Jeffery Brooks

Comparison of fatigue crack propagation behaviour in two gas turbine disc alloys under creep–fatigue conditions: evaluating microstructure, environment and temperature effects

Abstract Gas turbine disc materials should possess excellent fatigue and creep performance due to the severe in service conditions experienced. In this study, a comparison of fatigue crack propagation behaviour in two turbine disc alloys, i.e. N18 and low solvus high refractory (LSHR) superalloy, has been made in terms of the propagation rate and fractography observed under equivalent testing conditions. Temperatures of 650 and 725°C are compared for a trapezoidal dwell fatigue cycle (1–20–1–1) in both air and vacuum at an R ratio of 0·1. It is found that coarse grained LSHR superalloy has better fatigue crack propagation resistance than fine grained N18 in vacuum, which is ascribed to its better creep performance. Oxidation causes significant degradation of fatigue performance of these two alloys, especially in the LSHR superalloy at higher temperature (725°C), resulting in its inferior fatigue performance compared with N18. In the LSHR superalloy, it seems that oxidation is the principal contributor to the deterioration of fatigue resistance. This is supported by observations of transgranular fracture in vacuum and intergranular fracture in air. In contrast, creep is a greater contributor to the deterioration in fatigue performance of N18 (as indicated by the intergranular failure modes observed in vacuum). An apparent activation energy analysis is able to provide further insight into the underlying mechanisms of fatigue crack propagation under creep–oxidation–fatigue conditions in these two alloys.

The Importance of Materials Data and Modelling Parameters in an FE Simulation of Linear Friction Welding

Linear friction welding has become a key technology in the aeroengine industry due to its capability to produce blisk components. Finite element (FE) simulation of linear friction welding applications has been studied in recent years by a number of institutions, using a variety of software codes. Several codes have been demonstrated to be capable of predicting with reasonable accuracy some or all of the critical outputs of friction welding, namely, the thermal loading, plastic deformation, and residual stresses generated. The importance of reliable material data in performing these calculations is paramount. Available material data in the published literature is often restricted to lower temperatures and strain rate regimes. Extrapolation methods used on this data to estimate high temperature properties can lead to uncertainties in the modelled predictions. This paper reviews the approach to materials modelling, including material datasets and material constitutive laws, for FE simulation work in the literature regarding linear friction welding. Best-practice methods for materials constitutive laws, materials data-sets, and the associated experimental temperatures and strain rates used to gather data are suggested. Finally, successfully validated modelled outcomes—when a robust, reliable, and accurate material database has been selected—are demonstrated for a number of the FE methods considered.

Multiscale microstructure modelling for nickel based superalloys

Abstract The present paper is concerned with the development of multiscale modelling approaches for predicting the microstructural evolution and high temperature deformation characteristics of superalloys with special attention to creep and hot forming behaviour. A microstructure informed deformation model is presented that links rearrangements at the microscale to the overall macroscopic response of the material through a damage mechanics approach and results are presented on the application of the model to CMSX4. The control of microstructure, during the manufacture of nickel based superalloy components, is key to the development of the mechanical properties required for the high temperature applications typical of these materials. Results from empirical methods and a new physics based approach for modelling recrystallisation in polycrystalline superalloys are presented for the prediction of the grain size distributions produced during hot forming operations in Inconel alloy 718. A global macroscale modelling approach based on Neural Networks has been developed which includes the effects of composition, heat treatment and processing route and the effectiveness of the model for both property prediction and interpolation is demonstrated.

Adaptive numerical modelling of high temperature strength, creep and fatigue behaviour in Ni-based superalloys

Abstract The mechanical behaviour of high performance Ni alloys is required for many applications and where experimental data is not readily available then a suitable predictive approach would be beneficial. There are numerous routes to achieve this, however, here the data driven neural network method has been adopted to produce models for the tensile, creep and fatigue performance of nickel base alloys. These models have been successfully developed and tested against a range of criteria. The tensile and creep models have displayed excellent fidelity to known nickel alloy behaviour, while good correspondence was also achieved for the fatigue properties (both strain and stress controlled). Potential routes to further improve the performance of these models have been discussed.

Effect of Delta Phase on the Hot Deformation Behaviour and Microstructural Evolution of Inconel 718

In order to investigate the influence of -phase precipitation during high temperature forging of Inconel 718, hot axi-symmetric compression tests have been performed on specimens with two distinct initial microstructures: i) as-received material containing a dense population of  precipates at grain boundaries and along intragranular slip planes, and ii) material solution-treated to dissolve the  phase. Results indicate that the presence of  leads to a slight increase in peak stress and a proportionately greater post-peak reduction in flow stress, as compared to solution-treated material. For both types of microstructure flow softening is associated with grain refinement, but in different ways: in -free material conventional dynamic recrystallisation leads to the formation of new grains, whereas the presence of plate-like  appears to cause the mechanical break-up and segmentation of prior grains.

Friction during precision forging of high temperature aerospace materials

Abstract A programme of experimental work has been carried out to determine the viscosity and friction performance for a range of borosilicate based glass forging lubricants for conditions relevant to the closed die forging operations used in the manufacture of aerospace parts in titanium and nickel alloys. A simple experimental method for the measurement of glass viscosities has been described and the measured viscosities and the friction coefficients have been used to provide an interpretation of the behaviour observed. Isothermal friction tests in the temperature range from 800 to 1100°C were carried out to determine the basic performance of the glass followed by tests representative of forging operations with the workpiece at 925°C and the dies between 300 and 600°C. It has been found that both boundary layer and hydrodynamic lubrication regimes occur for the temperatures and strain rates encountered during the forging of aerospace materials coated with glass lubricants.

Mechanical performance of extruded Ti–46Al–5Nb–1W titanium aluminide

Abstract Titanium aluminide alloys offer considerable promise for use in high temperature applications, such as gas turbines. In this study an extruded Ti–46Al–5Nb–1W alloy has been examined, in terms of its tensile and creep behaviour. A reasonably fine and uniform microstructure was found in this bar product. This gave excellent properties, with tensile strengths up to ∼950 MPa at room temperature, along with 1% elongation. These properties were accompanied by a very good creep behaviour, with low primary strains at the lower stresses and very low secondary creep rates. Comparison of the creep properties of this titanium aluminide alloy with other similar compositions and some typical nickel alloys shows that it is significantly superior to first generation titanium aluminides but also nickel alloys, such as IN718 and Udimet 720Li. However, the strain controlled fatigue performance of the titanium aluminide alloy was significantly poorer than these same wrought nickel alloys.

Probabilistic Property Prediction of Aero-Engine Components for Fatigue

Service lives for critical rotating parts in aero engine gas turbines are declared using deterministic lifing calculations based on fixed point values of key mechanical properties and factors to allow for the scatter. However, novel probabilistic lifing algorithms have been developed, which are able to take into account the degree of scatter in the material properties throughout the component. Process simulation software has been developed to predict the material flow, residual stresses, microstructure and properties in components during the disc forging operations to ensure robust manufacturing routes. This allows the changes in the materials microstructure, and the mechanical property variation throughout the component, to be predicted as the crack initiation and propagation properties are significantly dependent on the grain structure. These two strains of research have been combined in an attempt to increase the reliability of service life predictions through modelling the scatter in the mechanical properties resulting from manufacturing variation. Results will be presented which indicate that significant life benefits can be obtained by adopting a location specific lifing method based on this approach.Copyright

Implementing CFD Modelling to Address Defect Formation in Core Injection Moulding

Ceramic cores are used in casting processes to create complex internal shapes within the final component. The work presented here uses Computational Fluid Dynamics (CFD) analysis to predict the filling and solidification behaviour of ceramic core material during the Ceramic Injection Moulding (CIM) process in the large, complex geometries now typical of modern cores. The aim of the study is to develop a predictive capability to identify key defects in the core that might otherwise only be observed after a number of expensive manufacturing processes. Manufacturing trials using short shots have been carried out in order to validate the transient flow patterns of the paste; this has highlighted the occurrence of jetting, weld lines and flow defects that are highly dependent on the injection parameters and runner designs employed. Analysis of the modelled solidification, shear rates, stagnation points and phase migration has driven die and process optimisation in a production environment.

An Integrated Modeling Approach for Predicting Process Maps of Residual Stress and Distortion in a Laser Weld: A Combined CFD–FE Methodology

Laser welding has become an important joining methodology within a number of industries for the structural joining of metallic parts. It offers a high power density welding capability which is desirable for deep weld sections, but is equally suited to performing thinner welded joints with sensible amendments to key process variables. However, as with any welding process, the introduction of severe thermal gradients at the weld line will inevitably lead to process-induced residual stress formation and distortions. Finite element (FE) predictions for weld simulation have been made within academia and industrial research for a number of years, although given the fluid nature of the molten weld pool, FE methodologies have limited capabilities. An improvement upon this established method would be to incorporate a computational fluid dynamics (CFD) model formulation prior to the FE model, to predict the weld pool shape and fluid flow, such that details can be fed into FE from CFD as a starting condition. The key outputs of residual stress and distortions predicted by the FE model can then be monitored against the process variables input to the model. Further, a link between the thermal results and the microstructural properties is of interest. Therefore, an empirical relationship between lamellar spacing and the cooling rate was developed and used to make predictions about the lamellar spacing for welds of different process parameters. Processing parameter combinations that lead to regions of high residual stress formation and high distortion have been determined, and the impact of processing parameters upon the predicted lamellar spacing has been presented.