| Aero-engine is the important component of aircraft,and its technical level is an important symbol to measure the national scientific and technological strength and military equipment level.Meanwhile,the high-quality manufacturing of its core components is one of the key problems that restrict the development of aero-engine in our country.Due to the superior service performance,nickel-base powder metallurgy(P/M)superalloys are the important material for high performance aeroengine turbine disks.Hot extrusion(HEX)is the key step in turbine disk manufacturing process,which can eliminate the prior particle boundaries(PPBs),inclusion and metallurgical defects.Meanwhile,the grain structure can be refined.However,due to the high content of alloying elements,large deformation resistance and narrow processing window of nickel-base P/M superalloys,the alloy is easy to crack during HEX process,which makes it difficult to realize the manufacture of high-quality turbine disk blank.Therefore,a novel type of nickel-base P/M superalloy(FGH4113A superalloy)for high-quality aero-engine turbine disk is studied in this thesis.The high temperature flow properties and microstructure evolution mechanisms of hot isostatic pressed(HIPed)FGH4113A superalloy are systematically studied.The microstructure evolution of this alloy during HEX is simulated.The HEX forming process of this alloy is optimized,and the feasibility of the process is verified by HEX engineering test.The grain size and distribution of the extruded part reach the design goal of initial microstructure of die forging billet.The main research contents are as follows:(1)The macroscopic flow behavior of HIPed FGH4113A superalloy are studied by thermal compression experiments.Dislocation accumulation and dislocations pinned byγ?phase are the main strengthening mechanisms of the alloy.Stratified faults shearing and the formation of microtwinnings are the main deformation mechanisms.The strain-compensated Arrhenius model and the back propagation artificial neural network model based on particle swarm optimization algorithm are established.The correlation coefficients between the predicted and the experimental true stress values of the two models are 0.972 and 0.999,respectively,indicating that the established models can accurately predict the high temperature deformation behavior of HIPed FGH4113A superalloy.Additionaly,the hot working diagram is established,and the hot forming process parameter window of HIPed FGH4113A superalloy is determined.(2)The effect of deformation parameters on the evolution ofγ′phase in HIPed FGH4113A superalloy is studied.Results show that the morphology ofγ′phase changes from the irregular,split-off branch,dendrite,butterfly-like,and cauliflower to spherical during hot compression.Increasing the deformation temperature or true strain can accelerate the dissolution ofγ′phase,while increasing the strain rate can limit the dissolution ofγ′phase.The interaction betweenγ′/γinterfacial energies,the coherent strain energy caused by lattice mismatch,and the interaction between dislocation andγ′phase,are the main reasons for the morphology evolution and dissolution ofγ′phase.Based on the experimental analysis,the dynamic dissolution kinetics equation ofγ′phase is established,the correlation coefficient between the predicted and experimental values is 0.991.The quantitative characterization between the dissolution and deformation parameters ofγ′phase is realized.(3)The formation mechanism of PPBs in FGH4113A superalloy during hot isostatic pressing(HIP)and the influence of deformation parameters on the evolution of PPBs are studied.During HIP,the diffusion of elements and the reaction of alloying elements with carbon/oxygen promote the precipitation ofγ′phase,carbide/oxide at the powder particles boundaries to form PPBs.During hot deformation,the dissolution/breakage degree of PPBs increases with increasing true strain/deformation temperature or decreasing strain rate.The elimination of PPBs is mainly related to interfacial energy,coherent strain energy,element diffusion promoted by dislocation,stress and vacancy,and dynamic recrystallization(DRX).Based on the experimental analysis,the evolution of PPBs in the HEX process is simulated,and the mathematical model of extrusion ratio and PPBs deformation is established.(4)The impacts of deformation parameters on the DRX behavior of HIPed FGH4113A superalloy are investigated,and the interaction mechanism betweenγ′_Ⅰ、γ′_Ⅱandγ′_Ⅲphases and the DRX behavior is revealed.The main DRX mechanism for the HIPed FGH4113A superalloy is discontinuous DRX(DDRX),and the auxiliary mechanisms are twin assisted nucleation andγ′phase excitation nucleation.Additionaly,the coarseγ′_Ⅰphase promotes the DRX by inducing the dislocation accumulation,while the fine sphericalγ′_Ⅲphase results in the formation of sub-grain boundaries and refine the grains.Based on the law of microstructure evolution,a two-stage form DRX kinetics model and a grain-growth model are developed.Compared with the experimental results,the established models can accurately predict the DRX behavior and grain size evolution of the alloy during hot forming process.(5)The flow stress constitutive model,the dynamic dissolution kinetics model ofγ′phase,the DRX kinetics model and the grain-growth model are developed into DEFORM software,and a simulation system is established to describe the microstructure evolution of HIPed FGH4113A superalloy during hot forming process.The impacts of extrusion temperature,extrusion speed and extrusion ratio on DRX behavior,average grain size distribution and dissolution ofγ′phase of FGH4113A superalloy are analyzed.Based on the idea of coordinated control of grain structure andγ′phase,the HEX forming parameters of FGH4113A superalloy are optimized by multi-objective particle swarm optimization algorithm.The HEX engineering experiment of HIPed FGH4113A superalloy is carried out.The average grain size of the hot extruded part is 3.72μm,which realizes the high-quality HEX forming of FGH4113A superalloy and lays an important foundation for subsequent isothermal forging of turbine discs. |
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