Imperial College London

ProfessorJianguoLin

Faculty of EngineeringDepartment of Mechanical Engineering

Head/Professor in Mechanics of Materials Division
 
 
 
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Contact

 

+44 (0)20 7594 7082jianguo.lin

 
 
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Assistant

 

Miss Valerie Crawford +44 (0)20 7594 7083

 
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Location

 

524City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

312 results found

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

© 2019 Elsevier B.V. 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

Zheng K, Dong Y, Zheng D, Lin J, Dean TAet al., 2019, An experimental investigation on the deformation and post-formed strength of heat-treatable aluminium alloys using different elevated temperature forming processes, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 268, Pages: 87-96, ISSN: 0924-0136

JOURNAL ARTICLE

Dragatogiannis DA, Kollaros D, Karakizis P, Pantelis D, Lin J, Charitidis Cet al., 2019, Friction stir welding between 6082 and 7075 aluminum alloys thermal treated for automotive applications, Materials Performance and Characterization, Vol: 8

Copyright © 2019 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. The automotive industry demands ecofriendly manufacturing processes and lightweight structures and materials to reduce CO2emissions and decrease weight and fuel consumption while optimizing overall performance. Aluminum alloys in the 7xxx and 6xxx series offer high potential for weight reduction in automotive and other transportation industries. 7xxx aluminum alloys have the best strength performance among all commercial aluminum alloys. Alloys in the 6xxx series generally have medium-to-high strength, high corrosion resistance, and good formability. Alloys in both the 7xxx and 6xxx series are strengthened by heat treatment. The increased strength-to-weight ratio and the thermal treatment processing open thepossibilitytouse thesealloysaspossible alternative materials instead of steel in the automotive industry for the fabrication of car body parts. However, their poor formability and weldability are the main drawbacks when these alloys are considered as substitutes of steel and other dissimilar joints during the fabrication of car body parts and the production chain. Recently, Hot Forming and in-die Quenching (HFQ), a patented hot stamping process, has been introduced to manufacture complex-shaped high-strength heat-treatable aluminum alloys. The work describedinthis articleisanexperimental investigation of the weldability between AA6082 and AA7075 by Friction Stir Welding (FSW) that considers thermal treatmentsused during HFQ for automotive applications. The aforementioned FSW base metals, 6082 and 7075, are heat treated according to the solution heat treatment adopted during HFQ to evaluate the effect of the HFQ thermal cycle on the quality of the produced joints. Optical microscopy has been used to characterize the microstructure of the produced joints. The defect-free welded joints are characterized by good mechanical mixing between the joined mater

JOURNAL ARTICLE

Li Y, Yang Y-L, Rong Q, Shi Z, Lin J, Said Ret al., 2019, Effect of initial tempers on mechanical properties of creep-aged AA2050, Manufacturing Review, Vol: 6, ISSN: 2265-4224

The evolution of mechanical properties of a third-generation Al–Cu–Li alloy, AA2050, with different initial tempers (as-quenched WQ, naturally aged T34 and peak-aged T84) during creep-ageing has been investigated in this study. A set of creep-ageing tests was carried out under 150 MPa at 155 °C with different durations for all initial temper conditions and tensile tests were performed subsequently to acquire the main mechanical properties of the creep-aged alloys, including the yield strength, ultimate tensile strength and uniform elongation. The evolution of these mechanical properties during creep-ageing has been discussed in association with precipitation behaviour of AA2050 alloys with different initial tempers. The results indicate that the T34 alloy is the best choice for creep age forming (CAF) applications among these initial tempers, as it provides better yield strength and uniform elongation concurrently after creep-ageing. In addition, a work hardening rate analysis has been carried out for all the creep-aged alloys, helping to understand the detailed dislocation/precipitate interaction mechanisms during plastic deformation in the creep-aged AA2050 alloys with WQ, T34 and T84 initial tempers.

JOURNAL ARTICLE

Li Y, Shi Z, Rong Q, Zhou W, Lin Jet al., 2019, Effect of pin arrangement on formed shape with sparse multi-point flexible tool for creep age forming, International Journal of Machine Tools and Manufacture, Vol: 140, Pages: 48-61, ISSN: 0890-6955

The effect of forming pin arrangement on formed shape accuracy with sparse multi-point flexible (SMPF) tool has been experimentally and numerically investigated for creep age forming (CAF) process. An analytical method has been introduced to predict shape, stress and strain distributions of blanks loaded by SMPF tool with different pin configurations. Experiments and FE simulations of loading and CAF processes by SMPF tool with various pin number/interval conditions have been performed and the formed shapes after loading and CAF have been quantitatively analysed. The results show that increasing pin numbers in SMPF tool decreases shape errors and stress variations in the loaded blank, leading to lower deflections of the formed blank after CAF. With increasing pin numbers, the formed shape approaches the shape formed with corresponding surface tool. The shape error percentage in loaded blanks is significantly enlarged after CAF with SMPF tool, from 3% to more than 20% for singly-curved tool shapes with aluminium alloy 6082, and detailed value varies with tool shapes. Stresses in loaded blanks directly affect CAFed shapes and it has been found for the first time that there is a same stress discrepancy level between loaded blanks with SMPF tool and corresponding surface tool to achieve a particular shape accuracy after CAF with different tool shapes. It is proposed that the stress discrepancy parameter in loaded blanks can be used as a new and more efficient design criterion for pin arrangement in SMPF tool for CAF process. In addition, an asymmetric pin pattern, which reduces half of pins in SMPF tool and increases efficiency, has been proposed and its effectiveness for CAF process has been tested and discussed.

JOURNAL ARTICLE

Zhou W, Yu J, Lin J, Dean TAet al., 2019, Manufacturing a curved profile with fine grains and high strength by differential velocity sideways extrusion, International Journal of Machine Tools and Manufacture, Vol: 140, Pages: 77-88, ISSN: 0890-6955

For structural and aerodynamic reasons, curved profiles are widely used in the transport industry for manufacturing lightweight structures. In the present work, a curved AA1050 bar with fine grains and high strength was manufactured by a novel forming technique, differential velocity sideways extrusion (DVSE). The evolution of grain structure and micro-texture during DVSE and the mechanical properties of the formed bar were studied, and the grain refinement mechanism was revealed. Due to the severe strains arising in the process, (greater than that for conventional one pass equal channel angular extrusion), significant grain refinement in the curved bar (grain size ∼3 μm) was achieved from the original billet (grain size ∼357 μm) in one extrusion operation. Coarse band-like structures containing subgrains with low angle boundaries in the shearing zone gradually transformed into fine shear band-like structures containing equiaxed (sub)grains with a mixture of low and high angle boundaries. The fine shear band-like structures inclined approximately along the shear intersection planes. Severe plastic deformation induced a high dislocation density that initiated subgrain walls with low angle boundaries, which gradually transformed into grain boundaries with high misorientation, indicating that refinement of AA1050 grains in the DVSE process is due mainly to continuous dynamic recrystallization (cDRX). Due to the appearance of greater effective strain on the inner bend of the extruded bar, the grain refinement degree and high angle boundary fraction of the material on the inner bend are slightly greater than those of the material on the outside. DVSE resulted in a weak C-type shear-texture component which can be determined by a proper rotation of the negative simple shear texture. Compared with the billet, significant increase of the hardness, yield strength and ultimate tensile strength by 134.8%, 354.0% and 116.8% respectively was achieved in the formed c

JOURNAL ARTICLE

Zheng J-H, Pan R, Wimpory RC, Lin J, Li C, Davies CMet al., A novel manufacturing process and validated predictive model for high-strength and low-residual stresses in extra-large 7xxx panels, Materials & Design, ISSN: 0264-1275

A novel manufacturing process, enabling the production of high quality (i.e. with low and controllable residual stress distributions and good mechanical properties) T-section 7xxx panels, has been established. This process provides a solution to residual stress induced distortion problems, which greatly concerns a range of industries and especially the aircraft industry. This process consists of three sequential steps — water quenching (WQ), cold rolling (CR) and constrained ageing (CA). The effectiveness of this process was experimentally verified, through applying this process to laboratory sized 7050 T-section panels. The RS was measured by neutron diffraction and X-ray techniques, in addition to deflections and hardness at each processing stage. An integrated Finite Element (FE) model, including all three steps, was developed to simulate this manufacturing process and predict both the RS and the final strength distributions. It has been concluded that this novel process can effectively reduce the residual stresses from ±300 MPa to within ±100 MPa and produce T-section panels with required mechanical properties (i.e. hardness: ~159 HV10). A cold rolling level of 1.5% was found most appropriate. The residual stress and yield strength distributions were accurately predicted by FE, providing a valuable prediction tool to process optimization for industrial applications.

JOURNAL ARTICLE

Rong Q, Li Y, Shi Z, Meng L, Sun X, Sun X, Lin Jet al., 2019, Experimental investigations of stress-relaxation ageing behaviour of AA6082, Materials Science and Engineering: A, Vol: 750, Pages: 108-116, ISSN: 0921-5093

Stress-relaxation ageing behaviour of peak-aged aluminium alloy 6082 (AA6082-T6) has been experimentally investigated with initial loading in both elastic and plastic regions in this study. The stress-relaxation ageing tests with various initial strain levels were carried out at 160 °C for up to 12 h and room temperature tensile tests were performed subsequently. Selected samples have been examined using transmission electron microscopy (TEM) to study microstructural evolution during the process. The results show that the total stress relaxed after 12 h test increases significantly with increasing initial strain levels. The initial strain levels contribute little effect on yield strength evolution during stress-relaxation ageing, as softening from the coarsening of precipitates is balanced by hardening from dislocations. Stress-relaxation mechanisms of the material have been analysed on the basis of creep stress exponent. In the elastic region, a creep stress exponent of 3 with a threshold stress of 70.1 MPa has been obtained, indicating a dislocation glide controlled creep mechanism; while in the plastic region, a decreasing threshold stress with increasing initial strain levels has been proposed to achieve a positive creep stress exponent.

JOURNAL ARTICLE

Pan R, Pirling T, Zheng J, Lin J, Davies CMet al., 2019, Quantification of thermal residual stresses relaxation in AA7xxx aluminium alloy through cold rolling, Journal of Materials Processing Technology, Vol: 264, Pages: 454-468, ISSN: 0924-0136

Residual stresses (RS) are often induced through quenching of aluminum alloys and present a potential risk of developing crack or distortion in subsequent manufacturing processes. Study of methods to minimise the RS in quenched components is of practical importance. In this paper, cold rolling (CR) has been carried out to remove the RS in quenched AA7050 blocks. The CR effect on relaxing RS in quenched AA7050 blocks has been evaluated via the neutron diffraction (ND), X-ray diffraction (XRD) and contour techniques. The results reveal that although CR transforms near-surface residual stresses from large compression to large tension along the rolling direction, it results in remarkable RS relief in the core part of the material. An integrated finite element model for RS evolution through the CR process was put forward and has been validated by the experimental results.

JOURNAL ARTICLE

Lyu F, Li Y, Huang X, Shi Z, Zeng Y, Lin Jet al., 2019, An investigation of creep age forming of AA7B04 stiffened plates: Experiment and FE modelling, Journal of Manufacturing Processes, Vol: 37, Pages: 232-241, ISSN: 1526-6125

Creep age forming (CAF) of aluminium alloy 7B04 (AA7B04) stiffened plates has been experimentally and numerically investigated in this study. Creep-ageing experiments of AA7B04-T651 were conducted under different tensile stress levels at 140 °C for up to 20 h, and a set of unified constitutive equations was calibrated based on the experimental results of the evolutions of creep strain, yield strength and precipitate size, which was implemented into ABAQUS for CAF process modelling. CAF experiments and corresponding simulations of AA7B04 stiffened plates were then carried out and the effect of stiffener height and die radius on springback and yield strength was studied. It was found that the springback percentage decreases with increasing stiffener height and decreasing forming die radius, and the yield strength is slightly lower in the stiffener than in the skin of the CAFed stiffened plates due to stress effect on ageing progression. A good agreement has been achieved between experimental and corresponding FE results, with maximum deviations of 6.7% and 3.3% respectively for springback and yield strength.

JOURNAL ARTICLE

Li Y, Rong Q, Shi Z, Sun X, Meng L, Lin Jet al., 2018, An accelerated springback compensation method for creep age forming, International Journal of Advanced Manufacturing Technology, ISSN: 0268-3768

Springback compensation is essential for tool design in creep age forming (CAF) process. In this study, a new accelerated springback compensation method integrating springback mechanism of a plate with creep-ageing behaviour of materials has been developed for CAF tool design to manufacture both singly and varyingly curved products. Springback compensation curves that relate the objective shapes and springback compensated shapes by their curvature, stress and strain states have been established, based on the numerical solution of springback behaviour of CAF process. For singly curved products, a one-step springback compensation method is proposed with reference to the springback compensation curves, and its effectiveness has been demonstrated by CAF test with a peak-aged aluminium alloy AA6082-T6. For products with varying curvatures, an accelerated method is developed for CAF tool design by integrating springback compensation curves with finite element (FE) assisted displacement adjustment techniques. The new accelerated method can significantly improve the tool design efficiency for CAF process when compared with conventional displacement adjustment techniques and has been verified by CAF manufacture of a varyingly curved product with AA6082-T6 material. The new accelerated springback compensation method developed in this study can be used for efficient tool design for CAF process of various products.

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

Li Y, Shao Z, Rong Q, Shi Z, Balint D, Sun X, Meng L, Lin Jet al., 2018, Development of similarity-based scaling criteria for creep age forming of large/extra-large panels, The International Journal of Advanced Manufacturing Technology, 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

Pan R, Zheng J, Zhang Z, Lin Jet al., 2018, Cold rolling influence on residual stresses evolution in heat-treated AA7xxx T-section panels, MATERIALS AND MANUFACTURING PROCESSES, Vol: 34, Pages: 431-446, ISSN: 1042-6914

JOURNAL ARTICLE

Lin J, Axinte D, 2018, Professor Trevor A. Dean and his contribution to the Journal, International Journal of Machine Tools and Manufacture, Vol: 133, Pages: 1-3, ISSN: 0890-6955

JOURNAL ARTICLE

Kopec M, Wang K, Wang Y, Wang L, Lin Jet al., 2018, Feasibility study of a novel hot stamping process for Ti6Al4V alloy, MATEC Web of Conferences

© The Authors, published by EDP Sciences, 2018. To investigate the feasibility of a novel hot stamping process for the Ti6Al4V titanium alloy using low temperature forming tools, mechanical properties of the material were studied using hot tensile tests at a temperature range of 600 - 900°C with a constant strain rate of 1s-1. Hot stamping tests were carried out to verify the feasibility of this technology and identify the forming window for the material. Results show that when the deformation temperature was lower than 700°C, the amount of elongation was less than 20%, and it also had little change with the temperature. However, when the temperature was higher than 700°C, a good ductility of the material can be achieved. During the forming tests, parts failed at lower temperatures (600°C) due to the limited formability and also failed at higher temperatures (950°C) due to the phase transformation. The post-form hardness firstly decreased with the temperature increasing due to recovery and then increased due to the phase transformation. Qualified parts were formed successfully between temperatures of 750 - 850°C, which indicates that this new technology has a great potential in forming titanium alloys sheet components.

CONFERENCE PAPER

Li Y, Yang Y-L, Rong Q, Shi Z, Lin J, Said Ret al., 2018, Effect of initial temper on mechanical properties of creep-aged Al-Cu-Li alloy AA2050, 5th International Conference on New Forming Technology - ICNFT 2018, Publisher: EDP Sciences, ISSN: 2261-236X

The evolution of mechanical properties of a third generation Al-Cu-Li alloy, AA2050, with different initial tempers (T34 and as-quenched (WQ)) during creep-ageing has been investigated and analysed in this study. A set of creep-ageing tests under 150 MPa at 155 °C for up to 24 h was carried out for both initial temper conditions and tensile tests were performed subsequently to acquire the main mechanical properties of the creep-aged alloys, including the yield strength, ultimate tensile strength (UTS) and uniform elongation. Precipitation behaviour of the T34 and WQ AA2050 alloys has been summarised and successfully explains the detailed evolutions of the obtained mechanical properties of the alloy with these two initial tempers during creep-ageing. The results indicate that the T34 alloy can be a better choice for creep age forming (CAF) process compared with WQ alloy, as it provides better yield strength and uniform elongation properties concurrently after creep-ageing. In addition, a work hardening rate analysis has been carried out for all the creep-aged alloys, helping to reveal the detailed dislocation/precipitates interaction mechanisms during plastic deformation in the creep-aged T34 and WQ AA2050 alloys.

CONFERENCE PAPER

Rong Q, Li Y, Sun X, Shi Z, Meng L, Lin Jet al., 2018, Experimental studies of the efficient use of flexible tool in creep age forming, 5th International Conference on New Forming Technology - ICNFT 2018, Publisher: EDP Sciences, ISSN: 2261-236X

Application of a newly developed flexible forming tool to creep age forming (CAF) process has been investigated in this study. The flexible tool mainly consists of sparsely distributed forming pins, splines and elastomeric sheet. The effect of key factors related to the forming tool on the shape of the formed parts has been studied through various CAF experiments. The key factors investigated in this study include: the interval between forming pins, the arrangement of pins, the accuracy requirement of pin height and the material of splines. It has been found that reducing the interval between pins can efficiently smooth the shape of CAFed plates. The feasibility of asymmetric arrangement of pins has been proven, which can decrease the number of used pins, reduce tool weight, and increase efficiency. The forming results are very sensitive to the pin height, thus the experimental set-up error should be carefully controlled. Additionally, compared with mild steel, spring steel is more suitable as the spline material.

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.

CONFERENCE PAPER

Lane C, Shao Z, Zheng K, Lin Jet al., 2018, Effect of the thickness reduction of specimens on the limit strains in thermomechanical tensile tests for hot-stamping studies, Manufacturing Review, Vol: 5, ISSN: 2265-4224

Sheet metal formability under hot stamping conditions has been evaluated using a novel planar testing system developed previously, being used within a Gleeble machine. Nevertheless, the specimen design with the central recess was not 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 accuracy of the calculation of forming limits based on the use of specimen with thickness reduction was highly dependent on the selection of the stage of the deformation of the specimen.

JOURNAL ARTICLE

Li Y, Shi Z, Lin J, Yang Y, Saillard P, Said Ret al., 2018, Effect of machining-induced residual stress on springback of creep age formed AA2050 plates with asymmetric creep-ageing behaviour, International Journal of Machine Tools and Manufacture, Vol: 132, Pages: 113-122, ISSN: 0890-6955

Effect of machining-induced residual stresses on springback of creep age formed (CAFed) AA2050 plates with asymmetric creep-ageing behaviour has been investigated experimentally and numerically in this study. A finite element (FE) model integrating unified asymmetric creep-ageing constitutive model for CAF of AA2050 has been developed through a commercial FE software-PAM-STAMP and a simplified machining-induced residual stress field for aluminium alloy plates has been proposed and implemented into the FE model. Different levels of residual stresses have been considered in the FE model and numerical results have been compared with corresponding CAF experiments and surface residual stress measurements. Results show that the springback prediction of the FE models has been significantly improved when the residual stress effect was considered. The best prediction can be achieved with a certain residual stress level for each plate thickness and the level increases with decreasing thickness of the plates produced by machining, which agrees well with the surface residual stress measurement results. For CAF of AA2050 with asymmetric creep-ageing behaviour, different springback behaviour has been observed with different residual stress levels: introduction of residual stress fields can either decrease or increase formed deflections of CAFed AA2050 plates, depending on the relative levels of residual stresses and the stresses caused by applied external forces. When the residual stress is large, subsurface of the plate may exceed the elastic limit after applying external force, leading to a significant decrease of springback in the CAFed plates.

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

Li Y, Shi Z, Yang Y-L, Lin J, Said Ret al., 2018, Experimental and numerical study of creep age forming of AA2050 plates with sparse multi-point flexible forming tool, Metal Forming 2018, 17th International Conference on Metal Forming, Publisher: Elsevier, Pages: 1016-1023, ISSN: 2351-9789

Creep age forming of an Al-Cu-Li alloy (AA2050-T34) with an asymmetric tension/compression creep-ageing behaviour has been experimentally and numerically investigated in this study. Creep age forming experiments of plates with different thicknesses have been carried out to form doubly curved shapes using a recently developed sparse multi-point flexible forming tool. Corresponding FE model for creep age forming, incorporating the material model that describes the stress-strain relation and asymmetric creep-ageing behaviour of the alloy, has been developed through PAM-STAMP. The flexible forming tool can generate various tool shapes for creep age forming and is ideal for innovative trials. Stress concentration was found in some forming pin areas of the loaded plate. With larger initial loaded plastic strain and higher creep strain generated in these local areas, bumps may occur in formed shapes after creep age forming under certain circumstances. It is shown that the FE model is able to predict the formed shape accurately for thicker plates and can be used for tool set-up optimisation.

CONFERENCE PAPER

Chung T-F, Yang Y-L, Hsiao C-N, Li W-C, Huang B-M, Tsao C-S, Shi Z, Lin J, Fischione PE, Ohmura T, Yang J-Ret al., 2018, Morphological evolution of GP zones and nanometer-sized precipitates in the AA2050 aluminium alloy, International Journal of Lightweight Materials and Manufacture, ISSN: 2588-8404

Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM) was employed to examine the phases in Al-Cu-Li alloy (AA2050), including GP(T1), GP(θꞌꞌ) and GPB zones with their subsequent nanometer-sized products, T1 (Al2CuLi), θꞌ (Al2Cu), and S (Al2CuMg) precipitates, respectively. Under the peak-aging condition, some solute-atom enriched clusters could still be found, and the newly-formed nucleus of GP(θꞌꞌ) with a mono-layer {100} plane of Cu atoms occurred at the adjacent area of the joint between θꞌ and S precipitates or the edge of an individual S precipitate. The transition of a single Cu-layer GP(θꞌꞌ) → θꞌ was presumed to be transformation via in-situ nucleation. The developing GP(θꞌꞌ) zones and θꞌ precipitates were easily subjected to soft impingement. However, hard impingement between two variants of θꞌ presumably occurred, wherein one θꞌ variant precipitate was blocked out by the other θꞌ variant. As for the creep-ageing forming (CAF) treated sample, some precipitates of T1 and θꞌ were found to have the cutting characteristic on specific ledges.

JOURNAL ARTICLE

Zheng J, Lin J, Pan R, Lee J, Li C, Davies CMet al., 2018, A novel constitutive model for multi-step stress relaxation ageing of a pre-strained 7xxx series alloy, International Journal of Plasticity, Vol: 106, Pages: 31-47, ISSN: 0749-6419

A novel set of unified constitutive equations has been developed and validated to describe stress relaxation ageing (SRA) behaviour of 7xxx series aluminium alloys. The model, based on dynamic ageing and power-law creep relations, can predict the stress relaxation, age hardening response and their interactions at different temperatures, through considering the microstructure evolutions (precipitate radius, volume fraction and dislocation density) during SRA. In addition, the model newly incorporates the effects of prior plastic strain. This model was verified through T74 multi-step SRA experiments for different pre-strain conditions. Excellent agreement was achieved between the predicted and experimental results for stress relaxation and yield strength variation. The evolution of micro-internal variables (e.g. normalised precipitate radius) within the model were calibrated by observing transmission electron microscopy (TEM) images performed in this work and available in literature. The advanced constitutive model developed predicts the mechanical properties and residual stresses in components after ageing. Therefore, the model provides a valuable tool to optimise manufacturing processes leading to many benefits including reduced scrap rates and financial losses.

JOURNAL ARTICLE

Xiao W, Wang B, Zheng K, Zhou J, Lin Jet al., 2018, A study of interfacial heat transfer and its effect on quenching when hot stamping AA7075, ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING, Vol: 18, Pages: 723-730, ISSN: 1644-9665

JOURNAL ARTICLE

Politis DJ, Politis NJ, Lin J, Dean TAet al., 2018, A review of force reduction methods in precision forging axisymmetric shapes, INTERNATIONAL JOURNAL OF ADVANCED MANUFACTURING TECHNOLOGY, Vol: 97, Pages: 2809-2833, ISSN: 0268-3768

JOURNAL ARTICLE

Pan R, Shi Z, Davies CM, Li C, Kaye M, Lin Jet al., 2018, An integrated model to predict residual stress reduction by multiple cold forging operations in extra-large AA7050 T-section panels, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol: 232, Pages: 1319-1330, ISSN: 0954-4054

A finite element (FE) model has been developed to determine the effectiveness of acold compression technique to reduce the large residual stresses generated fromquenching solution heat treated T-Section components of aluminium alloy AA7050. Tocompress long components, a multi-step process is required with some amount ofoverlap. A parametric study has been performed to determine the effect of thecompression ratio, friction coefficient, length of overlap and length of the T-sectioncomponent on the residual stress distribution post quenching and after subsequent coldcompression. Generally a percentage reduction in the peak residual stress of over 90%was found. The optimal parameters for residual stress relief by cold compression havebeen suggested from the cases considered.

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

Li Y, Shi Z, Lin J, Yang Y-L, Saillard P, Said Ret al., 2018, FE simulation of asymmetric creep-ageing behaviour of AA2050 and its application to creep age forming, International Journal of Mechanical Sciences, Vol: 140, Pages: 228-240, ISSN: 0020-7403

A finite element (FE) model has been developed and validated in this study for the first-time to simulate the asymmetric creep-ageing behaviour of an Al-Cu-Li alloy (AA2050) for creep age forming (CAF) applications. An implicit integration algorithm integrated with the Secant method was proposed to efficiently solve the creep-ageing constitutive model of AA2050 and a “maximum principal stress” technique was employed to characterise the asymmetric tension and compression creep behaviour for CAF FE simulation. The proposed algorithm has been implemented into the FE solver in PAM-STAMP via a user-defined subroutine and an implicit FE model has been developed for CAF of AA2050. The effectiveness of the developed FE model has been validated by four-point-bending creep-ageing experiments of AA2050 plates with different thicknesses. The springback behaviour of AA2050 plates after 18 h CAF with a doubly curved tool was then predicted using the validated FE model. The results show that springback levels of the CAFed plates decreased with increasing thickness. Significant springback was observed in all the CAFed AA2050 plates within elastic loading, for example, the 8 mm plate which was initially loaded to near yielding had a springback value of 87.1% after 18 h CAF at 155 °C. The implicit algorithm and the maximum principal stress technique can be employed for constitutive models for other alloys with asymmetric creep-ageing behaviour.

JOURNAL ARTICLE

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