| FGH98 is the third generation of high strength damage tolerant powder superalloy for aeroengine turbine disk with high thrust-weight ratio,which is being developed in China.The R&D of FGH98 turbine disk adopts the technical route of "hot isostatic pressing ? hot extrusion isothermal forging ? dual property heat treatment".Due to the insufficiency of the research on the microstructure and properties of FGH98 alloy and turbine disc during hot working and heat treatment,especially the hot extrusion cracking seriously affects the development of disc,it is urgent to study how to improve the thermoplasticity of alloy,optimize the hot working process and dual performance heat treatment process of disc.In this paper,the microstructure,properties and basic problems of FGH98 alloy under various processes were systematically studied,which provided theoretical basis and technical support for further research and development of hot working and heat treatment process and microstructure controlling,Basic research on hot deformation behavior of hot isostatic pressed FGH98 alloy was carried out by thermophysical simulation.In the range of true strain 0.2?0.6,temperature 1060?1165? and strain rate 0.01?10 s-1,it was found that strain-induced discontinuous dynamic recrystallization at 1080-1120? resulted in complete ?+?' microduplex structure with grain size of 1.2-6.8 ?m,and the thermoplasticity of the alloy was greatly improved.Through the observation of SEM and TEM,the interaction analysis of power dissipation factor with microstructures and flow stress,the formation mechanism of ?+?' microduplex structure was revealed,and the stability evolution and scientific control method of such special morphological structure were found,which provided a new way to overcome the hot extrusion cracking problem of large deformation of the alloy.The grain growth of y matrix in hot extrusion+isothermal forged alloys during solution treatment was studied by heat treatment experiments.The temperature was 1060?1191 ? and the time was 10?60 min.The grain growth behavior was different at various temperatures:the grain growth was slow when the temperature was low;the grain growth was fast when the temperature was close to or higher than the temperature of ?' solvus.It was found that grain growth behavior was mainly controlled by the grain boundary migration resistance of ?'.The ratio of the number to the size of y' was used to characterize the resistance.It was found that the apparent activation energy Q of grain boundary migration decreased gradually with the increase of temperature to the diffusion activation energy of grain boundary.Therefore,a modified grain growth model of y matrix was established,which can accurately predict the grain growth behavior of alloys in various temperature ranges,and thus provided a basis for obtaining grain size distribution satisfying the dual performance requirements.Differential scanning calorimeter(DSC)and thermophysical simulation were used to study the precipitation behavior of the ?' phase during solution cooling of hot extrusion+isothermal forged alloys.The solution temperature and cooling rate were 1191? and 0.1?10.8? s-1,respectively.It was found that the size,morphology and distribution of the ?' phase were closely related to the cooling rate,and then the quantitative relationship between the average size of the secondary/tertiary ?' phase and the cooling rate was obtained by linear regression method.It was found that the multimodal size distribution microstructures of the ?'phase can be obtained at 0.1?1.4? s-1.The micro-mechanism was controlled by the change of matrix supersaturation and the diffusion rate of ?' forming elements(Al,Ti).The secondary ?' phase splited and unstable protruded at the cooling rate of 0.1? s-1,and some tertiary ?' at grain boundaries unstable protruded due to the local point diffusion effect at high supersaturation.The results of the ?' precipitation behavior were used to guarantee the properties and to avoid quenching cracking of the disc.The multimodal microstructures formed at 0.1 ? s-1 cooling rate were aged at 815? for 16 to 100 h to study and evaluate the microstructural stability of the alloys in service.It was found that the secondary ?' particles formed by solution cooling further splited into smaller particles during aging,resulting in the decrease of the average size of the ?' with the increase of aging time,while the coarsening driving force of the ?' decreased when the split secondary ?' particles with the same size,and the shape of the secondary ?' particles tended to be rounded.The matrix supersaturation was almost exhausted during ageing.The tertiary ?' particles formed by solution cooling always distributed regionally between the secondary ?',and their surface protrusions disappeared and tended to be smooth spherical.Finally,an artificial neural network model was constructed based on the results of the previous basic research on hot deformation behavior,which solved the problem that the strain compensated Arrhenius constitutive model can not predict the hot deformation behavior of hot isostatic pressing FGH98 alloy under large strain conditions.Based on the large strain hot deformation behavior,grain growth of y matrix and high temperature precipitation of ?',the key processes of FGH98 alloy turbine disc technology route,that were "hot extrusion" and "dual property heat treatment" were studied. |
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