| As a light weight heat-resistant medium-high-strength titanium alloy,TA15 alloy is widely used in aerospace and military industries.However,the rapid development of our country’s transportation vehicles and defense military technology and the continued deterioration of the service environment,further improving the service performance and realizing the integrated manufacturing of titanium alloy components has become a key issue in the engineering field,and has become the frontier and main trend of research and development at this stage.As a typical hard-to-deform material,the problems of narrow processing window,large deformation resistance,uneven deformation structure,limited heat treatment strengthening,grain coarsing and surface oxidation seriously restrict the development and application of TA15 titanium alloy.Multi-directional forging technology,as a representative severe plastic deformation(SPD)technology,is urgently to be developed because it has strong microstructure refinement and performance improvement effects.And compared with other SPD technologies,it has more obvious advantages in process and industrialization,thus becomes an important way and method to realize the integrated forming of shape and performance of TA15 titanium alloy components.Microstructure control is a key problem in this process due to the decisive role of performance on microstructure.However,the multi-directional forging of the TA15 titanium alloy below the recrystallization temperature(called warm multi-directional forging)is a complex physical process with the action of multiple parameters and multiple processes.So how to prediction and precise regulation of its microstructure and the strengthening mechanism related to microstructure evolution after SPD become a challenging technical problem.To this end,a systematic and thorough investigation on the microstructure evolution and comprehensive performance in warm multi-directional forging of TA15 alloy has been carried out using finite element(FE)simulation combined with theoretical analysis and experimental study.A brief introduction to the work and its main achievements obtained are as follows.The deformation behavior and mechanism of microstructure development during multi-pass deformation of TA15 titanium alloy have been investigated with the assistance of isothermal compression tests.It is found that the TA15 alloy has flow softening in the.temperature range of 600℃～800 ℃.When deformed at 600℃,the softening mechanism of TA15 alloy is mainly the dynamic recovery of the primary αphase,supplemented by the spheroidization and dynamic recrystallization of the secondary a phase;When deformed from 700℃ to 800℃,the spheroidization and dynamic recrystallization of the secondary α phase are enhanced and gradually dominate.The effects of deformation temperature and strain rate on the volume fraction,grain size and aspect ratio of primary α phase are obtained.Deformation has little influence on the volume fraction of primary α phase but can refine microstructure under the action of dynamic crushing mechanism.And the lower the temperature,the more obvious the refinement effect,but the overall level of refinement is low.The grain size and aspect ratio of primary α phase are mainly affected by the deformation temperature.As the deformation temperature decreases,the average grain size decreases and the aspect ratio increases.According to the experimental results of warm deformation,the temperature application range of the high temperature deformation microstructure evolution model based on internal variable method of TA15 alloy was corrected.Comparing with the measured values of the average grain size of the primary α phase,the average deviation of the prediction is only 1.2%,and the reliability is verified.The warm multi-directional forging experiment for TA15 alloy was completed at 600℃～800℃.A variety of characterization methods are used to analyze the deformed structure,such as OM,EBSD and TEM.The results show that the size and aspect ratio of the primary α phase and secondary a phase decrease significantly with the increase of cycle number;As the deformation temperature decreases,the size and aspect ratio of the primary α phase show a downward trend,while the corresponding microstructure parameters of the secondary a phase show an overall upward trend.When the temperature rises to 800℃,the volume fraction of the primary a phase decreases significantly,deformation induced α→β phase transition occures.The phase transition temperature is lowered by a combination of continuous multi-pass deformation,relatively low strain rate and deformation temperature rise.The microstructure refinement mechanism of warm multi-directional forged TA15 titanium alloy is revealed.It is found that at 600℃,the grain refinement mechanism changes with the increase of the deformation cycle.In the case of 1 cycle,the dynamic mechanical fracture mechanism of the grain caused by the micro-deformation bands is dominant.Continuous dynamic recrystallization mechanism is less important.As the number of deformation cycle increases,the deformation-induced continuous dynamic recrystallization mechanism gradually dominates.Equiaxed fine grains with a relatively uniform size distribution was obtained,and the average grain size was about 1.21 μm.Applying the corrected thermal deformation microstructure evolution model by temperature application range to TA15 alloy multi-directional forging,it was found that the deviation between the calculated and measured values of the average grain size of primary α phase was as high as 56.4%under the condition of 600℃,60%and 3 passes.And as the deformation temperature increases,the calculated and measured value change in opposite directions.The reason for the low prediction accuracy of the corrected model is clarified.Based on the dislocation dynamics and the non-equilibrium large-angle grain boundary evolution theory,the internal variable model of the warm multi-directional forging of TA15 alloy is established,and the reliability of the model is verified.The mechanical properties of warm multi-directional forged TA15 alloy were tested by quasi-static tensile experiment.The results show that increasing the reduction and deformation cycle,or decreasing the deformation temperature can effectively increase the strength of the TA15 alloy,but the elongation decreases significantly.It has a tendency to change from ductile fracture to brittle fracture.The classical strengthening model is used to quantitatively estimate the mechanism of microstructure evolution on material properties.It is found that the increase in yield strength of TA15 alloy is mainly caused by the combination of grain boundary strenthening and dislocation strengthening after warm multi-directional forging.And as the deformation pass increases or the deformation temperature decreases,the contribution of grain boundary strenthening and dislocation strengthening increases.Based on the analysis of deviation between measured and calculated values of yield strength,It is proved that the special grain boundary structure and properties formed by SPD also have a significant influence on the yield strength of the material.For studying microstructure thermal stability,based on the static coasening theory,the growth kinetics of the long-term thermal insulation process of the primary a phase and the secondary a phase of the warm multi-directional forged TA15 titanium alloy at service temperature(450℃)were analyzed.The results show that the thermal stability of the primary α phase is poor,the growth dominated by the mechanism of diffusion coarsening along the dislocation core.But the secondary a phase shows better thermal stability.After 150 h,the average grain size is increased by 4%. |
PDF Full Text Request