Investigations Of Constitutive Behaviour For GH4169 Superalloy And The Evolution And Influence Of Residual Stress In Turbine Disc

The turbine disc,as one of the most important hot-end components in aeroengines,during the operation is subjected to the huge centrifugal inertia forces in conjunction with the thermal stresses,and the mechanical performances of the turbine disc are closely related to the processing technology.It is required to apply the quenching and aging treatments to GH4169 turbine disc after solution treatment,the quenching induced residual stresses are unlikely to be relaxed thoroughly attributed to the hard-to-deform and high-strength characteristics of the alloy.After quenching and ageing treatments,machining,or a certain period of use,the superalloy discs in aero-engines will have unexpected deformation caused by the relaxation or release of the residual stresses,which will lead to unacceptable changes in the component dimensions and affect the dynamical performance.In cases of overrun,it may lead to the failure issues and cause catastrophic accidents.At the present,on the side of the constitutive behavior studies for the GH4169 alloy,it is still in lack of understandings into the viscoplastic deformation mechanisms for the solid solution treated alloy and the alloy in the transition state from the solid solution state to the aging state.The existing constitutive models are unlikely to reflect fully and accurately the high temperature mechanical behavior of the alloys and discs.In addition,the evolutionary and working mechanisms of the residual stresses during the manufacturing and service processes,such as heat treatments,machining and high temperature rotating of the GH4169 discs are rather complex.The key governing factors have not been clarified yet.To solve these key issues,the mechanical behaviors of GH4169 superalloy are systematically elucidated in this work.In particular,a constitutive model is developed to fully describe the identified high temperature mechanical behavior of the alloy,and the model is further used to simulate successfully the formation and evolution of residual stresses during heat treatments for GH4169 discs.The effects of residual stresses on the machining deformation and the rotational creep deformation are also explored.The following four aspects of investigations are carried out in the present thesis.The high-temperature mechanical behavior and the deformation mechanism of the GH4169 alloys in the solid solution state and the aging state are experimentally studied.The uniaxial tensile tests,cyclic tension and compression tests,stress relaxation tests and creep tests at high temperatures are carried out.The experimental results indicate that the alloy of interest exhibits quite complex phenomena,such as the negative strain rate sensitivity,the serrated yield caused by the dynamic strain aging effect,the isotropic and kinematic combined hardening,the precipitation strengthening and the volume shrinking during the aging process.In order to describe those deformation and hardening mechanisms,a temperature-and strain-rate dependent viscoplastic constitutive model is developed based on the dislocation slip theory.The effects of the dynamic strain aging,the back stress evolution and the precipitation are incorporated into the model by introducing the relevant internal variables and the evolution equations of the internal variables.The material parameters in the constitutive model are identified by comparing the modelling results with the experimental data.The mechanical responses of the solid solution treated and the aged alloys under various thermo-mechanical coupling conditions can be accurately simulated.Based on the developed constitutive model,a thermal-mechanical sequentially coupled finite element model is presented to predict the formation and evolution of residual stresses for GH4169 discs during the quenching and aging processes.The simulated predictions are compared with the experimental measurements for the purpose of verifying the current model,and the results show that the relative deviation between the simulated and the experimental results are not more than 50%.The factors affecting the residual stresses during heat treatments are further examined,including the dynamic strain aging,the back stress,the precipitation,the convective heat transfer coefficients,and the cross-process transfer of the internal variables from the quenching process to the aging process.The results indicate that although the dynamic strain ageing can lead to the localised plastic strain rate band in the early stage of the quenching process,its effect on the quenching-induced residual stresses can be ignored.The kinematic hardening related back stress evolution can significatly affect the quenching-induced residual stresses for the discs with relatively large size.The effect is no strong for the relatively small discs.Due to the precipitation of the strengthening phases in the aging process,the deformation resistance is significantly improved,making the quenching-induced residual stresses be difficultly relaxed during the subsequent aging process.In addition,the non-uniform plastic strain field caused by quenching is found to be capable of affecting significantly the evolution of residual stress during the aging process.Based on the thermo-mechanical sequentially coupled finite element model developed for the heat treatment processes,the birth-death element method is further applied to simulate the machining process.The redistribution of residual stresses and the deformation of the disc during the machining process are simulated.The accuracy of the current modelling predictions is verified by comparing the simulated predictions with the experimental measurements.The numerical results indicate that the deformation during the maching depends significantly on the initial shape of the blank disc and the geometry of the cutting area.The machining-related deformation of the disc can be reduced by adjusting the heat treatment process based on the asymmetric morphology of the machined disc.The effect of heat treatment-induced residual stresses on the rotational creep behavior of discs at high temperature is studied by using the developed constitutive model.The modelling predictions are verified by comparing with the experimental measurements of the rotational creep deformation at high temperature.The real rotational creep load acting on the disc is composed of the centrifugal inertia force and the residual stresses.The modeling results indicate that the residual stresses induced by heat treatments have a significant impact on the rotational creep deformation of the disc.The influence of heat treatment methods on the distribution of residual stress and the rotational creep deformation is further investigated by the finite element model.It is found that the rotational creep deformation of the disc could be reduced more significantly by using the central-hole-forced air cooling method than that by the water quenching and the air cooling methods.