| With the development of aviation and aerospace vehicles towards light weight,high speed,high reliability and long life,there is an urgent need for high-performance and high-precision titanium alloy complex thin-walled components.TA15 titanium alloy has a high specific strength and high temperature strength at a service temperature of 450-500 ℃,and the structural benefit is very significant.Therefore,TA15 titanium alloy thin-walled components are widely demanded in the fields of aviation and aerospace.However,improving comprehensive performance of components poses new challenges to forming manufacturing technology.Due to the low room temperature formability of TA15 titanium alloy,superplastic forming has been used in the past,but the high superplastic forming temperature(above 850 ℃)and long forming time often lead to weakened microstructure and performance,and it is difficult to meet service performance requirements.To this end,this paper analyzes the micro-deformation mechanism of TA15 titanium alloy sheet through the crystal plastic finite element simulation method,it provides a technological basis for regulating the microstructure and properties of titanium alloy hot gas forming thinwalled components.In order to establish the constitutive model of TA15 titanium alloy during hot deformation,firstly,through the analysis of hot deformation mechanism(700-800 ℃+0.1-0.001 s-1),the kinetic equations of dislocation sliding and grain boundary sliding considering deformation temperature,deformation activation energy and mobile dislocation density was established.After that,dynamic recovery,dynamic recrystallization,and micro-cavity evolution models were established.The crystal plastic constitutive model was embedded into the ABAQUS finite element software through the UMAT subroutine,and the crystal plastic finite element simulation of the high-temperature micro-deformation behavior of TA15 titanium alloy was realized.The effects of dislocation density and initial texture on the dislocation slip mechanism were analyzed by crystal plastic finite element simulation and EBSD technology.It was found that high dislocation density can simultaneously increase the service temperature strength and high temperature deformation performance of TA15 titanium alloy.Therefore,in the TA15 titanium alloy hot forming process,a rolled sheet with a high dislocation density should be selected,and the component should be maintained in a state with a high dislocation density.The initial texture of the material significantly affects the dislocation slip mode by changing the Schmid factor of the dislocation slip system.In order to improve the high-temperature forming properties of the TA15 titanium alloy,the initial texture of the sheet should be fully utilized to promote the basal and prismatic slip with a lower critical shear stress.In addition,the strain state of TA15 titanium alloy during hot deformation determines the texture evolution behavior of the component,the texture state of the component can be adjusted by changing the strain state.The microscopic deformation behavior of two-phase polycrystalline of TA15 titanium alloy was analyzed by crystal plastic finite element simulation.It was found that three strain bands at 45 ° to the tensile direction appeared on the polycrystalline strain distribution cloud map: β-phase region,deformation band inside the grain and near grain boundary of α-phase,and the micro-strain distribution determines the micro-deformation behavior such as dislocation density distribution.In addition,during the mechanical property test at the service temperature(500 ℃),the strength of the grain boundary region is higher.As the grain size decreases,the volume fraction of the grain boundary region increases,and the strength of the service temperature gradually increases.However,as the deformation temperature increases and the strain rate decreases,the decrease in grain size will promote the grain boundary sliding mechanism,which appears as a softening of fine grains.In order to improve the hightemperature forming properties of the material and the mechanical properties of the service temperature of the component,the hot forming process of the TA15 titanium alloy should select a rolled sheet with a fine-grained structure and keep the component in a fine-grained state.The static recrystallization mechanism of TA15 titanium alloy was studied by quasi-in-situ characterization,and the microstructure evolution model including continuous and discontinuous dynamic recrystallization were established,which provided a theoretical basis for the control of microstructure properties of the titanium alloy thermoformed component.Under low Z-parameter deformation conditions,TA15 titanium alloy are prone to grain growth,resulting in an increase in grain size and a decrease in the mechanical properties of the service temperature of the component.Under high Z-parameter deformation conditions,the forming performance of TA15 titanium alloy is reduced,and microvoids are prone to occur,which also leads to a decrease in the mechanical properties of the service temperature of the component.In order to control the microstructure evolution behavior of TA15 titanium alloy during hot deformation,a high temperature variable-rate loading path is proposed,and the variable-rate bulging pressure loading path is calculated through the free bulging theory of the plate,and the variable-rate hot gas forming of TA15 titanium alloy hemispherical component was realized.By adjusting the strain rate,the dynamic recrystallization behavior of the TA15 titanium alloy component is reasonably controlled to ensure the high dislocation density and fine-grained state,and at the same time avoid the formation of microvoids,ensuring the microstructure and mechanical properties of the component. |
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