Publications
15 results found
Lv J, Zheng J, Yardley VA, et al., 2020, A review of microstructural evolution and modelling of aluminium alloys under hot forming conditions, Metals, Vol: 10, ISSN: 2075-4701
Microstructural evolution during hot forming of aluminium alloys plays a critical role in both the material flow behaviour during the deformation and the post-form mechanical properties in service. This paper presents a comprehensive review on the recrystallisation mechanisms, the interrelations between microstructures and macroscopic responses, and the associated modelling methods for aluminium alloys under hot forming conditions. Particular attention is focused on dynamic recrystallisation (DRX), which occurs during hot forming. The mechanisms, key features, and conditions of occurrence (forming temperature, strain rates, etc.) during hot forming for each type of DRX type are classified. The relationships between microstructures and macroscopic responses, including the flow behaviour, the post-form strength and ductility, are summarised based on existing experimental results. Most importantly, the associated modelling work, describing the recrystallisation and the viscoplastic behaviour under hot forming conditions, is grouped into four types, to enable a clear and concise understanding of the existing quantitative micro–macro interactions, which are particularly valuable for the future development of advanced physically based multi-scale modelling work for hot-forming processes in aluminium alloys.
Zhang K, Zheng J-H, Huang Y, et al., 2020, Evolution of twinning and shear bands in magnesium alloys during rolling at room and cryogenic temperature, Materials and Design, Vol: 193, Pages: 1-11, ISSN: 0264-1275
Twinning and shear bands are two main deformation structures in magnesium alloys at low temperatures, however, the relationship between these two deformation structures is still under debate. To clarify their relationship and behaviours at low temperatures, rolling tests to various thickness reductions at room temperature (RT) and liquid nitrogen temperature (LNT) were conducted for AZ31 magnesium alloys. The evolutions of shear bands and twinning, and their interactions with geometrically necessary dislocation (GND), were observed during the RT- and LNT-rolling process. Abundant shear bands, evolving from {101 ̅1}-{101 ̅2} double twins (DTWs), were observed in the RT-rolled samples, while a high quantity of twins, including {101 ̅2} tension twins (TTWs), twin-twin interactions and twinning sequence, were observed in the LNT-rolled samples. More importantly, a rarely observed twinning sequence behaviour, namely primary TTW-TTW interactions→ secondary TTW-TTW interactions, creating a 45° <202 ̅1 ̅> misorientation peak, was studied. Abundant GNDs accumulated around these twin-twin interactions, twinning sequence, DTWs and shear bands, while the GND density was low around TTWs. This research delivers a systematic investigation into the deformation structures in Mg alloys during the rolling process from RT to cryogenic temperature and provides insights into the newly discovered twinning sequence and twin-twin interactions.
Luan Q, Lee J, Zheng J, et al., 2020, Combining microstructural characterization with crystal plasticity and phase-field modelling for the study of static recrystallisation in pure aluminium, Computational Materials Science, Vol: 173, ISSN: 0927-0256
An in-depth understanding of the recrystallization process in alloys is critical to manufacturing metal parts with superior properties. However, the development of recrystallization model under various processing conditions is still in its early research stage and becoming an urgent demand for both the manufacturing industry and scientific research. In this work, a validated numerical model that is capable of predicting the recrystallized grain structure, incubation time for the grain nucleation and texture evolution, was developed using a Kobayashi, Warren and Carter (KWC) phase-field model coupled with crystal plasticity finite element (CPFE) analysis. Through characterising the microstructural evolution of static recrystallization (SRX) by quasi-in-situ Electron Backscatter Diffraction (EBSD) mapping, insights into dislocation density, grain nucleation position, grain growth rate and recrystallized grain orientation were established and compared with the computational model. This model enables a reliable and accurate prediction of recrystallized grain morphology and texture.
Zhang K, Zheng J, Shao Z, et al., 2019, Experimental investigation of the viscoplastic behaviours and microstructure evolutions of AZ31B and Elektron 717 Mg-alloys, Materials and Design, Vol: 184, Pages: 1-13, ISSN: 0264-1275
An insight into the thermo-mechanical behaviours of AZ31B and Elektron 717 magnesium alloys under the hot stamping conditions was established. High-temperature tensile tests (i.e. 350–450 °C) at a strain rate of 0.1 to 5/s were conducted to examine the material viscoplastic behaviours. Additionally, microstructure characterizations were performed, using the electron backscatter diffraction (EBSD), on the deformed samples to capture the underlying deformation mechanisms. Dynamic recrystallization (DRX) and texture formation were observed during the deformation at high temperature in both alloys and are the primary factors that affect the viscoplastic behaviours. The yield stress of both alloys reduced with increasing temperatures and reducing strain rates. More importantly, the ductility of the samples increased with both the temperatures and the strain rates. The higher ductility at higher strain rates was primarily attributed to finer grains and the slightly weakened textures, enabling a more uniform deformation. A maximum ductility of ~2 was observed in AZ31B under 450 °C at 1/s while ~0.9 in Elektron 717 under the identical condition. The addition of rare earth elements in Elektron 717 may suppress the active DRX. The recrystallization type was identified as discontinuous DRX. The research findings deliver understandings on the viscoplastic behaviours and the deformation mechanisms of AZ31B and Elektron 717 under the hot stamping conditions and provide scientific guidance for feasibility study on applying hot stamping technique to Mg-alloy for forming complex geometry components.
Zheng J-H, Dong Y, Zheng K, et al., 2019, Experimental investigation of novel fast-ageing treatments for AA6082 in supersaturated solid solution state, Journal of Alloys and Compounds, Vol: 810, ISSN: 0925-8388
Developing a fast-ageing treatment can significantly reduce the current processing time (180 °C × 9 h) of high strength AA6082 automotive components. In this study, a fast ageing treatment in supersaturated solid solution state was developed, such that the mechanical properties can be rapidly achieved after the paint bake (PB) treatment through introducing a pre-ageing (PA) treatment. The determined fast ageing method considered effects of temperature & time, heating rate and subsequent PB on the ageing response. Tensile tests and TEM observations of typical conditions were undertaken to examine evolved strength and precipitate distribution. Results showed that 210 °C was the optimum pre-ageing temperature as uniformly sized and distributed small precipitates were obtained. The final strength of about 280 MPa, that is 95% of the nominal strength for T6 temper, can be obtained within 15 min soaking for fast heating, and nearly this value for slow heating. More prolific nucleation occurred during slow heating, resulting in more finely distributed precipitates and a higher strengthening. It was observed that PB further increased the strength of over-aged alloy pre-aged at a high temperature of 240 °C. The subsequent PB enabled further nucleation of small clusters and growth of the pre-ageing-induced precipitates which were smaller than 20 nm. This resulted in an improvement in the material strength potentially to satisfy the safety requirements in automotive industry.
Zheng K, Dong Y, Zheng J-H, et al., 2019, The effect of hot form quench (HFQ (R)) conditions on precipitation and mechanical properties of aluminium alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 761, ISSN: 0921-5093
Zheng J-H, Pan R, Wimpory RC, et al., 2019, A novel manufacturing process and validated predictive model for high-strength and low-residual stresses in extra-large 7xxx panels, MATERIALS & DESIGN, Vol: 173, ISSN: 0264-1275
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Pan R, Pirling T, Zheng J, et 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.
Pan R, Zheng J, Zhang Z, et 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
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.
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.
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.
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
Li N, Zheng J, Zheng K, et al., 2017, A fast ageing method for stamped heat-treatable alloys, WO2017021742 A1
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.
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