Microstructure And Mechanical Properties Of Wire Arc Additive Manufacturing TC4/NiTi Heterogeneous Bionic Structural Materials

Ti6Al4V（TC4）is the most commonly used titanium alloy,and its excellent mechanical properties,very high specific strength,good biocompatibility and anti-corrosion properties are widely used in aerospace,biomedical,military and nuclear power,etc.NiTi alloys are widely used in many important industries such as bio-engineering,pharmaceuticals,energy,and microelectromechanical systems,by virtue of their unique shape-memory effect and superelasticity,good fatigue resistance,and corrosion-resistant properties,which make them play a key application in many important industries.The bimetallic coupling design and integrated manufacturing of TC4 and NiTi can give full play to the respective advantages of the two materials and greatly promote the multifunctionality of the applied products.However,due to the large differences in physical properties,linear expansion coefficients and thermal conductivity between different materials,the temperature field asymmetry during the integrated molding process is highly susceptible to the formation of large stress mismatches.In addition,high-temperature diffusion,local changes in chemical composition and the formation of brittle intermetallic compounds can also contribute to the formation of weakened regions at the interface between NiTi and TC4alloys,leading to premature failure of the bimetallic structure.At present,the heterogeneous components such as steel/nickel,steel/copper and nickel/copper prepared by traditional methods have shown significantly improved multi-material coupling characteristics,but there are still bottlenecks such as simple structure,limited functionality,and insufficient performance for bimetallic localized modular forming fabrication of TC4/NiTi.In contrast,the heterogeneous structures of living organisms in nature are inherent with low stress matching,perfect mechanical combination and excellent multifunctional properties,which can provide a natural blueprint for the local and global optimization design of heterogeneous metal structures,especially the rapid development of additive manufacturing technology,which makes the integrated manufacturing of complex heterogeneous bionic structures possible.Aiming at the above problems,this thesis takes the multi-phase multiphase structure of natural organisms such as mantis shrimp and snail as a bionic model,and the solid heterogeneous structure with alternating multilayers,strong-tough bonding and gradient transition as the basic framework of bionic design.Taking TC4,NiTi and Nb welding wires as the main constituent materials,the multi-wire arc additive manufacturing（MWAAM）technology is adopted to develop a manufacturing process of low-stress connection at the interface of multiple heterogeneous materials,and a number of heterogeneous metal components with biomimetic functional structures have been prepared and obtained,and the functionally graded design of TC4/NiTi heterogeneous components with integrated additive manufacturing has been realized step by step.On this basis,the elemental diffusion,microstructure evolution and the formation and control of interfacial intermetallic compounds between heterogeneous interfaces have been investigated in depth,revealing the crystallographic features and the functional mechanisms of the heterogeneous components in the multi-material system.The main research contents and conclusions are as follows:（1）A low-stress transition layer with a gradient transition in composition from TC4 titanium alloy to NiTi alloy was designed and prepared to realize the integrated additive manufacturing of TC4/NiTi bionic gradient heterogeneous components.It was found that a good metallurgical bond could be formed between the gradient deposited layers without generating macro-and microscopic cracks.NiTi₂ intermetallic compounds existed in the gradient layer,and the maximum microhardness was 669.6±12 HV.The maximum bending strength of the TC4/NiTi gradient heterogeneous member was 257.01±26 MPa,and the maximum compressive strength was 1533.33±26 MPa;and the recovery rate of the component after the 10th compression cycle was54.18%,which showed a certain superelasticity.Due to the large amount of NiTi₂intermetallic compounds,the above bimetallic heterogeneous members are difficult to obtain excellent tensile strength.（2）In order to enhance the interfacial mechanical properties of TC4/NiTi heterogeneous components,the pure Nb transition was used to reduce the elemental diffusion between TC4 and NiTi alloy and the formation of NiTi₂ intermetallic compounds.The interfacial bonding mode between TC4 and Nb was optimized using MWAAM technology,and the influence law of TC4/Nb gradient heterogeneous interfacial mechanical properties was revealed.It was found that the maximum compressive strength of the TC4/Nb gradient heterostructure was 2162.64±26 MPa,and the ultimate tensile strength of 514.21 MPa could be obtained in the direction perpendicular to the interface.The TC4/Nb/NiTi bionic layered heterostructures were successfully prepared by depositing pure Nb and NiTi alloys on the TC4/Nb heterostructures in sequence.It was found that the pure Nb transition layer played a good role in blocking the bidirectional diffusion of elements between TC4 and NiTi,and the proportion of NiTi₂ phase in the interface region was drastically reduced.The maximum compressive strength of the TC4/Nb/NiTi bionic layered heterogeneous component is 1419.87 MPa,and the recovery rate of the component after 10 cycles of compression is about 31%,which is a slight decrease in the superelasticity compared with that of the TC4/NiTi bionic gradient heterogeneous component.The ultimate tensile strength of 378.8 MPa was obtained at the interface perpendicular to the bionic laminar heterogeneous component.（3）NiTi/Nb bionic layered heterogeneous members with different layer spacings were designed and successfully prepared using composite wire arc additive manufacturing（WAAM）technology with NiTi and Nb as substrates to enhance the bonding strength at the interface between NiTi and Nb.In this paper,the interdiffusion of elements between NiTi and Nb interfaces with different layer spacings is thoroughly investigated,and the effects of Nb on the microstructure,phase evolution,and interfacial bonding strength of NiTi/Nb layered heterogeneous members are clarified.It is found that the enhancement of the strength of the pure Nb layer as well as the NiTi-Nb interface can be achieved by modulating the elemental interdiffusion behavior between the interfaces through the composite WAAM technique.The maximum tensile strength obtained perpendicular to the NiTi/Nb interface was 664.3±16 MPa,the maximum compressive strength was 2400.6±32 MPa,and the recovery rate was about40%after compression with 10 cycles of 10%strain cycling.The preparation of Nb deposited layer using CMT technology successfully alloyed some elements of NiTi alloy into the Nb deposited layer,which realized the improvement of the properties of pure Nb deposited layer.（4）In order to break through the bottleneck problem that the strength of pure metal transition layer heterostructures is limited by their own properties,NiTi/Nb/TC4 bionic layered heterostructures were designed and prepared by using composite WAAM technology.The dilution rate of the Nb transition layer was controlled by CMT technique,and appropriate amounts of Ni and Ti were introduced into the high melting point Nb transition layer,which consumed the diffused Ti and thus suppressed the formation of NiTi₂ brittle phase.It is found that the Ni content reaches a minimum at the top of the Nb deposition layer,and only a small amount of NiTi₂ phase is formed near the Nb-TC4 interface.the NiTi₂ phase is interwoven with the Ti Nb phase in a reticulated distribution,which reduces the local stress concentration.Meanwhile,the compressive strength of the composite WAAM NiTi/Nb/TC4 layered heterogeneous component obtained perpendicular to the interface was 1549.0±17 MPa,and the tensile strength was 460.7±16 MPa;the recovery rate after 10 cycles of compression was65.3%.