The residual stress exhibited in post-machined metallic components fabricated by directed energy deposition (DED) determines their final mechanical performance and reliability in mission-critical applications. This study develops a numerical model to predict the final surface residual stress after the orthogonal cutting of DEDproduced IN718, which integrates two critical factors: DED-induced initial residual stress states and microstructure properties. Using the developed modeling procedure, the penetration depth of post-machining into the initial residual stress distribution can be effectively quantified, which aligns with residual stress measurements through X-ray diffraction. The developed model is further employed to quantify the cumulative effects of initial residual stress states and grain size on cutting forces and final surface residual stress profiles. The results suggest that, under the given orthogonal cutting conditions of DED parts, variations in the initial residual stress states of the chip formation region have negligible effects on cutting forces. However, magnitudes of surface compressive residual stress in the longitudinal direction reduce by 21.8 %-52.3 % as the initial residual stress states shift from compressive-dominant to tensile-dominant, and decrease by 23.8 %-54.0 % as the built-in grain size (dg_x) increases from 10 mu m to 100 mu m. With a comprehensive understanding of post-machining DED processes using this numerical modeling procedure, post-treatment techniques can now be tailored to achieve surface residual stress profiles on DED-generated or other additively manufactured metallic components to meet various industrial requirements.

This study examines the dependence of fretting wear of a high strength steel using four non-conforming specimen configurations: cylinder-on-flat and crossed-cylinders, with both 6 mm and 160 mm radii cylinders. In each configuration, the wear of the two bodies is well described by their geometrical volume of intersection. A predictive model indicated significant differences in the evolution of the size and depth of the wear scar and contact pressure in the four cases considered; despite this, the size-dependent specific wear rates evaluated were similar to each other (between 24.5 and 43.5 mm4 MJ- 1). Furthermore, oxide debris formation was shown to depend on contact size rather than pressure; smaller contact sizes favoured protective debris-bed formation limiting subsurface plasticity in the specimens.

In this study, we developed a 2 mm thick deposit of CoCrFeNiMn high entropy alloy (HEA) from cold spray. After cold-spraying, annealing at 600, 800 and 1000 °C for 5 hrs was conducted to improve and consolidate the microstructure. The influence of the annealing treatment on the microstructure, hardness and tensile strength of the HEA deposit was studied. The results showed that annealing treatment increased the fraction of metallurgical bonded areas due to diffusion, which resulted in enhanced mechanical performances of the deposit. The examined fractured surfaces of the tensile test samples revealed that the annealing treatment changed the failure behavior of the as-sprayed deposit from mostly particle-particle interface failure to void coalescence (ductile failure). Interestingly, a distinct microstructure was observed for the deposited annealed at 600 °C; a partially recrystallized microstructure with a small volume fraction of Cr-rich phase formed along grain boundaries, whereas fully recrystallized microstructure at higher two temperatures. The strengthening effect of partial recrystallisation, with a small volume fraction of the Cr-rich phase led to a greater reduced modulus and tensile strength ( 196.7 GPa and 51.7 MPa) of the deposit annealed at 600 °C when compared with that annealed at 800 °C ( 182.5 GPa and 43.6 MPa). It is believed that the small volume fraction of the Cr-rich phase partly constrained the deformation of the surrounding FCC HEA matrix during mechanical loading, leading to better mechanical properties as compared to the deposit annealed at 800 °C.

The application of cold spray (CS) for additive manufacturing (CSAM) of structural components using metallic materials has recently attracted much attention. However, there are limited reports on developing thick deposits or components with high entropy alloys (HEAs) via CSAM and investigating the microstructural evolution and mechanical properties after deposition and subsequent annealing heat-treatment. This work investigated the microstructure and mechanical properties of asdeposited and heat-treated thick CoCrFeNiMn HEA deposit fabricated via CSAM. The microstructure of the HEA deposit and after heat-treatment were characterised using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and x-ray diffraction (XRD). The microstructural analysis reveals heterogeneous grain size distribution with ultrafine grains at the particle-particle interfacial regions and coarse grains at the particle interiors in the as-deposited sample. The as-deposited sample, characterised by moderate porosity, was consolidated following the heat treatment at different temperatures. Additionally, increasing the temperature increases grain sizes resulting from static recovery and recrystallisation, with annealing twin formed at higher temperatures. Most notably, phase decomposition of the deposit microstructure occurs at 600 ºC, with Cr-rich phase particles formed at regions of high dislocations and grain boundaries. Nano-and micro-hardness and tensile testing of micro-flat dogbones samples were performed on the as-deposited and heattreated samples. The effect of heat-treatment on the microstructure and mechanical properties of the cold-sprayed HEA deposit were analysed and discussed.

In this work, the Low Cycle Fatigue (LCF) performance of a weight optimized SLM Ti-6Al-4V bracket in a 'struts & connectors' shape has been investigated through the displacement controlled LCF testing at 200°C. The bracket was designed to operate under Thermo-mechanical loading, i.e. an elevated temperature environment under mechanical loading, during engine normal operation. The bracket was demonstrated to have met the LCF target cycles for the defined displacement loading, which simulated the maximum possible loading under aero-engine flight operations. Subsequently the same bracket was also subjected to a higher level of cyclic displacement loading to allow the bracket to experience some level of plastic strains in the structure, to assess the LCF performance of the bracket under the one-off extreme event such as a Fan Blade Off event. The successful outcome of the elevated temperature LCF testing of the bracket is a significant forward step towards adopting SLM technology for safety critical load bearing applications.

This work aims to establish the mechanical relationship between additive manufactured Titanium alloy and geometrically complex load-bearing aeroengine lightweight structure under cyclic loading. To achieve this, the constitutive behavior of Laser Powder Bed Fusion (LPBF) Ti-6Al-4V was characterized through a computational cyclic plasticity model derived based on the uniaxial strain-range controlled low cycle fatigue (LCF) test. An idealised three-dimensional (3D) finite element (FE) model was developed and experimentally validated to investigate the remote response and localised stress-strain nature in the bracket elements for risk evaluation and structural optimization. The predicted results indicated that the dominant cracking mode of the bracket under LCF test, was more dependent on the localised maximum principal stress rather than von Mises equivalent stress. Furthermore, the first failed region exhibited an opening crack (Mode-I failure), while the second and third failed regions showed a mixture of opening and shear crack (Mode-I and Mode-II failure). The microscopic observation further revealed that the three failed regions exhibited a ductile type of fracture at the cracking initiation and final fracture regions, and a brittle fracture at the propagation region.

The rapid development of cold spraying technology for additive manufacturing of engineering components has made it a viable option for developing thick deposits from high-entropy alloys (HEAs). The microstructure of cold-sprayed CoCrFeNiMn deposit was investigated in this study using electron backscattered diffraction, scanning electron microscopy, and finite element analysis (FEA). The limited studies on the impact deformation behavior of the HEA during cold spraying, limiting our understanding of impact phenomena, and interactions between the HEA particles under ultra-high strain rate deformation motivated this study. From the microstructural characterization, heterogeneous microstructure appears to be formed in the cold-sprayed HEA deposit, comprising of equiaxed ultrafine grains at the particle–particle interfacial regions and coarse grains at the particle interiors. The FEA reveals large strain (> 250%) and temperature (> 90% of the alloy solidus temperature), mainly at the splat’s interfaces. Adiabatic shear instability and rotational dynamic recrystallization resulting from heat accumulation and high strain are believed to be responsible for these observations during the ultra-high strain rate deformation of the HEA. The large deformation and grain refinement experienced by the HEA resulted in greater deposit hardness when compared with the sprayed powder, with the nanohardness increasing from 1.16 GPa in the powder to 5.14 GPa in the deposit. This study explores and provides an understanding of the deformation behavior of the HEA and the resulting microstructure during cold spraying.

In this paper, the Low Cycle Fatigue (LCF) performance of a weight optimized SLM Ti-6Al-4V bracket has been investigated. The bracket, a ‘struts & connectors’ shape for an aero-engine application, was designed to operate within the material elastic limit. The tensile and LCF fatigue data obtained from the coupon tests are discussed first which was then used to establish the loading levels for the LCF tests on the bracket. Cyclic softening was observed in the LCF coupon tests at strain levels higher than the material elastic limit, which was attributed to the pile-up of dislocations and formation of sub-grains in the SLM Ti-6Al-4V material. The SLM Ti-6Al-4V bracket met the LCF target cycles when operated near the material elastic limit and also when cycled at displacement levels causing plastic strain in the bracket elements. The carefully designed, weight optimized bracket has shown promising results in terms of its LCF performance and this provides a good encouragement to practicing engineers to adopt SLM technology for load bearing applications.

Due to the stringent certification requirements and complex engine operating environment, the design of AM components for aero-engine application remains a challenging task. The high cycle fatigue performance of a weight-optimised bracket made using the Laser Powder Bed Fusion (LPBF) process was studied through shaker table testing. The results are discussed together with the observed mechanical and fatigue strengths in conjunction with material characterisation of microstructure, surface roughness, defects, micro-hardness and fracture surfaces. The debits in mechanical and fatigue strengths are expected due to process dependent surface roughness and the density of defects present. From the shaker table test results, it was concluded that the proposed weight-optimised LPBF bracket is capable of meeting the performance targets. The fundamental vibration mode of the bracket assembly was 84 Hz, which was much higher than the 1st Low Pressure shaft speed (48 Hz) of the target engine and thus avoids potential resonance. The bracket achieved its target inertial g load capability of 20 g and it was demonstrated that the bracket had enough redundancy in its load transfer paths should a strut fail during service of the engine. Lack of fusion voids and micro-cracks present on or near the surface were the prime sites for crack initiation. It has been shown that the as-built surface can cause a significant reduction (up to 40%) in the fatigue strength when compared to machined Ti-6Al-4V. A safe life regime for LPBF component design has been presented based on the Kitagawa-Takahasi diagram, modified using the Chapetti curve, which effectively links the material fatigue properties and performance of LPBF parts with intrinsic defects.

The Low Cycle Fatigue (LCF) performance of an aero-engine bracket in a 'struts and connectors' design, made using the Laser Powder Bed Fusion (LPBF) process has been studied using strain controlled LCF tests at different Strain Ranges (SR). From the cyclic test results, the bracket has demonstrated that it meets the target LCF performance which was in excess of the normal expected operating loads that the LPBF bracket would experience. The failures in the bracket at different locations were observed only under the extreme loading conditions. The failure of the bracket was of a progressive nature with failure of different struts/connectors happening at different discrete cycles. Even after the first five failures, the LPBF bracket had 47% of the original load carrying capacity in the tensile part of the hysteresis loop, indicating a good redundancy in its load transfer paths. In general, the surface locations of the bracket struts/connectors, where there are lack of fusion voids, were the prime sites for crack initiation. If any of these locations also coincide with high stresses, i.e. the combination of 'size of defect' and 'magnitude of stress', then this will contribute to an increased likelihood of failure at these locations in the LPBF bracket.

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

The widespread use and development of inertia friction welding is currently restricted by an incomplete understanding of the deformation mechanisms and microstructure evolution during the process. Understanding phase transformations and lattice strains during inertia friction welding is essential for the development of robust numerical models capable of determining optimized process parameters and reducing the requirement for costly experimental trials. A unique compact rig has been designed and used in-situ with a high-speed synchrotron X-ray diffraction instrument to investigate the microstructure evolution during inertia friction welding of a high-carbon steel (BS1407). At the contact interface, the transformation from ferrite to austenite was captured in great detail, allowing for analysis of the phase fractions during the process. Measurement of the thermal response of the weld reveals that the transformation to austenite occurs 230 °C below the equilibrium start temperature of 725 °C. It is concluded that the localization of large strains around the contact interface produced as the specimens deform assists this non-equilibrium phase transformation.

The fatigue response of a weight optimised Selective Laser Melted (SLM) Ti-6A1-4V bracket was studied through shaker table testing until fracture. The bracket assembly was tested at its fundamental vibration mode at 84Hz, which was outside of the key excitation frequency (48Hz) of the intended aero-engine application. The study has shown that the SLM bracket has achieved its target inertial capability of '20g' and has been demonstrated to have redundancy in the load path transfer, should a strut fail during engine operation. An analysis of the initial fracture surface suggested that the crack initiation point was the surface breaking internal void acting as a stress raising feature and a point of micro -crack initiation. The results of the shaker table test suggested that the weight optimised SLM bracket has met its performance target.

Inertia friction welding (IFW) is a process used to create joints with high geometrical accuracy and near net shape form.To cope with the complex phenomena occurring during welding, the majority of available studies have analysed the interaction of the workpieces to be joined under simplified conditions, in which the influence of machine assembly tolerances, spindle dynamics and system compliance have been neglected.Among the dimensional properties, the headstocktailstock concentricity is particularly important to assess the conformity of the weld, for this reason, a novel approach was developed to investigate the physical causes behind the evolution of the radial misalignment between the two workpieces, conventionally referred to as radial runout.First an inverse approach to evaluate the equivalent pressure distribution at the weld interface and the equivalent process loads was implemented starting from the experimental data of radial runout, headstock angular speed and strain extracted with a custom monitoring system during a set of steel welds.The results showed a large variability of the pressure distribution in circumferential direction and non-axisymmetric load components in particular during the conditioning and burnoff phases.Then, the equivalent process loads were used as an input for a Timoshenko beam dynamic representation of the spindle.A good agreement between the model and the experimental data was observed with an average relative error in the radial runout of 0.085.From these results, it was possible to conclude that the lack of axisymmetry in the load components has to be attributed mainly to the misalignment between two workpieces, while the irregular runout to compliance of the system to the non-ideal process loads.

Inertia Friction Welding (IFW) is commonly approached by considering ideal conditions in which the two workpieces are in perfect contact, subjected to the nominal loads and the machine reacts ideally to the process loads. These conditions, however, are not represen-tative of a real weld, where fixturing issues, non-ideal interaction between the workpieces and compliance of the system under the process loads could significantly affect the quality and the repeatability of the weld. To fill this research gap, a novel monitoring system able to collect in-process data and a methodology for their analysis was developed. A set of run -down tests and steel w elds were performed on an industrial inertia welder to validate the rig. Then, the data extracted were used to study the interaction conditions between the spindle and fixture side of the machine and build dynamics models to understand the physical implication of specific events connected to the impact at part contact and the fly -wheel deceleration. The results showed a significant influence of the machine in the align-ment of the workpieces, with the runout between spindle and fixture that became larger and irregular during welding when the workpieces interact in non-ideal conditions. The quantitative comparison between the runout magnitude of rundown tests and welds showed an increase of more than four times that can be justified with the compliance of the machine, in which the spindle bearings representing the weakest element. The compar-ison of the outputs of the different sensors installed allowed to obtain, for the first time, a holistic view of the macroscopic phenomena occurring during the welding phase and observe how these could affect the final weld geometry and the dynamically evolving thermo-mechanical conditions of the weld. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

During cold-spraying processes, the deposited particles play a crucial role owing to the subsequent particle collisions with surface. Herein, using finite-element modeling, we numerically analyze the particle behavior under three models: impact of a single Ti6Al4V (TC4) particle, deposited particle hammered by a subsequent TC4 particle, and deposited particle impacted by a large shot peening particle (SP). For the single TC4 particle case, the particle deformation was limited and maximum interface temperature of the particle was lower than its melting point. The high-temperature region was mainly distributed in a limited area near the particle edge. Meanwhile, for subsequent impact in the second TC4 particle case, the upper half of previously deposited particle was deformed, although the change in maximum interface temperature was minimal. However, for particles subsequently impacted by large peening particle, the deformation of previously deposited TC4 particles increased significantly, and the temperature, both adjacent to and opposite the interface, exceeded the melting point of the titanium alloy in a large surface area, indicating that localized interfacial melting has occurred. In the third case, the change in interface temperature, stress, and energy with time, along with the experimental results, suggests that the bond between the particle and substrate enhanced.

The fatigue response of a weight optimised Selective Laser Melted (SLM) Ti-6Al-4V bracket was studied through shaker table testing until fracture. The bracket assembly was tested at its fundamental vibration mode at 84Hz, which was outside of the key excitation frequency (48Hz) of the intended aero-engine application. The study has shown that the SLM bracket has achieved its target inertial capability of ‘20g’ and has been demonstrated to have redundancy in the load path transfer, should a strut fail during engine operation. An analysis of the initial fracture surface suggested that the crack initiation point was the surface breaking internal void acting as a stress raising feature and a point of micro-crack initiation. The results of the shaker table test suggested that the weight optimised SLM bracket has met its performance target.

The use of the Electron Beam Melting (EBM) Additive Manufacturing (AM) process to fabricate parts for applications which require strength and reliability is limited. EBM parts suffer from manufacturing defects and poor surface finish, and its strength properties can be dependent on strain rates and temperature. In this paper, the effects of parameters such as the strain rate, temperature, surface finish and build orientation together with the role of defects on tensile properties of EBM Ti-6Al-4V alloy have been studied and discussed. Due to reduced material density and multiplication rate of dislocations, a significant decrease in material tensile strength but an increase in material ductility was observed for tests on machined specimens at higher temperatures. Surface nucleating micro-cracks (poor surface finish) and inherent internal defects caused a reduction in tensile strength and material ductility for the as-built specimens. Coalescence of small size spherical pores and the presence of large voids have a direct deleterious effect on the ductility of Ti-6Al-4V alloy. The orientations of voids, difference in thermal history resulting into different microstructures, presence of grain boundary alpha and alignment of prior columnar beta grains with respect to the loading direction for vertical and horizontal built specimens are the reasons for observed anisotropy in material strength of EBM Ti-6Al-4V alloy. The interaction effects of different parameters are also discussed, and it is suggested that these parameters should be optimised, in addition to the process parameters, to make AM parts using the EBM process which can reliably and safely be used for load bearing applications.

The increasing demand for more efficient and environmental-friendly gas turbines has driven the development of new strategies for material development. SiC/SiC ceramic matrix composites (CMCs) can fulfil the stringent requirements; however, they require protection from the operating environment and debris ingested during operation. Environmental barrier coatings (EBCs) are a protective measure to enable the CMCs to operate under harsh conditions. EBC-coated CMCs will enable an increased efficiency and reduced pollutant and CO2 emissions. In this review, the fundamentals of SiC/SiC ceramic matrix composites degradation in steam environments and under the presence of corrosive species, namely CaO-MgO-Al2O3-SiO2 (CMAS), are first presented. Then, a summary of EBCs along with a comprehensive summary of the current compositions and their interactions with steam and molten corrosive species is presented. Finally, an overview of the latest research directions for the potential next generation of EBCs are outlined.