Study On Equal Channel Angular Extrusion Of NiTi Shape Memory Alloy Tube

Due to its low elastic modulus, shape memory effect, superelasticity, corrosion resistance,biocompatibility and so on, NiTi shape memory alloy （hereinafter referred to as NiTi alloy） iswidely used in aviation, engineering and medical fields. The medical devices （such as stents,probe, catheter and so on） manufactured by NiTi alloy tube can be implanted in our bodies,aim to reduce or avoid the fracture of the medical devices in the bodies, NiTi alloymechanical properties should be improved. Equal channel angular extrusion （ECAE） cansimultaneously improve plasticity and toughness of the sample by reducing the grain size.Equal channel angular extrusion, which is applied to improve the mechanical properties ofblock samples by many scholars, is rarely applied to process tubular materials. In this thesis,Equal channel angular extrusion is applied to process NiTi alloy tube to improve itsmechanical properties.Near equiatomic NiTi alloy cylinders with the diameter of4mm and the height of6mmwere compressed at high temperature. The temperature of the hot compressive tests rangesfrom600℃to1000℃, and the strain rate ranges from0.001s^-1to1s^-1. The true stress-straincurves and microstructure were analyzed, decreasing stain rates or increasing temperatureslead to the increase of dynamic recrystallization. Hot compression tests provide the materialmodels for simulation of Equal channel angular extrusion. Because the dynamicrecrystallization leads reducing of the mechanical properties of materials, so the test of Equalchannel angular extrusion was ran in the high temperature condition about500℃to reducethe dynamic recrystallization.Extrusion dies are of use in achieving Equal channel angular extrusion, and thecalculation of extrusion pressure is the key to designing extrusion dies. Extrusion pressurewas calculated by means of the upper bound element method, the calculation result of the Bida sen extrusion model is115227N, and the calculation result of the Luri extrusion model is86907N. The larger extrusion pressure, the easier it is for sample to traverse channels, but themaximum load of our equipment is100000N at high temperature, so we choose the Luriextrusion model, and the circular arc of the crossings of channels can reduce the wear ofsample and model.In this thesis, the whole deformable body, which boundary conditions are complex, consists of four parts: coat, plug, core and NiTi alloy tube. Rigid-viscoplastic finite elementmethod （FEM） was used to simulate and analyze Equal channel angular extrusion based onthe complex boundary conditions. Simulation results reveal regularity of distribution of theload curve, the velocity field, the equivalent strain and equivalent stress. The influence ofdifferent friction coefficients （0.12,0.25and0.3）, different internal angles （90o,120oand135o）, different internal arc radius （0.5mm,1mm and2.5mm） and different external arcradius （2mm,3.2mm and3.8mm） on NiTi alloy tube was analyzed by means of finiteelement simulation. Due to the complexity of boundary conditions, stress is nonuniformbetween contacting surfaces, load curves of the whole deformable body and NiTi alloy tubeoscillate back-wards and forwards severely, velocity field, equivalent strain and equivalentstress is nonuniform. In the process, friction coefficient and internal angle have a large effecton NiTi alloy tube. With friction coefficient increasing or internal angle decreasing, loadsincrease gradually， equivalent strains increase gradually in the area between wall of beforeand after tube. With friction coefficient decreasing or internal angle decreasing, uniformity ofequivalent stress of area of NiTi alloy tube which has been deformed in decrease gradually.The radius of arc of internal and external have little influences on NiTi alloy tube.Though calculation of extrusion pressure and analysis of the simulation results,parameters of fixtures of Equal channel angular extrusion are determined:=120°,R_ext=3.2mm and R_int=1mm.At500℃, NiTi alloy tube was pressed by one pass Equal channelangular extrusion, the wall in the internal corner was sheared more acutely than the wall in theexternal corner, it was consistent with the simulation results. Though shearing deformation,elongated substructures were observed. Finally, due to dynamic recovery and static recoveryoccurring, the linear substructures was disappeared, the grains deformed were changed intothe original grains.