Research Of Interface Behavior And Strengthening Mechanism Of Surface Texture On Al/Steel Laser Welding Brazing Joints

With the continuous improvement of requirements for environmental protection,energy saving and emission reduction,the employment of light-weight structure has become the mainstream direction of the development of the multiple industries such as automobiles,rail transit,and aerospace industry.The composite structures composed of high-strength steel and lightweight aluminum alloy have wide application prospects in component fabrication.However,the weldability of Al and steel is poor,and the formation of brittle phases further limits the improvement of joint performance.Thus,it is necessary to use interface control methods to achieve high-quality joining of aluminum alloys and high-strength steel.In this work,6061 aluminum alloy with the thickness of 2mm and DP980 sheets with the thickness of 1.5mm were selected as base metal,and Al-Si5 was used as the filler metal.Firstly,the influence of the surface texture on the wetting and spreading behavior of Al-Si5 on the DP980 surface under the condition of static laser heating was explored,which laid the foundation for the subsequent research on the formability and interfacial reaction of laser welding-brazing joints.Subsequently,the Al/steel laser welding-brazing was performed to explore the influence of the surface texture on the joint appearance,microstructure and mechanical behavior of welded joints.Through the calculation of the stress distribution of the joints,the strengthening mechanism of the surface texture on Al/steel welding-brazing joints was concluded.First,the wetting and spreading behavior of Al-Si5 on DP980 steel with the condition of static laser heating was carried out.The results showed that the wettability was related to the laser process parameters and the morphology of surface texture.The wettability of the filler showed anisotropic on the surface of steel resulted from the effect of the surface texture.The wettability in vertical direction was significantly inhibited,while in the parallel direction,the spreading length increased with the decrease of the texture spacing,and the wetting angle reached the minimum when the spacing was 0.75mm.The calculation results of thermodynamics and dynamic of wetting behaviors showed that the surface texture could effectively reduce the wetting activation energy and normalized free energy,providing a greater driving force for wetting and spreading.However,in the vertical direction,the existence of texture led to the energy barrier to the wetting.Subsequently,research on laser welding-brazing of 6061 Al/DP980 steel was carried out.The relationship between process parameters and mechanical property was analyzed by response surface methodology.The optimal process parameters were determined as follows:the defocused distance of+25.5mm,the laser power of 3.14k W and the welding speed of 0.35m/min.Subsequently,the effect of surface texture on the microstructure evolution and mechanical properties of Al/steel laser welding-brazing joints was studied.The results showed that the addition of the surface texture could improve the surface formation of the joints;The surface texture did not affect the variety of intermetallic compounds,but affected theirs morphology distribution.The interface reaction layer was composed of Fe₂(Al,Si)₅ and Fe(Al,Si)₃,which distributed along the sidewalls inside the texture.The degree of diffusion of atoms in texture increased which resulted in the increment of the thickness of IMC layer.The surface texture with line spacing of 0.5mm-1.0mm could improve the mechanical properties of joints.When the spacing was 0.75mm,the mechanical property was best,increasing the strength of the joints without surface texture by 20.4%.The residual stress after welding in the area near the interface was calculated,and the results showed that the addition of thesurface texture effectively reduced the tensile stress of the intermetallic compound layer and promoted the uniform distribution of the residual stress.The sudden change of groove geometry and the reduction of tensile stress changed the propagation path of the crack on the one side of the groove,which hindered the rapid propagation of the crack.