| Titanium alloy/stainless steel composite structure could combine the advantages of high specific strength,good corrosion resistance for titanium alloy，and low price for stainless steel,so,it can fully meet the demand of structural weight reduction and diversification requirement of material performance in modern equipment manufacturing industry,and it possesses significant application in aerospace,energy chemical industry,engine,and biomedical engineering.At present,most researches about fusion welding of titanium alloy and stainless steel focused on high-energy beam welding techniques,such as laser welding and electron beam welding.However,due to the high cost and complex equipment,high-energy beam welding techniques are more suitable for the small batch production of specific component with high additional value.In contrast,gas tungsten arc welding,one of the most widely used fusion welding techniques,possesses advantages of high welding quality,easy operation,low production cost,flexibility to adapt different welding positions and working conditions.Therefore,the development of high-quality gas tungsten arc welding of titanium alloy and stainless steel will be beneficial to accelerate the promotion and application of titanium/steel composite structure.However,due to the large difference of physicochemical properties between titanium alloy and stainless steel,brittle intermetallic compounds and high welding residual stress easily form in dissimilar titanium/steel joint,resulting in cracks in the resultant joint.In this study,TC4 titanium alloy and 304 stainless steel sheets with 1 mm thickness were used as base materials,and the effect of welding parameters on joining mechanism,the influence of alloying elements in filler metal on intermetallic compounds in interfacial zone,and the distribution of residual stress were investigated.Results in this study also have important theoretical value and guiding significance for the fusion welding of titanium alloy and steel with different thickness and configuration.The main research content and conclusions are drawn as follows.（1）Firstly,the effect of welding current on joint forming,microstructure and mechanical properties of TC4/304SS joint filled with pure copper filler metal was studied.It revealed that TiFe2 brittle phase resulted in spontaneous cracking of TC4/304SS joint unfilled with filler metal.Filling pure copper filler metal in TC4/304SS joint could effectively prevent the formation of brittle TiFe2 phases and avoid joint cracking after welding.With the increase of welding current,the bonding mechanism of TC4/304SS joint transformed from brazing mode under low current welding process to partial fusion welding and fully fusion welding mode.Under the brazing mode with low welding current,the formation of TiFe2 brittle phases in the joint is completely suppressed,and the less brittle intermetallic compounds including Ti2Cu,TiCu,AlCu2Ti,TiCu4,and Ti2Cu3 formed in Ti/Cu interface,so,the tensile strength of the joint reached 261 MPa.With increasing of welding current,the copper/steel interface was partially melted,resulting in mechanical interconnection effect between α-（Fe,Cr）and Cu solid solution phases,and the tensile strength of the resultant joint increased to 363 MPa.Further increasing welding current to 60 A or above,large number of TiFe2 brittle phases formed at copper/steel interface and decreased the tensile strength of the resultant joint.（2）The formation and control of brittle intermetallic compounds is an important factor restricting the improvement of mechanical properties of titanium/steel dissimilar joints.As a result,the regulation and control of brittle phases in interfacial zone become an important way to optimize the microstructure and mechanical properties of resultant joints.In this work,Cubased filler metal was applied to weld TC4 titanium alloy and 304 stainless steel,and the effect of Si,Al,Ni alloying elements on formation of intermetallic compounds and mechanical properties of the joint was concluded.In the low current welding process,Ti5Si3 phases formed outside the Ti/Cu interfacial zone and prevented the diffusion between interfacial zone and weld zone,decreasing the TiCu dendrite phase in the Ti/Cu interface.Al in Cu-based filler metal promoted the formation of AlCu2Ti with lower microhardness.Si and Al promoted the atomic diffusion and significantly increased the thickness of diffusion layer at copper/steel interface.Increasing welding current,the effect of alloying elements is weakened,and large number of TiFe2 brittle phase formed in the interfacial region,becoming the dominant factor that restricts the tensile strength of fusion welded TC4/304SS joint.（3）Adding Ni in Cu-based filler metal promoted the formation of solid solution at copper/steel interface of TC4/304SS joint welded with low welding current,and the tensile strength of resultant joint raised with the increase of Ni content.In the fusion welding process with large current,TiFe2 brittle phases in copper/steel fusion zone were fined by adding Ni into Cu-based filler metal.While filling with CuNi30 filler metal,Cu-rich y-（Fe,Ni）solid solution formed at copper/steel fusion zone,and broke the continuous distribution of brittle TiFe2 phases,resulting in higher tensile strength of 413 MPa.（4）To further clarify the interfacial metallurgical reaction sequence and the influence of alloying elements on the formation of intermetallic compounds in titanium/steel fusion welded joint,the thermodynamic prediction model was built to calculate the formation enthalpy △H,Gibbs free energy G,and chemical potential μi based on Miedema theory.It revealed that Si,Al and Ni preferentially reacted with Ti,inhibiting the formation of brittle TiFe and TiFe2 phases in Ti/Cu interfacial zone.Si,Al,and Ni diffused toward stainless steel substrate driving by chemical potential,resulting in thicker diffusion layer at copper/steel interface.（5）Besides the brittle intermetallic compounds in the interfacial zone,the high residual stress is another important factor that restricts the mechanical properties of titanium/steel fusion welded joint.Finite element calculation was conducted to investigated the residual stress distribution in TC4/304SS joint in this work.The region of high stress transferred to HAZ on both sides,and the longitudinal residual stress in weld zone decreased obviously after filling pure copper filler metal.Increasing welding current,lots of fine intermetallic compounds distributed in weld seam and produced dispersion strengthening,resulting in increase of longitudinal residual stress in weld and more transverse cracks.When nickel-based filler metal is used,the von Mises equivalent stress in TC4/304SS joint decreased significantly,and the longitudinal residual stress in the weld is much lower than the tensile strength of filler metals,which is beneficial to restrain the transverse crack in the weld.（6）According to the simulation results of residual stress,Cu+Ni composite filler metal was designed for TC4/304SS fusion welding,and the TC4 titanium alloy/Cu coating/Ni-based weld/304 stainless steel composite structure was obtained.Filling with Cu+Ni composite filler metal,the transverse crack in the weld was successfully restrained and the welding process window was enlarged.The copper foil enfolded at the end face of titanium alloy sheet could reduce the dissolution of titanium alloy,and decrease the Ti-based brittle compounds in Ti-side interfacial zone.The Ti-side interfacial zone consisted of β-Ti,Ti2Ni,TiNi,TiNi3 brittle phases and（Cr,Mo）solid solution.Increasing welding current,Ti（Fe,Cr,Ni）2,and Ni-Fe-Cr-Ti phases formed in the Ti-side interfacial zone.FeCrNi solid solution formed between nickel-based weld and 304 stainless steel substrate without continuous brittle phases.All the joint filled with Cu+Ni composite interlayer fractured at the Ti-side interfacial zone,and the average tensile strength of TC4/304SS joint reached 432 MPa,and the highest tensile strength of single joint reached 485 MPa. |
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