G. Baxter

High temperature fatigue of friction welded joints in dissimilar nickel based superalloys

Abstract The high temperature fatigue performance of two dissimilar material joints made by inertia friction welding and linear friction welding is assessed in the present study. The fatigue strength of the welded joints is found to be comparable to that of the weaker parent material for the plainsided specimens. However, the fatigue crack growth resistance within the weld zone of both material systems is found to be reduced compared to that of parent material. This effect is more pronounced for high temperature Ni alloys than for the lower temperature nickel alloy. Environmental attack is confirmed to be the controlling failure mechanism for acceleration of crack growth, but the extent to which it is observed is sensitive to local microstructure, i.e. to grain size, redistribution of the main strengthening phase γ′, and to the grain boundary carbide morphology. Such local microstructural variations are the result of welding and subsequent post-weld heat treatment procedures.

Mechanical and microstructural assessments of RR1000 to IN718 inertia welds – effects of welding parameters

Abstract In the present study inertia friction welded RR1000 to IN718 joints have been investigated. Crack growth tests within 0·3 mm of the weld interface, conducted in air at 500 and 650°C, have shown that there is no difference in crack growth rate due to three different sets of welding parameters applied. The cracks were found to propagate from RR1000 through the weld line into IN718. In RR1000 the cracks pass a zone within 15–30 μm from the weld interface, allowing higher crack growth rates. Fractographic studies have shown that these higher crack growth rates are caused by a higher tendency to intergranular cracking, most likely due to oxidation damage along grain boundaries. The similar properties of the welds tested can be related to a similar weld process characteristic during the last second of the welding cycle for all three sets of welding parameters applied.

Effects of tooling on the residual stress distribution in an inertia weld

Abstract Neutron diffraction residual strain measurements have been made on a tubular structure formed by joining two nickel-based superalloy RR1000 parts by inertia welding. Residual strains in the radial, hoop and axial directions of the tube cross-section have been measured. The corresponding residual stress field has been calculated accounting for the stress-free lattice parameter variations in the region close to the weld line. Tensile residual stresses were observed near the inner diameter of the tube with magnitudes of the order of +500, +1100 and +1300 MPa in the radial, axial and hoop directions, respectively. By comparison near the outer diameter (OD) of the weld the corresponding stresses are of the order of −200, −1000 and 150 MPa. The final stress state reflects the influence of the gripping fixture tooling and thermal gradients during inertia welding. Additional X-ray (at the surface) and hole-drilling (at the near surface) measurements show a steep residual stress gradient in the near surface region. Tensile hoop and axial machining stresses at the surface indicate the potential for improving the inertia weld tooling and the machining parameters used when removing the flash.

Inertia friction welds between nickel superalloy components: Analysis of residual stress by eigenstrain distributions

Machining, surface treatment, plastic forming and stretching, welding, and other manufacturing processes introduce residual stresses and distortion into work pieces and engineering components. These phenomena exert a significant influence on the behaviour of components affecting the response to thermal/mechanical in-service loading, e.g. in terms of crack initiation and propagation under the conditions of creep and fatigue, thus ultimately affecting their durability. In the present study, the inertia friction welding process is considered that is used for butt joining of hollow cylindrical components, such as shafts and drums. An inverse eigenstrain framework is used for the interpretation of neutron diffraction measurements in terms of the underlying eigenstrain distributions. Eigenstrain distributions that describe the nature of permanent inelastic deformation are found by minimizing the sum-of-squares measure of the disagreement between model prediction and experimental measurements of residual elastic strains. Experimental data obtained from neutron diffraction measurements are used in an inverse solution scheme in order to determine the underlying eigenstrain (strains permanently ‘locked in’) that give rise to the residual stress state. Once these are found, approximate reconstruction of the complete stress tensor within the entire component becomes straightforward. Eigenstrain distributions are first obtained for reduced size test specimens which have been characterized in detail using neutron diffraction. Subsequently, the eigenstrain distributions are scaled and applied to more complex, full-size real engine components, with scaling factors adjusted to match surface hole drilling measurements.

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.

Residual Stress Prediction for the Inertia Welding Process

Predictions of residual stresses in inertia friction welded Ni-base superalloys have been carried out using a commercial finite element code (DEFORM) combined with an energy balance approach and a 2D axisymmetric formulation. A coupled thermal and mechanical finite element model has been created. During inertia welding trials of RR1000, a high γ ′ Ni-base superalloy, the rotational velocity and upset history curves were recorded. From these data the heat flux (the rate of energy input) through the weld interface was inferred and together with the upsetting rate input into the model as evolving boundary conditions. The coupled thermal and mechanical analysis leads to the prediction of the thermal history and the residual stresses generated during cooling. The validation of the model has been undertaken by comparing the predicted residual stresses with those measured using neutron diffraction. The comparison between predicted and measured stresses is in good agreement.

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.

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.