Research On The Annihilation Behavior Of Pore Defects And Evolution Of Microstructure Around Pore Defects During Hot Isostatic Pressing Of Ti6Al4V Titanium Alloy Castings

The unavoidable internal pore defects in Ti6Al4 V titanium alloy castings for aviation seriously restrict their high-quality production.Hot isostatic pressing can effectively repair such defects,realize the annihilation of pore defects,and improve the comprehensive mechanical properties of castings.The process of pore annihilation includes two stages of internal pore closure and micropores healing,both of which are carried out in a closed container with high temperature and high pressure.It is difficult to observe the annihilation of pores through experiments,and the influencing factors are complex.The annihilation of pore defects cannot be predicted based on experience.At the same time,there are obvious differences in the microstructure around and away from the pore during the hot isostatic pressing of Ti6Al4 V titanium alloy castings.Therefore,it is necessary to study the annihilation behavior of pores and the evolution of microstructure around the pore to optimize the hot isostatic pressing process and improve the performance of Ti6Al4 V titanium alloy castings.To meet the above requirements,the annihilation behavior of pores and the evolution of the microstructure around the pores during the hot isostatic pressing of Ti6Al4 V titanium alloy castings were systematically studied.The main work and results are summarized as follows.(1)A thermo-elastoplastic-creep constitutive model was constructed to describe the flow stress of Ti6Al4 V titanium alloy castings during hot isostatic pressing,and the model was verified by hot isostatic pressing interruption experiments of samples with artificial pore.Firstly,uniaxial hot compression experiment was designed,and the flow stress curves were fitted based on the Arrhenius equation.By correcting the material parameters,a straincompensated elastic-plastic constitutive model was constructed,and then the Norton creep constitutive model was used to fit the creep curves.Finally,a thermo-elastic-plastic-creep constitutive model was constructed,which was used in the finite element numerical simulation of pore closure in this thesis.The model was verified by the hot isostatic pressing interruption experiment.The results showed that the numerical simulation results were in good agreement with the experimental results.(2)The closure behavior of gas pores and shrinkage cavities during hot isostatic pressing of Ti6Al4 V titanium alloy castings was simulated,the closure mechanisms of gas pores and shrinkage cavities and the formation law of surface depressions during the hot isostatic pressing were explored.The results showed that creep was the main mechanism of pore closure during hot isostatic pressing.The effects of different hot isostatic pressing process parameters,the shape and position of pores,the wall thickness of the castings and other factors on the pore closure behavior were studied by simulation.It was found that the hot isostatic pressing process parameters was the direct factor affecting the pore closure.At the same time,the shape,size,location of the pore and the wall thickness of the sample had a certain influence on the pore closure behavior.By simulating and studying the effect of pores with different shapes and positions on the surface depression of the casting,it was found that when the shortest distance between the center of the pore and the surface of the model in two directions perpendicular to each other was small(x:y≤4:3),the position of the surface depression was determined by the shape of pores.When the shortest distance between the center of the pore and the surface of the model in two directions perpendicular to each other was greatly different(x:y≥7:3),the surface depression appeared on the surface closest to the center of the pore.(3)The kinetic process of micropore healing was simulated and studied,the healing behavior and mechanism of micropore during hot isostatic pressing were clarified,and the micropore healing time under hot isostatic pressing was predicted based on the sintering thermodynamic model.The results showed that for micropores at grain boundary,atomic diffusion along grain boundary was the main mechanism of micropore healing.For intragranular micropores,atomic lattice diffusion was the main mechanism of micropore healing.The contribution of diffusion to micropore healing decreased gradually with decreasing temperature.Micropore healing was more sensitive to the hot isostatic pressing temperature,and the healing rate of micropore at grain boundary was much higher than that of the intragranular micropore.For the hot isostatic pressing process of Ti6Al4 V titanium alloy castings(920 °C/120 MPa),the intragranular micropores could be completed healed when the the holding time was 56 min,and the micropores at grain boundary could be healed when the holding time was 7.5 s.(4)The evolution of microstructure around the pore during the hot isostatic pressing of Ti6Al4 V titanium alloy castings was studied,the spheroidization mechanism of α lamellae around pore was revealed,and the effect of hot isostatic pressing on the tensile mechanical properties of Ti6Al4 V titanium alloy at room temperature and 500 ℃ was explored.The results showed that the α lamellae around the pore was preferentially spheroidized at the colony junction and the kinking region during hot isostatic pressing.The spheroidization mechanism mainly included continuous dynamic recrystallization,lamella kinking and grain growth.The interface healing was achieved through discontinuous dynamic recrystallization and grain growth.Hot isostatic pressing could effectively improve the elongation of the material.The elongation of the samples treated by HIP-4 process was increased by 47.2% and 53.1% at room temperature and 500 ℃,respectively.