Publications
74 results found
Li N, Zheng J, Zhang C, et 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 2015), Publisher: EDP Sciences
For HFQ volume production in automotive applications, short heat treatmenttime is an important factor for efficient and cost effective manufacturing. In this study,fast, efficient and energy saving solution heat treatment (SHT) and ageing processes havebeen identified based on precipitation nucleation and growth mechanisms and a series ofexperiments. Material chosen was AA6082, which is an alloy favoured for automotive bodystructures. The effects of heating rate, soaking temperature and time on SHT processes,and the influence on post mechanical properties, have been characterised. A duplexageing process integrated paint bake process has been developed, with an optimum ageingcondition determined. In addition, the effect of pre-deformation has been analysed.
Mohamed M, Li N, Wang L, et al., 2015, An investigation of a new 2D CDM model in predicting failure in HFQing of an automotive panel, The 4th International Conference on New Forming Technology (ICNFT2015)
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.
Gao H, Li N, Ho H, et al., 2015, Determination of a Set of Constitutive Equations for an Al-Li Alloy at SPF Conditions, Materials Today: Proceedings, Vol: 2, Pages: S408-S413, ISSN: 2214-7853
Li X, Li N, Zhu Q, et al., 2015, A Testing System and Method to Generate Forming Limit Diagrams at Elevated Temperatures, CN102749253 B
Li N, Sun C, Guo N, et al., 2014, Damage Investigation of Boron Steel at Hot Stamping Conditions, Procedia Engineering, Vol: 81, Pages: 1744-1749, ISSN: 1877-7058
The uniaxial tension tests of 1500 MPa boron steel at strain rates of 0.01-5.0 s-1 and temperatures of 550-850 °C were performed. Considering the difference between the deformation necking cross section and the centre measuring section of specimen, a correction method of measuring strain and temperature at necking cross section are developed. For the method, the deformation of necking cross section was obtained which approximate for the trapezoidal change law near the necking zone. The stress strain curves are corrected based on the correction method of the strain and by considering temperature correction. A set of unified constitutive equations has been developed and calibrated. Correlation between the corrected and predicted true stress data from the constitutive equation is presented. The correlation coefficient is 0.969, and the absolute average relative error is 3.73% in the range of allowable experimental conditions.
Li N, Lin J, Dean TA, et al., 2014, Materials Modelling for Selective Heating and Press Hardening of Boron Steel Panels with Graded Microstructures, Procedia Engineering, Vol: 81, Pages: 1675-1681, ISSN: 1877-7058
Li N, Lin J, Dean TA, et al., 2014, Concept Validation for Selective Heating and Press Hardening of Automotive Safety Components with Tailored Properties, Key Engineering Materials, Vol: 622-623, Pages: 1124-1131, ISSN: 1013-9826
A new strategy termed selective heating and press hardening, for hot stamping of boron steel parts with tailored properties is proposed in this paper. Feasibility studies were carried out through a specially designed experimental programme. The main aim was to validate the strategy and demonstrate its potential for structural optimisation. In the work, a lab-scale demonstrator part was designed, and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Demonstrator parts of full martensite phase, full initial phase, and differentially graded microstructures have been formed with high dimensional quality. Hardness testing and three point bending tests were conducted to assess the microstructure distribution and load bearing performance of the as-formed parts, respectively. The feasibility of the concept has been validated by the testing results.
Li N, Gao W, Tang X, et al., 2013, A Tool Design and Method for Forming Automotive Metal Panel Components, CN103182452 A
Liu G, Zhu Q, Zhu G, et al., 2012, A Testing System and Method to Generate Forming Limit Diagrams at Elevated Temperatures, CN102749253 A
Li N, Li X, Dry D, et al., 2012, Investigation on the Mechanical Properties of As-Formed Boron Steels for Optimizing Process Strategies in Hot Stamping, Steel Research International
Li N, Mohamed MS, Cai J, et al., 2011, Experimental and Numerical Studies on the Formability of Materials in Hot Stamping and Cold Die Quenching Processes, The 14th International ESAFORM Conference on Material Forming, Publisher: American Institute of Physics, Pages: 1555-1561, ISSN: 0094-243X
Formability of steel and aluminium alloys in hot stamping and cold die quenching processes is studied in this research. Viscoplastic‐damage constitutive equations are developed and determined from experimental data for the prediction of viscoplastic flow and ductility of the materials. The determined unified constitutive equations are then implemented into the commercial Finite Element code Abaqus/Explicit via a user defined subroutine, VUMAT. An FE process simulation model and numerical procedures are established for the modeling of hot stamping processes for a spherical part with a central hole. Different failure modes (failure takes place either near the central hole or in the mid span of the part) are obtained. To validate the simulation results, a test programme is developed, a test die set has been designed and manufactured, and tests have been carried out for the materials with different forming rates. It has been found that very close agreements between experimental and numerical process simulation results are obtained for the ranges of temperatures and forming rates carried out.
Dai Y, Li N, 2007, BIW concept design in virtual testing lab applying computation grid network, SC '07: International Conference for High Performance Computing, Networking, Storage and Analysis, Publisher: ACM
Yi D, Li N, 2007, BIW Concept Design in Virtual Testing Lab Applying Computation Grid Network, 3rd China HPC Workshop, Publisher: ACM, Pages: 192-197
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