| Shape memory alloys have been widely used in medical fields due to their shape memory effect,super elasticity,corrosion resistance and good biocompatibility,showing good application prospects.At present,the main processing technology of NiTi shape memory alloy is mainly plastic processing.The processing objects are mostly rod and wire,and the processing methods are mostly extrusion and drawing.There are few researches on the processing technology ofNiTi shape memory alloy plate.However,one of the most important applications of NiTi shape memory alloy plates is to manufacture biomedical bone-plates,which are usually formed by hot stamping.Stamping forming process is a widely used plastic forming process with high production efficiency,good consistency of parts and low production cost,but it is rarely used in the processing of NiTi shape memory alloy sheet.The topic of the paper foucuses on forming NiTi shape memory alloy bracing bone-joining plate,where NiTi shape memory alloy bulk material is used.It provides important theoretical basis for forming NiTi shape memory alloy bracing bone-joining plate to establish the constitutive equations of NiTi shape memory alloy,analyze dynamic recovery and dynamic recrystallization mechanism of NiTi shape memory alloy at high temperature deformation,investigate hot working performance of NiTi shape memory alloy at high temperature,study rolling of NiTi shape memory alloy plate based on bulk material,and conduct numerical simulation research of stamping ofNiTi shape memory alloy bracing bone-joining plate based on the rolled plate.The sample used in this topic is a NiTi shape memory alloy cylinder(4mm in diameter and 6mm in height)with a nickel atomic content of 50.7at.%.After solution treatment at 850℃ for 2h high temperature compression tests at different deformation temperatures(650℃-950℃)and strain rates(0.0005 s-1～0.5s-1)are conducted to obtain stress-strain curves.According to the data obtained from high temperature compression experiment,the constitutive equation of NiTi shape memory alloy is established by linear fitting,and the high temperature constitutive equation of NiTi shape memory alloy based on strain compensation is established.Compared with the experimental data,the constructed constitutive equation can better predict the flow behavior of NiTi shape memory alloy at high temperature.NiTi shape memory alloy is sensitive to strain rate when it deforms at high temperature.The flow stress decreases with the increase of temperature and increases with the increase of strain rate.The constitutive equation of NiTi shape memory alloy provides an accurate material model for subsequent numerical simulation of forming process of NiTi shape memory alloy.Meanwhile,according to the true stresses corresponding to 0.3,0.6 and 0.9 true strains of NiTi shape memory alloy at different deformation temperatures(650℃-950℃)and different strain rates(0.0005 s-1～0.5 s-1),the hot processing map of NiTi shape memory alloy under different strains is established based on the dynamic material model.The instability zone and workable zone of NiTi shape memory alloy were determined,and the thermal workability of NiTi shape memory alloy is explained.In this paper,electron backscatter diffraction experiment is used to study the plastic deformation mechanism and microstructure evolution of NiTi shape memory alloy at different deformation temperatures(650℃～950℃)and strain rates(0.0005 s-1～0.5s-1).The main deformation mechanism of NiTi shape memory alloy at high-temperature plastic deformation is dislocation slip.Work hardening and dynamic softening exist simultaneously and affect each other.When the plastic deformation of NiTi shape memory alloy occurs at 650℃,the dynamic recovery plays a major softening role.With the increase of deformation temperature,the dynamic softening mechanism gradually is transformed into dynamic recrystallization,which is the result of both discontinuous dynamic recrystallization and continuous dynamic recrystallization.In order to obtain NiTi shape memory alloy sheet with excellent properties,it can be prepared by hot rolling process.In this paper,the finite element model for hot rolling of NiTi shape memory alloy is established.By using DEFORM-3D finite element software,the influence of main parameters such as different rolling temperatures(750℃～950℃),different rolling speeds(0.1m/s～0.3m/s)and different rolling reductions(10%-30%)on rolling forming is simulated.The temperature field of rolling deformation zone,edge crack damage and rolling load under different conditions is analyzed,and the edge crack damage of NiTi shape memory alloy sheet with different width ratios is analyzed.The temperature field distribution of the rolled parts shows that the plastic deformation work leads to the temperature rise,Furthermore,the greater the rolling reduction is and the faster the rolling speed is,the higher the temperature rise is.The rolling load of rolled parts shows that the rolling load is influenced by the rolling temperature,rolling speed and rolling reduction.The higher the rolling temperature is and the smaller the rolling speed is and the smaller the rolling reduction is,the smaller the required rolling load is,Conversely,the required load is the larger.Through the analysis of the edge crack damage of the rolled parts,it can be seen that the higher the deformation temperature is,the smaller the deformation speed is,the smaller the rolling down amount is,the smaller the edge crack damage is,and the larger the width to thickness ratio of the rolled plate is,the larger the rolling damage is.According to the above analysis,appropriate parameters are selected for multi-pass rolling of NiTi shape memory alloy plate to prepare the plate required for stamping.The rolling temperature is 950℃ and the rolling speed is 0.2m/s.After three passes of rolling,appropriate parts are cut for stamping.The finite element simulation of hot stamping of NiTi shape memory alloy plate was carried out.In this paper,ABAQUS finite element software is used to simulate the stamping forming of NiTi shape memory alloy plate.The corresponding stamping die is designed by three-dimensional software,the hot stamping finite element model of NiTi shape memory alloy bracing bone-joining plate is established,and the stamping die structure is optimized by numerical simulation results.The influences of different temperatures(650℃～850℃),different die fillet radiuses(2mm～4mm)and different friction coefficients(0.1-0.3)on the stamping formability of NiTi shape memory alloy bracing bone-joining plate are studied and analyzed.The springback control in stamping process is taken as the main objective,and the best parameters of stamping results are selected.The structure of stamping die is optimized by this parameter.According to the finite element simulation results ofNiTi shape memory alloy bracing bone-joining plate stamping,the higher the stamping temperature is,the smaller the die fillet clearance is,the larger the friction coefficient is,the smaller the springback of the stamping part is,the better the forming quality is.The experimental results show that the influence of sheet temperature on stamping performance is the largest,the influence of fillet radius is the second,and the influence of friction coefficient is the smallest... |
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