Prediction Of Hot Deformation And Microstructure Of Solid Solution TC18 Titanium Alloy

TC18 titanium alloy is widely used in aerospace landing gear bearing components because of its high strength,toughness and excellent fatigue resistance.Usually,TC18 titanium alloy is formed by isothermal die forging.However,due to the complex microstructure of TC18 titanium alloy,and the microstructure and rheological behavior of TC18 titanium alloy are very sensitive to the process parameters of hot deformation,it is difficult to predict the rheological behavior and adjust the structure of the forging in the actual die forging process.Therefore,this study took solid solution TC18 titanium alloy as an example to study its thermal deformation behavior and microstructure evolution law,and established relevant prediction model on this theoretical basis to clarify the quantitative relationship between thermal deformation process parameters,microstructure and rheological behavior,and explore the prediction and control methods of microstructure and rheological behavior in the thermal deformation process of TC18 titanium alloy.The main research contents and important conclusions of this paper are as follows:（1）The macroscopic rheological behavior in the two-phase region of solid solution TC18 titanium alloy was studied by thermal simulation experiments.A prediction model based on long and short memory artificial neural network was established,and the deformation history of materials was considered to predict the rheological behavior of materials.In addition,according to the established hot working diagram,the thermal working window of the alloy is ranges from deformation temperature 700^oC to 850^oC and strain rate 0.001s^-1 to 1s^-1.（2）The microstructure evolution of solid solution TC18 titanium alloy during thermal deformation was studied through microstructure observation experiments.It was found the microstructure evolution is greatly affected by thermal deformation parameters,The content of secondaryαphase increases with the increase of strain/the decrease of deformation temperature/the decrease of strain rate.The content of primaryαphase increases with the decrease of strain rate when the deformation temperature is lower than 820^oC,and decreases with the decrease of strain rate when the deformation temperature is higher than 820^oC.Forβmatrix,the dynamic recrystallization fraction increases with the increase of strain rate when the deformation temperature is 700-760^oC.When the deformation temperature is 790-850^oC,it decreases with the increase of strain rate.The average subgrain size ofβmatrix decreases with the decrease of deformation temperature/increase of strain rate/increase of strain.The average dislocation density ofβmatrix increases with the increase of strain/strain rate,but decreases with the increase of deformation temperature.（3）The mechanism of thermal deformation in the two-phase region of solid solution TC18 titanium alloy was studied by observing the microstructure.It was found that the dislocation would cause the dynamic precipitation and non-uniform nucleation of the secondaryαphase during the thermal deformation.At high temperature and low strain rate,the dynamic spheroidization of the secondaryαphase is mainly accomplished by the formation of subgrain boundaries and the diffusionα→βtransformation.At low temperature and high strain rate,the dynamic spheroidization is mainly accomplished by the formation of shear bands or twins.At low strain rates,the primary equiaxedαphase fuses with spheroidizedαphase with similar orientation nearby,resulting in coarsing of equiaxedαphase.The substructure formed by CDRX process can refine theβmatrix,while increasing the content of equiaxedαphase can increase the degree of refinement ofβmatrix.（4）Combined with the evolution law of microstructure and thermal deformation mechanism,a prediction model of deformation parameters-microstructure and flow stress based on physical mechanism was established.Physical mechanism model contains the microstructure evolution:the secondary precipitation of secondaryαphase,the coarsening of the primaryαphase,the dynamic spheroidizing of secondaryαphase,the continuous dynamic recrystallization ofβmatrix,the evolution of the average dislocation density ofβmatrix,the average grain boundary orientation angle ofβmatrix and the average subgrain size ofβmatrix,etc.The model is trained by experimental data,and the prediction accuracy is high.Basing on the model,It is further found that:In order to obtain smaller grain size or higher HAGBs fraction,the deformation should be done at lower deformation temperature and higher strain rate.In order to obtain more spheroidizedαphase or lower matrix dislocation density,it is necessary to deform at higher temperature and lower strain rate.The initialβgrain breakage for die forging should be high enough to make theβgrain size small enough after die forging.Increasing the content of primaryαphase can increase the degree of CDRX and the degree of refinement ofβmatrix during die forging.In order to accelerate the evolution rate of microstructure,the die forging with high initial dislocation density should be adopted as far as possible.（5）The effect of deformation parameters on microstructure and mechanical property after aging was studied by unified aging heat treatment and hardness testing experiment.It was found that the microstructure of TC18 titanium alloy showed the changes of equiaxed microstructure,two-state microstructure and three-state microstructure during the three stages of solution treatment,thermal deformation and aging treatment,and the microhardness increased and the microhardness deviation decreased continuously.The aging treatment results in the precipitation of a large number of fine dispersedαphase on the basis of the original deformed structure,which homogenizes the structure.The microhardness after aging was greatly affected by deformation parameters.With the increase of deformation temperature,the microhardness first decreased and then increased,and the inflection point appeared at760^oC.With the increase of strain rate,the microhardness increases first and then decreases,and the inflection point appears at 0.01s^-1.Basing nano indentation experiment,it can be found that the hardness ofαphase is higher than that ofβmatrix,and the plasticity ofβmatrix is better than that ofαphase,and the smaller the size ofαphase,the higher the hardness and the worse the plasticity.In addition,the microstructure-microhardness prediction model and deformation parameter-microhardness after aging prediction model are established,and the accuracy of the model is verified.