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  • Journal article
    Zheng J-H, Dong Y, Zheng K, Dong H, Lin J, Jiang J, Dean TAet al., 2019,

    Experimental investigation of novel fast–ageing treatments for AA6082 in supersaturated solid solution state

    , Journal of Alloys and Compounds, Vol: 810, Pages: 151934-151934, ISSN: 0925-8388
  • Journal article
    Zhang K, Zheng J, Shao Z, Pruncu C, Turski M, Guerini C, Jiang Jet al.,

    Experimental investigation of the viscoplastic behaviours and microstructure evolutions of AZ31B and Elektron 717 Mg-alloys

    , Materials and Design, ISSN: 0264-1275

    An insight into the thermo-mechanical behaviours of AZ31B and Elektron 717 magnesium alloys under the hot stamping conditions was established. High-temperature tensile tests (i.e. 350–450 °C) at a strain rate of 0.1 to 5/s were conducted to examine the material viscoplastic behaviours. Additionally, microstructure characterizations were performed, using the electron backscatter diffraction (EBSD), on the deformed samples to capture the underlying deformation mechanisms. Dynamic recrystallization (DRX) and texture formation were observed during the deformation at high temperature in both alloys and are the primary factors that affect the viscoplastic behaviours. The yield stress of both alloys reduced with increasing temperatures and reducing strain rates. More importantly, the ductility of the samples increased with both the temperatures and the strain rates. The higher ductility at higher strain rates was primarily attributed to finer grains and the slightly weakened textures, enabling a more uniform deformation. A maximum ductility of ~2 was observed in AZ31B under 450 °C at 1/s while ~0.9 in Elektron 717 under the identical condition. The addition of rare earth elements in Elektron 717 may suppress the active DRX. The recrystallization type was identified as discontinuous DRX. The research findings deliver understandings on the viscoplastic behaviours and the deformation mechanisms of AZ31B and Elektron 717 under the hot stamping conditions and provide scientific guidance for feasibility study on applying hot stamping technique to Mg-alloy for forming complex geometry components.

  • Journal article
    Wu M, Murphy J, Jiang J, Wilshaw P, Wilkinson Aet al., 2019,

    Microstructural evolution of mechanically deformed polycrystalline silicon for kerfless photovoltaics

    , physica status solidi (a), Vol: 216, ISSN: 1862-6300

    Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.

  • Journal article
    Li Y, Shao Z, Rong Q, Shi Z, Balint D, Sun X, Meng L, Lin Jet al., 2019,

    Development of similarity-based scaling criteria for creep age forming of large/extra-large panels

    , The International Journal of Advanced Manufacturing Technology, Vol: 101, Pages: 1537-1551, ISSN: 0268-3768

    A scaling method is developed for the creep age forming (CAF) process to downscale manufacturing of large/extra-large panels to lab-scale experimental trials for industrial application. Similarity theory is applied to identify both the geometrical and physical (non-geometrical) similarities between large-size prototypes and scaled-down models in all process stages of CAF, including loading, stress-relaxation and unloading (springback). A constitutive model is incorporated into the theory in order to identify the similarity in the highly non-linear stress-relaxation behaviour for aluminium alloy plates during CAF, and to obtain the effective scaling criteria for the CAFed plates after springback. The method was demonstrated by scaling down CAF manufacturing of both singly curved and doubly curved large plates under both proportional and non-proportional geometrical scaling conditions. The analytical results of the scaling method and numerical results obtained by CAF FE modelling were found to be in good agreement. Scaling diagrams linking the key deformation (springback) and structural (flexural rigidity) variables to scaling ratios under both proportional and non-proportional conditions were generated, and the developed scaling diagrams have been validated by corresponding CAF experiments. The scaling method developed in this study provides guidance on the design of scaled-down CAF experimental trials and will be used in the practical CAF process of large/extra-large panels.

  • Journal article
    Liang XZ, Dodge MF, Jiang J, Dong HBet al., 2019,

    Using transmission Kikuchi diffraction in a scanning electron microscope to quantify geometrically necessary dislocation density at the nanoscale

    , ULTRAMICROSCOPY, Vol: 197, Pages: 39-45, ISSN: 0304-3991
  • Conference paper
    Zhang R, Shao Z, Jianguo L,

    Applications of the digital image correlation (DIC) technique for high-temperature strain measurement: a review

    , 4th International Conference on Advanced High Strength Steel and Press Hardening (ICHSU2018)
  • Journal article
    Shao Z, Jiang J, Lin J, 2018,

    Feasibility study on direct flame impingement heating applied for the solution heat treatment, forming and cold die quenching technique

    , Journal of Manufacturing Processes, Vol: 36, Pages: 398-404, ISSN: 1526-6125

    The solution heat treatment, forming and cold die quenching (HFQ) process has been developed and adopted for forming high strength complex-shaped components of light alloys in the automotive industry. In order to exploit and increase the competitiveness of this technology, production cycle time and manufacturing costs need to be reduced to enable high productivity and energy efficiency. This can be realised by reducing the cycle time for heating a metallic sheet to its solution heat treatment temperature during the HFQ process, and by decreasing post ageing time. Rapid heating methods are capable of providing a solution to be integrated into this novel forming technique of HFQ. This paper presents feasibility study on the adoption of the direct flame impingement (DFI) heating method that has a high potential for non-ferrous blanks to achieve higher heating rate in HFQ processes, compared to convection heating in a conventional furnace. The adaptability of DFI heating for HFQ process has been validated, in terms of capability of high heating rate, quality of surface layer examination and lap-shear strength measurement of bonded samples.

  • Journal article
    Zhao C, Stewart D, Jiang J, Dunne FPEet al., 2018,

    A comparative assessment of iron and cobalt-based hard-facing alloy deformation using HR-EBSD and HR-DIC

    , Acta Materialia, Vol: 159, Pages: 173-186, ISSN: 1359-6454

    Three iron-based alloys (Nitronic 60, Tristelle 5183 and RR2450) and a cobalt alloy (Stellite 6) are studied using bend-testing to induce progressive straining and both high resolution DIC and EBSD are utilized to provide quantitative characterization of the deformation mechanisms. The roles of austenite, ferrite and carbide/silicide phases are investigated, together with how each contributes to slip activation and localisation, GND development and hardening through to particle pull-out and fracture. The observed mechanisms are discussed in the context of galling performance.The results suggest that a distribution of fine precipitates, both intra-granular and at grain/phase boundaries, promote more homogeneous and distributed slip, and the development of distributed higher densities of GNDs. The latter promotes hardening which in turn also facilitates homogeneity of deformation and potentially better galling resistance. A uniform size of fine precipitates is also helpful; large silicides lead to particle fracture and pull-out, likely highly damaging under conditions of sliding contact and galling.

  • Conference paper
    Jiang J, qinmeng L, 2018,

    Static recrystallization study on pure aluminium using crystal plasticity finite element and phase-field modelling

    , Metal Forming 2018, Publisher: Elsevier, Pages: 1800-1807, ISSN: 2351-9789

    In-depth understanding of the recrystallization process in alloys is critical for generating desirable small grains and weak textured microstructure, which provides high strength and toughness for metal formed parts. The manufacturing industry has a high demand for a valid computational model to accurately predict the level of recrystallization and recrystallized grain size under different strain paths and temperatures. However, current understanding and numerical calculation have not been linked properly for a reliable, physically based model to simulate the deformation and annealing process. Our phase-field model coupled with crystal plasticity simulations, which is based on the theory of stored energy minimization, enables a reliable prediction on the microstructure evolution under different processing routes. We hope that this modelling work provides a solution for the prediction of some long standing microstructure formation problems.

  • Journal article
    Lane C, Shao Z, Zheng K, Lin Jet al., 2018,

    Effect of thickness reduction on the limit strain measurement for thermomechanical tensile tests

    , Manufacturing Review, Vol: 5, Pages: 1-10, ISSN: 2265-4224

    Sheet metal formability under hot stamping conditions can be evaluated by using a novel planar testing system in the Gleeble. However, the specimen designs with the central recess have not been standardised, and the thickness reduction was not applied to the dog-bone type of specimen for testing at the uniaxial straining state. In this paper, effect of thickness reduction of dog-bone specimens on limit strain measurement under hot stamping conditions is investigated, and two types of dog-bone specimens without and with central recess are presented. Thermomechanical uniaxial tensile tests were performed at various deformation temperatures and strain rates, ranging from 370 – 510°C and 0.01 – 1/s, respectively, by using the developed biaxial testing system in the Gleeble. The distributions of temperature and axial strain along gauge region of the two types of specimen were measured and compared. The specimen with consistent thickness had a better uniformity of temperature and strain distributions compared to that with thickenss reduction. Forming limits for both types of specimen were also determined using the section-based international standard method. It is found that the calculation of forming limits by using specimen with thickness reduction was highly dependent on the selection of the deformation stage.

  • Journal article
    Zhang R, Shao Z, Lin J, 2018,

    A review on modelling techniques for formability prediction of sheet metal forming

    , International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 115-125, ISSN: 2588-8404

    With an increasing demand for lightweight design of vehicles in automotive and aircraft industries, sheet metals with low density and high strength have been widely and intensively used in forming lightweight structural panel components. Formability is a critical material property in describing deformation ability of sheet metals, and it is usually evaluated by a forming limit diagram (FLD) determined at various forming conditions. FLDs for metallic material are usually obtained experimentally, which is time-consuming and costly. Numbers of theoretical and numerical models have been developed and used to predict the formability of sheet metals. These modelling techniques are primarily developed based on bifurcation theory, geometrical imperfection theory and continuum damage mechanics. This paper covers a comprehensive review of modelling methods used for the formability prediction of lightweight materials for sheet metal forming applications.

  • Journal article
    Shao Z, Lin J, Ganapathy M, Dean Tet al., 2018,

    Experimental and modelling techniques for hot stamping applications

    , Procedia Manufacturing, Vol: 15, Pages: 6-13, ISSN: 2351-9789

    Hot stamping techniques have been developed for the production of complex-shaped components since the 1970s, increasingly used for the automotive industry. The application of these techniques includes hot stamping of boron steel for critical automobile safety components, and solution heat treatment, forming and cold die quenching (HFQ®) for forming complex-shaped high strength aluminium panels of automobile bodies and chassis structures. The developed forming techniques need dedicated experimental testing methods to be improved for characterising the thermomechanical behaviour of materials at the hot stamping conditions, and advanced materials modelling techniques to be developed for hot stamping applications. In this paper, requirements for thermomechanical tests and difficulties for hot stamping applications are introduced and analysed. The viscoplastic modelling techniques have been developed for hot stamping applications. Improved experimental methods have been proposed and used in order to obtain accurate thermomechanical uniaxial tensile test data and determine forming limits of metallic materials under hot stamping conditions.

  • Journal article
    Ahn J, He E, Chen L, Dear JP, Shao Z, Davies Cet al., 2018,

    In-situ micro-tensile testing of AA2024-T3 fibre laser welds with digital image correlation as a function of welding speed

    , International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 179-188, ISSN: 2588-8404

    In this paper, the influence of welding speed on tensile properties of AA2024-T3 fibre laser welds was investigated by monitoring the deformation behaviour during tensile loading. In-situ micro-tensile testing combined with a high-resolution optical microscope and DIC was used to measure strain distribution in narrow weld regions showing characteristics of fibre laser beam welding with limited metallurgical modifications. A chemical etching technique was used to generate a micro-scale random speckle pattern by revealing the weld microstructure. Such pattern enabled a sufficient spatial resolution of strain while keeping the weld seam visible during deformation. The results of microstructural and mechanical properties determined under numerous welding speeds indicated that increasing the welding speed led to the transition of weld pool shape from circular to elliptical to teardrop with a greater fraction of equiaxed dendrites. The weaker strength of the weld, as measured by local lower micro-hardness values, constrained significant plasticity development locally within the weld. Tensile tests revealed that increasing the welding speed resulted in greater yield strength and ultimate tensile strength, whereas, total elongation to failure dropped. The tensile properties improved with increasing welding speed as the fraction of equiaxed dendrites increased.

  • Journal article
    Zhao L, Zhang X, Deng T, Jiang Jet al., 2018,

    Develop an effective oxygen removal method for copper powder

    , Advanced Powder Technology, Vol: 29, Pages: 1904-1912, ISSN: 0921-8831

    At present, one of crucial limitations for the hot isostatically pressed (HIPed) Cu-3Ag-0.5Zr alloy, which is used on the combustion chamber liner of aerospace engine, is the high oxygen content, which easily results in the intergranular fracture under high temperature, pressure, liquid hydrogen and oxygen environment during operation. In this study, a novel effective oxygen control method is developed, for which vacuum degassing process is integrated with a flowing hydrogen reduction reaction at an elevated temperature before HIP. For this technique, a container is designed with two gas pipes for hydrogen inflow and outflow, so the hydrogen circulation can be established. Allowing hydrogen to react effectively with oxygen, the oxygen content of HIPed alloy is found to drop significantly from 140 ppm (raw powder) to 28 ppm, which is equivalent to the oxygen-free copper and copper alloys. As a result of the reduction, no prior particle boundaries could be observed in the low oxygen content material. Although the tensile strength of the materials with and without employing this technique does not vary significantly, the ductility of low oxygen content material has improved by about 70% at 500°C. This significant improvement of ductility is critical to ensure the safety critical PM components.

  • Journal article
    Shao Z, Li N, Lin J, Dean TAet al., 2018,

    Strain measurement and error analysis in thermo-mechanical tensile tests of sheet metals for hot stamping applications

    , Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, Vol: 232, Pages: 1944-2008, ISSN: 0954-4062

    In order to conduct uniaxial tensile tests for hot stamping applications, tests are normally performed by using a Gleeble thermo-mechanical materials simulator so that rapid heating and cooling processes can be obtained. However, temperature gradients in a specimen tested on Gleeble are inevitable due to resistance heating principles and heat loss to grips and water-cooled jaws. In this research, a pair of purpose-built grips made of stainless steel with low thermal conductivity and significantly reduced contacting area for clamping, as well as a flat dog-bone specimen with maximised parallel length (80 mm) were designed, for the purpose of improving the temperature uniformity within the concerned gauge section area of the specimen. Uniaxial tensile tests on AA6082 were performed, after controlled heating and cooling processes, at constant deformation temperatures in the range of 400 ℃–500 ℃ and at constant strain rate in the range of 0.1–4/s, to simulate its hot stamping conditions. The digital image correlation system was adopted to enable strain distributions in specimens to be measured. The temperature distributions in specimens were investigated and an effective gauge length of 14 mm was specified accordingly to ensure temperature gradients less than 10 ℃ within it at all tested temperatures. True stress–true strain curves of AA6082 were obtained based on results of strain measurements along the defined effective gauge length and used to calibrate a set of advanced material model. Error analysis was carried out by using thermo-electrical and thermo-mechanical FE models on ABAQUS, in which the calibrated material constitutive equations were implemented via subroutines. The error of stress–strain curves of AA6082 measured based on the specified gauge length was investigated and quantified by analysing the distribution of axial strain and axial stress.

  • Journal article
    Galindo-Nava EI, Jing YJ, Jiang J, 2018,

    Predicting the hardness and solute distribution during brazing of Ti-6Al-4V with TiZrCuNi filler metals

    , MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 712, Pages: 122-126, ISSN: 0921-5093
  • Journal article
    Jing Y, Gao X, Su D, Zhao C, Jiang Jet al., 2018,

    The effects of Zr level in Ti-Zr-Cu-Ni brazing fillers for brazing Ti-6Al-4V

    , JOURNAL OF MANUFACTURING PROCESSES, Vol: 31, Pages: 124-130, ISSN: 1526-6125
  • Journal article
    Chen B, Jiang J, Dunne FPE, 2017,

    Is stored energy density the primary meso-scale mechanistic driver for fatigue crack nucleation?

    , International Journal of Plasticity, Vol: 101, Pages: 213-229, ISSN: 0749-6419

    Fatigue crack nucleation in a powder metallurgy produced nickel alloy containing a non-metallic inclusion has been investigated through integrated small-scale bend testing, quantitative characterisation (HR-DIC and HR-EBSD) and computational crystal plasticity which replicated the polycrystal morphology, texture and loading. Multiple crack nucleations occurred at the nickel matrix-inclusion interface and both nucleation and growth were found to be crystallographic with highest slip system activation driving crack direction. Local slip accumulation was found to be a necessary condition for crack nucleation, and that in addition, local stress and density of geometrically necessary dislocations are involved. Fatemi-Socie and dissipated energy were also assessed against the experimental data, showing generally good, but not complete agreement. However, the local stored energy density (of a Griffith-Stroh kind) identified all the crack nucleation sites as those giving the highest magnitudes of stored energy.

  • Conference paper
    Jiang J, Hooper P, Li N, Luan Q, Hopper C, Ganapathy M, Lin Jet al., 2017,

    An integrated method for net-shape manufacturing components combining 3D additive manufacturing and compressive forming processes

    , ICTP 2017, Publisher: Elsevier, Pages: 1182-1187, ISSN: 1877-7058

    Published by Elsevier Ltd. Additive manufactured (AM) or 3D printed metallic components suffer poor and inconsistent mechanical properties due to the presence of a large number of micro-voids, residual stress and microstructure inhomogeneity. To overcome these problems, a new forming process has been proposed, which effectively combines AM and compressive forming. The aim of this study is to prove the feasibility of this newly proposed method by providing preliminary results. Thus, we compared the tensile performance of hot-forged additive manufactured stainless steel 316L samples to none-hot-forged additive manufactured ones. Significant improvement in mechanical properties has been found in the tensile tests as well hardness test. In addition, our EBSD characterized grain orientation maps at each stage of the process revealed the corresponding microstructure revolution which provides insights into underlying mechanistic.

  • Journal article
    Shao Z, Li N, Lin J, 2017,

    The optimisation of cruciform specimen for the formability evaluation of AA6082 under hot stamping conditions

    , Procedia Engineering, Vol: 207, Pages: 735-740, ISSN: 1877-7058

    The hot stamping and cold die quenching process is increasingly adopted to form complex-shaped structures of sheet metals in the automotive industry. However, it is difficult to obtain formability data of sheet metals under hot stamping conditions by using conventional experimental testing methods. In this study, a novel in-plane biaxial testing system, which is attached to a Gleeble materials thermo-mechanical simulator, had been developed for determining forming limit diagrams (FLDs) under hot stamping conditions. However, there is no standard of cruciform specimen geometries available for this type of biaxial tests. In this paper, the features of thickness reduction in the central region and slots in the arms of a type of cruciform specimen of aluminium alloy 6082 were verified first to increase strain uniformity of the biaxial loading zone on a cruciform specimen, based on the selective heating and cooling method. Finite Element (FE) thermo-electrical and thermo-mechanical models with UAMP and VUMAT subroutines were then implemented in ABAQUS 6.12 to optimise specimen dimensions so that fracture occurs in the concerned central region of the specimen during testing. By the use of the optimised specimen for AA6082 in the biaxial testing system, formability tests under the designated strain paths were conducted at specified hot stamping conditions. Strain fields in the gauge region of the cruciform specimens were measured using the digital image correlation (DIC) system and the experimental results were presented and analysed in order to verify the cruciform specimen design.

  • Conference paper
    Shao Z, Li N, Lin J, 2017,

    The comparison of two continuum damage mechanics-based material models for formability prediction of AA6082 under hot stamping conditions

    , 36th IDDRG Conference – Materials Modelling and Testing for Sheet Metal Forming, Publisher: IOP Publishing, ISSN: 1742-6588

    The hot stamping and cold die quenching process has experienced tremendous development in order to obtain shapes of structural components with great complexity in automotive applications. Prediction of the formability of a metal sheet is significant for practical applications of forming components in the automotive industry. Since microstructural evolution in an alloy at elevated temperature has a large effect on formability, continuum damage mechanics (CDM)-based material models can be used to characterise the behaviour of metals when a forming process is conducted at elevated temperatures. In this paper, two sets of unified multi-axial constitutive equations based on material's stress states and strain states, respectively, were calibrated and used to effectively predict the thermo-mechanical response and forming limits of alloys under complex hot stamping conditions. In order to determine and calibrate the two material models, formability tests of AA6082 using a developed novel biaxial testing system were conducted at various temperatures and strain rates under hot stamping conditions. The determined unified constitutive equations from experimental data are presented in this paper. It is found that both of the stress-state based and strain-state based material models can predict the formability of AA6082 under hot stamping conditions.

  • Journal article
    Jing Y, Su D, Yue X, Britton T, Jiang Jet al., 2017,

    The development of high strength brazing technique for Ti-6Al-4V using TiZrCuNi amorphous filler

    , Materials Characterization, Vol: 131, Pages: 526-531, ISSN: 1044-5803

    The brazing joint of the Ti-6Al-4V alloy was produced with a designed brazing filler alloy and the optimized brazing temperature which is lower than the β-phase transformation of the matrix. The strength and the ductility of brazing joined Ti-6Al-4V samples were evaluated by conventional tensile tests with a DIC 2D–strain field measurement. The Widmanstätten microstructure with no voids or cracks or intermetallic compounds was found throughout the joint with a width of β-lamellar as ~ 1 μm. Due to the fine acicular α-Widmanstätten and β-lamellar, and the uniformly diffused filler elements throughout the entire joint, the strength of the joint was as much as the matrix. In addition, the hardness test results agreed well with the tensile strength tests. All fractures occurred in the matrix rather than the brazing joints. Furthermore, the maximum local tensile strain was measured as 20% in the matrix, while under the same stress, the brazing joint only reached 6.3% tensile plastic strain. Thus, the mechanical properties of the joint with the associated microstructure demonstrated that a successful brazing filler alloy has been developed for the Ti-6Al-4V alloy.

  • Journal article
    Chen B, Jiang J, Dunne F, 2017,

    Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals

    , Journal of the Mechanics and Physics of Solids, Vol: 106, Pages: 15-33, ISSN: 1873-4782

    An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out for nickel single crystal (CMSX4) and oligocrystal (MAR002) alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation.The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the single and oligocrystals tests. In addition, once primary nucleated cracks develop and are represented in the crystal model using XFEM, the stored energy correctly identifies where secondary fatigue cracks are observed to nucleate in experiments. This (Griffith-Stroh type) quantity also correctly differentiates and explains intergranular and transgranular fatigue crack nucleation.

  • Conference paper
    Shao Z, Li N, Lin J,

    The comparison of two continuum damage mechanics-basedmaterial models for formability prediction of AA6082 underhot stamping conditions

    , Materials Modelling and Testing for Sheet Metal Forming IDDRG 2017
  • Journal article
    Shao Z, Li N, 2017,

    A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions

    , Journal of Visualized Experiments, Vol: 122, ISSN: 1940-087X

    This protocol proposes a novel biaxial testing system used on a resistance heating uniaxial tensile test machine in order to determine the forming limit diagram (FLD) of sheet metals under hot stamping conditions.

  • Conference paper
    Rounthwaite N, McGilvery CM, Jiang J, Williams R, Giuliani F, Britton TBet al., 2017,

    A chemical and morphological study of diesel injector nozzle deposits - insights into their formation and growth mechanisms

    , SAE 2017 World Congress and Exhibition, Publisher: SAE International, Pages: 106-114, ISSN: 1946-3960

    Modern diesel passenger car technology continues to develop rapidly in response to demanding emissions, performance, refinement, cost and fuel efficiency requirements. This has included the implementation of high pressure common rail fuel systems employing high precision injectors with complex injection strategies, higher hydraulic efficiency injector nozzles and in some cases <100µm nozzle hole diameters. With the trend towards lower diameter diesel injector nozzle holes and reduced cleaning through cavitation with higher hydraulic efficiency nozzles, it is increasingly important to focus on understanding the mechanism of diesel injector nozzle deposit formation and growth. In this study such deposits were analysed by cross-sectioning the diesel injector along the length of the nozzle hole enabling in-depth analysis of deposit morphology and composition change from the inlet to the outlet, using state-of-the-art electron microscopy techniques. Deposits produced in the injector nozzles of the industry standard fouling test (CEC F-98-08 DW10B bench engine) were compared with those formed in a vehicle driven on a chassis dynamometer, using a drive cycle more representative of real world vehicle conditions, to explore the effects of differing drive cycles and engine technologies. Fouling in all tests was accelerated with the addition of 1ppm zinc neodecanoate, as specified in the CEC DW10B test. This in-depth characterisation revealed a complex multi-layered system of deposits inside the diesel injector nozzle. Through analysing these layers the mechanisms enabling the initial deposit formation and growth can be postulated.

  • Journal article
    Jiang J, Dunne F, Britton T, 2017,

    Toward predictive understanding of fatigue crack nucleation in Ni-based Superalloys

    , JOM, Vol: 69, Pages: 863-871, ISSN: 1047-4838

    Predicting when and where materials fail is a holy grail for structural materials engineering. Development of a predictive capability in this domain will optimize the employment of existing materials, as well as rapidly enhance the uptake of new materials, especially in high-risk, high-value applications, such as aeroengines. In this article, we review and outline recent efforts within our research groups that focus on utilizing full-field measurement and calculation of micromechanical deformation in Ni-based superalloys. In paticular, we employ high spatial resolution digital image correlation (HR-DIC) to measure surface strains and a high-angular resolution electron backscatter diffraction technique (HR-EBSD) to measure elastic distortion, and we combine these with crystal plasticity finite element (CPFE) modeling. We target our studies within a system of samples that includes single, oligo, and polycrystals where the boundary conditions, microstructure, and loading configuration are precisely controlled. Coupling of experiment and simulation in this manner enables enhanced understanding of crystal plasticity, as demonstrated with case studies in deformation compatibility; spatial distributions of slip evolution; deformation patterning around microstructural defects; and ultimately development of predictive capability that probes the location of microstructurally sensitive fatigue cracks. We believe that these studies present a careful calibration and validation of our experimental and simulation-based approaches and pave the way toward new understanding of crack formation in engineering alloys.

  • Conference paper
    Shao Z, Li N, Lin J,

    A New Damage Model for Predicting Forming Limits under Hot Stamping Conditions

    , The International Conference on Plasticity, Damage, and Fracture 2017
  • Journal article
    Shao Z, Li N, Lin J, Dean Tet al., 2016,

    Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model

    , International Journal of Mechanical Sciences, Vol: 120, Pages: 149-158, ISSN: 0020-7403

    Hot stamping and cold die quenching has been developed in forming complex shaped structural components of metals. The aim of this study is the first attempt to develop unified viscoplastic damage constitutive equations to describe the thermo-mechanical response of the metal and to predict the formability of the metal for hot stamping applications. Effects of parameters in the damage evolution equation on the predicted forming limit curves were investigated. Test facilities and methods need to be established to obtain experimental formability data of metals in order to determine and verify constitutive equations. However, conventional experimental approaches used to determine forming limit diagrams (FLDs) of sheet metals under different linear strain paths are not applicable to hot stamping conditions due to the requirements of rapid heating and cooling processes prior to forming. A novel planar biaxial testing system was proposed before and was improved and used in this work for formability tests of aluminium alloy 6082 at various temperatures, strain rates and strain paths after heating, soaking and rapid cooling processes. The key dimensions and features of cruciform specimens adopted for the determination of forming limits under various strain paths were developed, optimised and verified based on the previous designs and the determined heating and cooling method [1]. The digital image correlation (DIC) system was adopted to record strain fields of a specimen throughout the deformation history. Material constants in constitutive equations were determined from the formability test results of AA6082 for the prediction of forming limits of alloys under hot stamping conditions. This research, for the first time, enabled forming limit data of an alloy to be generated at various temperatures, strain rates and strain paths and forming limits to be predicted under hot stamping conditions.

  • Journal article
    Li N, Shao Z, Lin J, Dean TAet al., 2016,

    Investigation of uniaxial tensile properties of AA6082 under HFQ® Conditions

    , Key Engineering Materials, Vol: 716, Pages: 337-344, ISSN: 1013-9826

    © 2016 Trans Tech Publications, Switzerland. For a metal forming process, the uniaxial tensile properties of a material are the most fundamental and important properties to investigate. Solution heat treatment, forming and in-die quenching (HFQ®) is a patented process to form complex shape panel components using aluminium alloys at high efficiency and low cost. A Gleeble materials thermo-mechanical simulator was used to conduct uniaxial tensile testing of AA6082 under HFQ® conditions. A set of grips were specially designed to reduce the heat loss of specimen during testing in a Gleeble and allow the strain measurement by using digital image correlation (DIC) system. A large dog-bone specimen with parallel length of 80mm was designed to minimise the temperature gradient along the gauge section. Temperature gradient was measured and uniaxial tensile tests were conducted at the range of deformation temperature of350-535 °C and the range of strain rate of 0.1-4/s. The uniaxial tensile properties of AA6082 at different temperatures and strain rates under HFQ® conditions were summarised and the viscoplastic response of the material was discussed.

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