Modeling And Simulation Of Hot Gas Forming-Aging Treatment For Ti₂AlNb Alloy Thin-Walled Components

The high-temperature structural materials represented by Ti₂AlNb alloy are characterized by high forming temperature,significant difficulty in forming,and complex microstructural evolution over forming and subsequent heat treatment.The formability of such materials were remarkably influenced by forming parameters（e.g.temperature and loading force）.The microstructure and mechanical properties of Ti₂AlNb alloy components were thus dominated by the forming parameters and subsequent heat treatment parameters.To optimize the forming parameters and heat treatment parameters of Ti₂AlNb components,the traditional trivial and error approach was adopted,which was not only time-energy consuming but also difficult to clarify the comprehensive influence of various factors on the microstructure and mechanical properties of the components.To meet the demands of digital,intelligence,low-carbon and environmental protection for manufacturing,based on the application of virtual manufacturing,the whole process simulation including forming and heat treatment provides a new method for shape and mechanical properties control of Ti₂AlNb alloy thin-walled components.The whole process simulation technology which can predict the shape,microstructure and mechanical properties of components will become an important development direction in the field of plastic forming.In order to realize the control of microstructure and properties of Ti₂AlNb alloy components,five kinds of typical microstructure of Ti₂AlNb alloy were obtained via different heat treatments,of which the bimodal microstructure was selected as the final control target due to its excellent comprehensive mechanical properties.The effects of solution temperature,aging temperature and time on the microstructure and mechanical properties of Ti₂AlNb alloy were studied in detail.Moreover,the kinetic equations for describing the change in O-phase content and coarsening behavior of lamellar O-phase during solution-aging treatment and the relationship between yield strength and thickness of O-phase were established.To further analyse influence of pre-deformation on the microstructure evolution during aging process,the Ti₂AlNb alloy was aged at 775-875℃ after deformation at different temperatures（950-990℃）,strains（0.15-0.75）and strain rates(0.001-0.1s^-1).The results showed that the strength of the pre-deformed Ti₂AlNb alloy were higher than counterparts of the non-deformed samples under the same aging conditions.The width of O-phase in the Ti₂AlNb alloy increased with increasing strain and strainrate.According to the evolution of microstructure and mechanical properties of Ti₂AlNb alloy under different pre-deformation states during aging process,the microstructure evolution model and the relationship model of microstructure and mechanical properties during aging process were established by choosing the phase content,the size ofα₂ phase and O phase,solid solubility,dislocation density and damage as internal variables.Model of microstructure evolution and properties prediction during the whole process of hot deformation and aging treatment based on physical internal variables was proposed via further modifying the uniform viscoplastic constitutive model of Ti₂AlNb sheet at high temperature and coupling with the heat treatment model.The parameters of the model were solved by genetic algorithm.The predicted results of the model demonstrated excellent agreement with the experimental results.At high temperature,the formability of Ti₂AlNb alloy was affected by complex factors,including forming temperature,strain and strain rate.The forming limit should firstly be determined to design the forming process of Ti₂AlNb alloy thin-wall components.In this dissertation,the forming limits at high temperatures were predicted based on the M-K theory during which the necessary conditions were proposed for the constitutive model.Then,M-K theory prediction model for high-temperature forming limit of Ti₂AlNb alloy was established by introducing the coupling effect of microstructure evolution and flow stress during deforming process into the M-K theory.Moreover,the effects of forming temperature and strain rate on forming limit of Ti₂AlNb alloy were analyzed.With the increasing temperature and strain rate,the forming limit of Ti₂AlNb alloy was improved on positive strain ratio zone.The forming limit at constant strain rate condition of 985℃&0.01s^-1 was tested.The experimental results were basically consistent with the calculated results.Based on the above developed microstructure evolution model describing the whole process of deformation-heat treatment and the predictive model determining the high-temperature forming limit of Ti₂AlNb alloy,the simulation toolbox predicting microstructure and mechanical properties of Ti₂AlNb alloy was not only developed but also combined with ABAQUS software for the whole process of forming-aging treatment.Consequently,the integrated simulation of deformation,microstructure and mechanical properties of Ti₂AlNb alloy thin-wall components could be achived by such simulation system.The whole process of forming and heat treatment of typical Ti₂AlNb alloy thin-wall components with bulging deformation,bending deformation and complex deformation was simulated and verified by experiments.The results showed that the method had good simulation effect,the maximum simulation error of microstructure parameters were 8.6%,and the maximum simulation error of and mechanical properties were 8.6%.Overall,the whole process simulation method established in this dissertation can guide the control of the shape,microstructure and properties of components.The whole process simulation of hot gas forming-pressing compound-forming and aging treatment of rectangular-section components with small fillet of Ti₂AlNb alloy was carried out by the whole process simulation system.The forming and heat treatment parameters of the rectangular-section components were designed and optimized according to the simulation results.The rectangular-section components with minimum relative fillet radius of 1.7,maximum wall thinning ratio of 8.3%and yield strength of more than 880MPa at 650℃ were obtained through hot forming and heat treatment experiments.The manufacturing process assisted by whole process simulation in this dissertation not only solves the challenge about controlling shape and mechanical properties of Ti₂AlNb thin-wall complex components but also reduces the production cost significantly and the research and development cycle.