Imperial College London

DrNanLi

Faculty of EngineeringDyson School of Design Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 8853n.li09 Website

 
 
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Location

 

M224Royal College of ScienceSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

59 results found

Reza Attar H, Li N, Foster A, 2021, A new strategy for developing component design guidelines for aluminium alloy corners formed through cold and hot stamping processes, Materials & Design, Vol: 207, ISSN: 0264-1275

In recent sheet metal forming research, efforts have been largely focused on determining optimum processing parameters, while intuitive guidelines to efficiently develop feasible component geometries are rarely considered. Consequently, there are currently no suitable design support tools that can guide component designers using the most recent, and therefore unfamiliar, sheet metal manufacturing technologies. This paper aims to address this bottleneck by proposing a strategy for the creation of early stage manufacturing design guidelines for the common limiting design requirement of deep corners. Aluminium alloys formed under both cold, and elevated temperature working conditions are considered. A new methodology to simplify the analysis of complex viscoplastic behaviour of aluminium alloys at elevated temperatures into an equivalent strain hardening response is presented. The effects and trends of the deep corner geometry and simplified material hardening characteristics on the post-form thinning distribution are identified. New equation sets are proposed which model the identified trends and enable the development of intuitive design maps. Following the approach proposed in this paper, an awareness of the available design envelope can be created at the early stages of a design process to guide component design, material, and manufacturing process selection decisions for deep corner geometries.

Journal article

Xu Q, Nie Z, Xu H, Zhou H, Attar HR, Li N, Xie F, Liu X-Jet al., 2021, SuperMeshing: a new deep learning architecture for increasing the mesh density of physical fields in metal forming numerical simulation, Journal of Applied Mechanics, Pages: 1-11, ISSN: 0021-8936

In stress field analysis, the finite element method is a crucial approach, in which the mesh-density has a significant impact on the results. High mesh density usually contributes authentic to simulation results but costs more computing resources. To eliminate this drawback, we propose a data-driven mesh-density boost model named SuperMeshingNet that uses low mesh-density as inputs, to acquire high-density stress field instantaneously, shortening computing time and cost automatically. Moreover, the Res-UNet architecture and attention mechanism are utilized, enhancing the performance of SuperMeshingNet. Compared with the baseline that applied the linear interpolation method, SuperMeshingNet achieves a prominent reduction in the mean squared error (MSE) and mean absolute error (MAE) on the test data. The well-trained model can successfully show more excellent performance than the baseline models on the multiple scaled mesh-density, including 2X, 4X, and 8X. Enhanced by SuperMeshingNet with broaden scaling of mesh density and high precision output, FEA can be accelerated with seldom computational time and cost.

Journal article

Wang X, Zhu L, Sun L, Li Net al., 2021, Optimization of graded filleted lattice structures subject to yield and buckling constraints, Materials & Design, Vol: 206, Pages: 1-17, ISSN: 0264-1275

To reduce the stress concentration and ensure structural safety for lattice structure designs, in this paper, a new optimization framework is developed for the optimal design of graded lattice structures, innovatively integrating fillet designs as well as yield and buckling constraints. Both relative strut radii and fillet parameters are defined as design variables, for BCC and PC lattices. Numerical homogenization is employed to characterize the effective elastic constants and yield stresses of the lattice metamaterials. Metamaterial models are developed to represent the relationships between the metamaterial effective properties and lattice geometric variables. Yield and buckling constraints, based on modified Hill’s yield criterion as well as Euler and Johnson buckling formulae respectively, are developed as functions of lattice geometric variables. A new optimization framework is proposed with both yield and buckling constraints integrated. A case study on minimizing the compliance of a Messerschmitt-Bolkow-Blohm beam, composed of either BCC or PC lattices, is conducted. The yield and buckling constraints guarantee the structural safety of the optimized lattice beams. The optimized beams composed of filleted lattices, compared with non-filleted lattices in the corresponding type, show reduced proportions subject to high modified Hill’s stress (

Journal article

Attar HR, Zhou H, Foster A, Li Net al., 2021, Rapid feasibility assessment of components to be formed through hot stamping: A deep learning approach, Journal of Manufacturing Processes, Vol: 68, Pages: 1650-1671, ISSN: 1526-6125

The novel non-isothermal Hot Forming and cold die Quenching (HFQ) process canenable the cost-effective production of complex shaped, high strength aluminiumalloy panel components. However, the unfamiliarity of designing for the newprocess prevents its widescale adoption in industrial settings. Recent researchefforts focus on the development of advanced material models for finite elementsimulations, used to assess the feasibility of new component designs for theHFQ process. However, FE simulations take place late in design processes,require forming process expertise and are unsuitable for early-stage designexplorations. To address these limitations, this study presents a novelapplication of a Convolutional Neural Network (CNN) based surrogate as a meansof rapid manufacturing feasibility assessment for components to be formed usingthe HFQ process. A diverse dataset containing variations in component geometry,blank shapes, and processing parameters, together with corresponding physicalfields is generated and used to train the model. The results show that nearindistinguishable full field predictions are obtained in real time from themodel when compared with HFQ simulations. This technique provides an invaluabletool to aid component design and decision making at the onset of a designprocess for complex-shaped components formed under HFQ conditions.

Journal article

Wang H, Liu H, Ding Z, Li Net al., 2021, Experimental and constitutive modelling studies of semicrystalline thermoplastics under solid-state stamp forming conditions, Polymer, Vol: 228, Pages: 1-17, ISSN: 0032-3861

Experimental characterisation and constitutive modelling studies on the thermomechanical behaviour of thermoplastics, under solid-state stamp forming conditions, are required for understanding and optimising the stampforming process. In this paper, two semicrystalline thermoplastics, Polyamide 6 (PA6, or Nylon 6) and PolyEther-Ether-Ketone (PEEK) are studied via uniaxial tensile tests at temperatures between their glass transitiontemperatures (Tg ) and melting temperatures (Tm), and at different strain rates (0.001–50 /s). The temperatureand strain rate effects are analysed quantitatively to further understand the thermomechanical response of thesesemicrystalline thermoplastics. The results show that temperature has significant effects on the thermomechanical behaviour of thermoplastic polymers, while the strain rate effects are relatively marginal in theinvestigated strain rate range. In addition, a new physically-based viscoelastic-viscoplastic constitutive model isproposed to simulate the thermomechanical behaviour of both materials. The model shows good predictionaccuracy on tensile stress responses and provides an insight into the microstructural evolution of the semicrystalline thermoplastics; thus, it can be used to analyse solid-state stamp forming of pure semicrystallinethermoplastics and thermoplastic polymer matrix composites (TPMCs).

Journal article

Zhou H, Xu Q, Nie Z, Li Net al., 2021, A study on using image-based machine learning methods to develop surrogate models of stamp forming simulations, Journal of Manufacturing Science and Engineering, Vol: 144, Pages: 1-41, ISSN: 1087-1357

In design for forming, it is becoming increasingly significant to develop surrogate models of high-fidelity finite element analysis (FEA) simulations of forming processes, to achieve effective component feasibility assessment as well as process and component optimizations. However, surrogate models using traditional scalar-based machine learning methods (SBMLMs) fall short on accuracy and generalizability. This is because SBMLMs fail to harness the location information available from the simulations. To overcome this shortcoming, the theoretical feasibility and practical advantages of innovatively applying image-based machine learning methods (IBMLMs) in developing surrogate models of sheet stamp forming simulations are explored in this study. To demonstrate the advantages of IBMLMs, the effect of the location information on both design variables and simulated physical fields is firstly proposed and analyzed. Based on a sheet steel stamping case study, a Res-SE-U-Net IBMLM surrogate model of stamping simulations is then developed and compared with a baseline multi-layer perceptron (MLP) SBMLM surrogate model. The results show that the IBMLM model is advantageous over the MLP SBMLM model in accuracy, generalizability, robustness, and informativeness. This paper presents a promising methodology in leveraging IBMLMs as surrogate models to make maximum use of information from stamp forming FEA results. Future prospective studies that are inspired by this paper are also discussed.

Journal article

Attar HR, Zhou H, Li N, 2021, Deformation and thinning field prediction for HFQ® formed panel components using convolutional neural networks, International Deep-Drawing Research Group Conference (IDDRG 2021), Publisher: IOP Publishing, Pages: 1-11, ISSN: 1757-8981

The novel Hot Forming and cold die Quenching (HFQ®) process can provide cost-effective and complex deep drawn solutions through high strength aluminium alloys. However, the unfamiliarity of the new process prevents its widescale adoption in industrial settings, while accurate Finite Element (FE) simulations using the most advanced material models take place late in design processes and require forming process expertise. Machine learning technologies have recently been proven successful in learning complex system behaviour from representative examples and have the potential to be used as design support tools for new forming technologies such as HFQ®. This study, for the first time, presents a novel application of a Convolutional Neural Network (CNN) based surrogate to predict the deformation and thinning fields for variable deep drawn geometries formed using HFQ® technology. A dataset based on deep drawn geometries and corresponding FE results is generated and used to train the model. The results show that near indistinguishable full field predictions in real time are obtained from the surrogate when compared with HFQ® simulations. This technique can be adopted in industrial settings to aid in both concept and detailed component design for complex-shaped panel components formed under HFQ® conditions, without underlying knowledge of the forming process.

Conference paper

Ding Z, Wang H, Luo J, Li Net al., 2021, A review on forming technologies of fibre metal laminates, International Journal of Lightweight Materials and Manufacture, Vol: 4, Pages: 110-126, ISSN: 2588-8404

Fibre metal laminates (FMLs), as a class of hybrid material taking advantages of both metals and composites, have shown great promise as lightweight structural materials in the transportation industry. Accordingly, manufacturing technologies of FMLs are attracting increasing research interests. This review emphasises the developing technologies of forming FML components, with other aspects related to FML materials being briefly introduced. First, we provide an overall review of the historical background and recent developments of FMLs, their classifications, sheet fabrication processes, and their advantages and disadvantages. Then, various forming technologies are introduced in detail, with a particular focus on stamp forming, which is considered to be the most promising approach for the high-volume production of complex-shaped FML components. Furthermore, the deformation modes and defects in forming FMLs are analysed and the challenges encountered in the existing research are thoroughly discussed. Finally, studies on modelling and process simulation of forming FMLs are reviewed and discussed. Based on the comprehensive appraisal of various aspects, current research progress and challenges related to FMLs and their forming technologies are summarised and an outlook of further developments is discussed.

Journal article

Wang X, Zhu L, Sun L, Li Net al., 2021, A study of functionally graded lattice structural design and optimisation, 2020 6th International Conference on Mechanical Engineering and Automation Science (ICMEAS), Publisher: IEEE, Pages: 50-55

Artificially designed lattice based structures, enabled by additive manufacturing are promising in various engineering applications due to their high stiffness and strength with low density and attractive multifunctional properties. In this work, a robust framework has been developed for structural optimisation by generating graded lattice structures. The goal of optimisation was to achieve the minimum structural weight while satisfying the stiffness requirement. Periodic representative volume element (RVE) homogenisation method was employed to calculate the effective mechanical properties of a unit cell of the lattice structure. A metamaterial model was determined to represent the relationship between the effective elastic constants and the geometric parameter, i.e. the strut radius of quasi-isotropic BCC lattice unit cell. Mesh effect analysis was carried out to capture the optimal Finite Element (FE) mesh size for numerical simulation, taking into consideration of the trade-off between accuracy and efficiency. The optimisation process was conducted through commercial software optistruct by applying the feasible directions (MFD) algorithm for optimisation, to achieve the optimal distribution of lattice strut radii. In post-processing, local maximum radius values were applied to joints of lattice unit cells to avoid sharp changes of strut radii between adjacent unit cells. Finally, a case study of 3-point bending beam was conducted to examine this framework and it was found that the proposed optimisation framework is valid for design and optimising graded lattice structures.

Conference paper

Ganapathy M, Li N, Lin J, Bhattacharjee Det al., 2020, A feasibility study on warm forming of an as-quenched 22MnB5 boron steel, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 277-283, ISSN: 2588-8404

In this paper, the feasibility of a newly proposed forming method, being the warm forming of as-quenched 22MnB5 boron steels, was studied through a series of proof of concept experiments. To assess the material thermo-mechanical behaviours under the proposed forming conditions, first, the as-received 22MnB5 boron steel was austenized and quenched to below the martensite transformation finish temperature to obtain a martensitic microstructure; second, uniaxial tensile tests of the as-quenched steel were conducted under proposed warm forming conditions on a Gleeble 3800 materials simulator. To evaluate the material post-form properties, first, tempering treatments on the as-quenched steel samples were performed to simulate the heat-treating conditions in the proposed warm forming process; second, the mechanical properties (hardness, strength, and ductility) of as-tempered samples were measured and a microstructure analysis was conducted. From the experimental results, it was found that, under the proposed warm-forming process conditions (420 °C–620 °C), the material showed significant strain softening, which would increase the tendency of necking during stamping and adversely affect its drawability. In addition, it was found that the heating of martensite in a 22MnB5 boron steel to higher temperatures (>400 °C) adversely affected its post-form strength and ductility due to the tempering effect. Therefore, according to the results obtained in this study, the warm forming of as-quenched 22MnB5 boron steel may reduce the strength of formed parts by more than 50% in comparison to the possible strength the material could achieve under the investigated process.

Journal article

Tian F, Li N, 2020, Investigation of the feasibility of a novel heat stamping process for producing complex-shaped Ti-6Al-4V panel components, Procedia Manufacturing, Vol: 47, Pages: 1374-1380, ISSN: 2351-9789

A novel cost-efficient Heat Stamping (HS) process, combining Heat treatment and fast Stamping, was proposed to produce complex-shaped titanium alloy panel components with low energy consumption and short cycle time. To investigate the feasibility of the HS process for forming Ti-6Al-4V, the stress-strain behaviours of the material under step quenching treatments in HS processes were investigated and compared with those under direct heating treatments in Hot Forming (HF) processes, through uniaxial tensile tests at the temperature range of 800 - 950 °C and the strain rate range of 0.01 - 5 /s. To reduce the strain softening, step quenching treatment was designed by soaking the specimen at 950 °C and fast quenching it to 800 °C for forming. It was found that strain softening in the step quenching tests was reduced as compared to direct heating tests at the strain rate of 1 /s; strain hardening was observed in step quenching test at the strain rate of 0.1 /s, achieved by enhancing the dynamic phase transformation from β phase to secondary α (αs) during deformation. Strain rate hardening of the material under step quenching treatment was found to be higher than those under direct heating treatment at the same temperature of 800 °C. To evaluate the novel HS concept, Heat Stamping experiments under step quenching treatments were carried out by using a drawability tool set. A cup-shaped demonstrator with the drawing ratio of 1.3 was produced to prove the feasibility of HS process.

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

Ganapathy M, Li N, Lin J, Abspoel M, Bhattacharjee Det al., 2019, Experimental investigation of a new low-temperature hot stamping process for boron steels, International Journal of Advanced Manufacturing Technology, Vol: 105, Pages: 669-682, ISSN: 0178-0026

This paper demonstrates the promise of a new low-temperature hot stamping process with pre-cooling for 22MnB5 boron steels. It is the first time for the new process being successfully implemented for producing an automotive demonstrator component assisted with thorough experimental studies. The studies mainly include hot forming experiments carried out on an industrial prototyping line, post-form examinations, and in-die quenching tests. Automotive B-Pillar components with two designed drawing depths (50 and 64 mm) were hot stamped at a wide range of temperatures and forming speeds, through both the conventional hot stamping processes and the new processes with pre-cooling applied. For the as-formed B-Pillars, 3D shape scanning was conducted to investigate the thickness distribution of the components; uniaxial tensile testing, hardness testing, and scanning electron microscopes (SEM) observation were conducted to assess the final mechanical properties and microstructures. To understand the benefit of the low-temperature hot stamping in reducing cycle time, a separate set of in-die quenching experiments were designed and carried out, with combinations of three different process parameters: workpiece start quenching temperature, initial tool temperature, and die-workpiece contact pressure. The results of this work confirmed that low-temperature hot stamping could be performed successfully in producing complex-shaped components, such as automotive B-Pillars, with much reduced cycle time.

Journal article

Zheng K, Zhu L, Lin J, Dean TA, Li Net al., 2019, An experimental investigation of the drawability of AA6082 sheet under different elevated temperature forming processes, Journal of Materials Processing Technology, Vol: 273, ISSN: 0924-0136

The performed research has, for the first time, investigated and compared the drawability of AA6082 at a comparable temperature range between two elevated temperature forming processes: termed (i)Low Temperature Hot Form and Quench (LT-HFQ®)or pre-cooled HFQ®, patented by Adam et al. (2015)and (ii)Direct Heating Aluminium Forming (DHAF)which represents a refinement of conventional warm forming targeting a higher temperature range. A series of uniaxial tensile and cylindrical deep drawing experiments were conducted. According to uniaxial tensile test results, the most obvious work-hardening and reasonable ductility was observed under LT-HFQ® at a deformation temperature of 350 °C and strain rate of 1 s−1, which can enhance drawability. For deep drawing experiments, it was found that preheating conditions of each process prior to forming significantly affected forming characteristics and post-formed hardness of the alloy; both the achieved maximum Draw ratio (DR)and limit Blank Holding Force (BHF)at some specific process parameters were increased under LT-HFQ®. Forming speed and temperature had significant effects on alloy deformation and thus drawability for both processes. In addition, by evaluating the post-formed hardness, process drawability and microstructural evolutions under different processes were simultaneously analyzed.

Journal article

Li N, Mohamed M, Lin J, 2019, METHOD FOR PREPARING AND FORMING SHEET MATERIAL, WO 2019/052966 A2

There is provided a method of preparing a sheet metal material for forming, the method comprising preparing the sheet o metal material to have a desired resistance profile along an axis; heating the sheet metal material to a desired temperature by passing acurrent along the axis; and cutting the sheet metal material to define a blank for forming into a predefined arrangement. There is alsoprovided a method of forming a sheet material.

Patent

Li N, Lin J, 2019, A method for forming sheet material components, WO 2019/038534 Al

A method for forming a component from a Ti-alloy or Ni-alloy sheet material. The method comprising heat treating thesheet material, wherein a final temperature of the sheet material is above 100°C below a b-transus temperature of the sheet material.The sheet material is formed into a predefined configuration between two dies. Forming is completed before the temperature of the o sheet material reaches a start temperature for b to martensite transformation within the sheet material and wherein the temperature ofthe dies is less than a finish temperature for b to martensite transformation within the sheet material.

Patent

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

Zhu L, Li N, Childs P, 2018, Light-weighting in aerospace component and system design, Propulsion and Power Research, Vol: 7, Pages: 103-119, ISSN: 2212-540X

Light-weighting involves the use of advanced materials and engineering methods to enable structural elements to deliver the same, or enhanced, technical performance while using less material. The concept has been extensively explored and utilised in many industries from automotive applications to fashion and packaging and offers significant potential in the aviation sector. Typical implementations of light-weighting have involved use of high performance materials such as composites and optimisation of structures using computational aided engineering approaches with production enabled by advanced manufacturing methods such as additive manufacture, foam metals and hot forming. This paper reviews the principal approaches used in light-weighting, along with the scope for application of light-weighting in aviation applications from power-plants to airframe components. A particular area identified as warranting attention and amenable to the use of light-weighting approaches is the design of solar powered aircraft wings. The high aspect ratio typically used for these can be associated with insufficient stiffness, giving rise to non-linear deformation, aileron reversal, flutter and rigid-elastic coupling. Additional applications considered include ultralight aviation components and sub-systems, UAVs, and rockets. Advanced optimisation approaches can be applied to optimise the layout of structural elements, as well as geometrical parameters in order to maximise structural stiffness, minimise mass and enable incorporation of energy storage features. The use of additive manufacturing technologies, some capable of producing composite or multi-material components is an enabler for light-weighting, as features formally associated with one principal function can be designed to fulfil multiple functionalities.

Journal article

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, International Conference on the Technology of Plasticity (ICTP 2017), Publisher: Elsevier, Pages: 1182-1187, ISSN: 1877-7058

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.

Conference paper

Ganapathy M, Li N, Lin J, Bhattacharjee Det al., 2017, Investigation of a new hot stamping process with improved formability and productivity, ICTP 2017, Publisher: Elsevier, Pages: 771-776, ISSN: 1877-7058

In order to improve the drawability of boron steel and also to increase the productivity of hot stamping process, a new hot stamping process with pre-cooling has been proposed. Stress-strain behavior at various temperatures was investigated and compared with that in traditional hot stamping processes. Detailed studies were carried out on the strain hardening parameter, n, at different temperatures and deformation rates. To evaluate this concept, hot stamping experiments were performed with both conventional (without pre-cooling) and new process (with pre-cooling) for a scaled down B-Pillar automotive component. The new hot stamping process with pre-cooling was able to produce the B-Pillar at low temperature (500°C) with less thinning than the hot stamping carried out without precooling at high temperature (765°C). Also the in-die quenching time was reduced by about 60%, by adopting the new hot stamping process with pre-cooling, which would increase the productivity significantly for automotive mass production without compromising the part quality.

Conference paper

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.

Journal article

Ganapathy M, Li N, Lin J, Abspoel M, Bhattacharjee Det al., 2017, A Novel Grip Design for High-Accuracy Thermo-Mechanical Tensile Testing of Boron Steel under Hot Stamping Conditions, Experimental Mechanics, Vol: 58, Pages: 243-258, ISSN: 0014-4851

Achieving uniform temperature within the effective gauge length in thermo-mechanical testing is crucial for obtaining accurate material data under hot stamping conditions. A new grip design for the Gleeble Materials-Simulator has been developed to reduce the long-standing problem of temperature gradient along a test-piece during thermo-mechanical tensile testing. The grip design process comprised two parts. For the first part, the new design concept was analysed with the help of Abaqus coupled Thermal-Electric Finite element simulation through the user defined feedback control subroutine. The second part was Gleeble thermo-mechanical experiments using a dog-bone test-piece with both new and conventional grips. The temperature and strain distributions of the new design were compared with those obtained using the conventional system within the effective gauge length of 40 mm. Temperature difference from centre to edge of effective gauge length (temperature gradient) was reduced by 56% during soaking and reduced by 100% at 700 °C. Consequently, the strain gradient also reduced by 95%, and thus facilitated homogeneous deformation. Finally to correlate the design parameters of the electrical conductor used in the new grip design with the geometry and material of test-piece, an analytical relationship has been derived between the test-piece and electrical conductor.

Journal article

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.

Conference paper

Shao Z, Li N, Lin J, 2017, Planar test system, WO2017025730A1

Patent

Shao Z, Li N, Lin J, 2017, 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

Conference paper

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.

Journal article

Zheng K, Lee J, Politis DJ, Li N, Lin J, Dean TAet al., 2017, An analytical investigation on the wrinkling of aluminium alloys during stamping using macro-scale structural tooling surfaces, International Journal of Advanced Manufacturing Technology, Vol: 92, Pages: 481-495, ISSN: 0268-3768

Structural surface texturing is believed to be a promising approach to modify tribological and thermal performances of tooling for sheet-stamping processes. However, a fundamental study on the surface-texturing design and resulting material deformation is currently lacking. In this paper, an advanced analytical buckling model specifically for the utilisation of textured tools at macro-scale, comprising dislocation-driven material model, isotropic yield criteria, bifurcation theory and Donnell-Mushtari-Vlasov (DMV) shell structure theory, was established. The developed analytical buckling model was validated by cylindrical deep-drawing experiments. Further finite element (FE) simulations with the implementation of material model via user-defined subroutine were also used to validate the bucking model for large surface texture designs. Effects of theoretical assumptions, such as yield criterion, boundary condition and test-piece geometry, on the accuracy of model prediction for wrinkling were investigated. It was found that the von Mises yield criterion and hinged boundary condition exhibited more accurate predictions. In addition, the DMV shell theory made this model most representative for large structural texturing designs. Furthermore, the implementation of induced shear strain component has an important effect on precisely predicting the wrinkling occurrence. The advanced analytical models developed in this study combine various classical mechanics, structure stability and material modelling together, which provides a useful tool for tooling engineers to analyse structural designs.

Journal article

Li N, Zheng J, Zheng K, Lin J, Davies Cet al., 2017, A fast ageing method for stamped heat-treatable alloys, WO2017021742 A1

Patent

Shao Z, Li N, Lin J, 2017, A New Damage Model for Predicting Forming Limits under Hot Stamping Conditions, The International Conference on Plasticity, Damage, and Fracture 2017

Conference paper

Shao Z, Li N, Lin J, Dean Tet al., 2017, 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

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