Study On Microstructure Control Of Interface Layer And Properties Of TC4 / 304L Joints Fabricated By CMT

TC4(Ti6Al4V)titanium alloy is widely used in aerospace,medical and chemical fields due to its low density and high strength.304L(00Cr19Ni10)stainless steel is also a commonly used steel in industry due to its comprehensive mechanical properties and good economy.With the development of industrial technology,the demand for materials is more diverse,such as nuclear waste processing equipment,storage devices,and engine fuel storage tanks of large nuclear power plants,which requires high-strength joining between titanium alloys and stainless steel.However,the large differences in physical properties and the unavoidable Ti Fe and Ti Fe2 brittle intermetallic compounds are the key issues that need to be addressed in the joining process.Therefore,this paper uses TC4/304L heterogeneous material cold metal transition(CMT)joints as the object of research and integrates energy regulation and metallurgical regulation to optimize the interface microstructure of the joints,so as to achieve high-efficiency,low-cost,and high-strength joints.In this paper,based on the Miedema enthalpy model and the Tanaka entropy model,the Gibbs free energy?G calculation model of the binary intermetallic compound is established.The Fe and Ti atoms are adjusted by energy regulation and the combination of Fe and Ti atoms are suspended by metallurgical regulation in the molten pool.Combined with the Toop model,the theoretical calculation model of the intermetallic compound Gibbs free energy?G and chemical potential??in the ternary system of Ti-Cu-Si and Ti-Ni-Cu are established.Based on this,three Ti-Fe intermetallic compound barrier solutions in the molten pool are designed:intermetallic compound-physical barrier solution(pure Cu welding wire),intermetallic compound barrier solution(CuSi welding wire)and intermetallic compound-solid solution barrier solution(CuNi welding wire).When pure Cu welding wire is used,the brittle Ti-Cu and Ti-Fe intermetallic compounds exist in the TC4/seam interface layer and the seam,so the maximum strength of the joint is only 201.7 MPa.When CuSi3 welding wire is used,Si element suppresses the formation of Ti-Fe intermetallic compounds by generating Ti₅Si₃ and Ti-Fe-Si,so Si element can reduce the thickness of the TC4/seam interface,but the diffusion layer in the interface layer is still mainly consisted of Ti-Cu and Ti-Cu-Al intermetallic compounds.Therefore,under the same parameters,the thickness of the TC4/seam interface layer obtained by DC CMT drops to 174.2?m,and the thickness of the diffusion layer drops to 45.8?m.However,the higher fusion ratio results in(Cu)solid solution and large-sized petal-like Ti-Fe-Si intermetallic compounds distributed in the seam.With the change of polarity CMT technology,as EP/EN decreases,the fusion ratio decreases,and the thickness of the TC4/seam interface layer decreases from 118?m(EP/EN=8:1)to 89?m(EP/EN=8:8)and then reduces to 81?m(EP/EN=1:8),where the thickness of the diffusion layer is reduced to less than 25?m,and the Ti-Cu intermetallic compounds are densely needle-shaped and slightly segregated.The distribution state of Ti₅Si₃ in the dissolved layer is sequentially changed from massive continuous distribution to massive aggregate distribution and linear continuous distribution.When the CuNi welding wire is used,although the Ti-Fe and Ti₅Si₃intermetallic compounds are suppressed,the thickness of the TC4/seam interface layer is increased,and the severe segregation of the Ti-Ni-Cu intermetallic compound and the intercrystalline low melting point eutectic microstructure can be found.The seam is composed of(Fe,Ni)solid solution and interdendritic(Cu)solid solution.With the introduction of the applied magnetic field and the increase of the magnetic field strength,the flow velocity of the molten pool is accelerated,so the thickness of the interface layer is reduced to 157.3?m,and the Ti Ni Cu equiaxed dendrites and(Cu)columnar dendrites that cause severe segregation in the interface layer disappear and become more dense and uniform.Combining the microstructure and mechanical properties of the three barrier solutions,it is found that when pure Cu welding wire is used,the interface layer is thick and the hardness is high,so the joint strength is the lowest.Although CuSi3 welding wire reduces the thickness of the TC4/seam interface layer,the microstructure composition does not change much,so the average nano hardness of the interface layer is still about 7.0 GPa,and the difference of Vickers hardness between the interface layer and the seam reaches 500 Hv,which limits the joint.With the increase of strength,under the effect of energy regulation,the strength of the joint is only increased from 201.7 MPa to 293.5 MPa.The CuNi10 wire can change the microstructure of the interface layer to Ti-Ni-Cu with a lower hardness,so the average nano hardness decreases to 6.6 GPa,the Vickers hardness difference between the interface layer and the seam decreases to about 200 Hv,and the joint strength increases to 350.8 MPa.The external applied magnetic field reduces the degree of segregation of the interface layer,suppresses high-rigidity equiaxed dendrites,and finally increases the tensile strength of the joint to 393.1 MPa,which reaches the level of diffusion welding,and realizes the dissimilar joining of titanium alloy to stainless steel with high efficiency and low cost.As the Ni content of the welding wire increases to 30%,the supercooling of the components in the liquid molten pool increases,so a large number of high-hardness equiaxed dendrites in the TC4/seam interface layer reappears,and the joint strength decreases.Through microstructure analysis and high-speed photography in-situ observation(200000 fps)of joint fracture behavior,it can be found that one of the reasons for crack initiation at the bottom of the weld is the connection between Ti₅Si₃(CuSi welding wire)or low-melting eutectic microstructure gathering at the bottom of the seam(CuNi welding wire)and micro cracks in the interface layer.