Imperial College London

DrPaulHooper

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 7128paul.hooper Website CV

 
 
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Location

 

456ACity and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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112 results found

Whiteman G, Lea LJ, Quinn RM, Cox MJ, Hooper PA, Williamson DMet al., 2023, High Strain-Rate Characterisation of Vanadium, Journal of Dynamic Behavior of Materials, ISSN: 2199-7446

The mechanical response of the body centred cubic metal vanadium has been studied across a wide range of strain-rates through uniaxial stress compression. Samples of polycrystalline material have been tested up to strain-rates of 105 s−1 using miniature split and direct impact Hopkinson bars. The strain-rate sensitivity of the yield strength is shown to be significant with an apparent increase around 102–103 s−1 and another increase ~ 105 s−1. This later increase is discussed as being consistent with uniaxial strain experiments and shown to compare favourably with a Preston-Tonks-Wallace model modified to fit data out to strain-rates of ~ 107 s−1. The benefits of the miniatured direct impact Hopkinson bar experiment are discussed along with the importance of velocimetry diagnostics and the requirement to understand and correct for friction at these high rates. Multiple repeat experiments and comparison of data taken with different loading platforms and different sample sizes are shown to produce a consistent and reliable data set which is complementary to existing sparse data on vanadium.

Journal article

Sandmann P, Keller S, Kashaev N, Ghouse S, Hooper PA, Klusemann B, Davies CMet al., 2022, Influence of laser shock peening on the residual stresses in additively manufactured 316L by Laser Powder Bed Fusion: A combined experimental-numerical study, Additive Manufacturing, Vol: 60, Pages: 1-14, ISSN: 2214-8604

Detrimental subsurface tensile residual stresses occur in laser powder bed fusion (LPBF) due to significant temperature gradients during the process. Besides heat treatments, laser shock peening (LSP) is a promising technology for tailoring residual stress profiles of additively manufactured components. A multi step process simulation is applied aiming at predicting the residual stress state after applying LSP to a cuboid shaped specimen manufactured by LPBF in two different building directions as well as comparing it with a post-build heat treatment. The validity of the numerical simulation is evaluated based on comparisons of residual stresses determined by incremental hole drilling technique within different stages of the multi step process: in the as-build condition, after subsequent heat treatment as well as after applying LSP to the as-build and heat treated specimens, showing overall a good experimental–numerical agreement throughout each of the process stages. Applying a heat treatment to the as-build LPBF sample at 700 °C for 6 h showed not to be effective in eliminating the surface tensile stress entirely, reducing the tensile residual stresses by 40%. However, the application of LSP on LPBF components showed promising results: LSP was able even to convert the detrimental near surface tensile residual stresses in the LPBF component into compressive residual stresses next to the surface, which is known to be beneficial for the fatigue performance.

Journal article

Simoes M, Harris JA, Ghouse S, Hooper PA, McShane GJet al., 2022, Process parameter sensitivity of the energy absorbing properties of additively manufactured metallic cellular materials, MATERIALS & DESIGN, Vol: 224, ISSN: 0264-1275

Journal article

Samieian MA, Cormie D, Smith D, Wholey W, Blackman B, Dear J, Hooper Pet al., 2022, A study on the bending of laminated glass under blast loading, Experimental Mechanics, Vol: 63, Pages: 385-400, ISSN: 0014-4851

Background:The bending behaviour of laminated glass plays an important role in determining its overall response to blast loading. It is costly and difficult to characterise the bending behaviour by carrying out full-scale blast tests, therefore an alternative method is required.Objective:The objective of this study is to understand the response of laminated glass under high-rate bending in the laboratory at rates representative of blast loading.Methods:In this paper a novel testing method is presented in which laminated glass strips of 700 mm long by 60 mm wide are tested up to speeds of 10 m/s in the laboratory. The laminated glass is accelerated to speeds comparable to blast loading and then brought to rest at its edges to mimic impulsive blast loading conditions. Different interlayer thickness, impact speeds, and boundary conditions were explored. Additionally, modelling methods were used to study the flexural rigidity of post-cracked laminated glass.Results:From the experiments it was found that the interlayer thickness plays a key role in determining whether the dominant failure mechanism is de-bonding of interlayer from the glass or interlayer tearing. In addition, it was found that by allowing the frame to bend under loading, the laminated glass can carry greater loads without failure. Finally, an iterative method was used to quantify the flexural rigidity of post-cracked laminated glass depending on the speed of travel. This is a novel finding as it is usually assumed that laminated glass behaves like a membrane in the post-cracked phase of the response.Conclusion:In modelling and design of laminated glass structures under blast loading, post-crack flexural rigidity must be taken into account. Additionally, having novel frame designs to add further load bearing capacity to the framing members, plays a key role in reducing the load intensity on the laminated glass structure.

Journal article

Gaikwad A, Williams RJ, de Winton H, Bevans BD, Smoqi Z, Rao P, Hooper PAet al., 2022, Multi phenomena melt pool sensor data fusion for enhanced process monitoring of laser powder bed fusion additive manufacturing, MATERIALS & DESIGN, Vol: 221, ISSN: 0264-1275

Journal article

Larsen S, Hooper PA, 2022, Deep semi-supervised learning of dynamics for anomaly detection in laser powder bed fusion, Journal of Intelligent Manufacturing, Vol: 33, Pages: 457-471, ISSN: 0956-5515

Highly complex data streams from in-situ additive manufacturing (AM) monitoring systems are becoming increasingly prevalent, yet finding physically actionable patterns remains a key challenge. Recent AM literature utilising machine learning methods tend to make predictions about flaws or porosity without considering the dynamical nature of the process. This leads to increases in false detections as useful information about the signal is lost. This study takes a different approach and investigates learning a physical model of the laser powder bed fusion process dynamics. In addition, deep representation learning enables this to be achieved directly from high speed videos. This representation is combined with a predictive state space model which is learned in a semi-supervised manner, requiring only the optimal laser parameter to be characterised. The model, referred to as FlawNet, was exploited to measure offsets between predicted and observed states resulting in a highly robust metric, known as the dynamic signature. This feature also correlated strongly with a global material quality metric, namely porosity. The model achieved state-of-the-art results with a receiver operating characteristic (ROC) area under curve (AUC) of 0.999 when differentiating between optimal and unstable laser parameters. Furthermore, there was a demonstrated potential to detect changes in ultra-dense, 0.1% porosity, materials with an ROC AUC of 0.944, suggesting an ability to detect anomalous events prior to the onset of significant material degradation. The method has merit for the purposes of detecting out of process distributions, while maintaining data efficiency. Subsequently, the generality of the methodology would suggest the solution is applicable to different laser processing systems and can potentially be adapted to a number of different sensing modalities.

Journal article

Remani A, Williams R, Rossi A, Peña F, Thompson A, Dardis J, Jones N, Hooper P, Senin N, Leach Ret al., 2022, MULTI-SENSOR MEASUREMENT FOR IN-SITU DEFECT IDENTIFICATION IN METAL LASER POWDER BED FUSION, Pages: 141-145

In metal laser powder bed fusion (L-PBF), the manufacturing process entails complex thermomechanical interactions that yield deviations from the nominal geometry. While these deviations are typically unwanted, not all are necessarily detrimental to the functionality of the part. In this paper, we describe a multisensing approach, where layer-by-layer inprocess and post-process measurements are collected as an initial step to reliably discriminate harmful defects from neutral anomalies in a manufactured part. Focus is particularly placed on in-situ measurements, with an account of initial results and the data fusion strategy.

Conference paper

Larsen S, Hooper PA, 2022, A DATA-DRIVEN DYNAMICS MODEL FOR DEFECT DETECTION IN LASER POWDER BED FUSION, Pages: 188-191

Quantifying melt pool stability is an essential part of in-situ monitoring. In this study, the melt pool dynamics are modelled from data with an autoregressive model. The residual error of predictions is then utilised as a process signature. The study shows that a simple model, which does not fully capture the variance of the data (R2=0.78), is still a highly effective method for detecting laser focal shift (AUROC=0.9998).

Conference paper

Hales T, Rønneberg T, Hooper PA, Davies CMet al., 2022, Ductile damage model development and validation of 316l laser powder bed fusion steel under multiaxial stress conditions, ISSN: 0277-027X

Laser powder bed fusion (LPBF) is an additive manufacture technique which builds components up in layers from a powder feedstock, using a scanning laser to selectively melt the powder into the required shape. The process of LPBF can often introduce defects into the structure of a part, since the powder may not fully melt and leave holes, or pores, in the sample. Excessive laser power may also cause the powder to vaporise and create pores. In whatever manner these pores are formed, they can significantly impact the properties of the finished component. Since pores and small defects already exist in LPBF components, the void growth and ductile fracture behaviour of LPBF components under multiaxial stress conditions needs to be characterised and predicted. In this work, notched bar tensile tests have been performed on samples with a range of notch acuities and hence multiaxial stress states. These tests have enabled ductile damage models to be calibrated and finite element (FE) simulations of the notched bar tests performed. The model was validated by comparison to the experimental results. The model agrees well with the results in many cases assessed in this work, but sometimes suffers from discrepancies and premature failure due to variability in material tensile properties, emphasising the need for sensitivity studies.

Conference paper

de Winton HC, Williams RJ, Hooper PA, 2022, Detection theory and the comparison and evaluation of in-situ monitoring systems in laser powder bed fusion, Pages: 202-203

In-situ monitoring offers an additive manufacturing specific opportunity to reduce the cost of measuring quality. The most direct path to industry adoption for in-situ monitoring is to demonstrate it can meet existing defect detection standards. In this work Non-Destructive Evaluation (NDE) metrics are used to compare three real co-axial meltpool monitoring systems in their ability to detect porosity. These systems are then evaluated against current industry performance requirements. The presented approach to system evaluation is designed to speed industry adoption of in-situ monitoring as a method of inspection for critical components.

Conference paper

de Winton H, Cegla F, Hooper P, 2021, A method for objectively evaluating the defect detection performance of in-situ monitoring systems, Additive Manufacturing, Vol: 48, Pages: 1-13, ISSN: 2214-8604

In-situ monitoring systems have the potential to assess material quality in additive manufacturing processes on-the-fly, paving the way for accelerated component qualification using defect digital twins. However, current systems vary widely in sensor technology and data analysis methods, leading to a lack of consensus in how the performance of these systems should be measured and compared. This work proposes a methodology and set of metrics, specifically Receiver Operating Characteristic (ROC) and Probability of Detection (POD) curves, to allow objective comparison of performance between any system, regardless of its underlying technology. We demonstrate this approach by comparing the ability to detect increases in part-wide porosity using two of the most common co-axial monitoring techniques in laser powder bed fusion; photodiodes and high-speed cameras. Using ROC curves, we show that melt pool metrics extracted from the camera offer a better trade off between detection and false alarms compared to the photodiodebased system when discriminating between samples at a 0.5% porosity threshold. POD curves were used to characterise detection capability across all porosity levels. It was found that the camera-based system can detect 43% of compromised parts (0.5% porosity), while the photodiode system detects 20%. However, for significantly compromised parts (5% porosity), the camera based method reaches 100%, while the photodiode only achieves 85%. The developed methodology shows that while the camera-based system is measurably superior, further improvement is needed before commercial implementation can be realised. Ultimately, the ROC-POD methodology allows objective assessments of detection performance, enabling quantifiable progress in the development of defect detection systems based on in-situ monitoring.

Journal article

Williams RJ, Davies CM, Hooper PA, 2021, In situ monitoring of the layer height in laser powder bed fusion, Material Design & Processing Communications, Vol: 3, Pages: 1-5, ISSN: 2577-6576

In situ process monitoring has frequently been cited as an critical requirement in certifying the performance of laser powder bed fusion (LPBF) components for use in high integrity applications. Despite much development in addressing this need, little attention has been been paid to monitoring the layer thickness during the process. In this paper, a laser displacement sensor has been integrated into the build chamber of an LPBF machine, and the height of the top surface layer of a component has been monitored during a build. This has permitted the deposited layer thickness to be measured throughout the build, and the effect on this of a change in processing conditions is characterised. The thermal contraction of the top layer in between successive laser scans has also been evaluated. This demonstrates the potential of utilising laser displacement sensory as a process monitoring tool in LPBF and provides insightful data for implementation in detailed process models.

Journal article

Lan B, Wang Y, Liu Y, Hooper P, Hopper C, Zhang G, Zhang X, Jiang Jet al., 2021, The influence of microstructural anisotropy on the hot deformation of wire arc additive manufactured (WAAM) Inconel 718, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 823, Pages: 1-14, ISSN: 0921-5093

Hybrid additive manufacturing, incorporating additive manufacturing (AM) and other thermo-mechanical processes, has been developed to improve AM mechanical properties by modifying the as-deposited microstructure and eliminating defects. Additive manufactured parts present strong anisotropic properties, as shown by the anisotropic columnar grain morphology and texture. Samples of AM Inconel 718 were tested at high temperature and under uniaxial compression over a range of conditions. The evolution of microstructural anisotropy and the viscoplastic behaviour under these hot deformation processes was studied. The microstructure and texture evolution were characterised with optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that the initial anisotropic microstructure had a negligible effect on flow stress and slip system activation during the hot deformation. The shape of original grains did, however, play a predominant role in determining the final microstructure. When the compression direction was perpendicular to the longitudinal of columnar grains, a more uniform microstructure was obtained under high-flow-stress conditions. This preferred compression direction provides guidance for hot deformation in hybrid additive manufacturing practice. Furthermore, for the nickel alloy studied, controlling the deformation direction to achieve a fine grain structure at a lower temperature (950 °C, lower than δ-solvus) brings practical benefits in the form of possible further δ grain refinement and less demanding thermal conditions during subsequent deformation processes.

Journal article

Yavari R, Williams R, Riensche A, Hooper PA, Cole KD, Jacquemetton L, Halliday HS, Rao PKet al., 2021, Thermal modeling in metal additive manufacturing using graph theory ? Application to laser powder bed fusion of a large volume impeller, ADDITIVE MANUFACTURING, Vol: 41, ISSN: 2214-8604

Journal article

Dong P, Vecchiato F, Yang Z, Hooper PA, Wenman MRet al., 2021, The effect of build direction and heat treatment on atmospheric stress corrosion cracking of laser powder bed fusion 316L austenitic stainless steel, ADDITIVE MANUFACTURING, Vol: 40, ISSN: 2214-8604

Journal article

Hopper C, Pruncu CI, Hooper PA, Tan Z, Yang S-T, Liu Y, Jiang Jet al., 2021, The effects of hot forging on the preform additive manufactured 316 stainless steel parts, Micron, Vol: 143, Pages: 103026-103026, ISSN: 0968-4328

Additive Manufacture (AM) offers great potential for creating metallic parts for high end products used in critical application i.e. aerospace and biomedical engineering. General acceptance of AM within these fields has been held back by a lack of confidence in the consistency of the mechanical properties of AMed parts associated by the occurrence of porosity, large columnar grains and texture. In this research, to counters this problem we have combined hot forging and subsequent heat treatment. Although, perhaps not best suited to components featuring fine detail, this technique should be well suited to the manufacture of forged components such as fan blades. Here, AM is able to create a near net-shape blank which is then hot forged to size, eliminating intermediate production stages and generating good mechanical properties in the final component. The material used in the current study is AM 316 L Stainless Steel. By altering the printing parameters of the AM machine, two batches of samples were built, each displaying a different porosity content. This allowed the influence of initial build quality to be illustrated. By comparing the two sample batches, it was possible to gain an insight into the possibilities of controlling porosity and material microstructure. The success of the proposed hot forging and heat treatment technique was validated by mechanical testing (i.e. tensile and hardness experiments) and microstructure evolution characterization (i.e. optical microscopy observation and electron backscatter diffraction (EBSD) techniques). The results revealed that the post processing strategy reduced material porosity and enabled the creation of a more robust microstructure, resulting in improved mechanical properties of the AM material.

Journal article

Williams RJ, Al-Lami J, Hooper PA, Pham M-S, Davies CMet al., 2021, Creep deformation and failure properties of 316 L stainless steel manufactured by laser powder bed fusion under multiaxial loading conditions, Additive Manufacturing, Vol: 37, Pages: 1-11, ISSN: 2214-8604

316 L stainless steel has long been used in high temperature applications. As a well-established laser powder bed fusion (LPBF) alloy, there are opportunities to utilise additive manufacturing in such applications. However, the creep behaviour of LPBF 316 L under multiaxial stress conditions must first be quantified before such opportunities are realised. Uniaxial and double notched bar creep tests have been performed and characterised using power-law relations to evaluate the creep strain and rupture properties of LPBF 316 L. The creep response was found to be anisotropic with specimen build orientation, with samples loaded perpendicular to the build direction (Horizontal) exhibiting 8 times faster minimum creep rates than samples built parallel to the build direction (Vertical) and significantly shorter rupture lives. This was mainly attributed to the columnar grain structure, which was aligned with the build direction of the LPBF samples. The multiaxial creep rupture controlling stress was determined and found to be a combination of the equivalent and max. principal stress. X-Ray CT measurements in selected samples illustrated that the samples were approximately 99.6% dense post-build and the quantity of damage post testing was determined. Optical and EBSD microstructural characterisation revealed intergranular creep damage present in the specimens, however rupture was ultimately trans-granular in nature and influenced by the presence and orientation of pre-existing processing defects relative to the sample build and loading direction.

Journal article

Yavari MR, Williams RJ, Cole KD, Hooper PA, Rao Pet al., 2020, Thermal Modeling in Metal Additive Manufacturing Using Graph Theory: Experimental Validation With Laser Powder Bed Fusion Using In Situ Infrared Thermography Data, JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 142, ISSN: 1087-1357

Journal article

Vecchiato FL, de Winton H, Hooper PA, Wenman MRet al., 2020, Melt pool microstructure and morphology from single exposures in laser powder bed fusion of 316L stainless steel, Additive Manufacturing, Vol: 36, Pages: 101401-101401, ISSN: 2214-8604

Journal article

Jin M, Piglione A, Dovgyy B, Hosseini E, Hooper P, Holdsworth S, Pham MSet al., 2020, Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion, Additive Manufacturing, Vol: 36, Pages: 1-15, ISSN: 2214-8604

The CrMnFeCoNi high-entropy alloy is highly printable and holds great potential for structural applications. However, no significant discussions on cyclic plasticity and fatigue damage in previous studies. This study provides significant insights into the link between print processes, solidification microstructure, cyclic plasticity and fatigue damage evolution in the alloy fabricated by laser powder bed fusion. Thermodynamics-based predictions (validated by scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the core of the solidification cells, whilst Mn and Ni to the cell boundaries in all considered print parameters. Both dislocation slip and deformation twinning were found to be responsible for plastic deformation under monotonic loading. However, the former was found to be the single dominant mechanism for cyclic plasticity. The surface finish helped to substantially delay the crack initiation and cause lack-of-fusion porosity to be the main source of crack initiation. Most significantly, the scan strategies significantly affect grain arrangements and grain dimensions, leading to noticeable effects on fatigue crack propagation; in particular, the highest resistance crack propagation was seen in the meander scan strategy with 0° rotation thanks to the most columnar grains and the smallest spacing of grain boundaries along the crack propagation path.

Journal article

Croteau J-F, Kulyadi EP, Kale C, Siu D, Kang D, Fontenla ATP, Valdivieso EG-T, Bieler TR, Eisenlohr P, Solanki KN, Balint D, Hooper PA, Atieh S, Jacques N, Cantergiani Eet al., 2020, Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 797, ISSN: 0921-5093

Journal article

Dovgyy B, Piglione A, Hooper P, Pham M-Set al., 2020, Comprehensive assessment of the printability of CoNiCrFeMn in laser powder bed fusion, Materials and Design, Vol: 194, ISSN: 0264-1275

This study assesses the printability including the consolidation, solidification microstructure, and mechanical properties of the CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion. A range of print parameters was used for a comprehensive assessment of printability, providing a basis to establish the relationship between process, microstructure, and mechanical properties. The study demonstrates a high relative density of the alloy fabricated with energy density in the range 62.7-109.8 J/mm3. It is shown that the scan strategy plays an important role in consolidation. For the same energy density, the rotation of 67° between two consecutive layers tends to yield higher consolidation than other considered strategies. Moreover, the scan strategy is found to be most influential in microstructure development. The scan strategy rotation angle controls the extent to which epitaxial growth can occur, and hence the crystallographic texture and the grain morphology. Amongst four considered strategies, the 0°- and 90°-rotation meander led to the strongest preferred texture while the 67°-rotation resulted in weaker texture. The 67°-rotation strategies led to broadened grains with lower aspect ratios. The understanding of texture and grain size provides explanations to the observed mechanical properties (such as flow stress and plastic anisotropy) of the alloy.

Journal article

Pruncu CI, Hopper C, Hooper PA, Tan Z, Zhu H, Lin J, Jiang Jet al., 2020, Study of the effects of hot forging on the additively manufactured stainless steel preforms, Journal of Manufacturing Processes, Vol: 57, Pages: 668-676, ISSN: 1526-6125

The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.

Journal article

Williams RJ, Vecchiato F, Kelleher J, Wenman MR, Hooper PA, Davies CMet al., 2020, Effects of heat treatment on residual stresses in the laser powder bed fusion of 316L stainless steel: Finite element predictions and neutron diffraction measurements, Journal of Manufacturing Processes, Vol: 57, Pages: 641-653, ISSN: 1526-6125

Heat treatments are used in laser powder bed fusion (LPBF) to reduce residual stress and improve service life. In order to qualify components for service, the degree of stress relaxation under heat treatment must be known. In this work, the effect of heat treatment on residual stress (RS) in LPBF 316L stainless steel was studied. Finite element (FE) models were developed to predict the RS distribution in specimens in the as-built state and subjected to heat treatment. The models simulated the thermo-mechanical LPBF build process, sample removal from the build plate and creep stress relaxation effects from a 2 h heat treatment at 700 C. The predictions were validated by neutron diffraction measurements in as-built and heat treated samples, in both build orientations. Large tensile RS of around 450 MPa were predicted at the vertical sample's outer gauge surfaces, balanced by high compressive stresses of similar magnitude at the centre. The residual stresses in the horizontal sample were significantly lower, by around 40%. The influence of sample removal from the base plate on the RS distribution was found to be strongly dependent on the sample orientation and geometry. The heat treatment preserved the unique microstructure of the LPBF process and reduced the peak RS by around 10% in the vertical sample and 40% in the horizontal sample. The FE model predictions were found in good agreement with the experimental measurements, thus providing an effective tool for RS predictions in LPBF components and proving the effectiveness of the heat treatment on RS relaxation.

Journal article

Quinn R, Zhang LH, Cox MJ, Townsend D, Cartwright T, Aldrich-Smith G, Hooper P, Dear Jet al., 2020, Development and validation of a Hopkinson bar for hazardous materials, Experimental Mechanics, Vol: 60, Pages: 1275-1288, ISSN: 0014-4851

Background: There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure. Objectives: A Split-Hopkinson pressure bar (SHPB) testing technique is presented which has been specifically developed for the characterisation of hazardous materials such as radioactive metals. This new SHPB technique is validated and a comparison is made with results obtained at another laboratory. Methods: Compression SHPB tests are performed on identical copper specimens using the new SHPB procedures at Imperial College London and confirmatory measurements are performed using the well-established configuration at the University of Oxford. The experiments are performed at a temperature of 20 °C and 200 °C. Imperial heat the specimens externally before being inserted into the test position (ex-situ heating) and Oxford heat the specimens whilst in contact with the pressure bars (in-situ heating). For the ex-situ case, specimen temperature homogeneity is investigated both experimentally and by simulation. Results: Stress-strain curves were generally consistent at both laboratories but sometimes discrepancies fell outside of the inherent measurement uncertainty range of the equipment, with differences mainly attributed to friction, loading pulse shapes and pulse alignment techniques. Small metallic specimens are found to be thermally homogenous even during contact with the pressure bars. Conclusion: A newly developed Hopkinson bar forhazardous materials is shown to be effective for characterising metals under both ambient and elevated temperature conditions.

Journal article

Ibrahim Y, Davies C, Maharaj C, Li Z, Dear J, Hooper Pet al., 2020, Post-yield performance of additive manufactured cellular lattice structures, Progress in Additive Manufacturing, Vol: 5, Pages: 211-220, ISSN: 2363-9512

In energy absorption applications, post-yield behaviour is important. Lattice structures, having low relative densities, are an attractive way to obtain effective material properties that differ greatly from that of the parent material. These properties can be controlled through the manipulation of the cellular geometry, a concept that has been made significantly more attainable through the use of additive manufacturing (AM). Lattice structures of various geometries were designed, additively manufactured and tested to assess their structural integrity as well as to investigate the effect of varying the cell geometry on the overall performance of the structures. Uniaxial tensile and compressive tests were carried out on bulk material AM samples made of 316L, followed by tests on the lattice structures. Finite element (FE) analysis was also carried out and the results compared to the experimental data. The FE simulations were able to accurately predict the elastic response of both structures; however, the post-yield behaviour did not closely match the experimental data due to inadequate beam contact resolution in the FE model. The FE model yield stress was also overestimated in the regular lattice due to the presence of manufacturing defects found only in the manufactured test samples. The stochastic structure, both experimentally and in the FE model, displayed a transition in the elastic stiffness from a lower to a higher stiffness in the elastic region. This is due to changing load paths within the lattice from the beams in contact with the compression platens to the rest of the structure. This phenomenon did not occur within the regular structure.

Journal article

Ronneberg T, Davies C, Hooper P, 2020, Revealing relationships between porosity, microstructure and mechanical properties of laser powder bed fusion 316L stainless steel through heat treatment, Materials and Design, Vol: 189, ISSN: 0264-1275

The understanding of relationships between processing, microstructure and mechanical properties in laser powder bed fusion is currently incomplete. Microstructure-property relations in 316L stainless steel are revealed in this study using isothermal heat treatments as an investigative tool. As-built material was heat treated to selectively remove microstructural features such as melt pool boundaries, microsegregations and the as-built grain structure to evaluate their influence on yield and failure behaviour. Anisotropic yield behaviour was found to be caused by microstructural features alone and not influenced by porosity. However, ductility and failure were dominated by lack of fusion porosity. The alignment of pores between tracks along layer boundaries was found to cause anisotropic ductility. Three strengthening mechanisms in as-built material were identified as grain boundaries, chemical segregation and dislocation density. Heat treatments were categorised into three regimes: recovery, homogenisation and annealing. The findings of this study show that the shape, size, orientation and distribution of pores are crucial parameters for evaluating the structural integrity of parts produced by laser powder bed fusion.

Journal article

Hooper P, Li N, JIANG JUN, LIN J, BAI Qet al., 2020, Method of creating a component using additive manufacturing, US20200055121A1

Images (4) Classifications B22F3/24 After-treatment of workpieces or articlesView 19 more classificationsUS20200055121A1

Patent

Pham M-S, Dovgyy B, Hooper P, Christopher G, Alessandro Pet al., 2020, The role of side-branching in microstructure development in laser powder-bed fusion, Nature Communications, Vol: 11, ISSN: 2041-1723

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (i.e. 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67 rotation between layers.

Journal article

Yavari R, Williams R, Cole K, Hooper P, Rao Pet al., 2020, THERMAL MODELING IN METAL ADDITIVE MANUFACTURING USING GRAPH THEORY: EXPERIMENTAL VALIDATION WITH IN-SITU INFRARED THERMOGRAPHY DATA FROM LASER POWDER BED FUSION, 15th ASME International Manufacturing Science and Engineering Conference (MSEC), Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

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