Study On Microstructure And Physical Properties Of NiTi-Based Shape Memory Alloy Fabricated By Multi-Wire + Arc Additive Manufacturing

NiTi shape memory alloy has excellent shape memory properties,superelasticity and fatigue resistance,and is widely used in automotive,biomedical and aerospace fields.However,in the process of preparing NiTi alloy by traditional preparation process,there are some problems such as high impurity content and difficult preparation of raw materials,which limit its further application.In this study,NiTi alloy was prepared by in-situ alloying using multi-wire arc additive manufacturing technology,and the microstructure evolution and phase transformation behavior were studied.Firstly,the preparation of NiTi shape memory alloys based on double-wire+arc additive manufacturing（D-WAAM）was studied.The formation mechanism of different phases of SMAs with different Ni contents at room temperature and their effects on microstructure and properties were clarified.The results show that Ni_50.8Ti_49.2 is composed of B2 phase,R phase,Ni₄Ti₃ phase and NiTi₂ phase,and Ni₅₃Ti₄₇ is composed of B2,Ni₄Ti₃,NiTi₂,Ni₃Ti and Ni₃Ti₂.The DSC heating curve of Ni_50.8Ti_49.2shows a shoulder peak,indicating the existence of R phase transformation.Ni₅₃Ti₄₇does not show a peak due to the presence of precipitated phase inhibiting martensitic transformation.The hardness values of Ni_50.8Ti_49.2 and Ni₅₃Ti₄₇ are 278 HV_0.2 and 596HV_0.2,respectively.Compared with Ni_50.8Ti_49.2,the fracture strain of Ni₅₃Ti₄₇ is reduced by about 51.7%,and the compressive strength is similar.The unrecoverable strain of Ni_50.8Ti_49.2 gradually approaches 4.5%,while the Ni₅₃Ti₄₇ component shows a linear growth trend.However,due to the wide phase transition temperature of the binary shape memory alloy,its application range is limited.Therefore,in this study,Cu element was added to the binary NiTi alloy,and the in-situ alloying of ternary Ti Ni Cu was carried out to reduce the phase transition thermal hysteresis of the alloy.The effect mechanism of different Cu content on the microstructure and properties of the alloy was clarified.The results show that the precipitated phases in the Ti₅₀Ni₄₅Cu₅ component show a network distribution,and the Ti₅₀Ni₃₅Cu₁₅ and Ti₅₀Ni₂₅Cu₂₅ components show island precipitation.The addition of Cu strengthened the parent phase,prevented the formation of dislocations in the alloy,reduced the friction of the interface,and narrowed the phase transition hysteresis.The Ti₅₀Ni₃₅Cu₁₅ component has the most excellent mechanical properties.The microstructure of the martensite is mainly（111）I twins.The fine（Cu,Ni）₂Ti precipitated phase can be cut through by the twin shear force,which hinders the movement of the twin boundary to a certain extent.The fine（Cu,Ni）₂Ti precipitated phase will affect the growth of the martensite lath.In summary,in this study,NiTi alloy and NiTi Cu ternary shape memory alloy were prepared by multi-wire arc additive manufacturing process,and their microstructure and properties were studied,which expanded the preparation process of NiTi-based shape memory alloy and promoted its wide application.It is of great significance to the expansion of multi-wire arc additive manufacturing technology and the development of metal materials.