| Shape memory alloy (SMA) is a new type of functional intermetallics, so it has become one hotspot in the field of functional material. Because of its excellent shape memory effect,superelasticity, corrosion resistance and good mechanical properties, Nickel-titanium based shape memory alloy (Ni-Ti based SMA) has become the most typical SMA and it has been successfully used for pipe joint. Ni-Ti based SMAs are usually difficult to manufacture at room and middle temperature. So isothermal precision plastic forming provides a new approach to manufacturing the tube billet for pipe connection. In order to analyze deformation laws of isothermal precision extrusion of Ni-Ti based SMA and optimize the process parameters more availablely, finite element simulation will provide a excellent approach to manufacturing the good tube billet. The present study reflects the combination of new materials and new process and thus it has a strong innovation and frontier. The paper will lay the theoretical foundation for enriching the plastic deformation mechanism of Ni-Ti based SMA, phase transformation theory and promoting the development of pipe industry in China.In this paper, by using the self-developed as-cast NiTi based SMA (equiatomic Ni-Ti alloy, Ni-Ti-Nb alloys and Ni-Ti-Fe alloys) as the original testing materials, microstructure,transformation temperature, the fracture morphology and mechanical properties are studied.The studies indicate that the microstructures of equiatomic Ni-Ti SMA consist of coarse dendrites. The matrix phase of the equiatomic Ni-Ti SMA is NiTi. The Ti2Ni phase is dispersoid distribution among the NiTi matrix phases, where a phase transition process occurs. The microstructures of the equiatomic Ni-Ti-Nb SMA consist of equiaxed grains, in which the matrix phase is NiTi(Nb) and the precipitated P-Nb phase increases with the content of Nb. The as-cast Ni-Ti-Nb SMA is characterized by quasi-cleavage fracture under compression deformation at room temperature, but it has a small plastic deformation before breakage. The phase transition process occurs in the range from - 150? to 150? in the DSC experiments. The microstructures of the equiatomic Ni-Ti-Fe SMA consist of coarse grains.Brittle fracture occurs under compression deformation at room temperature and is characterized by cleavage fracture, and there is no phase transition process in the range from-150? to 150? in the DSC experiments.Ni-Ti SMA samples were prepared for the hot compression test at the different temperatures (500?,650?,800?,950?,1100?) and different strain rates(0.0005s-1,0.005s-1,0.05s-1, 0.5s-1) by means of INSTRON 5500R testing machine. According to the true stress-strain curves, Ni-Ti SMA is sensitive to the strain rates and exsits dynamic recrystallization and dynamic recovery during the hot compression deformation. With the increase in the strain rates, the yield stress of Ni-Ti SMA goes up. The constitutive equation of Ni-Ti SMA under hot compression deformation is established in terms of Arrhenius model.It reveals that flow stress and microstructures of Ni-Ti SMA are influenced by deformation temperatures and strain rates. Furthermore,it provides the material model for numerical simulation of isothermal precision extrusion of Ni-Ti based SMA billet for pipe coupling.Accordingly, the hot processing map is established on the basis of Arrhenius constitutive equation. As a consequence, the effective process parameters are determined.Rigid-viscoplastic finite element method was applied to simulate and analyze isothermal precision extrusion of Ni-Ti SMA. The metal flow, the extrusion pressure as well as stress and strain distribution is analyzed according to the different process parameters, such as temperature (600?, 700?, 800?), friction coefficient (0.3 and 0.7) and remaining material position. The studies lay the theoretical and technological foundation for optimizing the process parameters to manufacture the tube billet for Ni-Ti based SMA pipe joint. |
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