fe Al₂Si_0.1 phases at the interface hinder the formation of brittle Fe-Al IMC,thus improving the weld joint mechanical properties.