Publications
409 results found
Politis DJ, Politis NJ, Lin J, et 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
Zheng J, Lin J, Pan R, et 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.
Xiao W, Wang B, Zheng K, et 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
Pan R, Shi Z, Davies CM, et 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.
Shao Z, Li N, Lin J, et 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.
Zheng K, Politis DJ, Wang L, et al., 2018, A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components, International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 55-80, ISSN: 2588-8404
Chung T-F, Yang Y-L, Huang B-M, et al., 2018, Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy, Acta Materialia, Vol: 149, Pages: 377-387, ISSN: 1359-6454
High resolution transmission electron microscopy (HRTEM) with nanometer-scaled energy-dispersive X-ray (EDX) was employed to investigate the transformation mechanisms of the GP zone → η′ → η precipitation sequence of AA7050, an Al-Zn-Mg-Cu alloy. Serial in-situ HRTEM frames revealed that separated nucleation of an η′ precipitate occurred elsewhere as the adjacent GPII zone dissolved. Evidence from HRTEM coupled with EDX showed that in-situ nucleation of a new η2 precipitate (one form of η) took place, wherein it gradually developed from the original η′ precipitate via a similar hexagonal structure with different compositions. The in-situ transition product was composed of two distinctive regions; one was identified as η′, and the other, as η.
Li Y, Shi Z, Lin J, et 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.
Zheng K, Lee J, Xiao W, et al., 2018, Experimental investigations of the in-die quenching efficiency and die surface temperature of hot stamping aluminium alloys, Metals, Vol: 8, ISSN: 2075-4701
The in-die quenching is a key stage in the hot stamping volume production chain which determines the post-formed strength of lightweight alloy components, tool life, and hot stamping productivity. In this paper, the performance of in-die quenching, reflected by the quenching efficiency (the time of work-piece held within stamping dies) and die surface temperature during the simulated hot stamping process of AA6082, was experimentally and analytically investigated. A range of in-die quenching experiments were performed for different initial work-piece and die temperatures, quenching pressures, work-piece thickness, and die clearances, under hot stamping conditions. In addition, a one-dimensional (1D) closed-form heat transfer model was used to calculate the die surface temperature evolution that is difficult to obtain during practical manufacture situations. The results have shown that the in-die quenching efficiency can be significantly increased by decreasing the initial work-piece and die temperatures. Die clearances are required to be designed precisely to obtain sufficiently high quenching rates and satisfying post-formed strength for hot-stamped panel components. This study systematically considered an extensive variety of influencing factors on the in-die quenching performance, which can provide practical guides for stamping tool designers and manufacture systems for hot-stamping volume production.
Zhou W, Lin J, Dean TA, et al., 2018, Analysis and modelling of a novel process for extruding curved metal alloy profiles, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, Vol: 138, Pages: 524-536, ISSN: 0020-7403
Zheng K, Lin J, Wu G, et al., 2018, Experimental investigation and modelling of hot forming B4C/AA6O61 low fraction volume reinforcement composites, Journal of Theoretical and Applied Mechanics, Vol: 56, Pages: 457-469, ISSN: 1429-2955
This paper presents an experimental investigation of the hot deformation behaviour of 15%B4C particle reinforced AA6061 matrix composites and the establishment of a novel correspondingunified and physically-based visco-plastic material model. The feasibility of hotforming of a metal matrix composite (MMC) with a low volume fraction reinforcement hasbeen assessed by performing hot compression tests at different temperatures and strain rates.Examination of the obtained stress-strain relationships revealed the correlation betweentemperature and strain hardening extent. Forming at elevated temperatures enables obviousstrain rate hardening and reasonably high ductility of the MMC. The developed unified materialmodel includes evolution of dislocations resulting from plastic deformation, recoveryand punching effect due to differential thermal expansion between matrix and reinforcementparticles during non-steady state heating and plastic straining. Good agreement has beenobtained between experimental and computed results. The proposed material model contributesgreatly to a more thorough understanding of flow stress behaviour and microstructuralevolution during the hot forming of MMCs.
Zhou W, Shi Z, Lin J, 2018, Upper bound analysis of differential velocity sideways extrusion process for curved profiles using a fan-shaped flow line model, International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 21-32, ISSN: 2588-8404
An analytical model for predicting the shapes of rectangular bars with variable curvatures along their lengths through a novel forming method, differential velocity sideways extrusion (DVSE), previously proposed by the authors, has been developed on the basis of the upper bound method. A new flow line function was presented to describe its deformation field. The plastic deformation zone (PDZ) was assumed to be fan-shaped, where the trajectory of the material flow within the PDZ had an elliptic shape. The proposed continuous flow line function was validated using finite element simulations. The flow patterns, extrusion pressure, curvature, and effective strain predicted by the analytical solutions agreed well with modelling results. Compared to the classical discontinuous simple shear model of channel angular extrusion (CAE) with a 90° die, the new approach was shown to predict the effective strain more closely.
Kopec M, Wang K, Politis DJ, et al., 2018, Formability and microstructure evolution mechanisms of Ti6Al4V alloy during a novel hot stamping process, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 719, Pages: 72-81, ISSN: 0921-5093
A novel hot stamping process for Ti6Al4V alloy using cold forming tools and a hot blank was presented in this paper. The formability of the material was studied through uniaxial tensile tests at temperatures ranging from 600 to 900°C and strain rates ranging from 0.1 to 5 s −1 . An elongation ranging from 30% to 60% could be achieved at temperatures ranging from 750 to 900°C respectively. The main microstructure evolution mechanisms varied with the deformation temperature, including recovery, phase transformation and recrystallization. The hardness of the material after deformation first decreased with the temperature due to recovery, and subsequently increased mainly due to the phase transformation. During the hot stamping tests, qualified parts could be formed successfully at heating temperatures ranging from 750 to 850°C. The forming failed at lower temperatures due to the limited ductility of the material. At temperatures higher than 900 °C, extensive phase transformation of α to β occurred during the heating. During the transfer and forming, the temperature dropped significantly which led to the formation of transformed β reduction of the formability and subsequent failure. The post-form hardness distribution demonstrated the same tendency as that after uniaxial tensile tests.
Zhou W, Lin J, Dean TA, et al., 2018, Feasibility studies of a novel extrusion process for curved profiles: Experimentation and modelling, International Journal of Machine Tools and Manufacture, Vol: 126, Pages: 27-43, ISSN: 0890-6955
The work described in this paper concerns a novel method for directly forming curved profiles/sections from billets in one extrusion operation using two opposing punches. Its mechanics are based on internal differential material flow, and it has been given the acronym, differential velocity sideways extrusion (DVSE). A tool set enabling sideways extrusion to be performed using opposing punches moving with different velocities was used for a series of experiments in which punch velocity ratio and extrusion ratio were process parameters. Plasticine was used as a model work-piece material and a series of compression tests were undertaken, to determine its constitutive properties and gain an estimate of work-piece die friction for use in process simulation. Curvature of extrudate can be controlled and varied using a difference between the velocities of the two punches, defined by velocity ratio. Greater curvature is achieved with lower velocity ratio. Curvature is also dependent on extrusion ratio, an increase in which increases curvature, although curvature is less sensitive to it than to velocity ratio. The extent of work-piece flow velocity gradient across the die exit orifice, which causes curvature, has been identified. Severe plastic deformation of the extrudate occurs in a way similar to channel angular extrusion (CAE), thus a greatly promoted effective strain level is achieved, though it is not always uniform across a section. The inner bending region of an extrudate experiences maximum localised effective strain, which decreases with decrease in curvature. To the authors' knowledge this is the first publication in which extrudate curvature is deliberately induced using opposing punches with differential velocities. Although only fixed velocity ratio values have been used in the work described in this paper the ability to change during operation exists and the process has the potential for the production of a profile with different curvature along its length.
Chavoshi SZ, Jiang J, Wang Y, et al., 2018, Density-based constitutive modelling of P/M FGH96 for powder forging, International Journal of Mechanical Sciences, Vol: 138-139, Pages: 110-121, ISSN: 0020-7403
A set of viscoplastic constitutive equations is presented in this study to predict hot compressive deformation behaviour and densification levels of powder metallurgy (P/M) FGH96 nickel-base superalloy during direct powder forging (DPF) process. The constitutive equations make use of the elliptic equivalent stress proposed in porous material models, and unify the evolution of relative density, normalised dislocation density, isotropic hardening and flow softening of the powder compact. A gradient-based optimisation technique is adopted to determine the material constants using the experimental data obtained from Gleeble isothermal uniaxial compression tests of HIPed FGH96 at different temperatures and strain rates. The developed constitutive equations are incorporated into finite element code DEFORM via user-defined subroutine for coupled thermo-mechanical DPF process modelling. The constitutive equations benefiting from the viscoplastic densification model of the calibrated Abouaf, among the six studied porous material models, compare favourably with the experimental data, while the equations integrating the porous material model of Shima and Oyane provide excellent agreement with experiments in the low density outer region of the powder compact.
Zheng JH, Pan R, Li C, et al., 2018, Experimental investigation of multi-step stress-relaxation-ageing of 7050 aluminium alloy for different pre-strained conditions, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 710, Pages: 111-120, ISSN: 0921-5093
A novel insight into the whole two-step stress relaxation ageing process during T74 multi–step ageing treatment (120 °C for 6 h and subsequently 177 °C for 7 h), which is typically experienced by extra-large aircraft components that contain high residual stresses, has been established. Stress relaxation ageing (SRA) tests, tensile tests and transmission electron microscopy (TEM) were performed on AA7050 samples to determine the relationship between internal microstructure and macroscopic behaviour during the stress relaxation and precipitate evolution process. Samples were subjected to SRA at different initial stresses (220–360 MPa) after being pre-strained to different extents (i.e. 0%, 1%, 3%). Room temperature tensile tests were then performed on interrupted SRA test specimens to examine the corresponding strengthening phenomenon. TEM was performed on a selection of peak–aged and T74 over–aged samples to study the precipitate distribution. At 120 °C typical stress relaxation behaviour was observed and the data follow ed a logarithmic curve. Subsequently at 177 °C, dislocation–creep dominated stress relaxation behaviour, with no apparent threshold stress, was observed. The absence of a threshold stress at 177 °C may be attributed to the continuous over-ageing phenomenon. The effect of pre-deformation levels and initial stresses on SRA has also been investigated. Pre-stretching, which creates uniformly distributed dislocations, promotes stress relaxation and ageing. No significant influence of initial stress level on SRA was observed at 120 °C, but noticeable effects were seen at 177 °C. The calculated stress exponent n at 177 °C is found independent of the initial stresses. These findings provide clear scientific guidance for residual stress reduction during the multi-step ageing process of AA7050 and provide the basis for residual stress prediction models.
Politis DJ, Politis NJ, Lin J, et al., 2018, An analysis of the tooth stress distribution of forged bi-metallic gears, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, Vol: 232, Pages: 124-139, ISSN: 0954-4062
Huo Y, Wang B, Lin J, et al., 2017, Hot compression deformation behavior and microstructure evolution rule of a high-speed railway axle steel, Indian Journal of Engineering and Materials Sciences, Vol: 24, Pages: 447-454, ISSN: 0971-4588
© 2017, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved. The high performance of high-speed railway axle (HSRA) depends on the stress-strain state and microstructure formed in the hot processes, it is necessary to investigate the deformation behavior and microstructure evolution during hot uniaxial compression. Hot compression test was performed on specimen of a HSRA steel 25CrMo4 at a deformation temperature of 1040-1160oC at a strain rate of 1.0-10.0 /s using a Gleeble thermal mechanical simulator. During hot compression test, samples were compressed to different true strain: 0, 0.2, 0.4, 0.6 and 0.8. Hot compressive deformation behaviors and effects of processing parameters, including forming temperature, strain rate and deformation degree, on microstructure evolution of HSRA steel 25CrMo4 are investigated and studied by metallurgical analysis. Experiments results show that the peak value of flow stress increases about 30 MPa at a certain deformation temperature when the strain rate increases from 1.0 /s to 10.0 /s. At a certain strain rate, the peak value of flow stress increases about 20-30 MPa when the forming temperature decreases about 60oC. The average grain sizes increases with the increasing of forming temperature at a certain strain rate and a given strain. For a given forming temperature, grain size decreased before the critical strain of 0.4 and increased after strain of 0.4 due to grain growth, especially at lower strain rate. Therefore, grain size at lower strain rate is larger than that at higher strain rate when true strain reached 0.8.
Liu J, Wang A, Zheng Y, et al., 2017, Hot stamping of AA6082 tailor welded blanks for automotive applications, Procedia Engineering, Vol: 207, Pages: 729-734, ISSN: 1877-7058
Friction stir welded (FSWed) AA6082 tailor welded blanks (TWBs), with gauge combinations of 2.0-2.5 and 3.0-5.0 mm, have been prepared and successfully formed into automotive panel components. Experimental results indicated that the post-form strength, in terms of hardness, varied from location to location on the final parts. The strength is highly dependent on the blank gauges, with the average hardness values being HV 110 and HV 98 for the 2.0-2.5 and 3.0-5.0 mm TWB parts, respectively. Conventional FE simulation was built in PAM-STAMP and the prediction results were validated from experimental data in terms of strain distribution and temperature evolution. A typical continuous cooling precipitation (CCP) diagram for AA6082 was implemented into the verified simulation data to explain the strength variations. It is deemed that the temperature history during the stamping and quenching stages has played a major role on the post-form strength of the final parts.
Shi Z, Wang L, Mohamed M, et al., 2017, A new design of friction test rig and determination of friction coefficient when warm forming an aluminium alloy, International Conference on the Technology of Plasticity, ICTP 2017, Publisher: Elsevier, Pages: 2274-2279, ISSN: 1877-7058
To facilitate reduced fuel consumption and increase environmental friendliness, in recent years, demands for lightweight vehicles have been increasing, and interest in hot or warm forming of sheet aluminium alloys for use in vehicle body structures, has grown. For better understanding and optimisation of the forming processes, knowledge of friction coefficient between tooling and work-piece, at elevated temperature, is critical. However, because of difficulties with measurement at elevated temperature, most studies on friction are limited to room temperature. In this study, a friction rig was designed for isothermal tests at elevated temperature. The test rig enables pure sliding between pins (made of a tool steel) and a metal sheet. The friction behaviour of Forge Ease 278, a water based solid lubricant pre-applied to aluminium alloy AA5754, was investigated, under isothermal warm forming conditions, using the test rig. The effects of testing temperature, sliding speed and applied pressure on the friction coefficient were studied. It was found that Forge Ease produced a low friction coefficient of around 0.05, above room temperature and below 250 °C. The lubricant performance degrades at 350 °C and the friction coefficient increases markedly. Both sliding speed (up to 150 mm s -1 ) and applied pressure (up to 12.8 MPa) had no significant effect on friction coefficient of Forge Ease.
Huang X, Wang B, Lin J, et al., 2017, Effect of mandrel diameter on non-circularity of hollow shafts in cross wedge rolling, International Conference on the Technology of Plasticity, ICTP 2017, Pages: 2376-2381
© 2017 The Authors. Published by Elsevier Ltd. Because of the need for lightweight design, the application of hollow shaft parts rapidly increases. The mandrel plays an important role in the process of forming hollow shafts using cross wedge rolling (CWR). This paper presents the experimental and numerical analysis on the effect of the mandrel diameter on non-circularity in CWR for the hollow shafts with mandrel. A thermomechanical finite element (FE) model was developed to simulate the CWR process of hollow shaft. The experiments and numerical analysis suggest that the diameter of mandrel has a great effect on the non-circularity of hollow shafts. During the CWR process, because of the effect of the mandrel, the workpiece is compressed in radial direction and extended along the circumferential direction, which lead to the increase of the non-circularity. With increasing diameters, the workpieces show larger non-circularity. The non-circularity slowly increases when mandrel diameter is small. However, when the mandrel diameter exceeds a certain value, the non-circularity increases rapidly. Besides, the non-circularity of the inner hole is larger than that of the outer circle. According to the strain curves during the forming process, it can be found that circumferential compressive strain plays an important role in the rounding forming stage. Larger compressive strains decrease the non-circularity of the rolled pieces.
Zheng J-H, Davies CM, Lin J, et al., 2017, Constitutive modelling of a T74 multi-step creep ageing behaviour of AA7050 and its application to stress relaxation ageing in age formed aluminium components, International Conference on the Technology of Plasticity, ICTP 2017, Pages: 281-286
© 2017 The Authors. Published by Elsevier Ltd. This work focuses on validating a model, which is defined using load controlled creep ageing data, from strain-controlled stress relaxation ageing tests. A set of phenomenological constitutive equations were proposed. The material constants within the equations were determined from multi-step (120 °C× 6 h +177 °C ×7 h) creep-ageing test data. Temperature effects on the internal state variables (i.e. precipitate size, volume fraction and dislocation density) were also considered. This model was further used to predict the stress relaxation phenomena under the same ageing condition. Although similar stress reduction trends and features were observed, the stress reduction was significantly over-predicted. The over-prediction of the stress reduction magnitude suggests that improvements should be applied to the current modelling method for simulating spring-back after creep age forming.
Zhou W, Lin J, Dean TA, et al., 2017, A novel application of sideways extrusion to produce curved aluminium profiles: Feasibility study, International Conference on the Technology of Plasticity, ICTP 2017, Pages: 2304-2309
© 2017 The Authors. Published by Elsevier Ltd. Conventional curved aluminium alloy profile forming methods which depend on apparatus, additional to extrusion tooling, to bend extruded, straight sections inevitably decrease manufacturing efficiency and have to deal with springback and cross-sectional distortion. In this article, a novel energy-efficient forming method based on internal differential material flow, differential velocity sideways extrusion (DVSE), is proposed, which can directly form billets into curved profiles/sections by extrusion alone, using two opposing punches. A detailed study has been conducted and cold extrusion/bending of round bars and tubes was examined. It was found that curved profiles/sections with no distortion are formed, and adjustable profile curvature can be achieved by changing the velocity ratio of a slower lower punch to a faster upper punch and also extrusion ratio. Lower velocity ratio and larger extrusion ratio tend to increase the material flow velocity gradient at the die exit and lead to greater curvature. Effect of velocity ratio is greater than that of extrusion ratio, especially when velocity ratio is less than 1/3. Effect of the extrusion ratio is small when velocity ratio is greater than 0.5, after which the effect of extrusion ratio becomes more significant as velocity ratio decreases.
Zhao L, Yasmeen T, Gao P, et al., 2017, Mechanism-based constitutive equations for superplastic forming of TA15 with equiaxed fine grain structure, International Conference on the Technology of Plasticity, ICTP 2017, Pages: 1874-1879
© 2017 The Authors. Published by Elsevier Ltd. In this study, a set of mechanism-based viscoplastic constitutive equations has been established to predict the viscoplastic flow of TA15 alloy sheets in superplastic forming (SPF) processes. Internal variables are introduced in these constitutive equations to represent individual physical features of the material with equiaxed fine grain structure, such as dislocation density, isotropic hardening, recrystallization and dynamic recovery. 13 material constants in the constitutive equations have been determined from experimental data at a range of temperatures and strain rates. A gradient based optimisation method was applied for the calibration of the equations. Good agreement between the computational and experimental results has been obtained. These newly determined constitutive equations can be used for product and process design through superplastic forming processes.
Ganapathy M, Li N, Lin J, et 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.
Rong Q, Shi Z, Li X, et al., 2017, Experimental studies and constitutive modelling of AA6082 in stress-relaxation age forming conditions, International Conference on the Technology of Plasticity, ICTP 2017, Publisher: Elsevier, Pages: 293-298, ISSN: 1877-7058
AA6082 is a popular material for automotive body structure because of its good formability and weldability. In this study, stress relaxation and ageing behaviour of AA6082 at T6 condition were investigated through stress relaxation tests with three different initial stresses at 160 °C for 12 h. Interrupted tests for 1, 3, 5 and 12 h and subsequent uniaxial tensile tests were carried out to investigate the effect of ageing time on yield strength of the material. The results show that after 12 h stress relaxation tests with initial stresses of 173, 198, and 225 MPa, the stresses relaxed were respectively 19.4 %, 24.3 %, and 25.6 % of the initial stresses. Minor change of yield strength of the material throughout the ageing time was observed. Moreover, a set of unified constitutive equations was calibrated to model the stress relaxation behaviour of AA6082 during creep age forming process, and related material constants for the AA6082 were determined. A close agreement between experimental data and computed results has been achieved for stress relaxation behaviour of the material. This study helps to understand the stress relaxation behaviors of AA6082 material, and facilitates the CAF applications of AA6082 material in industrial field.
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.
Jiang J, Hooper P, Li N, et 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.
Ma W-Y, Wang B-Y, Lin J-G, et al., 2017, Influence of process parameters on properties of AA6082 in hot forming process, TRANSACTIONS OF NONFERROUS METALS SOCIETY OF CHINA, Vol: 27, Pages: 2454-2463, ISSN: 1003-6326
Zheng J, Davies CM, Lin J, 2017, Comparison of creep deformation rates during load and strain controlled multi-step creep ageing tests on AA7050, 20th International ESAFORM Conference on Material Forming, Publisher: AMER INST PHYSICS, ISSN: 0094-243X
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