Study On Process Characteristics And Joining Mechanism Of Laser Welding For Aluminum Alloy/TWIP Steel

Aluminum alloy and advanced high-strength steels are preferred materials for achieving lightweight in vehicle structures and functional components,and their joining can maximize the performance advantages of each material.Therefore,the welding reliability of Al/steel dissimilar metals welded joints has become an urgent problem to be solved.In this dissertation,the thin 5083 aluminum alloy and Twinning-induced plasticity（TWIP）steel plates were selected as the base materials,and the Cu foil and Ni foil were employed as the interlayer materials,respectively.The butt joints of Al/steel dissimilar metals with 2 mm in thickness were prepared by laser welding.The laser welding process characteristics of Al and steel were studied in depth.The effects of process parameters and interlayer materials on the surface and cross-section forming,element diffusion,microstructure and mechanical properties of Al/steel dissimilar joints were discussed.The grain boundary characteristics,recrystallization behavior and joining mechanism of joints were compared and analyzed.It provides a theoretical basis and technical support for improving the quality of laser welded Al/steel dissimilar metals.Firstly,the laser welding properties of TWIP steel,5083 aluminum alloy as homogeneous materials were investigated to provide a theoretical basis for the joining of dissimilar materials.The results show that the maximum tensile strength of the TWIP steel joint obtained under appropriate welding process parameters was 915 MPa.The weld seam consisted of columnar dendrites,cellular dendrites,and equiaxed dendrites.For the laser welded joint of 5083 aluminum alloy under appropriate welding process parameters,the maximum tensile strength was 214 MPa.The weld seam center was composed of a uniform and fine equiaxed grain microstructure,while the grains near the fusion line grew in a planar manner,followed by the growth of cellular dendrites and columnar dendrites.Next,the butt joints of aluminum alloy/TWIP steel dissimilar materials were carried out by the laser offset welding method.The effects of process parameters on the microstructure and mechanical properties of the joints were systematically discussed,and the joining mechanism of joints was revealed.The results show that the maximum tensile strength of the Al/steel joint obtained under appropriate welding process parameters was155 MPa.The weld seam mainly consisted of a Fe-Al IMCs layer（400μm）and a Fe-based solid solution layer.The EBSD analysis revealed that the proportion of low-angle grain boundaries（LAGBs）inside the weld seam was negatively correlated with the tensile strength of the joints,and the proportion of recrystallized structures inside the weld seam was positively correlated with the tensile strength of the joints.Then,the melting ratio of Al and steel during laser welding was regulated by adding Ni or Cu interlayers with different thicknesses,and the purpose of reducing Fe-Al IMCs content was achieved.The results show that the addition of Ni interlayer has a significant effect on the connection type and phase composition of Al/steel butt joints.The maximum tensile strength of the fusion welded joint obtained by adding a Ni interlayer with a thickness of 0.03 mm was 131 MPa,and the main phases of this joint at the aluminum interface included Al Ni,（Fe,Ni）solid solution,Fe Ni₃,Fe₄Al₁₃,and Fe Al₂.A joint with a Ni interlayer thickness of 0.20 mm exhibited dual characteristics of fusion welding and brazing,effectively suppressing the formation of Fe-Al IMCs.The weld seam consisted of an Al-Ni reaction zone,an unmelted Ni interlayer,and a Fe-based solid solution layer,and the major phases at the aluminum interface included Ni,Al,Al Ni,Al_0.9Ni_1.1,and Al₃Ni.However,the addition of Cu interlayer can effectively inhibit the formation of Fe-Al IMCs in Al/steel laser welded joints and improve the tensile strength of the joints.The maximum tensile strength of the fusion welded joint obtained by adding a Cu interlayer with a thickness of 0.02 mm was 176 MPa,and the weld seam was mainly composed of Fe-Al/Al-Cu IMCs layers and a Fe-based solid solution layer.Compared to the Al/steel laser welded joint without an interlayer,the tensile strength of the joint with Cu interlayer was increased by 13.5%.The joints obtained when the thickness of the added Cu interlayer was 0.30 mm and 0.50 mm also had the dual characteristics of fusion welding and brazing.The weld seam primarily consisted of Al₂Cu,AlCu,and AlCu₃ compound layers,an unmelted Cu layer,（Fe,Cu）solid solution layer,and a small amount of Fe-Al IMCs.The EBSD analysis demonstrated that the addition of Cu element could reduce the dislocation density in the joint,increase the proportion of recrystallized microstructure,and improve the tensile strength of the joint.Finally,in order to further increase the mechanical properties of Al/steel laser welded joint,the design concept of Al/steel laser weld utilizing Ni and Cu as composite interlayers was put forward in this dissertation.The aim was to establish a bimetallic solid phase layer at the aluminum side and utilize partially melted Ni and Cu elements to adjust the metallurgical behavior of the weld seam.A metallic Ni solid phase layer was formed in the joint with the addition of Cu/Ni composite interlayer.The joint exhibited dual features of fusion welding and brazing.The weld seam was mainly composed of Fe-Al/Al-Cu/Al-Ni IMCs layers（fusion zone）,an unmelted Ni layer,and a Fe-based solid solution layer.Furthermore,the weld seam of the joint with fusion characteristics obtained by adding Cu/Ni composite interlayer was mainly composed of Al-Ni reaction zone,needle-like phase(Fe₄Al₁₃),Fe-Al IMCs layer,a small amount of unmelted Ni layer and Fe-based solid solution layer.The maximum tensile strength of the fusion welding joint with Cu/Ni composite interlayer was 186 MPa,which was 19.8%higher than that of the Al/steel joint without interlayer material.With the addition of Ni/Cu composite interlayer,the Al/steel joints had both fusion and brazing features,and the Ni/Cu bimetallic solid layer was formed in the joint.The weld seam was mainly composed of Al-Ni IMCs,unmelted Ni/Cu layer,Fe-based solid solution layer with a lower Al（at.%）content,Fe-based solid solution layer with a higher Al（at.%）content,and a small amount of Fe-Al/Al-Cu IMCs.However,the fully fusion joint obtained after adding the Ni/Cu composite interlayer primarily comprises IMCs layers,Fe-based solid solution layer,and a small amount of austenite.Compared with the tensile strength of the Al/steel joint without interlayer,the tensile strength（176 MPa）of the fusion welding joint was increased by 13.2%,and the fracture mode of joint was ductile fracture+brittle fracture.The EBSD results show that the addition of Cu/Ni,Ni/Cu composite interlayer was beneficial in releasing the internal stress in the weld seam,reducing the deformation degree of the joint,and improving the tensile strength of the Al/steel laser welded joint.