Study On The Evolution Of Three-Dimensional Residual Stress Field In Turbine Discs By Using Neutron/X-Ray Diffraction And Finite Element Method

Ni-based superalloy turbine discs have been developed and employed in aerospace and land-based power-generation applications.Residual stress fields will be introduced into the turbine discs during processing,such as hot deformation and heat treatment.Under critical service conditions such as thermal cycle and high/low mechanical cyclic loads,residual tensile stress in the component is usually detrimental when superimposed with the service loads.It will also cause the macroscopic stress redistribution and local stress concentration.In recent decades,neutron diffraction and portable X-ray diffraction techniques were widely used in measuring the near-surface/internal residual stress and its gradient in industrial components.In order to optimizing preparation process and improving service performance of the turbine discs,it is of great significance to reveal the evolution of three-dimensional macroscopic residual stress in nickel-based superalloy during hot deformation and heat treatment process.In this paper,two kinds of ?'-strengthened nickel-based superalloy discs including wrought superalloy GH4738 and powder metallurgy superalloy FGH96 were studied,after isothermal forging(IF),solution treatment(ST)and aging treatment(AT)process.After optimizing the neutron diffraction parameters and instrument set-up for the turbine discs,characterization of the three-dimensional macroscopic residual stress field and stress-free interplanar spacing(d0)were carried out.Meanwhile,the synchrotron-based High Energy X-ray Diffraction(HEXRD)technique was employed to investigate both d0 and the micromechanical behavior of FGH96 alloy.Through the verification of experimental results,the thermo-mechanical coupled Finite Element Method(FEM),which could accurately predict the evolution of the macroscopic residual stress field,was proposed.The main content is summarized as follows:(1)By using HE-XRD,neutron diffraction and laboratory X-ray diffraction techniques,do were characterized using the 'comb-shaped' stress-free samples.The do results obtained from {311} crystal plane by HE-XRD in-situ and ex-situ method,are employed in residual stress measurement and confirmed with neutron diffraction results with an average deviation less than 30 MPa and 45 MPa,respectively.Due to the warpage deformation caused by electrical discharge machining and linear fitting error,the deviations between laboratory XRD experimental results and neutron diffraction exceed 570 ??.(2)The evolution of the microstructure,do and intergranular/interphase mechanical behavior in IF,ST and AT state FGH96 samples were studied.The morphology and volume fraction of ?' phase are considered to have negligible impact on the accuracy of the neutron/X-ray measurement and FEM simulation.The d0 of the super-solvus ST material is 300 ?? larger than the IF and AT sample.Besides,the interphase mismatch was calculated from ?'-{100} superlattice and y{200} reflection data,and the mismatch values continually increased after IF,ST and AT process.The HE-XRD in-situ experiment were carried out to study on the lattice strain response to uni-axial tensile loads.It was shown that the ?' phase with proper content and size after AT could obviously hinder the dislocation movement,and thus improve the yield strength of FGH96 material.(3)The neutron diffraction method was applied to obtain the three-dimensional macroscopic residual stress distribution inside the disc after IF,ST and AT processes.Incremental hole drilling method and layer removal XRD method were used to obtain the surface/near-surface stress distribution.In the FGH96 disc,it is shown that the near-surface/internal stress gradient induced by the thermal deformation process was completely relieved after super-solvus ST.Then,residual stress redistributed during the air-cooling process after ST.After AT process,compressive residual stress located closer to the center of disc and its magnitude decreased.The GH4738 discs were air-cooled and water-quenched after sub-solvus ST,separately.During the water-quenching process,the realistic interface heat transfer process caused diffferent relief of surface thermal stress due to non-uniform transient plastic deformation.A steep stress gradient was also introduced into near-surface layer with thickness of 0.4 mm.After the standard AT/air-cooling process,the surface residual stress distribution remained in the GH4738 discs,while both surface and internal residual stresses decreased.Yet,in the ST/water-quenched disc,the internal residual stress gradient along the depth direction increased significantly after AT/air-cooling process.It indicatesd that residual stress strongly affected the distribution of thermal stress along the direction perpendicular to maximum thermal gradient when cold-shrinked deformation occured in the center of disc.(4)After obtaining heat transfer coefficient using inverse heat transfer calculation program,the diffusional ?' phase transformation latent heat was added to the ABAQUS program phenomenologically.Thermoelastic-plastic assumption and Norton creep law were adopted in FEM.Accordingly,the thermal stress evolution and residual stress relaxation behaviors during the heat treatment process of superalloy discs were simulated thermomechanically.The average deviation between the three-dimensional residual stress field calculated by FEM and the neutron measured value is less than 12%.During AT,although the ST residual stress would relieve to a certain extent,it still significantly affected the thermal stress evolution during followed cooling process.After experiencing sub-solvus ST and 800? AT/water-quenching process,the evolution of residual stress field can be concluded as:I.If water were used as cooling medium after both ST and AT,the ST residual stress would offset the AT thermal stress of distinctive stress state when the near-surface region was cold-shrinked at first.The peak thermal stresses decreasing when the internal region was cold-shrinked,which resulting in generating greater AT residual stresses.II.It is shown that the characteristic of ST+air-cooled residual stress distribution remained in the AT/water-quenched disc.When cold-shrinked deformation first occured in the near-surface region,compressive state internal residual stress existed at the edge of component(even with a small magnitude)would superimpose with the same state thermal stress.Thus,the delay of statechange time will lead to lower thermal stress when cold-shrinked deformation occured in the internal region.