Microstructure And Mechanical Properties Of NiTi Shape Memory Alloy Fabricated By Laser Additive Manufacturing

Selective Laser Melting（SLM）forming NiTi shape memory alloy has an important application prospect in aerospace and medical fields,and the control of its phase transition behavior is the key to realize the functional characteristics of memory alloy.In addition to SLM process,Ni content also has a great influence on the phase transformation behavior of NiTi memory alloys.However,the existing literature mainly studies the SLM forming of NiTi alloys with fixed Ni content(such as Ni_50.8Ti at%).The purpose of this paper is to explore the phase transformation behavior and mechanical properties of NiTi alloy formed by SLM under different Ni content.In this paper,SLM forming of common Ni_50.8Ti alloy and Ni₅₁Ti and Ni_51.5Ti alloy is compared.The main research results are as follows:（1）The effect of printing parameters on the microstructure,phase and mechanical properties of Ni_50.8Ti alloy formed by SLM was studied.Under the optimized parameters（laser power 135 W and scanning speed650 mm/s）,the elongation of the sample is 8.2%and the tensile strength is 714 MPa.Both B2 austenite and B19’martensite phases are found in Ni_50.8Ti alloy with different parameters at room temperature.The phase transition temperature of Ni_50.8Ti alloy is significantly affected by the forming parameters.Low laser energy density is beneficial to reduce the phase transition temperature of Ni_50.8Ti alloy.（2）The effects of SLM forming parameters on the microstructure,phase,mechanical properties and superelasticity of Ni_51.5Ti alloy were studied by increasing Ni content.Compared with Ni_50.8Ti alloy,the transformation temperature of Ni_51.5Ti alloy is lower than that of Ni_50.8Ti alloy,and most of the formed samples are austenitic phase at room temperature,and only martensite phase is formed at low scanning speed（180-260mm/s）.The tensile fracture is brittle fracture with the highest elongation of 8.0%and tensile strength of 695 MPa.It is superelastic at high scanning speed（≥340 mm/s）,and it has better superelastic at laser power 150 W,and the residual strain is less than 0.15%after 2%deformation unloading.（3）The effect of Ni content on the microstructure and properties of NixTi（x=50.8,51,51.5）alloy formed by SLM was studied.With the increase of Ni content,the martensitic transformation temperature decreases,the critical yield stress of martensitic increases,and the deformation ability of the alloy is insufficient.Therefore,it is easy for cracks to germinate and expand due to stress concentration,resulting in interlayer cracks,which reduces the effective forming parameter window of powder.The mechanical properties of Ni₅₁Ti alloy after composition optimization are relatively better,with tensile strength of 759 MPa and elongation of 8.3%.