Broaching of udimet 720 Liassessment of surface integrity combining experimental and finite element modelling approaches

Abstract

Inconel 718 is the most widely used nickel-based alloy in the manufacture of low-pressure turbine discs for the aerospace industry. However, the need to reduce the emissions of jet engines is driving manufacturers to improve engine efficiency, which is mainly limited by the capacity of the turbine material to withstand the high thermomechanical loads involved in the combustion process. In this regard, the Udimet 720 Li alloy has emerged as a promising alternative to Inconel 718, due to its improved thermomechanical properties. Nonetheless, any change of material requires extensive study of the manufacturing conditions to ensure component quality, and at present, there is little published data available on the manufacturing characteristics of Udimet 720 Li. Broaching is commonly employed in the manufacture of turbine disc fir tree roots, which facilitate the blade to disc connection. Fir trees are subjected to high local stresses induced by centrifugal forces and thermal stresses. Broaching is therefore considered one of the most critical machining processes of the manufacturing route of turbine discs, because its considerable influence on surface integrity has a direct impact on the fatigue life of the component. Nevertheless, there is limited knowledge about both this machining process and how surface integrity is generated. At present, the definition of process window parameters requires numerous experimental tests, which is costly and time consuming. One further disadvantage is that such tests provide limited information about the physics involved in surface integrity generation. Interestingly, predictive modelling is emerging as one of the most promising approaches to address these issues. Against this backdrop, the main goal of this thesis is to assess the influence of cutting conditions and tool geometry on surface integrity when broaching Udimet 720 Li, combining experimental and finite element approaches. To this end, an orthogonal cutting finite element model was developed to predict fundamental outputs of the cutting process (forces and chip morphology) and surface integrity outcomes (residual stresses and microstructural damage). The required input parameters that describe the material and the tool-chip friction behaviour were characterised. A comprehensive material characterisation of Udimet 720 Li including microstructural, thermomechanical, ductile failure, and thermal properties is presented in this thesis. A key contribution of this work is the development of a new flow stress model that accurately reproduces the Dynamic Recrystallisation phenomena and the coupling between strain rate and temperature. A novel ductile failure model is also proposed, that considers the non-monotonic influence of temperature and stress triaxiality, and the coupling between strain rate and temperature. A combination of Digital Image Correlation and numerical simulation techniques was employed, with the result that the proposed ductile failure model correctly predicts fracture strain. The friction between Udimet 720 Li and the tool material T15 (High-Speed Steel) under conditions close to broaching was characterised, using a newly developed experimental methodology. Broaching tests were carried out to validate the modelling results and gain understanding of the scientific variables (forces and chip morphology) and surface integrity aspects (topography, residual stresses, and microstructural damage). In these tests, the cutting conditions (cutting speed and lubrication) and tool geometry (rake angle and rise per tooth) were varied. The topography characterisation includes 2D and 3D roughness indicators. The residual stresses were analysed by combining the X-Ray Diffraction technique with Hole Drilling to obtain a complete map of the stress state of the machined surface. Finally, in the microstructural damage analysis, the primary defects were found to be strain hardening and surface drag. The analysis was conducted using Electron Backscatter Diffraction combined with Kernel Average Misorientation, local misorientation, and Grain Orientation Spread, and revealed that the affected layer thickness was significantly higher of that observed with the optical microscope. The trends and quantitative values obtained from the simulation were in good agreement with the experimental results in terms of fundamental variables and surface integrity outcomes. The simulated forces and chip morphology faithfully reproduced the experimental trends and values. The model accurately predicted the residual stresses from the surface to their evolution through the depth until the bulk material, as well as the thickness of the stressed layer. Finally, the correlation between the plastic strain obtained from the simulation and the defects observed experimentally was established, where the model also reproduced the experimental trends obtained with the optical microscope and Electron Backscatter Diffraction