Study On Numerical Simulation Of Ball Spinning Of NiTiNb Shape Memory Alloy Tube

As a new type of shape memory alloy, NiTiNb shape memory alloy is on the basis of NiTi shape memory alloy and incorporates Nb element into the ingredients. In addition to the excellent shape memory effect, superelasticity, high corrosion resistance and outstanding mechanical properties, the addition of Nb elements contributes to the special characteristic of broad transformation hysteresis of NiTiNb alloys. As a characteristic application in fabricating pipe joint, NiTiNb alloys have good applicational prospects in the field of aerospace.However, since there are, in some way, some difficulties and complexes in manufacturing NiTiNb alloys, appropriate processing technologies are needed to process thin-walled tube of NiTiNb shape memory alloy. As a kind of successively and locally plastic forming process,ball spinning can tackle the issue of hard machinability of thin-walled NiTiNb shape memory alloy tube. In addtion, ball spinning is able to improve the precision and quality of thin-walled tube. In order to quickly grasp a deep insight into the deformation law of ball spinning,effectively optimize the process parameters, and finally manufacture high quality thin-walled tube, finite element method, as a widely adopted simulation method with low cost and time consuming, is undoubtedly an effective way for this analysis. Firstly in this study, based on continuum mechanics, a phenomenological macroscopic constitutive model of NiTiNb alloy under high temperature is established on the basis of compression experiments. Secondly, by combing the constitutive equations with rigid-viscoplastic finite element method, simulations of ball spinning on thin-walled tube of NiTiNb alloy are carried out to obtain the optimal parameters during ball spinning, which contributes to laying a theoretical foundation for manufacturing thin-walled tube of NiTiNb shape memory alloy.In this thesis, all specimens of NiTiNb alloy are taken from a large bar of commercial NiTiNb alloy manufactured by means of rolling process. The atoms percent (at.%) of the material is as follows: Ni:Ti:Nb=47:44:9. The phase transition temperatures of as-received NiTiNb alloy in this study are Ms=-86.3?, Mf=-116.5?, As=-52.9?, Af=-18.7?. The initial microstructures of specimens are characterized by means of optical microscopy. The result shows that appearance of dendrite is the main feature of this specimen. The samples with diameter of 6mm and the height of 9mm are cut by means of electro-discharge machining(EDM). Subsequently, uniaxial compression experiments under a combination of temperatures of 700?.800?.900?.1000? and strain rates of 0.0005s-1?0.005s-1?0.05s-1?0.5s-1 are carried out, respectively. According to the analyses of true stress-strain curves, it indicates that the plastic deformation of NiTiNb alloy is sensitive to the condition of temperature and strain rate.By extracting the peek stress of individual stress-strain curves and using the method of linear fitting, the revised hyperbolic sine Arrhenius constitutive equation is finally established and the related parameters in this equation are as follows: A=1.642836x109 ,a=7.188x10-3(MPa-1), n=4.19566, Q=2.4235×105 (J/mol). Meanwhile, based on the theory of dynamic material model (DMM), the processing map of NiTiNb alloy is constructed on the basis of extracted stress values under different strain condition(0.2, 0.5, 0.8). Through the analysis of the processing map, it can draw a conclusion that the optimal processing temperature is 850 ?.On the basis of rigid-viscoplastic finite element method, a finite element code(DEFORM3D) is adopted in this thesis to simulate the process of ball spinning of thin-walled tube. The emphases are putted on three technological parameters during ball spinning, such as ball diameters, wall thickness reduction per pass and the feed ratio, in order to obtain the optimal parameters during ball spinning of thin-walled tube. The best process parameters with respect to hot spinning are finally determined as follows: the ball diameter is 22mm, the number of ball is 5, the wall thickness reduction is 0.4mm per pass and the feed ratio is 1.6mm/r. In addition to the aforementioned simulations, the simulation of multi-pass ball spinning with wall thickness of 2mm is conducted. The process consists of three passes, and in each pass, the reduction is 0.3mm, which corresponds to a total of 45% reduction. The simulation results show that the adopted parameters contribute to alleviating upheaval limitation in each pass with relatively huge thickness reduction. By combining the previous analysis, finally, a simulation of ball spinning of composite tube of NiTiNb alloy is carried out.In this simulation, each tube possesses thickness of 1mm. In consideration of the bonding quality between inner and outer tubes and slipping-off distribution, the optimal parameters of multi-pass ball spinning of composite tube are finally obtained by comparison of the results of simulations applying different combination of ball diameters, wall thickness reduction per pass and the feed ratio. The best fit parameters are as follows: the ball diameter is 10mm, the number of ball is 11, the wall thickness reduction is 0.4mm per pass and the feed ratio is 2mm/r.