Study On Hot Deformation Behavior And Drawing Finite Element Simulation Of NiTi Shape Memory Alloy Wire

Near-equiatomic NiTi alloys are widely used in aerospace,biomedical and engineering machinery fields due to their excellent shape memory effect,superelasticity and corrosion resistance,etc.However,NiTi-based alloys have poor plasticity at room temperature and are difficult to cold working.Hot working methods often used to obtain high-quality NiTi alloy products in industrial production.Therefore,it is of great significance to study the hot deformation behavior of NiTi alloys.In addition,compared with the traditional method of optimizing the processing technology through repeated experiments,the finite element numerical simulation technology has the characteristics of low cost and short cycle,etc.Therefore,the use of this technology can greatly improve the production efficiency of NiTi alloys.In this paper,the hot deformation behavior,microstructure evolution and fracture morphology of Ti-50Ni（at.%）alloy wire were studied through isothermal tensile experiments and related test methods.Based on the experimental data,macroscopic phenomenological constitutive equations for the hot deformation behavior of the alloy were constructed.Applying the optimal constitutive equation,the hot-drawing finite element simulation was performed on the alloy wire,and the effects of process parameters on the equivalent stress,equivalent strain and drawing force during the drawing process were analyzed.Then the process parameters were optimized and it has undergone production verification.The main works and conclusions are as follows:According to the true stress-true strain curves,the dynamic softening effect of Ti-50Ni alloy enhanced,the elongation increased and the flow stress decreased with the increase of deformation temperature under the deformation temperature of 573⁸73K,and it has the negative temperature sensitivity characteristic;the elongation of the alloy changed irregularly,the flow stress increased with the increase of strain rate under the strain rate of 0.01⁰.2s^-1,and it has the positive strain rate sensitivity characteristic.Hot tensile fracture analysis showed that the fracture mechanism of Ti-50Ni alloy was mainly dimple fracture.As the deformation temperature increases,the size of the dimples increased,the number decreased,and the plasticity of the alloy increased;as the strain rate increases,the size of the dimples decreased,the number increased,and the plasticity of the alloy decreased.Microstructure and performance analysis after hot tensile indicated that at lower temperatures（≤773K）,the structures of Ti-50Ni alloy were fibrous,the grains were stretched and thinned,and incomplete dynamic recrystallization also occurred at 773K;at higher temperature（873K）,complete dynamic recrystallization occurred,and a large number of fine dynamic recrystallization grains were observed;the hardness of the alloy was mainly affected by the deformation temperature,the microhardness gradually decreased and the decrease gradually increased with the increase of deformation temperature.Three macroscopic phenomenological constitutive equations were established to predict the hot deformation behavior of Ti-50Ni alloy.Among them,the Field-Backofen constitutive equation cannot be used to predict the hot deformation behavior of the alloy;the Grosman constitutive equation can accurately predict the hot deformation behavior of the alloy under the conditions of573⁷73K and 0.01⁰.2s^-1;multiple linear regression constitutive equation can predict the hot deformation behavior of the alloy under the conditions of 573⁸73K and 0.01⁰.2s^-1,and the predicted values and experimental values have a high degree of fitting and small error.The hot drawing finite element model of Ti-50Ni alloy wire was constructed and the process parameters were optimized.Simulation results suggested that:（1）As the pass deformation increases,the degree of wire deformation increased,and the strain inhomogeneous index decreased;the maximum equivalent stress gradually increased,and the equivalent stress distributions were uniform within the range of 10²0%deformation;the drawing force gradually increased,and the increase were the largest between 20%and 25%deformation.（2）As the die angle increases,the degree of wire deformation increased,and the strain inhomogeneous index increased;the maximum equivalent stress gradually increased,and the equivalent stress distributions were uniform at all die angles studied;the drawing force decreased firstly and then increased at all deformations studied,among them,when the deformations were 20%and 25%,the optimal die angle was 12°.According to the above conclusions,the better drawing process parameters are:pass deformation?=20%,die angle 2α=12°.Based on the simulation results,the multi-pass hot drawing process program for Ti-50Ni alloy wire was developed and the drawing experiment was performed.The results showed that the surface quality of the wire after drawing was good and the dimensional accuracy was high,which met the production requirements;the structure was fibrous,and the composition phase at room temperature was mainly martensite phase;the mechanical properties reached industry production standards and have excellent shape memory performance,which verified the rationality of process parameters after simulation optimization.