Imperial College London

DrNanLi

Faculty of EngineeringDyson School of Design Engineering

Lecturer
 
 
 
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Contact

 

n.li09

 
 
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Location

 

City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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40 results found

Ganapathy M, Li N, Lin J, Abspoel M, Bhattacharjee Det al., 2018, 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

JOURNAL ARTICLE

Shao Z, Bai Q, Li N, Lin J, Shi Z, Stanton M, Watson D, Dean Tet al., 2018, Experimental investigation of forming limit curves and deformation features in warm forming of an aluminium alloy, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART B-JOURNAL OF ENGINEERING MANUFACTURE, Vol: 232, Pages: 465-474, ISSN: 0954-4054

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: 1994-2008, ISSN: 0954-4062

JOURNAL ARTICLE

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

Ganapathy M, Li N, Lin JG, Abspoel M, Bhattacharjee Det al., 2017, Test-Piece and Test-Rig Designs to Obtain High Accuracy Experimental Data for Boron Steel at Hot Stamping Conditions, 3rd International Conference on Advanced High Strength Steel and Press Hardening (ICHSU), Publisher: WORLD SCIENTIFIC PUBL CO PTE LTD, Pages: 109-113

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.

CONFERENCE PAPER

Kong XX, Li N, O'Keeffe R, Lin JGet al., 2017, Experimental Investigation on the Accuracy of Uniaxial Tensile Data of AA6082 Under HFQ (R) Conditions, 3rd International Conference on Advanced High Strength Steel and Press Hardening (ICHSU), Publisher: WORLD SCIENTIFIC PUBL CO PTE LTD, Pages: 390-394

CONFERENCE PAPER

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, 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, The optimisation of cruciform specimen for the formability evaluation of AA6082 under hot stamping conditions, ICTP 2017, Publisher: Elsevier, 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, 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

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

JOURNAL ARTICLE

Zuo B, Wang B, Li Z, Li N, Lin Jet al., 2017, An investigation of involute and lead deflection in hot precision forging of gears, INTERNATIONAL JOURNAL OF ADVANCED MANUFACTURING TECHNOLOGY, Vol: 88, Pages: 3017-3030, ISSN: 0268-3768

JOURNAL ARTICLE

Li N, Lin J, Balint DS, Dean TAet al., 2016, Experimental characterisation of the effects of thermal conditions on austenite formation for hot stamping of boron steel, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 231, Pages: 254-264, ISSN: 0924-0136

JOURNAL ARTICLE

Li N, Lin J, Balint DS, Dean TAet al., 2016, Modelling of austenite formation during heating in boron steel hot stamping processes, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 237, Pages: 394-401, ISSN: 0924-0136

JOURNAL ARTICLE

Li N, Politis D, Foster AD, Lin J, Wang L, Szegda Det al., 2016, Prediction of thinning behavior for complex-shaped, lightweight alloy panelsformed through a hot stamping process*, 2nd International Conference on Advanced High Strength Steel and Press Hardening (ICHSU2015):, Publisher: World Scientific

This paper provides an overview of hot stamping of light alloys using a method known as HFQ®forming. We present a new method of forming complex shaped automotive components through theuse of a hot stamping process for high strength AA6082 alloy. In particular, the component selectedfor study is a complex automotive door inner component that could not be conventionally formedfrom such a strong alloy. The forming process is described and the post-formed thicknessdistribution is presented. An FE simulation is given to demonstrate the potential to predict theforming process and to provide guidance as to the final component thicknesses.

CONFERENCE PAPER

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.

JOURNAL ARTICLE

Li N, Sun C, Guo N, Mohamed M, Lin J, Matsumoto T, Liu Cet al., 2016, Experimental investigation of boron steel at hot stamping conditions, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 228, Pages: 2-10, ISSN: 0924-0136

JOURNAL ARTICLE

Lin J, Zheng K, Wang L, Li N, El-Fakir O, Dean T, Foster A, Adam Get al., 2016, Material and Process for Preparing and Forming an Aluminium Alloy Material, WO/2016/067045

PATENT

Matsumoto T, Li N, Shi X, Lin Jet al., 2016, An Investigation of Deformation Effects on Phase Transformation in Hot Stamping Processes, SAE INTERNATIONAL JOURNAL OF MATERIALS AND MANUFACTURING, Vol: 9, Pages: 501-505, ISSN: 1946-3979

JOURNAL ARTICLE

Shao Z, Li N, Lin J, Dean TAet al., 2016, Development of a New Biaxial Testing System for Generating Forming Limit Diagrams for Sheet Metals Under Hot Stamping Conditions, EXPERIMENTAL MECHANICS, Vol: 56, Pages: 1489-1500, ISSN: 0014-4851

JOURNAL ARTICLE

Shao Z, Li N, Politis D, Bai Q, Lin Jet al., 2016, A Review on of Forming Limit Prediction and Determination at Elevated Temperatures, 2nd International Conference on Advanced High Strength Steel and Press Hardening (ICHSU2015), Publisher: World Scientific

The evaluation of the formability of sheet material is of great importance for forming complex-shapecomponents in automotive applications. However, for hot and warming forming conditions, thisusually requires formability tests with specialist devices and testing procedures and is difficult toobtain accurate and comparable results due to the lack of testing standards. In this paper, the conceptand development of strain-based forming limit diagram (FLD) are introduced. The testing methodsto experimentally determine the FLD, classified into formability tests and planar tensile tests, arewidely reviewed. The applications of both types to obtain FLD at elevated temperatures for differentforming processes are introduced, and the advantages and disadvantages of the methods areanalysed.

CONFERENCE PAPER

Yang L, Li N, Wang B, Lin J, Zhao H, Ma Wet al., 2016, Unified constitutive modelling for two-phase lamellar titanium alloys at hot forming conditions, MANUFACTURING REVIEW, Vol: 3, ISSN: 2265-4224

JOURNAL ARTICLE

Ganapathy M, Li N, Lin J, Abspoel M, Guido H, Bhattacharjee Det al., 2015, Analysis of new Gleeble tensile specimen design for hot stamping application, 4th International Conference on New Forming Technology (ICNFT), Publisher: E D P SCIENCES, ISSN: 2261-236X

CONFERENCE PAPER

Gao H, Li N, Ho H, Zhang Y, Zhang N, Wang L, Lin Jet al., 2015, Determination of a set of constitutive equations for an Al-Li alloy at SPF conditions, MATERIALS TODAY-PROCEEDINGS, Vol: 2, Pages: 408-413, ISSN: 2214-7853

JOURNAL ARTICLE

Li N, Lin JG, Dean TA, 2015, Development of Unified Viscoplastic-Damage Model for Crashworthiness Analysis of Boron Steel Safety Components with Tailored Microstructures, Applied Mechanics and Materials, Vol: 784, Pages: 427-434, ISSN: 1662-7482

Hot stamped boron steel panels with tailored properties are popular as car safety components for maximised energy absorption. In this study, dynamic and quasi-static tensile tests (strain rate: 0.001/s – 500/s) combined with microstructural observation were carried out to study the mechanical properties of press hardened boron steel with various microstructures (martensite volume fraction: 0 – 100%) at room temperature. Based on the test results, a physically-based unified viscoplastic-damage constitutive model has been developed and determined, which takes the volume fraction of martensite into account. Thus the crashworthiness and failure mode of boron steel parts having graded microstructure distributions can be described through a single set of equations.

JOURNAL ARTICLE

Li N, Zheng J, Zhang C, Zheng K, Lin J, Dean TAet al., 2015, Investigation on fast and energy-efficient heat treatments of AA6082 in HFQ processes for automotive applications, 4th International Conference on New Forming Technology (ICNFT), Publisher: E D P SCIENCES, ISSN: 2261-236X

CONFERENCE PAPER

Li X, Li N, Zhu Q, Li B, Zhu G, Liu G, Luo J, Lin Jet al., 2015, A Testing System and Method to Generate Forming Limit Diagrams at Elevated Temperatures, CN102749253 B

PATENT

Mohamed M, Li N, Wang L, El Fakir O, Lin J, Dean T, Dear Jet al., 2015, An investigation of a new 2D CDM model in predicting failure in HFQing of an automotive panel, 4th International Conference on New Forming Technology (ICNFT), Publisher: E D P SCIENCES, ISSN: 2261-236X

CONFERENCE PAPER

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