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Residual Stress Characterisation in Forgings for Aero-Engine Application
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
Residual stresses are the stresses which are present within a component without any external load. They can be introduced through any number of manufacturing processes and in-service conditions, meaning that they are almost ubiquitous in engineering components. The characterisation of residual stress is an important field of research particularly in an engineering context since the effects of residual stress sum with the loads. As a result, the performance of a component can be greatly enhanced, or significantly reduced, by the presence of residual stress depending on the sign of the stress and the applied load.In this EngD thesis the focus has been on the development of residual stress through the manufacturing processes of aero-engine forgings, specifically the turbine disc. The forgings studied were sub-scale geometries of the disc, forged from the nickel-base superalloy RR1000. The overall aim of this work is to improve the understanding of the residual stress generation and relaxation through implementation of advanced experimental characterisation techniques, with a view to improving current stress predicting process modelling capabilities. With this in mind the work has focussed on the use of neutron diffraction and the contour method to characterise residual stress experimentally, while residual stress predictions have been made using finite element modelling.The findings of this research indicated that very large residual stresses were generated as a result of the quenching process, and that these stresses were then relaxed and redistributed by ageing heat treatments and material removal by machining. The results obtained through the two experimental techniques exhibited very strong agreement, indicated a robust experimental process. Comparisons to the finite element predictions highlighted some issues with the current model; in particular it was found that the simulation of quenching could be improved by better definition of the heat transfer at the surface. Furthermore, the level of stress relaxation during ageing was consistently over predicted in the model. This result is thought to be the result of an over-prediction of the level primary creep in the alloy. Subsequent studies will investigate this behaviour further using the newly developed in-situ heat treatment capabilities which have developed as part of this research.