Interfacial Reactions And Joining Mechanism During Laser Welding-brazing Of Mg To Steel Assisted With Coating Layer

With the development of social economy and industrial technology,more and more attention has been paid to environmental protection and energy efficiency.The reliable joining between magnesium alloy and steel is one of the focuses in the lightweight production of automobile.However,the physical properties of the two metals are really different.They are insoluble and can not undergo metallurgical reactions.Therefore,the selection of appropriate welding methods and intermediate regulatory elements are important means to solve the problem of magnesium/steel welding.Laser welding-brazing is the preferred process to solve this problem.In this paper,the welding wire of AZ92 magnesium alloy was selected as the filling material,and the laser welding-brazing experiment assisted by Cu and Ni coating was conducted on DP590 steel and AZ92 mag nesium alloy with a thickness of 1mm.The joint formation,microstructure composition and distribution at the interface and mechanical properties of the joint were studied.The research experiment on laser welding-brazing process of magnesium/steel was designed,and the influence of laser power on weld appearance,interfacial structure and mechanical properties was studied under the control of single coating and hybrid coating.The increase of power was conducive to the wetting-spreading ability,but the excessive power caused the burn loss of magnesium alloy,and the optimal power was 1300 W.The effects of Cu and Ni content on the microstructure and distribution of interface were studied.For the direct irradiation region of the magnesium/steel joint under the control of these two elements,only Fe Al reaction layer was generat ed on the interface,and the range of diffusion zone increased with the thickening of the coating.The transition zone of the joint was similar to the interface products in weld toe region.The magnesium/steel joint regulated by Cu plating was mainly compo sed of the blocky Mg-Al-Cu ternary phase dispersed in the reticular(?-Mg+Mg2Cu)eutectic structure,while the dendritic(?-Mg+Mg2Ni)eutectic structure in the magnesium/steel joint regulated by Ni plating had a large number of rhombic Al Ni phases distributed on the outside.In addition,Fe(Ni)solid solution was generated at the interface of the joint regulated by Ni coating,which acted as the connection between the eutectic structure of transition phase strengthening(?-Mg+Mg2Ni)and the steel matrix.A large number of intermetallic compounds were produced in the transition zone of magnesium/steel laser welded joints under the control of Cu and Ni elements.The hardness distribution of the interface in the middle region indicated that the hardness of intermetallic compounds was relatively high.After the interface bonding in the direct irradiation region reached sufficient strength,the hard and brittle phases gathered in the middle region may cause stress concentration,which was one of the reasons restricting the mechanical properties of the joint and making the joint crack expand from the fusion zone.Finally,the finite element temperature field simulation provided the temperature basis for the thermodynamic calculation of each region of the joint.The prediction model of the chemical potential and free energy of the Mg-Cu-Al-Fe quaternary system and Mg-Ni-Al-Fe quaternary system was established.The evolution process of element diffusion and interfacial reaction phase in various regions of magnesium/steel laser welded joints under the control of Ni and Cu coatings was clarified,and finally the joining mechanism of magnesium/steel joints under the control of intermediate elements was reasonably explained.