Research On The Microstructure And Properties Of NiTi-based Shape Memory Alloys Fabricated By Selective Laser Melting(SLM)

NiTi-based alloys have excellent shape memory properties,pseudo elasticity,corrosion resistance,etc.They are the best performing and most widely used type of known shape memory alloys(SMA).Different from traditional processing methods that produce simple shapes such as rods,wires and plates,Selective Laser Melting(SLM)technology can efficiently and individually form complex structural parts.It is the most studiedand additive manufacturing technology for preparing shape-memory alloy materials so far,which is of great significance to promote the development and application of intelligent components.This paper focuses on the optimization process of forming Ni-Ti binary alloy and Ni-Ti-Zr ternary alloy by SLM technology,reveals the evolution of micro-defects,organization,phase transformation behavior and mechanical properties in SLM forming NiTi-based SMAs through the characterization of microstructure and properties,and explores the method of hot isostatic pressing(HIP)to solve the cracking of Ni-Ti-Zr ternary alloy formed by SLM.It can provide theoretical reference and data support to broaden the application of NiTi alloy under high temperature conditions and realize the SLM forming of high performance NiTi-based SMAs.The specific contents and conclusion are as follows:(1)Study on the process of SLM forming Ni-Ti binary alloy.Through two groups of specimens with low laser power and high laser power,the effect of laser energy density on alloy structure and properties is compared and analyzed,and the best combination of process parameters for SLM forming Ni-Ti binary alloy was obtained,and the maximum density of the formed specimens could reach 99.91%.The precipitation of Ti₂Ni phase increases the phase transition temperature,hardness and strength of the sample,but leads to a decrease in plasticity.Finally,the tensile fracture morphology of the sample at room temperature exhibits mixed fracture with less than 3%fully reversible deformation.(2)Study on the process of SLM forming Ni-Ti-Zr ternary alloy.The microstructure,phase composition,phase transformation behavior and mechanical properties of the alloy under different laser energy densities and preheating temperatures are studied,and the characteristics and formation mechanism of crack defects are summarized.The phase composition of ternary alloy is relatively rich,and its room temperature organization is mainly austenite,lath-shaped and spearhead martensite.The intracrystalline is precipitated by the finer?1 and(Ti,Zr)₂Ni phases,and the grain boundaries are large Ni Zr phases and long strips of(Ti,Zr)2Ni phases because of the Zr riched.The precipitated phase will cause local changes in stress and composition,thereby affecting its properties.The DSC curves showed that the introduction of Zr could effectively raise the phase transition temperature.Compared to the Ni_50.2Ti_49.8(at%)alloy,the peak temperature and the thermal hysteresis of the Ni₄₈Ti₄₄Zr₈(at%)alloy has increased by 52.16°C and 30.99°C respectively on average.(3)Study on the effect of hot isostatic pressing post-treatment process on the defects and properties of Ni-Ti-Zr alloy formed by SLM.By comparing three groups of specimens of SLM formed Ni-Ti alloy,Ni-Ti-Zr alloy and Ni-Ti-Zr alloy treated by HIP,it was found that HIP treatment did not change the phase composition of the specimens,which could effectively close the crack defects inside the specimens,promote the uniform distribution of the elements and the structure inside the sample,and significantly improve the hardness,tensile strength and plasticity of the alloy.The tensile strength of the ternary alloy specimens treated at 1050°C and 150 MPa was 960.53 MPa on average,and the elongation at break was 5.34%,which was higher than that of the Ni-Ti binary alloy.However,the agglomeration of precipitates would hinder the lattice distortion and suppress the martensitic phase transformation.Compared with the sample before treatment,the critical stress of stress-induced martensitic phase transformation was reduced by 293.24 MPa,the unloading reversible strain of the compression curve was reduced by 0.01%,and the shape reversion rate after heating was reduced by 0.33% on average.