Investigation Of Thermal Deformation Behavior And Microstructure Evolution Simulation Of Ti-62a Alloy

As a new type of damage tolerance titanium alloy,Ti-62A has good hydromechanical properties,corrosion resistance,welding stability and hot formability,so it is widely used in aircraft structural parts and other areas with high requirements for damage tolerance.The final properties of titanium alloy are mainly determined by the internal structure,and the microstructure evolution in the process of hot deformation is often accompanied by dynamic recrystallization and dynamic recovery.In this paper,the hot deformation behavior of Ti-62A alloy is studied,the flow stress curve and the evolution of microstructure under different parameters are analyzed,combined with hot working diagram,finite element numerical simulation analysis technology,metallographic microscopy and transmission electron microscopy Scientific methods such as technology,the internal structure of Ti-62A alloy is analyzed deeply,and the better hot working process parameters are obtained.In this paper,the isothermal compression experiment of Ti-62A alloy was carried out on gleeble3800 thermal simulator.Based on the stress-strain data of Ti-62A alloy in different phase regions,the rheological stress constitutive equations of the alloy at 800～950℃（α+βtwo-phase region）and 1000～1050℃（βphase region）were established.The activation energy Q of hot deformation is 244.122k J/mol and 282.797k J/mol,respectively.The flow stress of Ti-62A alloy increases with the decrease of deformation temperature and the increase of strain rate.The abnormal phenomenon that the dynamic softening rate of Ti-62A alloy decreases with the increase of deformation temperature is related to the decrease ofβstable elements such as Mo and Cr with the increase of temperature and the increase ofβin comparison with the case.During the multi-pass temperature reduction and thermal compression deformation of Ti-62A alloy,the decrease of the pass deformation temperature leads to the gradual increase of the flow stress.The deformation temperature and the accumulated deformation of passes have a significant effect on the distribution and quantity of secondaryαphase in the final tissue.For the hot working of Ti-62A alloy,it is necessary to pay attention to the two factors of hot working temperature and pass cumulative deformation control.The experimental stress-strain data were then employed to derive the constitutive equation of different phase regimes（α+βand singlephase）considering compensation of strain.Correlation coefficient（R）and average absolute relative error（AARE）were introduced to verify the validity of the developed constitutive equation.On the basis of dynamic material model,the hot working diagram of Ti-62A alloy is constructed based on Prasad and Murty criterion respectively.The analysis results show that the working diagram drawn based on Murty criterion is more accurate to predict the safe deformation zone of Ti-62A alloy.Based on the Murty criterion,three regions are distinguished in the processing diagram,namely,deformation temperature is 800～882℃,strain rate is 0.16～10s^-1;deformation temperature is 882～1050℃,strain rate is2.24～10s^-1;deformation temperature is 1006～1045℃,strain rate is 0.0072～0.063s^-1.Since the instability area mainly occurs in the processing area with high strain rate,special attention should be paid to avoid the area with high strain rate when processing in the deformation temperature of 800～1050℃.The results show that there are two optimal parameters of thermal deformation in the processing area,which are deformation temperature of 800～950℃,strain rate of 0.001s^-1,deformation temperature of 1000～1050℃and strain rate of 0.1s^-1.The dynamic recrystallization kinetics model and dislocation density model of Ti-62A alloy in the hot deformation process ofα+βtwo-phase region were established based on the rheological stress data of hot compression test.In DEFORM-3D simulation software,the cellular automata method was used to study the evolution of the microstructure of Ti-62A alloy at different strain rates at 900℃.When the strain rate is0.01s^-1 and the deformation temperature is 900℃,recrystallization takes place preferentially in the easily deformed zone of the specimen.Until the strain increases to0.9,almost completely dynamic recrystallization occurs in the easily deformable area,72.6%dynamic recrystallization takes place in the difficult deformed zone and 86.5%dynamic recrystallization takes place in the free deformed zone.When the deformation temperature is 900℃,the recrystallization grain size decreases with the increase of the strain rate.The simulation results are in good agreement with the experimental results,and reflect the deformation law accurately.