Study On Microstructures And Properties Of IN625/TC4 Bimetallic Structure By Laser Melting Deposition

Compared with the structural parts of a single IN625 Nickel-based high-temperature alloy or TC4 titanium alloy,the preparation of the two alloys into an integrated bimetallic structure（BS）can give full play to their respective characteristics and also reduce the weight of the components,which can help improve the thrust-to-weight ratio and performance of the aerospace vehicles.However,the physical and chemical properties of Ni and Ti,the main elements of the two alloys,are different,and the direct preparation of IN625/TC4 BS will generate a large amount of Ti-Ni intermetallic compounds and large internal stresses in the transition region,which will easily produce cracks and other metallurgical defects.In recent years,laser melting deposition（LMD）technology has played an important role in the manufacturing of BS.LMD technology is a digital,intelligent,low-cost,and short-cycle advanced manufacturing technology for near-net forming,which can meet the demanding requirements of manufacturing and weight reduction of complex components in the aerospace field.LMD is also possible to control the heat input more precisely,thus reducing the internal stress of the component;at the same time,it is possible to realize the composition design and control during the component forming process to reduce the formation of brittle phases,thus reducing the crack sensitivity of the component.Therefore,it is of great theoretical significance and application value to study the microstructure and mechanical properties of LMD IN625/TC4 BS.First,the direct transition of LMD IN625/TC4 BS was investigated.The results show that a large amount of Ti-Ni intermetallic compounds,massive（Cr,Mo）phases,and substantial internal stresses led to grain-piercing cracking in the transition region.The phase composition and cracking tendency of x IN625+y TC4（x=90%-10%,y=10%-90%）mixed alloy powder deposition layer with a 10%TC4 volume ratio as the incremental step was analyzed.Finally,90%IN625+10%TC4 and 80%IN625+20%TC4 with lower crack sensitivity were selected as gradient transition interlayers,and LMD IN625/TC4 BS without cracks and other metallurgical defects were successfully prepared.When depositing 90%IN625+10%TC4,Ti and Ni elements combine to form Ti Ni₃ phases,while Cr and Mo elements precipitate and aggregate into（Cr,Mo）phases.Simultaneously,due to the higher content of IN625,a significant amount ofγ-Ni remain in the deposit.When depositing 80%IN625+20%TC4,the content of Ti Ni₃ phases and（Cr,Mo）phases increases.Depositing TC4,with its higher melting point,required a higher laser output power,resulting in the diffusion of Ni,Cr,Mo,and Ti elements,leading to the formation of Ti₂Ni andβ-Ti phases within the melt pool.Continuing to deposit TC4,the diffusion effect of elements weakens,leading to a reduction in the content of Ti₂Ni andβ-Ti phases and the generation of theα-Ti phases.The room temperature tensile strength of the gradient transition BS is approximately 211 MPa.The gradient transition promotes microstructural changes in the transition region,reducing its crack sensitivity.However,there are still numerous Ti-Ni intermetallic compounds,which limit the improvement of the bonding properties of the BS.The influence of different thicknesses of Cu interlayer on the microstructure and properties of LMD IN625/TC4 BS has been investigated.The results show that when Cu is deposited,there is mutual diffusion between Cu and Ni elements,leading to the formation of a solid solution and a good metallurgical bond.Subsequently,during the deposition of TC4,with a Cu interlayer,a small amount of Ti diffuses to the lower part of the transition region and combines with Ni to form a limited amount of Ti Ni₃ phase.With two and three Cu interlayers,Ti elements only diffuse to the upper part of the transition region,forming various Ti-Cu intermetallic compounds.As the deposition of TC4 continues,the diffusion effect of elements weakens,resulting in a reduction in the types and quantities of Ti-Cu intermetallic compounds.The room temperature tensile strengths of BSs with one,two,and three Cu interlayers are approximately 229 MPa,272 MPa,and 258 MPa,respectively.The introduction of Cu interlayers effectively suppresses the formation of Ti-Ni intermetallic compounds but leads to the formation of Ti-Cu intermetallic compounds,which to some extent limits the improvement of the bonding properties of the BS.The influence of different thicknesses of V interlayer on the microstructure and properties of LMD IN625/TC4 BS has been studied.The results indicate that when V is deposited,due to its higher melting point,a higher laser output power is used in the manufacturing process.This leads to a significant diffusion of V elements with Ni and Cr elements,resulting in the formation of mutual diffusion and the creation ofγ-Ni and（V,Cr）ss.Subsequently,during the deposition of TC4,with a V interlayer,Ti elements diffuse to the lower part of the transition region and combine with Ni elements to form Ti-Ni intermetallic compounds.As the thickness of the V interlayer increases,the downward diffusion of Ti elements decreases,further suppressing the formation of Ti-Ni intermetallic compounds.As the deposition of TC4 continues,the diffusion effect of elements weakens,and the types and quantities of Ti-Ni intermetallic compounds decrease.The room temperature tensile strengths of BSs with one,two,and three V interlayers are approximately 250 MPa,268 MPa,and 279 MPa,respectively.The introduction of V interlayers helps to avoid direct bonding between IN625 and TC4 to some extent.However,Ti-Ni intermetallic compounds still form in the transition region,limiting the improvement of the bonding properties of the BSs.The influence of different thicknesses of Cu/V multi-interlayer on the microstructure and properties of LMD IN625/TC4 BS has been studied.The results indicate that when Cu is deposited,Cu combines with Ni elements to form a solid solution,resulting in a good metallurgical bond.When V is deposited,V and Cu elements mutually diffuse,and they do not form intermetallic compounds between them.During the deposition of TC4,due to the unlimited solubility of V and Ti elements,Ti and V atoms diffuse sufficiently at the interface.With a layer of Cu/V multi-interlayer,due to the insufficient thickness of the Cu interlayer,V and Ti elements diffuse downward and form（V,Cr）ss and Ti-Ni intermetallic compounds with Cr and Ni elements.With two layers of Cu/V multi-interlayer,the formation of Ti-Ni intermetallic compounds is completely suppressed,but due to the insufficient thickness of the V interlayer,Ti₂Cu intermetallic compounds are formed in the transition region.When three layers of Cu/V multi-interlayer are used,the Cu/V multi-interlayer blocks the contact between Ti and Ni elements,while the V interlayer blocks the contact between Ti and Cu elements,completely suppressing the formation of intermetallic compounds.The room temperature tensile strengths of BSs with one layer of Cu/V,two layers of Cu/V,and three layers of Cu/V multi-interlayer are approximately 496 MPa,359MPa,and 314 MPa,respectively.The introduction of Cu/V multi-interlayer further suppresses the formation of Ti-Ni and Ti-Cu intermetallic compounds,resulting in excellent bonding properties.