Fabrication Of NiTi Alloy Tube With Hot Extrusion And Improvement Of Its Properties Under Environmental Field

NiTi Shape memory alloys have the properties of high strength, low density, corrosion resistance and high temperature resistance, which makes them widely used in the fields of military and civil industry, such as pipe" connections, glasses frame, et al. However, NiTi alloys have high resistance to deformation, low thermo-conductivity, non-uniform deformation during hot processing. Thus, it is very difficult to obtain extruded tubular products of this alloy by extrusion. This also limits the application of the alloy. The research was intended to break though the limitation of hot extruding of NiTi alloys, to obtain high quality tubular products, and also try to improve the mechanical properties of NiTi alloys by using DC electric fields.The research of this article can be included as follows.Hot extrusion processing of NiTi alloy tubes was simulated using DEF0RM-3D software. Using finite element numerical simulation the equivalent stress,, the equivalent strain, the temperature field, the velocity field and the variation of the parametersduring the extrusionwere visualized,which provided optimized processing parameters for the experiment research. With the reference of the numerical simulating results the experimental tests were conducted successfully. The s withquality of inner and outer surfaces were manufacturedby the isothermal extrusion. The outside diameter of extruded pipe is φ20 mm and its inside diameter is φ14 mm. The microstructure and the mechanical properties of extruded pipes, as well as the blanks, were alsoanalyzed. The results demonstrate that there are a lot of fibred tissues in extruded pipes which will increaseits mechanical properties along axial direction. And the strength of thedeformed material is much higher than the former.The effects of a DC electric field on the deformation behavior of NiTi alloys was determined at a constant strain rate of 2.6x10~-2 s~-1 and a temperature of 600 ℃. The results demonstrate that the application of electric field results in the decrease of the flow stress and the tensile strength, as well as a little increase of tensile elongation. The existence of the electric field also suppressed the nucleation and growth of internal cavities in tension. When NiTi alloy was tensiled at 600 ℃ and at a lower strain rate, the dynamic recrystallization occurred. Without electric field the stress-strain curves were characterized by periodical undee graph, while under the electric field they presented steady state. When deformation under the electric field, both strain hardening and recrystallization take place simultaneously, thus the flow stress curves exhibits steady state. Further analyses also demonstrate that theapplication of the electric field promotes the activation and movement of the dislocations in tension at elevated temperature.