Study On Weldability Of Titanium/steel Dissimilar Metals Based On High-entropy Alloy Interlayer

Titanium-steel composite structure not only has the excellent corrosion resistance of titanium,but also has the advantanges of high strength and low cost of steel.It has great application prospect in petrochemical industy,aerospace industry,transportation industry,power plant boiler industry and engineering machinery industry.The most efficient way to join titanium and steel is welding,however,in the actual welding process,it is difficult to form a reliable connection because of the great difference between the physical and chemical properties of titanium and steel and the easy formation of intermetallic compounds in the welding seam.Due to the special high entropy effect of multi-principal element high-entropy alloy,the intermetallic compounds easily produced are inhibited,and the weld system is more likely to form a simple disordered solid solution structure,which can greatly reduce the weld brittleness,and is conducive to obtaining excellent mechanical properties.In this paper,high-entropy alloy was used as filling layer and transition layer respectively in titanium-steel pulse TIG welding,and the microstructure and mechanical properties of the welded joints were studied,which can provide a theoretical basis for high entropy of titanium-steel dissimilar metal welds.The experimental results shows that:（1）According to the design criterion of high-entropy alloy and the tensile strength of welded joint as the evaluation index,Co₁₃Cr₂₈Cu₃₁Ni₂₈ high-entropy alloy was optimized,the microstructure consists of equiaxed crystal,dendritic and columnar crystal.Cu element is segregated in intercrystal,and the other three elements are uniformly distributed in the crystal according to their theoretical values.Its phase composition is simple two phase FCC structure.The compression test was carried out at room temperature,the deformation reached 68%,and the strength was 642 Mpa,which proves that the alloy is a plastic material.（2）The titanium-steel welded joint obtained by using high-entropy alloys as filler material has no defects such as pores,cracks,inclusions,etc.The internal phase structure of the welded joint is dominated by FCC and BCC solid solutions,and the FCC phase mainly includesα-Cu,（Cu,Ni）solid solution,（Ni,Ti）solid solution,etc.BCC passed mainlu includes Cu₃Ti,Fe Ni,etc.Intermetallic compounds such as Ti Fe₂,Ti Fe are also formed in the weld.The average hardness of the weld（466.3 HV）is lower than that of the titanium-steel joint by direct welding（716.6 HV）.The tensile strength of the welded joint is 186 MPa,and the fracture has obvious river pattern,which indicates that it is brittle fracture.（3）In order to further improve the mechanical properties of welded joints,the high-entropy alloy was applied as a metal transition layer in titanium-steel pulse TIG welding.Firstly,the process parameters were optimized,the optimal process parameters for welding Q235 side are peak current 150 A,duty cycle 53%,welding speed 1.4 mm/s,puls-e frequency 2 Hz,argon flow 20 L/min.The optimal process parameters for welding TA2 tianium alloy side are peak current 110 A,duty cycle 50%,welding speed 1.5 mm/s,pulse frequency 2 Hz,argon flow 20L/min.Co₁₃Cr₂₈Cu₃₁Ni₂₈ as a metal transition layer can directly avoid the direct contact of Fe and Ti.The whole welding joints is composed of two parts of welds,which are well combined with the base metal on both sides.The welds on the side of Q235 are mainly composed of simple columnar and dedritic crystals,and the phase structure is highly consistent with the base me-tal of high-entropy alloy,so it is a simple FCC structure.TA2 of the weld on one side of the organization is more complex,weld center area form columnar crustal,the branch crystal and the equiaxial crystal composition,is mainly composed simple FCC and BCC phase structure,the main phase is BCC.Near the high-entropy alloy transition layer precipitation Cr₃O₈ is the highest hardness,the weld joint was broken in here.On observation of the fracture,there are shallow dimples and large cleavage surface,which is a mixed fracture,however,brittle cleavage fracture is the main.The tensile strength is 224 MPa,which is increased by 20.4%compared with the high-entropy alloy as the filling material（186 MPa）.