Characterization And Verification Of The Hot Working Processing Windows Of GH2907 Superalloy

GH2907 superalloy is a low-expansion superalloy based on iron-cobalt-nickel,which is comprehensively strengthened with bismuth,titanium,silicon and trace amounts of boron.Due to its many advantages such as high strength,good thermal fatigue properties as well as low expansion coefficient and elastic modulus below 650?,the alloy is suitable for the manufacture of a variety of aerospace engine rings and casings with the controllable gap size in this temperature range.Mixed crystals often appear in the ring rolling forming process and they are difficult to homogenize in the later heat treatment because of the existence of two sizes of?'strengthening phase and carbide in GH2907 superalloy.By establishing a thermal processing window,the thermodynamic parameters?strain,strain rate and deformation temperature?are related to the microstructure characteristics?grain size,grain range,recrystallized grain ratio and so on?,thus providing a reasonable method for the structure control.In this dissertation,the thermal deformation characteristics of GH2907 superalloy and the microstructure evolution were investigated by thermal simulation compression test,pre-forging heating experiment,near isothermal compression test and solution heat treatment experiment.Moreover,the constitutive relationship of GH2907 superalloy,thermal processing drawings,and thermal processing window were established.The flow stress curves of the alloy with the conditions of deformation temperature of950?¹100?and strain rate of 0.01s^-110s^-11 were obtained by thermal simulation compression experiment,besides the analysis of the effect of thermodynamic parameters on the flow stress of the alloy was also carried out.The thermal deformation activation energy Q of GH2907 superalloy is 463.043 kJ·mol^-1through applying the Arrhenius equation and theZener-Hollomon parameter.At the same time,the constitutive equations of GH2907superalloy in two stages which including hardening-dynamic recovery as well as dynamic recrystallization were established by the mechanism constitutive relation model,and the accuracy of the constitutive equations was verified.Based on the DMM model,the power dissipation diagram of GH2907 superalloy under distinct strains was plotted,and the thermal processing diagram of the alloy with distinct strains was drawn using the Prased instability criterion.According to the microstructure analysis of the alloy,the evolution law of dynamic recrystallization microstructure was obtained,and recrystallization kinetics model of the alloy was also established for avoid selecting the process parameters of instability deformation.The microstructure evolution law of GH2907 superalloy at distinct temperatures was explored by pre-forging heating experiment.The results show that the dissolution temperature of the?'phase of the alloy is at the range of 1020?¹040?,and the diminution of the pinning effect of the grain growth caused by the dissolution of?'phase as well as the acceleration of the crystal grains growth occurs at the same time.The near isothermal compression test was applied for obtaining the evolution law of the alloy microstructure with distinct thermodynamic parameters?deformation temperature and strain?,and the solid solution heat treatment was carried out for gaining the evolution law at distinct heat treatment temperatures after forging.The thermal processing window of GH2907 superalloy was established by investigating the properties of gain containing growth,recrystallized grain ratio,as well as size distinction with distinct thermodynamic parameters?deformation temperature,strain rate,and strain?.It is found that a relatively uniform microstructure can be obtained by adopting a deformation scheme of high-temperature deformation?temperature 1020?,strain 0.4+solution treatment980??by analyzing the hot working process window.The reliability of the GH2907superalloy hot working process window was verified by Simulfact finite element simulation ring rolling results.According to the application of the alloy hot working process window and the ring rolling simulation result,the uniform microstructure ring piece can be produced by applying the ring rolling forming experiment.