Forming Mechanism Study Of Tri-modal Microstructure During Near-isothermal Local Loading Forming Of Large-scale Component Of Titanium Alloys

Large-scale complex component of titanium components are crucial parts for high-end equipments in aviation and aerospace fields,which effectively combines the advantages of materials and structures.Near-isothermal local loading forming technology provides a feasible method to form the large-scale complex component.Therefore,it will be an advanced forming technology urgently needed to be developed.The critical issue for the application of this technology is to realize integrated forming of shape and performance and obtain desirable tri-modal structure to meet high service performance.Nevertheless,due to the process characteristic of near-isothermal local loading forming technology,the components will undergo complex thermal path and strong heterogeneous deformation during deformation,which causes complex microstructure evolution.This makes the prediction of macro-micro deformation behaviors and microstructure morphology control of this component very difficult.To this end,a systematic and in-depth investigation on the forming mechanism of tri-modal structure of near-isothermal local loading forming of large-scale component has been carried out by experimental study theoretical analysis,and finite element(FE)modeling.The main research contents and results obtained are as follows.The evolution of primary alpha(?_p)and secondary alpha(?_s)phase and competitive growth laws between them are revealed under different thermal paths by continuous cooling experiments and quantitatively metallographic analysis.As to epitaxial growth of?_p phase,the reason for underestimation of classical diffusion model is analyzed and a model based on a new additivity rule is established to predict the?_pgrowth effectively by considering thermal history-related the diffusion field around the?_p phase.By EBSD orientation analysis,a sympathetic nucleation mode of?_s phase is proposed and the mechanism of competitive growth between?_pand?_s phases is revealed by combining mean concentration field theory with soft impingement effect.By designing three hot-working routes(deformation followed by cooling,concurrent deformation and cooling,and controlled cooling to prescribed temperature followed by deformation),the microstructure evolution and related deformation behavior are revealed.It is found that the morphology of?_p phase is not changed by applying deformation and stress-induced???transformation takes places at the high temperature.However,the morphology change of?_s phase is significant and sensitivity to thermal path and strain rates.The reasonable match of large deformation and strain rate could promote the heterogeneous nucleation on dislocation and isotropic growth behavior of?_s phase,which leads to the change in the morphology of the?_s,from lamellar to equixed.It is also found that the flow stress under the non-isothermal condition is decreased by 20%compared with that under the isothermal condition as a result of the formation of fine equiaxed?_s phase.Considering the competitive growth mechanism between?_p and?_s phases under different temperature paths,a kinetic model of phase transformation is established to predict the epitaxial growth of?_p phase and precipitation of?_s phase.By considering the interaction between mobile dislocation,developed equiaxed?_s phase,epitaxial growth of?_p phase and plastic deformation,and introducing evolution equations of mobile dislocation,equiaxed?_s phase and???dynamic transformation,a internal-state-variable based constitutive model is established for non-isothermal deformation of two-phase titanium alloys coupling microstructure evolution.The unified prediction of grains size,phase amount,microstructure morphology of?_p and?_s phases and plastic responses is realized.By combining heat conduction experiments capturing the heat transfer characteristic between workpiece and die with classical inverse algorithm,the effect of pressure,glaze thickness and surface roughness on interfacial heat transfer coefficient(IHTC)is studied.Thus the key technology for heat transfer boundary condition during near-isothermal forming of large-scale titanium component is solved.By implementing the unified material model for coupling stress responses and microstructure evolution,a coupled macro-micro through-process FE is established for near-isothermal local loading forming of large-scale complex titanium component.The reliability of established FE model is verified from both macro and micro perspective via comparing the shape and microstructure evolution of experimental component and the simulated one,and the evolution characteristic of?_p and?_s phases is also obtained during different stages of near-isothermal local loading forming.Based on the established FE model,the effect rules of die movement speed,billet heating temperature and forming pass on the evolution of?_p and?_s phases are investigated.It is found that the precipitation of?_s phase is promoted by increasing heating temperature and decreasing die loading speed.Moreover,the reasonable combination of large deformation and fast cooling would lead to the formation of fine equiaxed?_s phase.Finally,the coupling effects of heterogeneous nucleation,isotropic growth behavior of?_s phase and strain-induced lamellae globularization are the main reason for microstructure morphology diversity,which provides an important foundation for the morphology control in near-isothermal local loading forming of titanium alloys.