Microstructural Evolution And The Effect On High Temperature Tensile Properties Of Hot Worked TC17 Titanium Alloy

As one of the most important componets of aero-engine with high thrust-weight ratio,the design and manufacture of titanium blisk has been regarded as the key technology by many countries.Conventionally,the blisk consists blades and disk as a whole.The central part of the blisk(disk)is lamellar microstructure;meanwhile,the outer edge(blades)would be globularized.Such special design can not only realize the full potential of titanium alloy,but also satisfy the high-property demand of aero-engine.Thus,it can be seen that quantitative control the globularization behavior of lamellar microstructure is the core technique for manufacturing the dual-property titanium blisk.In consequence,in virtue of microstructure observation and metallographic analysis,to take a more in-depth look at the mechanism of microstructure evolution during the dynamic globularization behavior;to build the finite element mode for predicting the effect of thermo mechanical parameters on dynamic globularization process;to research the microstructure evolution of lamellar microstructure and discuss the corresponding mechanism during the heat treatment after forging;to establish the neural network model between microstructure characteristic parameters and high temperature tensile properties,all of these have important scientific and reference values for the design and manufacture of titanium blisk.To solve the above problems,in this paper,TC17 titanium alloy with initial lamellar microstructure has been regarded as research object,using experiment and theoretical analysis combined with finite element numerical simulation,the globularization behavior of the lamellar microstructure during the hot working has been deeply researched and discussed.Main research contents and corresponding results are shown as follows:The microstructure evolution and the change of Burgers orientation relationship between α and β phase of TC17 titanium alloy with initial lamellar microstructure were analyzed and researched by SEM,EBSD and TEM.The results show that the dynamic globularization of α phase contained the formation of subboundary and the fragmentation of α phase.The fragmentized α phase was eventually transformed to globularized particle.Meanwhile,dynamic recrystallization of β phase was occured during hot deformation process.Besides,the strict Burgers orientation relationship between α and β phase was destroyed,which was due to the obvious change of orientation for α and β phase.A finite element method(FEM)model for predicting dynamic globularization behavior of TC17 titanium alloy was established.For obtaining the microstructure evolution during dynamic globularization under varying processing parameters,isothermal hot compression test and quantitative metallographic analysis were conducted on TC17 titanium alloy with initial lamellar microstructure.The prediction model,which quantitatively described the non-linear relationship between the dynamic globularization fraction and the deformation strain,temperature and strain rate,was developed on the basis of the Avrami equation.Then the developed model was incorporated into DEFORM software as a user-subroutine.Finally,the large-sized step-shaped workpiece was isothermally forged and corresponding FEM simulation was conducted to verify the reliability and accuracy of the integrated FEM model.The reasonable coincidence of the predicted results with experimental ones indicated that the established FEM model provided an easy and practical method to predict dynamic globularization for TC17 titanium component with complex shape.The microstructure evolution and its mechanism of hot worked TC17 titanium alloy with initial lamellar microstructure during heat-treatment were investigated.Therefore,TC17 alloy was isothermally forged to different height reductions at 800℃ and subsequently heat treated for times ranging from 10 min to 8h at 800℃.It is found that the microstructure evolution is sensitive to the prestrain and time.With the increasing of prestrain and time,the globularization fraction increased.There are two competing mechanisms for the microstructure evolution during heat-treatment: the boundary splitting mechanism and microstructure coarsening mechanism.The boundary splitting mechanism is significant at the initial stage of heat treatment,while the microstructure coarsening mechanism plays an important role during the prolonged heat treatment.Based on mechanical properties testing and quantitative microstructure analysis,the prediction model for quantitative analysising and characterizeing the correlation between the microstructure and high temperature tensile property was developed by BP-ANN technology.The input and output parameters of the model were the microstructural features parameters and the high temperature tensile propertys,respectively.Subsequtently,the accuracy and precision of the established model was verified by the experimental result of non-training samples.The validation results shown that the model achieves desirable predicting performance,and the modeling approach was given a way to describe the complicated non-linear relationship between the microstructural features parameters of titanium alloy and its mechanical properties.