Showing papers by "Rolls-Royce Holdings published in 2020"

Salt Template Assisted BN Scaffold Fabrication toward Highly Thermally Conductive Epoxy Composites.

Abstract: With the trend of device miniaturization and higher integration, polymer composites with high thermal conductivity are highly desirable for efficient removal of accumulated heat to maintain high performance of electronics. In this work, epoxy composites embedded with three-dimensional hexagonal boron nitride (BN) scaffold were fabricated. The BN-poly(vinylidene difluoride) (PVDF) scaffold was prepared by the salt template method using PVDF as the adhesive, while the corresponding epoxy composite was manufactured with vacuum-assisted impregnation. The epoxy/BN-PVDF composite exhibits high thermal conductivity with low loading of BN. The thermal conductivity of epoxy/BN-PVDF composite achieved 1.227 W/(m K) with 21 wt % BN, contributed by the constructed BN pathway held together by PVDF adhesive. In addition, PVDF could be further converted into carbon by thermal treatment, further enhancing the thermal conductivity of epoxy/BN-C composites through alleviating the phonon scattering at the interfaces, eventually obtaining thermal conductivity of 1.466 W/(m K). This type of epoxy-based composite with high thermal conductivity is promising to be used as thermal management materials in advanced electronic devices.

A state of the art review of hydroforming technology: its applications, research areas, history and future in manufacturing

Abstract: Hydroforming is a relatively new metal forming process with many advantages over traditional cold forming processes including the ability to create more complicated components with fewer operations. For certain geometries, hydroforming technology permits the creation of parts that are lighter weight, have stiffer properties, are cheaper to produce and can be manufactured from fewer blanks which produces less material waste. This paper provides a detailed survey of the hydroforming literature of both established and emerging processes in a single taxonomy. Recently reported innovations in hydroforming processes (which are incorporated in the taxonomy) are also detailed and classified in terms of “technology readiness level”. The paper concludes with a discussion on the future of hydroforming including the current state of the art techniques, the research directions, and the process advantages to make predictions about emerging hydroforming technologies.

Solving Recent Challenges for Wrought Ni-Base Superalloys

Abstract: This paper reviews the status of technology in design and manufacture of new wrought polycrystalline Ni-base superalloys for critical engineering applications. There is a strong motivation to develop new alloys that are capable of operating at higher temperatures to realize improvements in thermal efficiency, which are necessary to achieve environmental targets for reduced emissions of harmful green-house gases. From the aerospace sector, the development of new powder metallurgy and ingot metallurgy alloys is discussed for disk rotor and static applications. New compositions for powder metallurgy contain about 50 to 55 pct of gamma prime (γ′) strengthening precipitates to ensure components operate successfully at temperatures up to 788 °C (1450 °F). In contrast, new compositions for ingot metallurgy aim to occupy a design space in temperature capability between Alloy 718 and current powder alloys that are in-service, and show levels of γ′ of about 30 to 44 pct. The focus in developing these alloys was design for manufacturability. To complement the aerospace developments, a review of work to understand the suitability of candidate alloys for multiple applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by Detrois, Jablonski, and Hawk from the National Energy Technology Laboratory. In these power plants, steam temperatures are required to reach 700 °C to 760 °C. The common thread is to develop alloys that demonstrate a combination of high-temperature properties, which are reliant on both the alloy composition and microstructure and can be produced readily at the right price. For the AUSC applications, the emphasis is on high-temperature strength, long-term creep life, phase stability, oxidation resistance, and robust welding for fabrications. Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to time-dependent crack growth, phase stability, and resistance to environmental damage, while extending the current strength levels, which are shown by existing alloys, to higher temperatures.

Predicting dwell fatigue life in titanium alloys using modelling and experiment.

Abstract: Fatigue is a difficult multi-scale modelling problem nucleating from localised plasticity at the scale of dislocations and microstructure with significant engineering safety implications. Cold dwell fatigue is a phenomenon in titanium where stress holds at moderate temperatures lead to substantial reductions in cyclic life, and has been implicated in service failures. Using discrete dislocation plasticity modelling complemented by transmission electron microscopy, we successfully predict lifetimes for 'worst case' microstructures representative of jet engine spin tests. Fatigue loading above a threshold stress is found to produce slip in soft grains, leading to strong dislocation pile-ups at boundaries with hard grains. Pile-up stresses generated are high enough to nucleate hard grain basal dislocations, as observed experimentally. Reduction of applied cyclic load alongside a temperature excursion during the cycle lead to much lower densities of prism dislocations in soft grains and, sometimes, the elimination of basal dislocations in hard grains altogether.

A novel method to continuously map the surface integrity and cutting mechanism transition in various cutting conditions

Abstract: This research proposes an innovative method to study the machined surface integrity and the variation of cutting mechanisms during a continuous rapid transition between different cutting conditions with an example for a nickel-based superalloy as workpiece material. For the first time, a machined surface covering a wide range of cutting speeds in a single cutting test has been generated, able to capture a clear variation (from serrated to continuous) of chip morphologies and provide a way for in-depth understanding of cutting phenomena. Different material characterisation techniques, including Scanning Electron Microscope (SEM), Electron Back-Scattered Diffraction (EBSD), and X-ray diffraction (XRD), were used to quantitatively evaluate the plastic deformation of the machined surface captured at various cutting speed in a single test. The results show a high potential for the application of this method to continuously study the cutting mechanism transition between different cutting conditions and rapidly characterise the machining behaviour of advanced materials, from the point of views of surface integrity and portioning of cutting energy.

Influence of grit geometry and fibre orientation on the abrasive material removal mechanisms of SiC/SiC Ceramic Matrix Composites (CMCs)

Abstract: SiC/SiC Ceramic Matrix Composites (CMCs) have been identified as a key material system for improving aero engine performance as they offer low density, high strength and stiffness, and superior environmental resistance at high temperatures. Nevertheless, due to their heterogeneous, hard and brittle nature, these materials are considered among the most difficult-to-machine, and grinding arises as one of the preferred choices for their processing. Therefore, understanding of the material removal mechanism and influence of the abrasive grit geometry when grinding CMCs is a critical enabler for achieving high component quality at highest efficiency and minimum cost. With the aim to reduce the uncertainties associated with the stochastic nature of the abrasive particles, grits of different shapes and sizes have been accurately created by Pulse Laser Ablation (PLA). In order to reproduce the grinding process kinematics, scratch tests in a circular trajectory have been carried out with single abrasive grain with different geometries and sizes, and arrays of overlapped grains to determine the influence of shape, size and spacing on the surface integrity of SiC/SiC CMCs after grinding. The morphology of the various constituents of the workpiece has been assessed regarding the direction of the scratch with respect to the orientation of the fibres. Results reflect a higher influence on the process forces by the grain shape rather than fibre orientation. Moreover, after the inspection of the abraded individual CMC constituents, a change in the mechanisms governing the process for the different abrasive grain geometries have been identified, despite the brittle material removal mode displayed by all of them. Explanation of the ground surface morphology in an analytical and comprehensive manner through a contact mechanics approach shows that the crack onset location is governed by the grains shape but its direction of propagation depends on the fibre orientation.

Superconducting motors for aircraft propulsion: the Advanced Superconducting Motor Experimental Demonstrator project

Abstract: The European Union-funded Advanced Superconducting Motor Experimental Demonstrator (ASuMED) project started in May 2017 with the purpose of demonstrating the benefits of a new, fully superconducting motor for reaching the targets established by the Flightpath 2050 plan. The project aims at a motor power density of 20kW kg$^{-1}$ using a high-temperature superconducting (HTS) stator. The rotor will use HTS stacks operating like permanent magnets. A highly efficient cryostat for the motor combined with an integrated cryogenic cooling system and associated power converter will be used. This article provides a general overview of the prototype that is currently being assembled and that will be tested soon.

In situ radiographic and ex situ tomographic analysis of pore interactions during multilayer builds in laser powder bed fusion

Abstract: Porosity and high surface roughness can be detrimental to the mechanical performance of laser powder bed fusion (LPBF) additive manufactured components, potentially resulting in reduced component life. However, the link between powder layer thickness on pore formation and surface undulations in the LPBF parts remains unclear. In this paper, the influence of processing parameters on Ti-6Al-4 V additive manufactured thin-wall components are investigated for multilayer builds, using a custom-built process replicator and in situ high-speed synchrotron X-ray imaging. In addition to the formation of initial keyhole pores, the results reveal three pore phenomena in multilayer builds resulting from keyhole melting: (i) healing of the previous layers' pores via liquid filling during remelting; (ii) insufficient laser penetration depth to remelt and heal pores; and (iii) pores formed by keyholing which merge with existing pores, increasing the pore size. The results also show that the variation of powder layer thickness influences which pore formation mechanisms take place in multilayer builds. High-resolution microcomputed tomography images reveal that clusters of pores form at the ends of tracks, and variations in the layer thickness and melt flow cause irregular remelting and track height undulations. Extreme variations in height were found to lead to lack of fusion pores in the trough regions. It is hypothesised that the end of track pores were augmented by soluble gas which is partitioned into the melt pool and swept to track ends, supersaturating during end of track solidification and diffusing into pores increasing their size.

Microstructure and microhardness of dissimilar weldment of Ni-based superalloys IN718-IN713LC

Abstract: Ni-based superalloys IN718 and IN713LC have been joined through linear friction welding (LFW) in this study. The variation of microstructure across the weld line developed during linear friction welding and after post weld heat-treatment (PWHT) has been investigated. Their effects on microhardness have also been studied. A clean weld region which is free of micro-porosity, micro-cracking and oxides, was achieved. Dynamic recrystallisation (full and partial) occurred on both sides of the weld, which produced much finer grains in the recrystallised zone. Dissolution of ‘parent’ γʹ/γʹʹ, towards the weld line was observed on each side of the weld. However, reprecipitation of γ′ was only found in the as-welded IN713LC. All these were found to have a huge impact on the hardness profile. A softer heat affected zone (HAZ) was found in the IN718 side with the lowest hardness value achieved at an axial position 0.6 mm from the weld line. The increased dissolution of γ'/γ'' towards the weld line resulted in decreasing hardness towards the weld line. However, the formation of refined grains closer to the weld line increased the hardness towards the weld line from axial position 0.6 mm. In contrast, a harder HAZ was found in the IN713LC side, which resulted from the formation of finer reprecipitated γ′ and recrystallised grains. PWHT brought about reprecipitation and/or further reprecipitation of γʹ/γʹʹ in the IN718 and IN713LC HAZs, resulting in stronger HAZs.

Detection and Classification of Turn Fault and High Resistance Connection Fault in Permanent Magnet Machines Based on Zero Sequence Voltage

Abstract: Health monitoring and fault detection are becoming more and more important in electrical machine systems due to the increasing demand for reliability. Winding turn fault is a common fault in permanent magnet (PM) machines, which can cause severe damages and requires prompt detection and mitigation. High resistance connection fault, which results in phase asymmetry may also occur but does not require immediate shutdown. Thus, apart from the fault detection, the classification between the two faults is also required. In this paper, a new technique for detecting and classifying turn fault and HRC fault by utilizing both the high and low frequency components of the zero sequence voltage is proposed with the enhanced sensitivity. The dependence on the operating conditions is minimized with the proposed fault indicators. The effectiveness of fault detection and classification has been verified by extensive experimental tests on a triple redundant fault tolerant PM assisted synchronous reluctance machine. The robustness of the turn fault detection in transient states and under no load conditions has also been demonstrated.

Characterization of plastic deformation induced by machining in a Ni-based superalloy

Abstract: The surface integrity characteristics of Udimet 720Li subjected to slight damage and damage machining conditions have been studied using a complementary range of techniques such as FIB-SEM, EBSD, TKD, TEM-EDS and nano-indentation. The results indicate the existence of nano-sized grains and no observable tertiary γ′ in regions of severe plastic deformation in the machined surface. The correlation between the machining condition and the resulting plastic deformation is established. Grain refinement of this alloy via machining was achieved by dislocation slip. The nanohardness of the surface of damage machined sample is 40% higher than that of bulk material, which is attributed to the formation of nano-sized grains and high density of dislocations in the superficial layer.