Study On Hot Deformation And Recrystallization Behavior Of Pure Titanium In EB Furnace Process

With the improvement of metallurgical quality requirements for titanium and titanium alloys,electron beam melting technology has developed rapidly.Compared with the grain size of the ingots produced by traditional vacuum arc furnace,the grain size of the ingots produced by electron beam melting furnace was much coarser,which even reached centimeter level.Researchers gradually showed great interest on the related research on the abnormal coarse and as-cast microstructure of pure titanium in the EB furnace.In this paper,the fine-grained VAR furnace process pure titanium was used as the comparative material.The thermal simulation method was used to study the effects of different deformation parameters,including deformation temperature and strain rate on the hot deformation behavior of pure titanium in the EB furnace process.Electron Backscattered Diffraction?EBSD?technique was used to investigate the dynamic recrystallization mechanism of pure titanium in EB furnace process.The effects of different strains,deformation temperature and strain rate on static recrystallization of pure titanium in EB furnace process were studied by stress relaxation method,and the static recrystallization kinetics equation was established.The analysis of true stress-true strain curve shows that the work hardening rate curve of pure titanium in EB furnace process at different deformation temperature and strain rate shows a typical three-stage characteristic:in the first stage,the work hardening rate decreases rapidly;in the second stage,the work hardening rate of pure titanium starts to rise after falling to a minimum value,and gradually increases to a peak with increasing stress,which is related to the hardening that occurs in the second stage of the twins;in the third stage,the work hardening rate curve falls from the peak.The results of dynamic recrystallization of pure titanium show that the hot deformation behavior of pure titanium in EB furnace process has strong sensitivity to deformation temperature and strain rate.As the deformation temperature decreases and the strain rate increases,the flow stress of pure titanium increases.The dynamic recrystallization nucleation rate increases with the increase of deformation temperature and strain rate.The higher the deformation temperature and the higher the strain rate is,the more likely the dynamic recrystallization of ultra-thick grain pure titanium occurs.The results of EBSD analysis indicate that the dynamic recrystallization mechanism of pure titanium in the EB furnace process is mainly discontinuous dynamic recrystallization.The optimal thermal deformation process range of EB furnace process pure titanium is:deformation temperature 830⁸70°C,strain rate 1¹0s^-1,strain 0.51⁰.92.The results of static recrystallization of pure titanium show that with the increase of deformation,deformation temperature and strain rate,the recrystallization kinetics curve of pure titanium in EB furnace process shifts to the left,the value of t_0.5.5 decreases and the rate of recrystallization increases.Compared with the kinetic curve of VAR furnace process titanium,under the same deformation parameters,the recrystallization kinetic curve of pure titanium in the EB furnace process is on its right.Because the coarse original grain size has a certain inhibitory effect on recrystallization,which delays the recrystallization kinetics.Based on the experimental results,the static recrystallization kinetic equation of pure titanium in the EB furnace process is established.