Study On The Rotary Swaging Process Of Medical Ni-Ti Shape Memory Alloy

Ni-Ti alloy has a wide range of applications in tube fittings,fasteners and medical equipment because of its special shape memory,superelasticity,corrosion resistance and biocompatibility.However,due to the rapid work-hardening rate and the strong sensitivity of the chemical composition to the phase transition temperature,the Ni-Ti shape memory alloy is expensive and difficult to process,and the production of the small diameter thin-wall tube is more difficult.Therefore,a new method is needed to solve this problem.In this paper,Ni-Ti shape memory alloy tube is studied by using the rotary swaging process.By building the constitutive equation of Ni-Ti alloy and the FEM simulation of the rotary swaging process,it provided the theoretical guidance for the small diameter thin-wall Ni-Ti tube.The microstructures,mechanical properties and the phase transformation behavior of Ni-Ti shape memory alloy with 50.5at%Ni solution treated at 650? for 2h were investigated by metallography,DSC,XRD,thermal simulation and Deform finite element method.It can be observed from the metallographic experiment that the structure is homogeneous,the grain keeps equiaxed and the average grain size is about 30?m.The phase transformation temperature of Ni-Ti alloy sample is as follows:As=3.8??Af=31.0??Ap=19.4??Ms=-3.5??Mf=-34.6??Mp=-19.0?;The Ni-Ti alloy samples were characterized by XRD.The results show that the Ni-Ti alloy samples are NiTi phase at room temperature before compression,the martensite and austenite coexist.The features of the fracture surface of the Ni-Ti alloy specimen are obtained under room temperature by scanning analysis.The Ni-Ti shape memory alloy cylindrical specimens(size ?8×12mm)were prepared by solid solution treatment,Using these specimens in thermal compression experiments at different temperature conditions(650??1050?,temperature gradient 100?),different strain rate conditions(0.01s-1?0.1s-1?1s-1)and different deformation conditions(20%?40%?60%)to obtain the true stress-strain curves of the Ni-Ti shape memory alloys.According to the Arrhenius constitutive model,the constants of the constitutive equation are calculated by linear fitting and cubic spline interpolation.The results are as follows:a=6.544×10-3(Mpa-1)?A=6.2348×1010?Q=2.57×105(J/mol)n=4.656.The flow stress constitutive equation of Ni-Ti shape memory alloy was constructed to provide a more accurate material model for the FEM simulation.The microstructure of the specimen was analyzed by thermo-simulation.The results show that the microstructure is equiaxed before compression,and the dynamic recovery and dynamic recrystallization of the specimen are observed after compression.Compression at 650?,the microstructure showed fibrous,is the typical dynamic recovery characteristics.Compression at 750??850?,the sample began dynamic recrystallization,and product small grains.Compression at 950 ?,there are a large number of equiaxed crystal,the main change is dynamic recrystallization.Compression at 1050? the sample grain grows serious.The finite element model of the rotary swaging process was established by using Deform-3D FEM software.It can simulate the influence of different temperature(550??650??750?),different deformation degrees(23%?38%)and different feeding speeds(3.0mm/s?5.0mm/s?8mm/s)to the rotary swaging process,and the process was validated by rotating forging Ni-Ti shape memory alloy wire.The results show that the optimum process parameters of Ni-Ti shape memory alloy tube and wire are:temperature 650??700?,deformation 20%?25%,feeding speed 4.0?5.0mm/s.By using this process,we can obtain small diameter thin-wall tube of Ni-Ti shape memory alloy with good microstructure and performance.