| Ball spinning belongs to the successively and locally plastic deformation process, and it is a new attempt for ball spinning to manufacture the thin-walled NiTi shape memory alloy tube. In order to analyze deformation law of ball spinning of the thin-walled NiTi shape memory alloy tube and optimize the process parameters more effectively, the finite element simulation technology undoubtedly provides a good approach for manufacturing the required thin-walled NiTi shape memory alloy tube. In this thesis, by means of using as-cast NiTi shape memory alloy as the original experimental materials, microstructure, mechanical properties and transformation temperature of NiTi shape memory alloy are studied, and deformation behavior of NiTi shape memory alloy under compression at various temperature and strain rates are analyzed, which lays the necessary foundation for ball spinning of the thin-walled NiTi shape memory alloy tube and provide the material model for numerical simulation of ball spinning of the thin-walled NiTi shape memory alloy tube. Finally, rigid-viscoplastic finite element method was used to simulate and analyse the ball spinning of the thin-walled NiTi shape memory alloy tube.As-cast NiTi shape memory alloy ingot with the diameter of 80mm and the height of 80mm was prepared by means of the consumable electrode vacuum furnace. The microstructures of the as-cast NiTi shape memory alloy consist of coarse dendrites and equiaxed grains. The matrix phase of the as-cast NiTi shape memory alloy is NiTi and the Ti2Ni phase is distributed among the NiTi matrix phases. The transformation temperatures of the as-cast NiTi shape memory alloy are as follows:Ms=34.2℃, Mf=19.2℃, As=45.8℃, Af =65.7℃. The crystal structures of the as-cast NiTi shape memory alloy have monoclinic and cubic characteristics simultaneously at the room temperature. The yield strength of this alloy is 172.9 MPa, and Young's modulus is 68.1GPa. The as-cast NiTi shape memory alloy is characterized by cleavage fracture under tensile deformation at low temperature and room temperature, because no obvious plastic deformation occurs before fracture. The as-cast NiTi shape memory alloy shows good plasticity under tensile deformation at high temperature and has the blade shape fracture.The true stress-strain curves of NiTi shape memory alloy under compression at high temperature is characterized by dynamic recrystallization and dynamic recovery. Furthermore, NiTi shape memory alloy is sensitive to the strain rates under compression at high temperature, where the yield stress of NiTi shape memory alloy increases with strain rates. The strain rates have little influence on the stress values at low temperature. The fracture pattern is typical of shear fracture and shows the cleavage fracture features, where the crack expands inward by means of transcrystalline.Rigid-viscoplastic finite element method was used to simulate and analyze the backward ball spinning of the thin-walled NiTi shape memory alloy tube. The influence of different wall thickness reduction, different ball diameter, different feed ratio and different wall thickness of tube blank on deformability of the spun part was analyzed by means of finite element simulation. The best process parameters with respect to hot spinning was obtained, where the number of the ball is 7, and the ball dimeter is 10mm, and wall thickness reduction per pass is 0.05mm, and the feed ratio is 1.6mm/r.It is not proper to adopt too big wall thickness reduction per pass in terms of the given ball diameter during ball spinning of the thin-walled NiTi shape memory alloy tube, so the multi-pass ball spinning is a necessary way to obtain the desired spun part. Finite element simulation of ball spinning of NiTi shape memory alloy tube was conducted by means of three spinning passes, where the wall thickness reduction per pass is 0.05mm, and the total wall thickness reduction rate is 30%. Finally, the qualified spun tubular part with 0.35mm wall thickness was obtained. |
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