Study On The Microstructure And Mechanism Of Laser Welding Joint For Aluminum/steel Surface With Simultaneous Adding Powder

Automobile lightweight is one of the most effective measures to reduce emissions and fuel consumptions,hybrid structures of aluminum and steel have great potential.However,it is difficult to high combine steel with aluminum owing to the discrepancy of their material properties and the formation of brittle intermetallic compounds at the interface,To explore a new method of aluminum/steel connection,this paper based on the thought of the little mixability between of the upper aluminum liquid and lower steel liquid in the method of laser welding of the aluminum to steel which in favour of formation of Fe-Al intermetallic compounds is limited.DP590 dual phase steel and 6016 aluminum alloy widely used in car body are selected as the research objects.It was presented that the experimental scheme of laser lap welding of aluminum to steel by smearing AlSi₁₂Mg_1.5 on aluminum alloy and adding Sn powder between steel and aluminum.The improvement mechanism of the welding with powder addition was explored by study on weld formability,the microstructure,fracture morphology,distribution and content of phase,grain size,interface combining situation,hardness,mechanical properties of welded joints and the spectral information of plasma.ANSYS software is also used in the paper to simulate the transient temperature field distributions,and the first-principle calculation based on density fuction theory to calculate the thermodynamic properties of each compounds.The results expected to enrich and perfect the theory of dissimilar metal welding,and to provide new ideas and theoretical basis for the popularization and application of laser welding of aluminum/steel structure.In order to obtain the optimal weld surface morphology,the laser welding process parameters of aluminum to steel is optimized.When the welding laser power is 2750W,welding speed is 32mm/s,the focal distances is-2mm,and the Ar flow is 15L/min,it is found that the welded joint is formed worse without powder.After smearing AlSi₁₂Mg_1.5 on aluminum alloy,the welded joint is formed well,the average thickness of the layer was approximately 50～100μm,a layer composed of Fe₂Al₅ is close to the side of the dual phase steel phase,and another composed of FeAl₃ near the pool side,the average tensile strength of joint is 33.67N/mm.Smearing AlSi₁₂Mg_1.5 on aluminum alloy and adding Sn powder between steel and aluminum,the average thickness of the layer was approximately 10～20μm,witch are the FeSn,Fe₃Sn and the white organization that destroyed the formation of FeAl₃ layer,the average tensile strength of joint is 54.16N/mm.Based on the numerical simulation of aluminum to steel lap laser welding,it is found that the mixability between of the upper aluminum liquid and lower steel liquid is low.Compared with the without powder,the depth of molten and weld width are apparent increases by smearing AlSi₁₂Mg_1.5 on aluminum alloy,and which is beneficial to absorb the laser energy.Smearing AlSi₁₂Mg_1.5 on aluminum alloy and adding Sn powder between steel and aluminum,simultaneous adding powders can increase the fusion zone between of the upper aluminum liquid and lower steel liquid,the temperature field distribution of the joint have been changed large.Based on the phase detection results in the interface region of aluminum and steel welded joint,the thermodynamic properties and the intrinsic mechanical properties of ductility and brittleness of Fe₂Al₅,FeAl₃,FeSn and Fe₃Sn compounds are calculated by using first-principles method,welding specimen fracture morphology and EDS analysis of micro zone,and the relationship of the interfacial intermetallic compounds and the mechanical properties of the joints are been discussed.It is found that the generated new phases FeSn and Fe₃Sn have good ductility and structure stability at high temperature when smearing AlSi₁₂Mg_1.5 on aluminum alloy and adding Sn powder between steel and aluminum,rapid solidification limit the upper aluminum liquid and lower steel liquid mixed each other,all of this factors limited formation of Fe-Al intermetallic compounds.The aluminum/steel surface with simultaneous adding powders improved the mechanical properties of the welding joint.