Robert Lancaster

Structural Integrity of an Electron Beam Melted Titanium Alloy

Advanced manufacturing encompasses the wide range of processes that consist of “3D printing” of metallic materials. One such method is Electron Beam Melting (EBM), a modern build technology that offers significant potential for lean manufacture and a capability to produce fully dense near-net shaped components. However, the manufacture of intricate geometries will result in variable thermal cycles and thus a transient microstructure throughout, leading to a highly textured structure. As such, successful implementation of these technologies requires a comprehensive assessment of the relationships of the key process variables, geometries, resultant microstructures and mechanical properties. The nature of this process suggests that it is often difficult to produce representative test specimens necessary to achieve a full mechanical property characterisation. Therefore, the use of small scale test techniques may be exploited, specifically the small punch (SP) test. The SP test offers a capability for sampling miniaturised test specimens from various discrete locations in a thin-walled component, allowing a full characterisation across a complex geometry. This paper provides support in working towards development and validation strategies in order for advanced manufactured components to be safely implemented into future gas turbine applications. This has been achieved by applying the SP test to a series of Ti-6Al-4V variants that have been manufactured through a variety of processing routes including EBM and investigating the structural integrity of each material and how this controls the mechanical response.

An Empirical Approach to Correlating Thermo-Mechanical Fatigue Behaviour of a Polycrystalline Ni-Base Superalloy

Assessment of thermo-mechanical fatigue behaviour of the polycrystalline nickel alloy RR1000 reveals a significant effect of phase angle on fatigue life. The current paper explores two scenarios: the first where the mechanical strain range is held constant and comparisons of the fatigue life are made for different phase angle tests; and secondly, the difference between the behaviour of In-phase (IP) and −180° Out-Of-Phase (OOP) tests over a variety of applied strain ranges. It is shown that different lifing approaches are currently required for the two scenarios, with a mean stress based approach being more applicable in the first case, whereas a Basquin-type model proves more applicable in the second. However, it is also demonstrated that the crack propagation phase should also be considered in these types of tests for high strain ranges and projects that future modelling approaches should attempt to unify mean stress, stress range and a crack propagation phase.

Non-invasive temperature measurement and control techniques under thermomechanical fatigue loading

Abstract A non-invasive temperature measurement, control and profiling technique has been investigated for use with thermomechanical fatigue loading. The technique utilises an infrared thermography camera and Rolls–Royce developed thermal paint to control and monitor cyclic temperature. Thermal paint is used to maintain a stable surface emissivity upon the test piece. The accuracy of the technique is compared against type N thermocouples and a pyrometer for both temperature control and monitoring purposes. Diverse test specimen geometries and alloy compositions are used over a 100–700°C temperature range. Effects on temperature measurement accuracy such as thermocouple shadowing are highlighted and quantified. The non-invasive technique has proved accurate to within ±2°C of the reference thermocouples when in combination with the thermal paint coating.

High Temperature Deformation Mechanisms in a DLD Nickel Superalloy

The realisation of employing Additive Layer Manufacturing (ALM) technologies to produce components in the aerospace industry is significantly increasing. This can be attributed to their ability to offer the near-net shape fabrication of fully dense components with a high potential for geometrical optimisation, all of which contribute to subsequent reductions in material wastage and component weight. However, the influence of this manufacturing route on the properties of aerospace alloys must first be fully understood before being actively applied in-service. Specimens from the nickel superalloy C263 have been manufactured using Powder Bed Direct Laser Deposition (PB-DLD), each with unique post-processing conditions. These variables include two build orientations, vertical and horizontal, and two different heat treatments. The effects of build orientation and post-process heat treatments on the materials’ mechanical properties have been assessed with the Small Punch Tensile (SPT) test technique, a practical test method given the limited availability of PB-DLD consolidated material. SPT testing was also conducted on a cast C263 variant to compare with PB-DLD derivatives. At both room and elevated temperature conditions, differences in mechanical performances arose between each material variant. This was found to be instigated by microstructural variations exposed through microscopic and Energy Dispersive X-ray Spectroscopy (EDS) analysis. SPT results were also compared with available uniaxial tensile data in terms of SPT peak and yield load against uniaxial ultimate tensile and yield strength.

Effect of Build Orientation and Post Processing of a Direct Laser Deposited Nickel Superalloy as Determined by the Small Punch Creep Test

Direct Laser Deposition (DLD) is a modern Additive Layer Manufacturing (ALM) technology that offers the possibility of lean manufacture and the ability to produce near-net shape components with complex geometries. Anisotropic microstructures are typically produced due to thermal cycles that occur during the layer by layer process, resulting in epitaxial grains forming along the build direction. Therefore, build direction, whether horizontal (0°) or vertical (90°), may have a pronounced effect upon mechanical properties. While, it is generally accepted that the mechanical properties of cast materials are well understood, the same cannot be said for materials produced using DLD. Although, mechanical testing of materials usually dictates the use of round bar specimens, due to the cost of manufacture and fundamental nature of this study a miniaturised test technique better lends itself to characterise the cast and DLD built alloys’ properties. The Small Punch (SP) creep test is a widely utilised miniaturised test technique for characterising and ranking the creep response of metallic material properties when large quantities may not be readily available. This paper will apply the SP creep test to characterise the properties of DLD variants of the nickel based superalloy C263 in comparison to the traditional cast material. Tests were performed at elevated temperatures akin to those experienced in service. Interpretation of the microstructures and SP creep results has been carried out; relating build direction, microstructures, minimum displacement rate and time to rupture.

Assessment of Infrared Thermography for Cyclic High-Temperature Measurement and Control

Infrared thermography (IRT), a non-invasive temperature measurement technique has been investigated and developed for use with cyclic high temperature loading. The technique utilises an infrared thermography camera (IRTC) and RollsRoyce HE23 black thermal paint (TP). The TP is applied to a test piece surface to provide a stable emissivity value and accurate temperature measurement for the duration of thermal cycling. Spot welded type N thermocouples (NTCs) are utilised for accuracy validation of the IRTC technique for both temperature monitoring and temperature control. An evaluation of the technique has been employed upon diverse test specimen geometries and alloy compositions at temperatures between 100 and 700°C. Unfavourable effects during cyclic temperature measurement such as thermocouple shadowing are also highlighted and quantified. In combination with HE23 TP, IRTC control and measurement has proven accurate to within ±2°C NTCs, a validated cyclic high temperature measurement technique.

Modelling the small punch tensile behaviour of an aerospace alloy

The small punch (SP) test is a widely accepted methodology for obtaining mechanical property information from limited material quantities. Much research has presented the creep, tensile and fracture responses of numerous materials gathered from small-scale testing approaches. This is of particular interest for alloy down selection of next-generation materials and in situ mechanical assessments. However, to truly understand the evolution of deformation of the miniature disc specimen, an accurate and detailed understanding of the progressive damage is necessary. This paper will utilise the SP test to assess the tensile properties of several Ti–6Al–4V materials across different temperature regimes. Fractographic investigations will establish the contrasting damage mechanisms and finite element modelling through DEFORM software is employed to characterise specimen deformation. This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

Mechanical Property Characterisation of Electron Beam Melted (EBM) Ti-6Al-4V via Small Punch Tensile Testing

Small punch (SP) tensile testing provides several advantages over conventional test techniques for mechanical property characterisation of components produced using novel manufacturing processes. Additive layer manufacturing (ALM) is becoming more widespread, particularly in high value manufacturing sectors such as the gas turbine industry as it allows near net shape manufacture of near fully dense components with complex geometries. One such ALM process which is receiving attention from the gas turbine industry is electron beam melting (EBM), a powder bed process which uses an electron beam energy source. The additive nature of ALM processes including EBM results in the microstructures produced differing significantly to those produced by conventional processing techniques. As well as being influenced by the input parameters, the microstructure and hence mechanical properties are also affected by the geometry of the component being manufactured, primarily due to the effect this has on the cooling characteristics. SP testing of material manufactured by EBM allows the mechanical property characterisation of local component representative geometries which wouldn’t be possible using conventional uniaxial testing techniques. This work is aimed towards developing and validating the SP tensile technique for this application; different Ti-6Al-4V material variants manufactured using EBM as well as conventional methods have been characterised with a range of test conditions.

A Novel Approach to Small Punch Fatigue Testing

Miniaturised mechanical test approaches are now widely recognised as an established means of obtaining useful mechanical property information from limited material quantities. To date these methods have largely been adopted to characterise the creep, tensile and fracture characteristics of numerous material systems from a range of industrial applications. One method developed for miniaturised testing is the small punch test. Many international institutions and research faculties have now made a significant investment in realising the potential that small punch testing has to offer. However, limited success has been made in replicating a miniaturised test approach for determining the cyclic fatigue properties of a small punch disc due to the complex biaxial stress field that typically occurs in any small punch test. Therefore, to realise such an approach and to interpret the fatigue behaviour of small scale components, the mechanical test arrangement must clearly be of a highly bespoke nature. This paper will discuss the ongoing research and progress in developing a novel small punch fatigue testing facility at the Institute of Structural Materials in Swansea University. Several experiments have been performed on the titanium alloy Ti-6Al-4V at ambient room temperature and effort has been made to understand the complex damage mechanism.

Small Punch Testing of Powder Bed Direct Laser Deposits

Additive Layer Manufacturing (ALM) technologies, such as Powder Bed Direct Laser Deposition (PB-DLD), have gained increasing popularity within the aerospace industry due to the advantages they hold over conventional processing routes. Among the advantages are the ability to produce more sophisticated cross-sectional geometries, a decrease in production lead times and an improvement to the buy-to-fly ratio. However, build quality and microstructural characteristics have a dependency on the process variables such as build direction. In order to understand the influence of grain size and build orientation on tensile behaviour, the Small Punch Tensile (SPT) testing technique has been applied to variants of the nickel based superalloy C263, manufactured using the PB-DLD method. The test technique utilises miniaturised samples, requiring only small volumes of material and is therefore a desirable test method to employ. SPT testing has characterised the mechanical properties between vertically and horizontally built PB-DLD C263 in comparison with the cast material derivative. Differences in mechanical performance between each variant have been revealed and found to be associated with microstructural variations. The deformation behaviour across each material variant have been exposed by interrupted tests. SPT results have also been accompanied by fractography, fracture energy calculations along with comparisons with uniaxial data.