Study On Ball Spinning Of Thin-walled Tube Of NiTi Shape Memory Alloy

Due to its excellent shape memory, superelasticity, corrosion resistance and goodbiomedical compatibility, Nickel-titanium shape memory alloys （NiTi SMA, hereinafterreferred to as NiTi alloys） have a wide application scope and great perspective in bothcoupling and medical treatments. NiTi SMA tube is difficult to process, so it is necessary tofind a effective forming methods. Ball spinning, which belongs to successively and locallyplastic deformation process, has been applied to manufacturing thin-walled tube with highprecision and high mechanical properties. However, there is no paper on manufacturing ofNiTi SMA tube by using ball spinning. In this thesis, it is a new attempt by using backwardball spinning to produce high quality thin-walled NiTi alloy tube. Dynamic recovery anddynamic recrystallization of NiTi alloy under hot compression was discovered and theconstitutive equation of NiTi alloy was established. Rigid-viscoplastic finite element method（FEM） is adopted to simulate the ball spinning processing of the thin-walled NiTi SMAtube,which lays the theoretical foundation for ball spinning of the thin-walled NiTi SMAtube.In this thesis, the Ni50.9at.%Ti49.1at.%alloy specimens with the diameter of4mm and theheight of6mm,which were solutionized at850℃and held for2hours in vacuum condition,were prepared for the hot compressive tests.The temperature of the hot compressive testsranges from600℃to1000℃, and the strain rate ranges from0.001s^-1to1s^-1, while thecompression deformation degree of samples ranges from10%to90%. Based on the truestress-strain curves of NiTi alloy, the constitutive equation of NiTi alloy with respect to theZener-Hollomon parameter was established according to different stress level at varioustemperatures by using the method of linear fitting, the value of related parameters isQ=2.30×10⁵（J/mol）, α=4.7×10^-3(MPa^-1), A=5.26889×10¹⁰, n=4.26211. The constitutiveequation accurately describes the deformation behavior of NiTi alloy during hot working, andprovides material model for numerical simulation of ball spinning of the thin-walled NiTiSMA tube.In the paper, the deformation behavior and microstructural evolution of NiTi alloys weresysmetically investigated by means of techniques and methods, such as optical microscopy,X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscope （TEM）, energy dispersive X-ray spectroscopy （EDS） and so on. The solutionizedNiTi SMA is characterized by quasi-cleavage fracture under deformation at roomtemperature.The crystal structures of the solutionized NiTi SMA matrix mainly beong to theaustenitic cubic structure （B2type ordered structure） with a small amount of TiC phase atroom temperature. The transformation temperatures of the solutionized NiTi SMA are asfollows: M_s=-21.7℃, M_f=^-38.5℃, A_s=^-12.5℃, A_f=1℃.The microstructure of uncompressed NiTi sample exhibits a characteristic of theequiaxed grains, while the microstructures of the compressed NiTi samples at600℃and650℃are mainly dominated by dynamic recovery, where the grains are elongatedconsiderably. In the meanwhile, the small grains occur in the elongated grains at700℃, whichreveals that dynamic recrystallization has taken place. However, the complete dynamicrecrystallization arises at800℃, and the grains size is much larger when the temperatureexceeds800℃, which reveals that the grains have grew up seriously. Both the stain rates andthe deformation degree have an important influence on dynamic recrystallization of NiTiSMA. Decreasing the stain rates leads to the increase of the time of dynamic recrystallization,which contributes to the larger equiaxed grains. There exists the critical deformation degreeduring dynamic recrystallization of NiTi SMA, beyond which the larger deformation degreecontributes to obtaining the finer equiaxed grains.In order to obtain both the deformation law and the optimized process parameters ofbackward ball spinning, the DEFORM^-3D finite element simulation software is adopted. Thefinite element model of backward ball spinning is firstly built.The temperature field,the stressfield, the strain field and the load prediction are obtained through simulating the primaryparamenters, such as feed ratio （0.2mm/r⁰.8mm/r）, wall thickness reduction （0.1mm⁰.3mm）and forming temperature （700℃⁸00℃） by means of FEM.The distributions temperature ofthe mandrel and balls are obtained also.The simulation results reveal that there is atemperature increase in the principal deformation zone of the spun part due to deformationenergy increasing. It can be found from the stress fields that the plastic deformation zone is ina three-dimensional compressive stress state. The radial strain and the tangential strain arecharacterized by the compressive strain, while the axial strain belongs to the tensile strain.The load prediction shows that NiTi SMA tube is easier to meet stable flowing.