Imperial College London

DrJunJiang

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

jun.jiang

 
 
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524City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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67 results found

Luan Q, Wang J, Huang Y, Balint D, Jiang Jet al., 2021, How would the deformation bands affect recrystallization in pure aluminium?, Materials and Design, Vol: 209, ISSN: 0264-1275

Deformation bands (DBs), formed after plastic deformation, are known to have an impact on the recrystallization (RX) process. The exact mechanisms of how DBs influence grain nucleation and grain growth remain unclear. In this paper, deformed single and multicrystal pure aluminium samples are annealed to explore the likely effects of DBs on the grain nucleation and the subsequent grain growth. Regarding the prediction of the recrystallized (RXed) texture, it is noticeable that the orientations of nucleated grains nearby DB are originated from the orientation in DB. Regarding the nucleated positions, it is demonstrated that potential nucleation sites are more likely located in DBs in comparison with the initial grain boundary. Regarding the rate of RX, the number of nucleated grains is also predicted to have a strong positive correlation with the area fraction of DBs, which would consequently affect the kinetics of the grain growth in the deformed microstructure. All the above observations imply that the RX process is strongly controlled by the ensemble characteristics of DBs rather than the initial grain boundaries.

Journal article

Wang Y, Liu Y, Lan B, Jiang Jet al., 2021, A study of solid-state bonding-by-hot-deforming mechanism in Inconel 718, Journal of Materials Processing Technology, Vol: 295, ISSN: 0924-0136

Solid-state bonding shows irreplaceable advantages in joining similar and dissimilar materials with poor weldability compared with fusion welding methods. To widen its applications in manufacturing safety-critical structural parts, a sound bonding quality must be achieved under less strict conditions. In this work, the metallurgical bonding is formed by hot compressing two Inconel 718 parts to different strain levels at the temperature of 1150 °C, strain rate of 0.1s−1 under a low vacuum. Full tensile strength and ductility comparable to the parent materials have been achieved for the first time by 0.5 engineering strain. The severe plastic deformation at hot temperature rapidly bonds the two parts by effectively closing the interface micro-voids, breaking up the oxide film, and in the meantime, accelerating the grain boundary(GB) migration through the dynamic recrystallization(DRx). Based on these observations, a theoretical model is proposed to describe the bonding process under hot deforming condition and the achieved relative interface bond quality, in which the cohesion between oxide-oxide and oxide-metal is assumed, and impairing effect of remaining dispersed oxides is minimized with the attempt of introducing a strain-amplifying factor. The insights and model provide the basis for further understanding of the solid-state bonding-by-hot-deforming under practical conditions and explore its wider application with ideal joint integrity.

Journal article

Guo L, Yuan J, Pei J, Zhao Y, Zhang K, Jiang Jet al., 2021, Study of the microstructure, bonding evolution and mechanical properties of continuously extruded magnesium AZ31 sheet, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 819, ISSN: 0921-5093

Journal article

Jing Y, Ren XY, Shang Y, Xiong H, Jiang Jet al., 2021, Develop a novel high-strength vacuum brazing technique for γ-TiAl intermetallic, International Journal of Lightweight Materials and Manufacture, Vol: 4, Pages: 237-245

Limited strength and ductile joints for bonding γ-TiAl intermetallics are one of the main concerns on γ-TiAl applications in the aerospace industry. In this study, a new ductile brazing filler with TiZrCuNi system was developed by an established model for calculating the electrons distribution of the unit cell for the filler. Three kinds of fillers with fixed amount of Cu and Ni as 25 wt.%, but different Zr level (25, 18, 10 wt.%) were designed. It was found that the filler diffusion was affected by the filler composition, which was insufficient with high Zr content up to 37.5 wt.%. And the defects such as the voids and the cracks were observed with medium Zr level as 25 wt.% and brazing temperature as 1000 °C. The sound joints by Zr as 18 wt.% and 10 wt.% filler at 950 °C and 980 °C respectively were achieved with tensile strength as 520 MPa–570 MPa at room temperature. This tensile strength was remained as 495 MPa in average at 600 °C. The strong joints were attributed to the fine, randomly oriented Ti3Al grains and evenly distributed elements at the interfaces.

Journal article

Zheng J-H, Pruncu C, Zhang K, Zheng K, Jiang Jet al., 2021, Quantifying geometrically necessary dislocation density during hot deformation in AA6082 Al alloy, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 814, ISSN: 0921-5093

Journal article

Zhang K, Zheng J-H, Hopper C, Sun C, Jiang Jet al., 2021, Enhanced plasticity at cryogenic temperature in a magnesium alloy, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 811, ISSN: 0921-5093

Journal article

Hopper C, Pruncu CI, Hooper PA, Tan Z, Yang S-T, Liu Y, Jiang Jet al., 2021, The effects of hot forging on the preform additive manufactured 316 stainless steel parts, Micron, Vol: 143, Pages: 103026-103026, ISSN: 0968-4328

Additive Manufacture (AM) offers great potential for creating metallic parts for high end products used in critical application i.e. aerospace and biomedical engineering. General acceptance of AM within these fields has been held back by a lack of confidence in the consistency of the mechanical properties of AMed parts associated by the occurrence of porosity, large columnar grains and texture. In this research, to counters this problem we have combined hot forging and subsequent heat treatment. Although, perhaps not best suited to components featuring fine detail, this technique should be well suited to the manufacture of forged components such as fan blades. Here, AM is able to create a near net-shape blank which is then hot forged to size, eliminating intermediate production stages and generating good mechanical properties in the final component. The material used in the current study is AM 316 L Stainless Steel. By altering the printing parameters of the AM machine, two batches of samples were built, each displaying a different porosity content. This allowed the influence of initial build quality to be illustrated. By comparing the two sample batches, it was possible to gain an insight into the possibilities of controlling porosity and material microstructure. The success of the proposed hot forging and heat treatment technique was validated by mechanical testing (i.e. tensile and hardness experiments) and microstructure evolution characterization (i.e. optical microscopy observation and electron backscatter diffraction (EBSD) techniques). The results revealed that the post processing strategy reduced material porosity and enabled the creation of a more robust microstructure, resulting in improved mechanical properties of the AM material.

Journal article

Zhang K, Shao Z, Daniel CS, Turski M, Pruncu C, Lang L, Robson J, Jiang Jet al., 2021, A comparative study of plastic deformation mechanisms in room-temperature and cryogenically deformed magnesium alloy AZ31, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 807, ISSN: 0921-5093

Journal article

Wang CH, Sun CY, Qian LY, Cai W, Jiang J, Xiao Yet al., 2021, Dynamic mechanical behaviour induced by adiabatic temperature rise of Fe-Mn-Al-C steel, MATERIALS SCIENCE AND TECHNOLOGY, Vol: 37, Pages: 280-291, ISSN: 0267-0836

Journal article

Zhou X, Shao Z, Zhang C, Sun F, Zhou W, Hua L, Jiang J, Wang Let al., 2020, The study of central cracking mechanism and criterion in cross wedge rolling, International Journal of Machine Tools and Manufacture, Vol: 159, ISSN: 0890-6955

Cross wedge rolling (CWR) is an innovative metal forming process to manufacture axisymmetric stepped shafts used in the transport industry. Central cracking, also called the Mannesmann Effect, consistently occurs in the central region of the CWR workpiece. This results in reduced product quality and increased costs due to rejected and failed parts. However, the understanding of central cracking mechanism and criterion is limited due to the complex stress states in CWR and the experimental limitations. A large number of CWR tests and different die geometries are required in the identification of the potential mechanistic factors such as the axial tensile stress, secondary tensile stress, shear stress and cyclic loading. Also, there is as yet no efficient method of determining the material constants associated with the central cracking fracture criteria. These problems are addressed in the present study. A physical model was built to reproduce the industrial CWR process. A newly designed model material (plasticine/flour composite) was used to mimic the material flows and internal fracture behaviours found in commercial CWR workpieces. This allowed a variety of die shapes to be rapidly and cost-effectively 3D printed, thereby enabling specific stress states to be achieved within the workpiece. Via experimental observations and the corresponding finite element modelling under different die geometries, the maximum shear stress was identified as the dominant factor for central cracking. The fracture criterion involving the maximum shear stress was quantitatively verified to be accurate and robust in predicting central cracking moments and locations. A novel approach using simplified die geometries to determine the associated material constants was proposed and validated. The high accuracy and cost/time efficiency of this new approach will be a significant benefit to fundamental research and also in industrial applications.

Journal article

Sun F, Pruncu CI, Penchev P, Jiang J, Dimov S, Blackman BRKet al., 2020, Influence of surface micropatterns on the mode I fracture toughness of adhesively bonded joints, International Journal of Adhesion and Adhesives, Vol: 103, Pages: 1-11, ISSN: 0143-7496

Surface patterning has been used to enhance the fracture toughness of adhesive joints. In this study, the effect of the variable bondline thickness introduced by the patterns and the effect of pattern geometry on the fracture behaviour of adhesive joints were analysed. Surface patterns including longitudinal grooves, transverse grooves, dimples and grids were fabricated by means of laser texturing. The patterned surfaces were bonded using a tough structural adhesive and the mode I fracture toughness was measured using the J-integral method. The toughness of the patterned joints was compared with the results from bonding with control surfaces and with grit blasted in combination with chromic acid etched (GB-CAE) surfaces. It was shown that both longitudinal and transverse grooves led to the highest value of toughness. Grids patterns yielded a toughness close to the GB-CAE treatment, both of which were higher than the dimple patterns. It was also shown that the variable bondline thickness due to the existence of surface patterns, which influences the size of the plastic zone, reduced rather than increased the fracture toughness.

Journal article

Zinong T, Bing Z, Jun J, Zhiqiang L, Jianguo Let al., 2020, A study on the hot roll bonding of aluminum alloys, Procedia Manufacturing, Vol: 50, Pages: 56-62, ISSN: 2351-9789

Joining of aluminum alloys through plastic deformation and short-time diffusion bonding has significant applications in a wide range of engineering sectors. It is also important in the elimination of voids and defects in casting ingots through hot rolling processes. An aluminum alloy, AA1060, was selected for the roll-bonding experimental research at vacuum conditions. Parameters including grain size and orientation, dislocation distribution, oxide film breakage, interface welding ratio were studied and analyzed at different hot rolling conditions. Analytical techniques were also developed to analyze the deformation, stress and strain states of the material at hot rolling conditions. The results show that under the condition of 580℃ and 60% deformation, the welding interfaces disappears, and the alloy matrix structure is not overburned.

Journal article

Zhang K, Shao Z, Jiang J, 2020, Effects of twin-twin interactions and deformation bands on the nucleation of recrystallization in AZ31 magnesium alloy, Materials & Design, Vol: 194, ISSN: 0264-1275

Investigating recrystallization is essential to optimize the microstructure including texture weakening and grain refinement in the rolling of magnesium alloys, thus to improve the mechanical properties of magnesium sheets for industrial applications. This research has gained an in-depth understanding of the effects of deformation bands and twin-twin interactions on recrystallization, which will potentially lead to improved manufacturing processes and mechanical properties of magnesium alloys. To study their individual effects, the recrystallization mechanisms of the room-temperature (RT)-rolled and liquid-nitrogen-temperature (LNT)-rolled samples during the annealing process were analysed with the quasi-in-situ electron backscatter diffraction method, respectively. It is found that recrystallization mainly occurred in deformation bands in the RT-rolled sample, which enhanced the initially formed texture, due to oriented and inhomogeneous grain growth. However, it is of great interest to see that the recrystallized sites were mainly located around the (101 ̅2)-(011 ̅2) twin-twin interactions with high kernel average misorientation (KAM) values in the LNT-rolled samples, resulting in rather weaker texture, finer grain size and more homogeneous microstructure, because of the randomized orientations of recrystallized grains and uniform grain growth, while almost no recrystallization was observed around the single tension twin variant.

Journal article

Pruncu CI, Hopper C, Hooper PA, Tan Z, Zhu H, Lin J, Jiang Jet al., 2020, Study of the Effects of Hot Forging on the Additively Manufactured Stainless Steel Preforms, Journal of Manufacturing Processes, Vol: 57, Pages: 668-676, ISSN: 1526-6125

The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.

Journal article

McGilvery C, Jiang J, Rounthwaite N, Williams R, Giuliani F, Britton Tet al., 2020, Characterisation of carbonaceous deposits on diesel injector nozzles, Fuel: the science and technology of fuel and energy, Vol: 274, Pages: 1-9, ISSN: 0016-2361

Diesel injector nozzles are highly engineered components designed to optimise delivery of fuel into the combustion chamber of modern engines. These components contain narrow channels to enhance spray formation and penetration, hence mixing and combustion. Over time, these injectors can become clogged due to fouling by carbonaceous deposits which may affect the long-term performance of a diesel engine. In this paper we explore the chemical composition and structure of deposits formed within the nozzle at the nanometre scale using electron microscopy. We focus on comparing deposits generated using a chassis dynamometer-based test with Zn fouled fuel with a DW10B dirty up test. We have developed and applied a method to precisely section the deposits for ‘top view’ scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis of the morphology and relative accumulation of deposits formed during chassis dynamometer and engine based dirty-up tests. We extend this analysis to finer length scales through lift-out of ~70 nm thick electron transparent cross section foils, including both the metal substrate and deposit, using focussed ion beam (FIB) machining. These foils are analysed using scanning transmission electron microscopy (STEM) and STEM-EDS. These thin foils reveal thin-film growth and chemical stratification of Zn, C, O and other elements in the organic deposit layers developed during growth on the steel substrate during industry standard fouling tests.

Journal article

Zhang K, Zheng J-H, Huang Y, Pruncu C, Jiang Jet 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.

Journal article

Yasmeen T, Zhao B, Zheng J-H, Tian F, Lin J, Jiang Jet al., 2020, The study of flow behavior and governing mechanisms of a titanium alloy during superplastic forming, Materials Science and Engineering: A, Vol: 788, Pages: 1-19, ISSN: 0921-5093

TA15 (Ti–6Al–2Zr–1Mo–1V) is a near-α titanium alloy and has wide applications in the aerospace industry because of its high strength to mass ratio, good weldability, and superior creep resistance at high temperatures up to 550 °C, compared to other titanium alloys. This study investigates the flow behavior and microstructural evolution as functions of temperatures and strain rates during deformations under the superplastic conditions at 880 °C/0.01s−1, 900 °C/0.01s−1, 880 °C/0.001s−1, and 920 °C/0.0005s−1. Results showed that this alloy exhibit excellent superplastic behavior for all selected temperatures and strain rates. The maximum tensile elongation of 1450% is achieved at 880 °C with a strain rate of 0.001s−1. Flow softening is observed under deformation conditions of 880 °C/0.01s−1 and 900 °C/0.01s−1, while strain hardening is observed at deformation conditions of 880 °C/0.001s−1 and 920 °C/0.0005s−1. These complex flow behaviors are rationalized by characterizing the underlying microstructures on the interrupted tensile samples using electron backscatter diffraction (EBSD) and backscattered electrons (BSE). The geometrically necessary dislocations (GNDs) density, which is caused by lattice rotation and misorientations and plays a vital role in the plastic constitutive behaviors, was for the first time, systematically revealed. Together with other key microstructures, i.e. grain sizes, texture, phase fractions, the results show that the dominant deformation mode changes at initial, intermediate, and final stages of the deformation. The probable deformation mechanisms, such as grain boundary sliding (GBS) under different deformation conditions, are discussed in terms of grain morphology, GNDs, and texture evolution. Also, it is observed that the β-phase transformation is accelerated during deformation and contributes to the enhancemen

Journal article

Ren X, Ren H, Shang Y, Xiong H, Zhang K, Zheng J, Liu D, Lin J, Jiang Jet al., 2020, Microstructure evolution and mechanical properties of Ti2AlNb/TiAl brazed joint using newly-developed Ti-Ni-Nb-Zr filler alloy, PROGRESS IN NATURAL SCIENCE-MATERIALS INTERNATIONAL, Vol: 30, Pages: 410-416, ISSN: 1002-0071

Journal article

Bhaduri D, Penchev P, Dimov S, Essa K, Carter LN, Pruncu CI, Jiang J, Pullini Det al., 2020, On the surface integrity of additive manufactured and post-processed AlSi10Mg parts, 5th CIRP CSI 2020, Publisher: Elsevier BV, Pages: 339-344, ISSN: 2212-8271

The research centres on the evaluation of surface integrity of AlSi10Mg parts produced via laser-based powder bed fusion (LPBF) process, followed by vibratory surface finishing. The alloy is chosen for its applications in lightweight components used in electronic packaging, automotive and aerospace sectors. Initial experiments involve optimisation of key LPBF process parameters by analysing the surface roughness and density data of the built parts. A Taguchi L18 orthogonal array is used for the optmisation trials with variations in the laser power (P), beam scanning speed (v), hatch spacing (H) and island size (I). Latter experimental phase deals with microhardness and microstructure assessment of heat treated LPBF specimens that are produced using the optimised LPBF parameters, i.e. P: 250 W, v: 1500 mm/s, H: 75 µm and I: 2 mm. Microhardnesses of the annealed samples reduce by ~12% with respect to the as-built parts and the values remain almost unchanged from the annealed state following solution treatment and ageing. The fish-scale like melt-pools observed on the unheat treated samples begin to fade off in the annealed specimens and completely disappear after solution treatment and ageing, with silicon particles dispersed all over the aluminium matrix. The final experimental phase involves vibratory surface finishing of the as-built LPBF parts using a vibrating ceramic media mixed with different acid and amine based liquid compounds for 1-6 hours, followed by vibrating in a maize based media for another 1-6 hours. The process aids in reducing the parts’ roughness, Sa by ~35-70%, however the effect is more prominent when using the ceramic media.

Conference paper

Zhou X, Shao Z, Pruncu CI, Hua L, Balint D, Lin J, Jiang Jet al., 2020, A study on central crack formation in cross wedge rolling, Journal of Materials Processing Technology, Vol: 279, ISSN: 0924-0136

Cross wedge rolling (CWR) is an innovative roll forming process, used widely in the transportation industry. It has high production efficiency, consistent quality and efficient material usage. However, the continual occurrence of crack formation in the centre of the workpiece is a critical problem excluding the CWR technique from more safety-critical applications, in particular, aerospace components. The mechanisms of central fracture formation are still unclear because of a combination of complicated stress and strain states at various stages of CWR. Thus, the aim of this study is to understand the stress/strain distribution and evolution during the CWR process and identify the key variables which determine central crack formation. A comprehensive investigation was then conducted to simulate 27 experimental cases. The stress and strain distributions in the workpiece were evaluated by finite element analysis. Various damage models from literature were applied and compared. A new fracture criterion was proposed, which was able to successfully determine the central crack formation in all 27 experimental cases. This criterion can be applied in CWR tool and process design, and the enhanced understanding may enable the adoption of CWR by the aerospace industry.

Journal article

Jing Y, Xiong H, Shang Y, Wang J, Cheng Y, Jiang Jet al., 2020, Design TiZrCuNi filler materials for vacuum brazing TA15 alloy, Journal of Manufacturing Processes, Vol: 53, Pages: 328-335, ISSN: 1526-6125

TA15 alloy with near α microstructure offers excellent high temperature mechanical properties for future gas turbine blades. To developing for the thin-wall and complex structure of this material, brazing is preferred. However, a new brazing material and process are required. In this study, a new theoretical modeling approach for designing Ti-based filler material for TA15 is presented by simulation on the bond strength in the crystal unit for the brazing filler material (BFM). The BFM chemical composition was determined as Ti-(17∼19)Zr-15Cu-15Ni wt.% to balance its strength, the toughness and relative lower melting temperature for TA15. All the fracture happened in the matrix rather than the brazing joints. It means that the strength of the brazed TA15 joint at room temperature was at least as strong as that of the matrix. Furthermore, under its service temperature (600℃), the strength of the brazing joint maintained almost 70 % of strength. The high joint strength and the ductile connection were attributed to the formed Widmanstätten structure and the sufficient diffusion of Cu and Ni solutes from the BFM across the interface. No intermetallics compound was formed in the joint. This model is established to optimizing a Ti-based filler material for achieving the excellent properties of brazed joints.

Journal article

Zhou X, Shao Z, Tian F, Hopper C, Jiang Jet al., 2020, Microstructural effects on central crack formation in hot cross-wedge-rolled high-strength steel parts, Journal of Materials Science, Vol: 55, Pages: 9608-9622, ISSN: 0022-2461

Central cracking in cross-wedge-rolled workpieces results in high wastage and economic loss. Recent cross-wedge rolling tests on two batches of steel showed that one batch formed central cracks, while the other was crack-free. The batches were both nominally of the same chemical composition and thermomechanical treatment history. In addition, both batches had passed all the standard quality assessments set for conventional forging processes. It was suspected that the different cracking behaviours were due to differences in microstructure between the two as-received steel billets, and the material in cross-wedge rolling (CWR) was more sensitive to the initial microstructure compared with other forging processes due to its specific loading condition including ostensibly compression and large plastic strain. Nevertheless, no previous study of this important problem could be identified. The aim of this study is, therefore, to identify the key microstructural features determining the central crack formation behaviour in CWR. The hot workability of the as-received billets was studied under uniaxial tensile conditions using a Gleeble 3800 test machine. Scanning electron microscope with energy-dispersive X-ray spectroscopy and electron backscatter diffraction was applied to characterise, quantitatively analyse, and compare the chemical composition, phase, grain, and inclusions in these two billets, both at room temperature and also at the CWR temperature (1080 °C). Non-metallic inclusions (oxides, sulphides, and silicates) in the billets were determined to be the main cause of the reported central cracking problem. The ductility of the steels at both room and elevated temperatures deteriorated markedly in the presence of the large volumes of inclusions. Grain boundary embrittlement occurred at the CWR temperature due to the aggregation of inclusions along the grain boundaries. It is suggested that a standard on specifying the inclusion quantity and size in CWR billets b

Journal article

Shao Z, Lee J, Wang J, Lin J, Jiang Jet al., 2020, A study of various heating effects on the microstructure and mechanical properties of AA6082 using EBSD and CPFE, Journal of Alloys and Compounds, Vol: 5 nov 2019, Pages: 1-13, ISSN: 0925-8388

The solution heat treatment (SHT) process resolving hardening precipitates in high strength aluminium alloys is a critical step for high-efficient forming processes, such as Hot Form Quench (HFQ®). SHT largely determines the overall cycle time of a forming process. However, effects of heating process parameters, such as the heating rate and soaking time, on the microstructure and the associated mechanical properties of aluminium alloy 6082, one of the most commonly used aluminium alloys, for HFQ applications have not been systematically investigated. The aim of this study is to explore and understand the relationships among heat treatment conditions, grain microstructure and associated mechanical properties for AA6082. A series of uniaxial tensile tests conducted under various SHT conditions revealed significant variation on mechanical behaviour characterised by stress-strain curves. To correlate these stress-strain relationship with underlying microstructure, the grain and orientation distribution of each heat-treated sample were characterised by the electron backscatter diffraction (EBSD) technique. Due to the presence of a large number of microscopic variables, such as grain size, morphology, texture, grain boundary and etc., the crystal plasticity finite element (CPFE) modelling was employed to identify the key microscopic factors which determine the differences in the observed strength and ductility for all samples. A new CPFE model integrated with local strain criterion was proposed and validated to correlate the ductility and the strength with the material microstructure. This rigorous investigation provides more insights on how microstructure (grain size and texture) affects the mechanical behaviour for AA6082, which enables to enlarge the capability of HFQ for industrial applications.

Journal article

Luan Q, Lee J, Zheng J, Hopper C, Jiang Jet 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.

Journal article

Luan Q, Jiang J, 2020, How would the deformation bands affect recrystallization in pure aluminium?, Publisher: Elsevier BV

Deformation bands (DBs), formed after plastic deformation, are known to have an impact on the recrystallization (RX) process. The exact mechanisms of how DBs influence the grain nucleation and grain growth remain unclear. In this paper, deformed single and multicrystal pure aluminium samples are annealed to explore the likely effects of DBs on the grain nucleation and the subsequent grain growth. Regarding the prediction of the recrystallized (RXed) texture, it is noticeable that the orientations of nucleated grains nearby DB are originated from the orientation in DB. Regarding the nucleated positions, it is demonstrated that potential nucleation sites are more likely located in DBs in comparison with the initial grain boundary. Regarding the rate of RX, the number of nucleated grains is also predicted to have a strong positive correlation with the area fraction of DBs which would consequently affect the kinetics of the grain growth in the deformed microstructure. All the above observations imply that the RX process is strongly controlled by the ensemble characteristics of DBs rather than the initial grain boundaries.

Working paper

Luan Q, Xing H, Zhang J, Jiang Jet al., 2020, Experimental and crystal plasticity study on deformation bands in single crystal and multi-crystal pure aluminium, Acta Materialia, Vol: 183, Pages: 78-92, ISSN: 1359-6454

Deformation bands (DBs) formed in metals even in single crystals are known to give rise to the microstructural heterogeneities, thus contributing to some long-standing microstructure formation problems, such as the occurrence of recrystallization on the basis of deformed microstructure. Previous experimental transmission electron microscope (TEM) work has identified two types of DBs in the microscopic scale, i.e. kink bands and bands of secondary slips, showing the importance of understanding the slip activation for DBs. To extend the theory in mesoscale, single crystal and multi-crystal pure aluminium, as well as their corresponding crystal plasticity finite element (CPFE) models, are used in this paper to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs. It is demonstrated that slip band intersection of primary and secondary slips is predicted to constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs regarding the wall of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slips. A sufficient amount of primary slips activated inside grains would be the key to the formation of distinct DBs with high area fraction and aspect ratio.

Journal article

Zhang K, Zheng J, Shao Z, Pruncu C, Turski M, Guerini C, Jiang Jet 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.

Journal article

Yasmeen T, Shao Z, Zhao L, Gao P, Lin J, Jiang Jet al., 2019, Constitutive modelling for the simulation of the superplastic forming of TA15 titanium alloy, International Journal of Mechanical Sciences, Vol: 164, ISSN: 0020-7403

Titanium alloy, TA15, has a high strength-to-weight ratio, high weldability, and superior creep resistance at high temperatures up to 550°C. TA15 is difficult to deform, especially for forming complex-shaped large-scale web–rib components, due to its low plasticity, large inhomogeneous deformation and narrow processing window. The objective of this research is to model the superplastic mechanisms in TA15 alloy with equiaxed, fine grain structure, and applying the proposed constitutive model to investigate the maximum grid aspect ratio, that can be achieved in superplastic forming (SPF), for a TA15 sheet with an initial thickness of 1.2 mm. Thermo-mechanical tensile tests are conducted first to characterize the superplastic behaviour of the material in the temperature range of 880– 940°C and the strain-rate of 0.0005 – 0.01s−1. A set of mechanism-based unified visco-plastic constitutive equations has been proposed and calibrated based on the results of stress-strain data. A gradient-based optimization method is applied for the calibration of constitutive equations. The constitutive model is incorporated into FEA code through creep subroutine to check the validity of the proposed material model against the experimental SPF test of a multi-box die. Predicted sheet thickness and thinning in a die entry radius region at the end of forming are examined in detail. Preliminary results show a good agreement between the computational and experimental results.

Journal article

Zheng J-H, Dong Y, Zheng K, Dong H, Lin J, Jiang J, Dean TAet 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.

Journal article

Birosca S, Liu G, Ding R, Jiang J, Simm T, Deen C, Whittaker Met al., 2019, The dislocation behaviour and GND development in a nickel based superalloy during creep, International Journal of Plasticity, Vol: 118, Pages: 252-268, ISSN: 0749-6419

In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) were investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculation were used, namely, EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above the yield stress of Waspaloy) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions on at least two octahedral (111) slip systems for easy dislocation movement. This allowed dislocations to reach the grain boundary without significant geometrically necessary dislocation accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700 °C), the GND accumulation was relatively low due to the insufficient macroscopic stress level. However, the regions near grain boundaries showed high GND densit

Journal article

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