Imperial College London

ProfessorJianguoLin

Faculty of EngineeringDepartment of Mechanical Engineering

Head/Professor in Mechanics of Materials Division
 
 
 
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Contact

 

+44 (0)20 7594 7082jianguo.lin

 
 
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Assistant

 

Miss Valerie Crawford +44 (0)20 7594 7083

 
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Location

 

522City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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366 results found

Zhou W, Lin J, Balint D, Dean TAet al., 2021, Clarification of the effect of temperature and strain rate on workpiece deformation behaviour in metal forming processes, International Journal of Machine Tools and Manufacture, Vol: 171, Pages: 1-6, ISSN: 0890-6955

In analysing metal forming processes the deformation mechanism map (elastic-plastic, elastic-viscoplastic, or creep type behaviour) for a particular process is commonly built solely in relation to temperature; which can be acceptable for a defined modest strain rate range. However, for a given temperature, if strain rate variation is large, the deformation mechanism could vary significantly. In this paper, a deformation mechanism map is proposed to clarify the interacting effect of deformation conditions (temperature and strain rate) on workpiece behaviour in metal forming processes. Rate type deformation equations which can be used to comprehensively model the effect of temperature and strain rate on deformation mechanism characteristics are elucidated and as examples, determined for Ti–6Al–4V and Al–Mg alloy.

Journal article

Zhang R, Shi Z, Shao Z, Yardley VA, Lin J, Dean TAet al., 2021, Biaxial test method for determination of FLCs and FFLCs for sheet metals: validation against standard Nakajima method, International Journal of Mechanical Sciences, Vol: 209, Pages: 1-14, ISSN: 0020-7403

Recently, a biaxial test method comprising a cruciform specimen design and spatio-temporal method to determine the limit strains has been proposed for the determination of forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for sheet metals. However, this test method has not yet been validated against the existing standard methods. In the present work, this biaxial test method has been applied to the aluminium alloy AA5754 for formability evaluation at room temperature and results from the biaxial test method have been compared with those from the standard Nakajima method. Theoretical analysis has been carried out to compare equi-biaxial tension cases for the two methods; a similar variation of thickness strain with radial distance normalised by the radius of the gauge area is found between the two methods. In the biaxial tests, decreasing the radius of the through-thickness dome profile, with which the gauge area is thinned, leads to fracture nearer the specimen centre but produces a less uniform strain distribution. Importantly, the major strains at necking on the FLC, as determined using the biaxial and the standard test methods, are almost the same in the plane-strain state, while in other strain states, the major strains are slightly lower for the biaxial method than that for the Nakajima method. An FFLC for AA5754 has also been determined using the biaxial test method, in which the major strain at fracture decreases with increasing strain ratio β from −0.5 to 0, while it changes only slightly when β > 0.

Journal article

Tong C, Zhu G, Rong Q, Yardley VA, Shi Z, Li X, Luo J, Lin Jet al., 2021, Investigation of austenitising behaviour of medium-Mn steel in the hot-stamping heating process, Journal of Materials Processing Technology, Vol: 297, Pages: 1-12, ISSN: 0924-0136

In this study, the austenite transformation behaviour in a medium-Mn (MMn) steel is investigated during heat treatments that replicate those occurring in low-temperature hot stamping (LTHS), with the aim of better understanding this behaviour to optimise heat-treatment design. The austenitisation behaviour and critical phase transformation temperatures during the LTHS heating process and their dependence on heating conditions are investigated using dilatometry with a Gleeble 3800 thermal-mechanical simulator, covering heating rates of 1–25 °C s−1 and soaking temperatures of 630–900 °C. Both a higher heating rate and higher soaking temperature are found to be beneficial to shorten the time required for obtaining a given austenite fraction. The martensite start temperature (Ms) shows a rapid increase with increasing soaking temperatures when austenitisation is partial, and a slower increase after full austenitisation. Excellent ultimate tensile strength values of around 1750 MPa and total elongation values of around 9.3 % are obtained for the material after the LTHS heating process. A physically based model describing the austenitic transformation under these conditions has been adopted and calibrated. The model shows good agreement with austenitic transformation diagrams constructed from experimental data, and thus can function as a guide for selecting optimum heat-treatment parameters for LTHS.

Journal article

Lu X, Yu J, Yardley VA, Liu H, Shi Z, Lin Jet al., 2021, Solid-state welding and microstructural features of an aluminium alloy subjected to a novel two-billet differential velocity sideways extrusion process, Journal of Materials Processing Technology, Vol: 296, Pages: 1-14, ISSN: 0924-0136

A novel process for fabricating cross-sectional shapes of curved profiles in industrial applications, two-billet differential velocity sideways extrusion (DVSE), is proposed in this study. The feasibility of the process is demonstrated by fabricating solid bars of the aluminium alloy AA1070 through the solid-state welding of two billets at elevated temperature. Microstructural examination has shown that the weld formed between the two billets consists of an unsound area in the dead metal zone within the die and a sound bonding area in the welding zone. Along the welding path, the effective and normal strains gradually increase while the shear strain decreases, leading to the transformation of grains from equiaxed to bamboo-like structures and increases in the hardness, average grain size, and fraction of low angle grain boundaries. The shear strength of the welded extrudate is larger than that of the material without welding. The effective strain in the welding zone is larger than that in other zones. Increasing the temperature or speed of extrusion decreases the unsound bonding area length and the shearing angle, thus improving the uniformity of distribution of the shear strain. The grain diameter is refined from ∼500 μm in the initial billet to ∼47 μm in the welding zone of the extrudate formed at 0.05 mm/s and 450 ℃. The hardness of the extrudate formed at 400 °C and 0.1 mm/s is ∼19 % larger than that of the initial billet, and is decreased by decreasing the extrusion speed or increasing the temperature.

Journal article

Zhou W, Yu J, Lu X, Lin J, Dean TAet al., 2021, A comparative study on deformation mechanisms, microstructures and mechanical properties of wide thin-ribbed sections formed by sideways and forward extrusion, International Journal of Machine Tools and Manufacture, Vol: 168, Pages: 1-18, ISSN: 0890-6955

Extruded profiles/sections are increasingly used in the transport industry for lightweight structures. In this paper, a wide thin-ribbed aluminium profile with asymmetric Z-shape, was manufactured by a novel sideways extrusion process proposed by the authors. A comparative study was conducted by utilising the direct/forward extrusion process at the same extrusion temperature and speed, in which the different process mechanics, resulting microstructures and mechanical properties of profiles have been investigated by experiments and finite element modelling. It was revealed that, compared with sideways extrusion, although the design of a die pocket in forward extrusion induces preform and avoids the use of the large-diameter billet and extrusion container/press needed for extruding wide profiles, it requires a greater extrusion force due to work-piece upsetting necessary to fill the die pocket and leads to a lower effective strain in the profile rib. EBSD characterisation of the regions with an equal effective strain indicated that an increased shear strain is more efficient for obtaining fine grains with a higher average misorientation angle. In the same region of the profile rib made from the two different processes, sideways extrusion results in greater grain refinement due to greater effective strains, and a slightly greater texture intensity was found due to the intensive shear deformation. Tensile tests on formed profiles revealed that sideways extrusion leads to a higher yield strength (YS) and ultimate tensile strength (UTS) but a relatively lower elongation to failure, due to the combined effects of grain refinement, GND and texture intensity enhancement. Compared with the billet, the profile formed by forward and sideways extrusion has a YS increased by about 60% and 79% respectively, and an UTS increased by about 74% and 80% respectively in the extrusion direction, demonstrating an advantage of the sideways extrusion process in improving material strength u

Journal article

Rong Q, Shi Z, Li Y, Lin Jet al., 2021, Constitutive modelling and its application to stress-relaxation age forming of AA6082 with elastic and plastic loadings, Journal of Materials Processing Technology, Vol: 295, Pages: 1-12, ISSN: 0924-0136

A novel constitutive model has been proposed in this study that predicts the different stress-relaxation ageing (SRA) behaviour of AA6082-T6 with elastic and plastic loading strains, extending the applications of stress-relaxation age forming (SRAF) from conventional elastically loaded panels to complex-shaped panels under plastically loaded conditions. The particular contributions of loading strain levels in elastic or plastic regions on the evolution of microstructural variables (i.e., inter-particle spacing, dislocation density, and precipitate length), yield strength and stress-relaxation behaviour during SRA process are concurrently modelled. The decreasing creep threshold stress and the increasing dislocation recovery effect from annealing with increasing initial strain in the plastic region have been proposed and introduced in the model. TEM analysis has been performed to quantify the effect of loading strain and ageing time on the evolution of β″ precipitates, and further calibrate the material model. Furthermore, the established model has been implemented into FE simulation to optimise the tool surface design of a train body panel component with complex and large curvatures, and corresponding SRAF tests have been conducted with the optimised tool surface. The maximum shape deviation from the objective shape of a component with a dimension of 820*300*3 mm3 has been controlled within 3 mm, demonstrating the feasibility of the developed material model for SRAF in industrial applications, especially for highly demanded complex-shaped components.

Journal article

Zhou W, Shi Z, Li Y, Rong Q, Zeng Y, Lin Jet al., 2021, Elastic-plastic buckling analysis of stiffened panel subjected to global bending in forming process, Aerospace Science and Technology, Vol: 115, Pages: 1-14, ISSN: 1270-9638

A new method has been proposed in this study for the elastic-plastic buckling analysis of stiffened panels under global bending. In this method, a simplified model of the stiffened panels has been built with the application of two theories of plasticity, the incremental theory (IT) and the deformation theory (DT). The effect of transverse shear deformation through the stiffener thickness has been considered using the Mindlin-Reissner plate theory. The governing differential equations have been solved by the differential quadrature (DQ) method and an iteration process has been adopted due to the non-linearity of material properties in the elastic-plastic buckling analysis. Non-linear finite element (FE) modelling of elastic-plastic buckling analysis has been carried out, and the FE results are between those based on DT and IT in general. When the reciprocal of strain hardening exponent increases to 20, the FE results are in a good agreement with DT results. Based on the proposed method and FE simulations, the effect of geometric parameters of stiffened panels (stiffener thickness to height ratio, stiffened panel length to height ratio, width to height ratio, and skin thickness to stiffener thickness ratio) on buckling behaviour in the elastic-plastic region has been investigated and discussed. The proposed method provides an efficient way for parameter optimisation in the structure design of stiffened panels for the aerospace applications.

Journal article

Altıparmak SC, Yardley VA, Shi Z, Lin Jet al., 2021, Challenges in additive manufacturing of high-strength aluminium alloys and current developments in hybrid additive manufacturing, International Journal of Lightweight Materials and Manufacture, Vol: 4, Pages: 246-261, ISSN: 2588-8404

Additive Manufacturing (AM) processes, also known as 3D printing, enable geometrically complex parts to be produced layer by layer on the basis of three-dimensional (3D) data generated either by scanning physical objects or using design software. Compared to conventional manufacturing processes, AM offers the elimination of production steps, allowing rapid and relatively easy prototyping of physical objects from 3D model designs, and reproduction of existing objects. Over the last two decades, AM processes have become widespread for the manufacturing of complex-shaped components in numerous industrial sectors, one of their main areas of application being in the aerospace industry. This sector makes extensive use of high-strength aluminium alloys because of their high strength-to-weight and stiffness-to-weight ratios and excellent machinability. However, the applicability of AM processes to high-strength aluminium alloys is still limited by the presence of several types of non-negligible issues and defects in additively manufactured (AMed) aluminium components. Over the years, significant research efforts have been directed at minimising or eliminating these defects and thereby expanding the range of applications of AM in high-strength aluminium alloys. This paper reviews the state of the art in AM of high-strength aluminium alloys for aerospace. The focus is on defects and issues in AMed 2xxx and 7xxx series alloys and recent developments in novel hybrid AM processes to minimise or eliminate the defects.

Journal article

Chung T-F, Yang Y-L, Tai C-L, Shiojiri M, Hsiao C-N, Tsao C-S, Li W-C, Shi Z, Lin J, Yang J-Ret al., 2021, HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al-Zn-Mg-Cu aluminium alloy, Materials Science and Engineering: A, Vol: 815, Pages: 1-16, ISSN: 0921-5093

High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) hasrevealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjectedto creep-age forming treatment (with a constant stress lower than its room-temperature yield strength duringageing). Along the zone axes of [110]Al//[2110]η of η1 and η12, [112]Al//[2110]η of η2 and [100]Al//[2110]η ofη13, atomic projections of (2110)η have been investigated. In those types of η, elongated hexagonal lattice defects(labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings.Furthermore, in η12, elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects)are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in thecourse of creep-age forming. Additionally, in η1 and η12, the features of a Penrose tiling defect connecting with TypeI defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting ina nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η14,consequently bring about a new orientation relationship: (513)Al//(0001)η14 and [112]Al//[2110]η14. Moreover, inan atomic STEM image of η14, the significant Z-contrast gradient adjacent to the transformation front of η14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {111}Al planes at the interface.

Journal article

Zhou W, Shao Z, Yu J, Lin Jet al., 2021, Advances and trends informing curved extrusion profiles, Materials, Vol: 14, Pages: 129-1603, ISSN: 1996-1944

Curved profiles/sections have been widely used for manufacturing lightweight structures with high stiffness and strength due to aerodynamics, structural properties, and design reasons. Structural components fabricated using curved aluminum profiles satisfy the increasing demands for products used in many high-technology industries such as aerospace, shipbuilding, high-speed rail train, and automobile, which possess the characteristics of lightweight, high strength/stiffness relative to weight, superior aerodynamics performance, and aesthetics. In this paper, the advances and trends in forming techniques of curved extrusion profiles of metal alloys have been reviewed. The curved profile forming techniques are classified into three major categories: conventional cold bending technique, stress/moment superposed cold bending technique, and extrusion-bending integrated forming technique. Processes for innovative development in the field of forming curved profiles are identified; the extrusion-bending integrated technique which can directly form the billets into curved profiles by one single extrusion operation possesses the full potential for further innovation. Due to the nature of the research to date, much of the work referred to relates to hollow circular and rectangular tube cross-sections.

Journal article

Liu S, Xia Y, Shi Z, Yu H, Li Z, Lin Jet al., 2021, Deep learning in sheet metal bending with a novel theory-guided deep neural network, IEEE/CAA Journal of Automatica Sinica, Vol: 8, Pages: 565-581, ISSN: 2329-9266

Sheet metal forming technologies have been intensively studied for decades to meet the increasing demand for lightweight metal components. To surmount the springback occurring in sheet metal forming processes, numerous studies have been performed to develop compensation methods. However, for most existing methods, the development cycle is still considerably time-consumptive and demands high computational or capital cost. In this paper, a novel theory-guided regularization method for training of deep neural networks (DNNs), implanted in a learning system, is introduced to learn the intrinsic relationship between the workpiece shape after springback and the required process parameter, e.g., loading stroke, in sheet metal bending processes. By directly bridging the workpiece shape to the process parameter, issues concerning springback in the process design would be circumvented. The novel regularization method utilizes the well-recognized theories in material mechanics, Swift's law, by penalizing divergence from this law throughout the network training process. The regularization is implemented by a multi-task learning network architecture, with the learning of extra tasks regularized during training. The stress-strain curve describing the material properties and the prior knowledge used to guide learning are stored in the database and the knowledge base, respectively. One can obtain the predicted loading stroke for a new workpiece shape by importing the target geometry through the user interface. In this research, the neural models were found to outperform a traditional machine learning model, support vector regression model, in experiments with different amount of training data. Through a series of studies with varying conditions of training data structure and amount, workpiece material and applied bending processes, the theory-guided DNN has been shown to achieve superior generalization and learning consistency than the data-driven DNNs, especially when only scarce

Journal article

Zhang R, Shao Z, Shi Z, Dean TA, Lin Jet al., 2021, Effect of cruciform specimen design on strain paths and fracture location in equi-biaxial tension, Journal of Materials Processing Technology, Vol: 289, Pages: 1-16, ISSN: 0924-0136

Hot stamping technologies require new methods for evaluating formability of sheet metal under various forming conditions. Biaxial tensile testing method using a cruciform specimen has been used for the applications, but a suitable cruciform specimen design has not yet been accepted. One of the challenges in designing a specimen for formability tests is to ensure proportional equi-biaxial strain paths arise at the location of fracture initiation. In this study, after reviewing existing cruciform specimen designs, three different geometries of cruciform specimen, named Type I, Type II and Type III, were proposed. Using numerical analysis and practical experiments, fracture initiation locations and corresponding strain paths in the specimens were investigated under equi-biaxial tension. Numerical simulations were performed to optimise the dimensions of Type I specimen to achieve a relatively high strain level near the centre point of the specimen. Based on the optimised dimensions, equi-biaxial tensile tests were carried out on cruciform specimens with different geometries, and strain paths at the fracture initiation locations were compared and analysed. It was found that in all cruciform specimens, equi-biaxial strain state appears only near the centre point. In the Type I and Type II specimens, fracture never initiates near the centre point, but at a location in the fillet transition zone where major strain is higher than that at the centre point. The Type III specimens have the ability to initiate fracture near the centre point, and to produce proportional strain paths with strain ratio β close to 1 in equi-biaxial tension, 0 in plane-strain tension, and -0.5 in uniaxial tension at the locations of fracture initiation. The research provides a cruciform specimen design, Type III, which has high potential to be used for evaluating formability for sheet metal.

Journal article

Zhang R, Shi Z, Shao Z, Yardley V, Lin Jet al., 2021, An effective method for determining necking and fracture strains of sheet metals, MethodsX, Vol: 8, ISSN: 2215-0161

Biaxial tensile testing methods using cruciform specimens have been developed in the last few decades for the determination of forming limit diagrams (FLDs) and fracture forming limit diagrams (FFLDs) for sheet metals. One of the difficulties associated with this test geometry is the lack of a widely accepted method to determine the necking and fracture strains which are necessary to construct these diagrams. In this study, a novel spatio-temporal method has been proposed for the determination of necking and fracture strains. In the method, two rectangular zones: the base zone (BZ) and the reference zone (RZ) are selected at the location where fracture initiates. The zone RZ includes the zone BZ and both zones have the same side length in the direction parallel to the necking band but different side length in the perpendicular direction. By plotting the thickness reduction within RZ against that in BZ, the onset of localised necking can be determined by finding the intersection of the two straight lines fitted separately using the data in the initial and final stages of deformation. The corresponding limit strains are then determined using the strains within the zone BZ. The method has been successfully applied to uniaxial tensile tests on AA6082 and boron steel dog-bone specimens, and to equi-biaxial tensile tests on AA5754 cruciform specimens.

Journal article

Politis DJ, Politis NJ, Lin J, 2021, Review of recent developments in manufacturing lightweight multi-metal gears, PRODUCTION ENGINEERING-RESEARCH AND DEVELOPMENT, Vol: 15, Pages: 235-262, ISSN: 0944-6524

Journal article

Zhang R, Shi Z, Shao Z, Dean TA, Lin Jet al., 2021, A novel spatio-temporal method for determining necking and fracture strains of sheet metals, International Journal of Mechanical Sciences, Vol: 189, ISSN: 0020-7403

Forming limit diagrams (FLDs) and fracture forming limit diagrams (FFLDs) have been widely used to evaluate formability of sheet metals. There are many existing methods for determining localised necking strain and fracture strain necessary to construct these diagrams, however, none has been widely accepted and applied to the range of available formability testing methods, e.g. Nakajima tests and biaxial tensile tests. In this study, a novel spatio-temporal method is proposed and developed for determining the localised necking strain and the fracture strain in deformed sheet metals. In the method, localised necking is assumed to appear at the beginning of an increasing difference between average thickness strain within two rectangular zones where localised necking occurs. The effects of dimensions of the two zones on determined localised necking strains were investigated using uniaxial tensile tests for three sheet metals: AA7075, boron steel and AA6082, and the optimal dimensions are recommended to ensure accurate determinations. In comparison with several widely used existing methods, it was concluded that the novel method has greater simplicity, stability and accuracy in determining the localised necking strains. The method was also successfully applied to determine the localised necking strain and the fracture strain for AA5754 in biaxial tensile tests and it was demonstrated to be unaffected by noise and the Portevin–Le Châtelier (PLC) effect.

Journal article

Rong Q, Shi Z, Li Y, Lin Jet al., 2021, Stress-Relaxation Age Forming of a Component with Complex and Large Curvatures: Simulation and Manufacturing, 13th International Conference on the Technology of Plasticity (ICTP 2021), Pages: 921-934, ISSN: 2367-1181

Stress-relaxation age forming (SRAF) of a component (820 × 300 × 3 mm3 ) with complex and large curvatures has been simulated and a corresponding SRAF test of an Al–Mg–Si alloy, AA6082-T6, has been carried out in this study. An FE model is established to simulate the stress-relaxation ageing (SRA) behaviour of AA6082-T6 during SRAF at various stress levels, ranging from elastic to plastic regions, and a unified constitutive model for SRA of the material has been implemented in the finite element (FE) software ABAQUS via the user-defined subroutine CREEP. An optimised tool surface was determined from the FE simulation through springback predication and compensation and was used for manufacturing test. A good agreement of the formed shape has been achieved between the FE simulation and the SRAF test, with a maximum shape error below 4 mm, satisfying the industrial requirement. The hardness results from the formed plate showed an insignificant change of strength during SRAF, and are in good agreement with the simulation. The effects of the initial plastic strain and creep strain generated during loading and stress-relaxation stages on the formed shape have also been analysed.

Conference paper

Li Y, Lyu F, Shi Z, Zeng Y, Huang X, Lin Jet al., 2021, Experimental Study and Modelling of Stress Relaxation Ageing Behaviour and Post-form Mechanical Properties in Creep Age Forming of Al-Zn-Mg Alloy, 13th International Conference on the Technology of Plasticity (ICTP 2021), Pages: 877-889, ISSN: 2367-1181

The stress relaxation ageing behaviour and post-form mechanical properties of an Al-Zn-Mg alloy, AA7B04, have been experimentally investigated and modelled. The stress relaxation ageing tests were carried out under different initial stress levels and durations at 165 °C and subsequent tensile tests were performed at room temperature. The detailed effects of stress and time on stress relaxation and main post-form mechanical properties (yield strength, ultimate tensile strength, and uniform elongation) have been analysed and discussed. Based on the results and analysis, a set of unified constitutive equations has been developed for the first time to simultaneously predict stress relaxation ageing behaviour of the aluminium alloy during the creep age forming (CAF) process and the main mechanical properties of the products after CAF. The model comprises three sub-models, including microstructure, stress relaxation ageing and post-form mechanical properties, and has successfully predicted corresponding behaviour. The developed model provides an effective tool to not only predict the forming process but also support possible industrial applications for CAF products.

Conference paper

Zhao H, Xi J, Zheng K, Shi Z, Lin J, Nikbin K, Duan S, Wang Bet al., 2020, A review on solid riveting techniques in aircraft assembling, Manufacturing Review, Vol: 7, Pages: 1-18, ISSN: 2265-4224

Solid riveting is the most widely used joining technique in aircraft assembly, and the current key problems affecting practical application and reliable lifting are concentrated on static strength and fatigue. This paper aims to present a practical review on current practice and novel techniques of solid riveting for aircraft applications in order to obtain a thorough understanding of the underlying mechanisms of defect development to assist industrial users to find pragmatic solutions for safe life extension of components. At first, the current status of solid riveting processes is reviewed, and the key influencing factors on static/fatigue failure of riveted joints are identified. Effects of solid riveting design parameters, manufacturing parameters, residual stress, load transfer and secondary bending on static and fatigue strengths of riveted lap joints are discussed, followed by a review of the state-of-the-art solutions that deal with static/fatigue failures. Furthermore the new development in solid riveting techniques, including the use of different materials and riveting processes, is addressed. Finally, future research perspective and applications industrial riveting is presented.

Journal article

Zheng K, Tong C, Li Y, Lin J, Kolozsvari ZC, Dean TAet al., 2020, An experimental and numerical study of feasibility of a novel technology to manufacture hot stamping dies with pre-constructed tube network, The International Journal of Advanced Manufacturing Technology, Vol: 111, Pages: 2919-2937, ISSN: 0268-3768

Journal article

Zhang R, Lin J, Shi Z, Shao Zet al., 2020, A new technique for characterising mechanical properties of materials under hot stamping conditions, 39th International Deep Drawing Research Group Conference, Publisher: IOP Publishing, ISSN: 1757-8981

In order to characterise mechanical properties of materials (e.g. formability) under hot stamping conditions, significant efforts have been made to the development of the biaxial tensile testing method using cruciform specimens. However, no method for necking strain determination and no cruciform specimen design have been widely accepted. In this study, a new technique for characterising mechanical behaviour of materials under hot stamping conditions has been proposed. It includes two main parts: 1) a novel spatio-temporal method for determining necking and fracture strains, and 2) a cruciform specimen design for formability evaluation using biaxial testing method. In the first part, the theoretical base of the novel spatio-temporal method has been discussed, and the method has been validated by applying to uniaxial tensile tests on AA6082 specimens. The method has also been compared with several existing popular methods, in the determination of limit strain at onset of localised necking. It is found that the novel method has greater simplicity, stability and accuracy for the determination of localised necking strain. In the second part, a proposed cruciform specimen of AA5754 has been tested under the equi-biaxial tension, and both the necking initiation location and the strain path at the location where necking initiates, have been analysed. Furthermore, the novel spatio-temporal method has been applied to the biaxial tensile test for the determination of necking and fracture strains. The results show that the designed cruciform specimen enables to initiate fracture at the centre of the specimen and realisation of linear strain path under equi-biaxial tension.

Conference paper

Lv J, Zheng J, Yardley VA, Shi Z, Lin Jet 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.

Journal article

Li Y, Gu B, Jiang S, Liu Y, Shi Z, Lin Jet al., 2020, A CDRX-based material model for hot deformation of aluminium alloys, International Journal of Plasticity, Vol: 134, Pages: 1-17, ISSN: 0749-6419

During hot deformation, continuous dynamic recrystallisation (CDRX) is believed to occur, and even dominates microstructural evolution in many metallic materials with high stacking fault energy, such as aluminium alloys. A unique material model for hot deformation of aluminium alloys is proposed in this paper, based on consideration of two processes: (i) increase of dislocation density, induced by plastic deformation, leading to generation, rotation and migration of low angle grain boundaries (LABs) and their transformation into high angle grain boundaries (HABs); (ii) migration of HABs leading to annihilation of both LABs and HABs. At large strain, the above counteracting processes, guided by different mechanisms, lead to saturation of HABs fraction. The model is applied to hot deformation of AA5052 and AA7050 alloys under various temperatures and strain rates, and calculated flow stress, HABs fraction and grain size evolution for both alloys agree well with the corresponding experimental data. The capability of predicting saturation of HABs fraction and average subgrain misorientation angle of both alloys under large strains demonstrate the potential applicability of the model to a wide range of hot forming process conditions.

Journal article

Shi Z, Liu S, Lin J, Li Zet al., 2020, Reinforcement learning in free-form stamping of sheet-metals, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 444-449, ISSN: 2351-9789

Sheet-metal free-form stamping technology deforms sheet-metals with simple and low costs universal tools on a working bench, which is normally an anvil. This traditional forming method is praised for its high forming flexibility but complained due to its reliance on individual experience thus low repeatability. In this paper, a python-based overall learning algorithm, which incorporates a reinforcement learning (RL) algorithm, for a designed sheet-metal free-form stamping case is developed. A neural network system, known as deep Q-network (DQN), was used to approximate the action-value function (Q function) in the Deep Q-learning algorithm. The DQN was trained using mini-batch training method, with the computational experiment data provided through Finite Element (FE) simulations. The overall learning algorithm was instantiated and evaluated by training the RL model to convergence, which is able to predict the optimal forming route to achieve the desired shape. This algorithm achieves the intellectualisation of the traditional free-form sheet-metal stamping process for the first time, without prior expertise for guidance.

Conference paper

Lu X, Yu J, Lin J, Shi Zet al., 2020, Investigation of material flow behaviour and microstructure during differential velocity sideway extrusion, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 226-230, ISSN: 2351-9789

Differential velocity sideway extrusion (DVSE) process is a cutting-edge technology to manufacture curved profiles with solid or hollow cross-sections. Extrusion welding is inevitable to form hollow cross-section profiles during DVSE. In the present work, two billets (AA1070) were welded into a bar in the chamber of an extrusion die through DVSE. The material flow behaviour, grain structure and its development in the extruded bar were studied. Based on material flow behaviour, the flow plane of material in the chamber of extrusion die can be divided into metal dead zone (MDZ), shearing intensive zone (SIZ), and metal flow zone (MFZ, including the welding zone). A sound weld without any bonding interface can be obtained by DVSE welding at 500 ℃ and 0.1 mm/s. Before material arrives at the exit of extrusion die, banded structures form along the metal flow direction with the increase of deformation and the mean grain size continually decreases due to dynamic recrystallisation (DRX). Grains significantly grow after exiting from the extrusion die.

Conference paper

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 R, Shao Z, Shi Z, Lin Jet al., 2020, A study on ratio and linearity of strain path in in-plane biaxial tensile test for formability evaluation, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 584-588, ISSN: 2351-9789

In-plane biaxial tensile test is an alternative to determine the forming limit diagram (FLD) for evaluating the formability of metal sheets, in which cruciform specimens are deformed under the plane stress condition. Given that strong dependence of an FLD on both the strain state and the strain path, it is critical to realise the deformations under various proportional strain paths in the in-plane biaxial tensile test. In this study, three different stretching modes in a previously developed planar biaxial tensile rig, called stretching model-I, stretching model-II and stretching model-III, were applied to deform one type of cruciform specimen for AA5754 under an expected strain state of the equi-biaxial tension, the plane-strain tension and the uniaxial tension, respectively. The digital image correlation (DIC) technique was adopted for strain field measurement. By analysing the ratio and the linearity of the strain paths in the different zones within the gauge area of the cruciform specimens, it was found that, by using the stretching mode-I, the equi-biaxial strain state was obtained only in the central zone, and the corresponding strain path is linear. The plane-strain states were not achieved in any zones within the gauge area by using the stretching mode-II, and the corresponding strain paths are nonlinear. By using the stretching mode-III, the fracture occurred in a zone within the gauge area where the strain state is uniaxial and the corresponding strain path is linear, while the strain state in the central zone is close to the pure shear and the strain path is nonlinear.

Conference paper

Gu B, Jiang S, Shi Z, Lin Jet al., 2020, On the lower limit of misorientation of grain boundaries in hot forging of AA7050, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 744-748, ISSN: 2351-9789

The goal of this work is to find a reasonable norm to measure the area of low angle grain boundaries (LABs) using EBSD data, which is useful for the metal forming community involved in developing appropriate constitutive models for dynamic recovery. Typically, LABs are defined as the grain boundaries with misorientation between 2° and 10°. However, it is found in this study that reasonable variations of area per volume of LABs (Slab) with strain level (ε), strain rate (έ) and temperature is not be observed for AA7050 with this definition. There is a competition among different processes, including sub-grain formation and coarsening, rotation of sub-grain/cell boundary and migration of LABs and high angle grain boundaries (HABs) during recovery and continuous dynamic recrystallisation. By analysing the evolution of Kernal Angle Misorientation (KAM), it is shown that recovery has more effect on the lower angle part (< 1°) of local misorientation. If the lower limit of LABs is shift to 0.5°, the evolution of Slab with ε and έ shows clear pattern and increasing trend of Slab with temperature is consistent with temperature effect on sub-grain formation process. With such definition, we can have a better understanding of the microstructure evolution in the dynamic restoration of AA7050.

Conference paper

Zhou W, Afshan S, Lin J, 2020, An investigation of damage healing in high temperature compressive forming process, International Conference on Metal Forming

Conference paper

Ganapathy M, Li N, Lin J, Bhattacharjee Det al., 2020, A feasibility study on warm forming of an as-quenched 22MnB5 boron steel, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 277-283, ISSN: 2588-8404

In this paper, the feasibility of a newly proposed forming method, being the warm forming of as-quenched 22MnB5 boron steels, was studied through a series of proof of concept experiments. To assess the material thermo-mechanical behaviours under the proposed forming conditions, first, the as-received 22MnB5 boron steel was austenized and quenched to below the martensite transformation finish temperature to obtain a martensitic microstructure; second, uniaxial tensile tests of the as-quenched steel were conducted under proposed warm forming conditions on a Gleeble 3800 materials simulator. To evaluate the material post-form properties, first, tempering treatments on the as-quenched steel samples were performed to simulate the heat-treating conditions in the proposed warm forming process; second, the mechanical properties (hardness, strength, and ductility) of as-tempered samples were measured and a microstructure analysis was conducted. From the experimental results, it was found that, under the proposed warm-forming process conditions (420 °C–620 °C), the material showed significant strain softening, which would increase the tendency of necking during stamping and adversely affect its drawability. In addition, it was found that the heating of martensite in a 22MnB5 boron steel to higher temperatures (>400 °C) adversely affected its post-form strength and ductility due to the tempering effect. Therefore, according to the results obtained in this study, the warm forming of as-quenched 22MnB5 boron steel may reduce the strength of formed parts by more than 50% in comparison to the possible strength the material could achieve under the investigated process.

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

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