| High-performance aerospace components are serving in the harsh environments and their high heat-resistance,lightweight,high-reliability,and high-load requirements pose a serious challenge to manufacturing materials and forming methods.The traditional manufacturing methods,like casting,forging and welding,have obvious shortcomings although they are technically mature.It is difficult for them to meet the current requirements for the production of complex parts and components of aerospace.The powder hot isostatic pressing technology can form an overall near-net shape of complex parts with comprehensive mechanical properties by coupling powder metallurgy with mold manufacturing technology,which is of great potential for aerospace high performance complex parts manufacturing.However,the technology of powder hot isostatic pressing is immature worldwide,the essence of powder hot isostatic pressing is that the particles in the discrete state transform into a densified continuum under the action of pressure.This process shows macroscopically that the plastic deformation of the capsule drives the densification of the internal powder.Microscopically,it is characterized by the movement of internal powders,and at the microscopic level,it is the microstructure evolution caused by the powder particle diffusion connection.No single-scale research method can fully explain its formation process.The classical continuous elastic-plastic finite element method can predict the overall macroscopic deformation and densification,but ignore the particle characteristics during the forming process.However,the discrete finite element method can analyze the movement and deformation of particles in the process of hot isostatic pressing from the meso-level,but it can not reflect the process of tissue evolution.The cellular automaton method mathematical modeling of the physical nature of the microstructure transformation can simulate the microstructure evolution process.Therefore,this paper intends to combine numerical simulation and experimental methods to analyze the related issues in hot isostatic pressing of Ti6Al4V alloy powder,we have used the finite elemet method to study the macroscopic deformation,the coupled discrete element method to study the mesoscopic particle movement and deformation,and the cellular automata method to study the micro dynamic recrystallization.Specific researches and results are as follows:Multiscale experiments were designed.As to the macroscopic controllment,the corresponding alloy samples were prepared by the cylindrical capsule interrupt experiments,and the uniaxial and Gleeble thermocompression experiments were used to modify the macro-finite model,and the cubical component was used to verify the established model;At the mesoscale,the formation of the microstructure and the phase transformation of the particles due to thermoplasticity during the hot isostatic pressing of powders were investigated.At the same time,the tensile properties of the formed parts at different stages of hot isostatic pressing were studied.At the microscopic scale,the mechanism of the microstructure evolution during powder hot isostatic pressing was studied,mathematical methods were used to process the softening curve to modify the KM dislocation density model,and a corresponding SEM experiment was designed to study the bending/breaking phenomenon of?-sheets in the?+?two phase region.Multi-scale mathematical models were established.As to the macroscopic numerical model,the strain regularity and yield strength of powder alloy parts at different densities were obtained by uniaxial experiments.The Shima modified yield criterion for Ti6Al4V powder hot isostatic pressing process was established based on data fitting.The classic two-stage Arrhenius model was used to establish a high-temperature flow stress model for the thermal processing of reactive Ti6Al4V alloys.Based on the hypothesis of continuum medium,a hot isostatically-elastic-plastic finite element model of powder was constructed.On the mesoscale,the hot isostatic pressing process was simplified to the powder compaction process under high temperature.Three different initial accumulation methods of powder particles were constructed to study the movement and deformation of the particles under high temperature pressing conditions.On the microscopic scale,based on the classical KM model of dislocation density evolution,the work hardening and dynamic softening parameters of Ti6Al4V alloy hot isostatic pressing process were modified based on experimental data.In the study of overall macroscopic deformation,A typical cubic sample was fabricated by the powder hot isostatic pressing to verify the established macroscopic finite element model.A two-stage numerical model was established to describe the densification mechanism of plastic deformation,creep and diffusion.Based on this,a hot isostatic pressing diagram was established which can reflect effects of temperature and pressure on the densification process at different reaction times.The influence of"capsule-core-temperature-pressure-initial density"on the shape control in the process of hot isostatic pressing has been analyzed.The results show that:The capsule shielding effect on the isostatic pressure is proportional to the wall thickness and the yield strength of the material,the bigger stiffness of the graphite core is better for control,but the local density is lower.Longer insulation platforms can achieve higher densification but can easily lead to coarse grains and increased micro cracks.Inhomogeneous initial density distribution will cause non-uniform deformation of the capsule and shearing of the internal powder to break the particles.At the meso level,based on the hot isostatic pressing interruption test,combined with the finite element and discrete element analysis,the dynamic evolution of microstructure and mechanical properties during hot isostatic pressing were studied.Results show that the initial Ti6Al4V alloy powder mainly consists of acicular?'phase and dispersed?phase.During the hot isostatic pressing,the?'phase transforms into?phase and?phase in two ways.In the process of hot isostatic pressing,the size of the particles affects the powders deformation law,and the deformation of the smaller size particles is more uniform.The plastic deformation of the powder particles mostly concentrates on the boundaries of the powder particles,and the larger plastic strain energy accumulates at the boundaries.XPS analysis of the particle surface shows that the surface of the Ti6Al4V powder is covered with a layer of pollutant film mainly composed of TiO2 and Al2O3 with the surface oxide thickness less than 10 nm.The presence of oxide on the surface of the particle can provide a nucleation substrate for the heterogeneous nucleation,and the release of the cumulative plastic strain energy at the particle boundary can further promote the formation of dynamic recrystallization.The final microstructure is a typical grid-like structure;the size of the grid is close to the size of the powder particles.The interior of the grid is a lath-like structure.The boundaries of the grid are distinguished by the equiaxed structure.The overall structure is composed of the equiaxed?-phase and the lath?and?.In the condition of 930?/120MPa,the degree of densification of alloy powder has been close to the theoretical value,with the holding time extension,although the grain will be grown,the diffusion between the particles are more fully.Yield strength of the prepared alloy sample is 922MPa;tensile strength is 1048MPa,13.9%elongation after break,this has reached to the level of forgings.At the micro level,the thermodynamics and grain boundary driving mechanism of initial grain growth have been established in conjunction with the physical laws of microstructure evolution and realized the initial grain growth simulation.A numerical model of dynamic nucleation,growth and transformation of the dynamic recrystallization grains was established using the dislocation density as a variable,and the dynamic simulation of the dynamic recrystallization process of Ti6Al4V alloy powder hot isostatic pressing was realized.The simulation results show that the main factors affecting the dynamic recrystallization are the temperature,the strain and the strain rate during hot isostatic pressing.Among them,strain rate and strain amount affect the morphology of recrystallized grains by changing the nucleation and growth time,and reasonable regulation of deformation and strain rate can achieve the effect of grain refinement;Increasing the hot isostatic pressing temperature can reduce the incubation period required for nucleation and accelerate the nucleation of the crystal grains,however,the high temperature is easy for the grains to grow coarse.For the Ti6Al4V alloy with a lamellar microstructure,the bending fracture of the lamellar structure during high temperature thermoforming also causes the softening effect.The lamellar structure at relatively low strain rates is coarser than the structure at high strain rates.As the strain rate increases,the number of initial lamellar structures buckling/kinking increases and the initial layer structure tends to be stretched rather than spheroidized.Therefore,at higher strain rates,the degree of softening behavior depends on the portion of the curved laminar structure. |
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