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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376 results found

Tong C, Yardley VA, Shi Z, Rong Q, Li X, Zhang B, Xu D, Lin Jet al., 2022, Investigation of the effect of initial states of medium-Mn steel on deformation behaviour under hot stamping conditions, Materials Science and Engineering: A, Vol: 855, Pages: 1-11, ISSN: 0921-5093

Medium-Mn (MMn) steels have received much research attention recently because their low austenitisation temperature enables low-temperature hot stamping (LTHS). However, the effect of the initial state of the material on the hot stamping performance is still unknown. In this study, the effect of different initial states on the deformation behaviour of a typical MMn steel during uniaxial tensile testing under LTHS conditions (deformation at 500–600 °C under strain rates of 0.01–1 s−1) are investigated using a Gleeble 3800 materials simulator; the final mechanical properties after austenitising and quenching are also examined. The microstructure of each material state before and after the LTHS heating cycle is characterised using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Of the three states investigated the hot rolled and annealed (HRA) state shows the best hot deformation performance represented by larger strain hardening exponent and higher total elongation, followed by the cold-rolled (CR) state, with the cold-rolled and annealed (CRA) state exhibiting the worst performance. The final mechanical properties, however, are very similar among the three states. In addition, the yield point phenomenon is found during hot deformation in both the CR and CRA states, and absent in the HRA state. The hot deformation behaviour has been discussed in terms of differences in microstructural properties, namely the grain size and its degree of heterogeneity.

Journal article

Zhang R, Shi Z, Yardley VA, Lin Jet al., 2022, A CDM-based unified viscoplastic constitutive model of FLCs and FFLCs for boron steel under hot stamping conditions, International Journal of Damage Mechanics, Vol: 31, Pages: 1373-1395, ISSN: 1056-7895

Forming limit curves (FLCs), which are constructed using the limit strains at localised necking, are the most widely used tools for the evaluation of the formability of sheet metals. Fracture forming limit curves (FFLCs) are more recently developed, complementary tools for formability evaluation which are instead constructed using the limit strains at fracture. Since the formability depends strongly on forming conditions such as strain state, temperature and strain rate, models for predicting FLCs and FFLCs are essential for the optimisation and further application of hot forming processes in which these forming conditions vary significantly with both position and time. However, no model has so far been developed to predict FFLCs either alone or in conjunction with FLCs for sheet metals such as boron steel under hot stamping conditions. In this study, a set of unified viscoplastic constitutive equations for the prediction of both FLCs and FFLCs based on continuum damage mechanics (CDM) has been formulated from a set of recently developed constitutive equations for dislocation-based hardening, in combination with two novel coupled variables characterising the accumulated damage leading to localised necking and fracture. The novel variables take into account the effects of strain state, temperature and strain rate on the formability of sheet metals. The material constants in the CDM-based constitutive equations have been calibrated using experimental data comprising true stress-true strain curves and limit strains of a 22MnB5 boron steel obtained at a range of temperatures and strain rates. Investigation of the effect of varying selected parameters in the coupled damage variables on the resulting computed FLCs and FFLCs has demonstrated the flexibility of the model in enabling curves of different shapes and numerical values to be constructed. This indicates the potential of the CDM-based constitutive model for application to other materials for warm or hot stamping pr

Journal article

Zhang R, Lin J, 2022, New Challenges on Developing Experimental Methods for Innovative Metal Forming Techniques, ICEM 2022, Publisher: MDPI

Conference paper

Tong C, Rong Q, Yardley VA, Shi Z, Li X, Zhang B, Xu D, Lin Jet al., 2022, Investigation of deformation behaviour with yield point phenomenon in cold-rolled medium-Mn steel under hot stamping conditions, Journal of Materials Processing Technology, Vol: 306, Pages: 1-15, ISSN: 0924-0136

Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60–600 s, deformation temperatures of 500–700 °C and strain rates of 0.01–5 s−1. A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.

Journal article

Liu S, Xia Y, Liu Y, Shi Z, Yu H, Li Z, Lin Jet al., 2022, Tool path planning of consecutive free-form sheet metal stamping with deep learning, Journal of Materials Processing Technology, Vol: 303, Pages: 117530-117530, ISSN: 0924-0136

Sheet metal forming technologies, such as stamping and deep drawing, have been widely used in automotive, rail and aerospace industries for lightweight metal component manufacture. It requires specially customised presses and dies, which are very costly, particularly for low volume production of extra-large engineering panel components. In this paper, a novel recursive tool path prediction framework, impregnated with a deep learning model, is developed and instantiated for the forming sequence planning of a consecutive rubber-tool forming process. The deep learning model recursively predicts the forming parameters, namely punch location and punch stroke, for each deformation step, which yields the optimal tool path. Three series of deep learning models, namely single feature extractor, cascaded networks (including state-of-the-art deep networks) and long short-term memory (LSTM) models are implemented and trained with two datasets with different amounts of data but the same data diversity. The learning results show that the single LSTM model trained with the larger dataset has the most superior learning capability and generalisation among all models investigated. The promising results from the LSTM indicate the potential of extending the proposed recursive tool path prediction framework to the tool path planning of more complex sheet metal components. The analysis on different deep networks provides instructive references for model selection and model architecture design for sheet metal forming problems involving tool path design

Journal article

Zhou W, Shi Z, Lin J, Dean TAet al., 2022, An upper bound solution for deformation field analysis in differential velocity sideways extrusion using a unified stream function, International Journal of Mechanical Sciences, Vol: 224, Pages: 107323-107323, ISSN: 0020-7403

An analytical model providing a detailed description of the material flow and deformation behaviour of extruded curved profiles produced by the novel differential velocity sideways extrusion (DVSE) process, has been developed on the basis of a unified stream function and the upper bound theorem. Plasticine experiments and finite element (FE) modelling were carried out to validate the proposed analytical model. The derived streamline equation contains a shape parameter n describing the degree of curvature of a flow line and the coordinate parameters x0 and y0 defining entering and leaving positions respectively of the flow line, from the plastic deformation zone (PDZ). The analytical model was able to closely model the material flow eccentricity ratio ξ (the relative amounts of work-piece material entering the deformation zone from two opposing directions), and flow lines obtained from experiments under different velocity ratios and extrusion ratios. The predicted value of ξ was found to be independent of n value and hardening of the material. The n value was found to increase from the corner near the die orifice to the corner around the dead material zone (DMZ). In addition, the n value increased with the increase of extrusion ratio and ratio of velocities of the two opposing extrusion rams, which enabled the representation of a decreased area of DMZ and more localised PDZ containing 1–99% accumulated effective strain. The predicted field distributions of the localised effective strain rate in the PDZ and inhomogeneous effective strain in the extrudates were consistent with FE modelling results.

Journal article

Zhang R, Shi Z, Yardley VA, Lin Jet al., 2022, Experimental studies of necking and fracture limits of boron steel sheet under hot stamping conditions, Journal of Materials Processing Technology, Vol: 302, ISSN: 0924-0136

Boron steel is the most widely used material in hot stamping applications for forming automotive body panels with complex shapes and ultra-high strength. Due to the high austenitic transformation temperatures and the complex thermal cycle required for hot stamping, however, it is difficult to evaluate the formability of the material using standard punch test methods developed for room-temperature testing. In this study, a high-temperature modification of a recently developed biaxial test method has been used to determine, in a single test procedure and for the first time, forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for 22MnB5 boron steel sheet with a thickness of 1.5 mm under thermal conditions that are representative of industrial hot stamping processes. A direct resistance heating strategy has been developed, and a recently proposed cruciform specimen design has been modified for high-temperature use. For tests with target temperatures in the range of 750 to 925 °C, the resulting test specimens had the highest temperature at the specimen centre and a temperature difference of less than 45 °C in the gauge area and fracture occurred close to the centre of this area under all test conditions investigated. Limit strains at the onset of necking and at fracture for the material have been determined by applying digital image correlation (DIC) to obtain full-field strain measurements, providing an experimental foundation for constructing both FLCs and FFLCs for industrial applications.

Journal article

Li Y, Shi Z, Lü F, Rong Q, Li D, Lin Jet al., 2022, Mechanism and Modelling Studies of Elastic-plastic Stress Relaxation of Aluminium Alloys Based on Thermally Activated Plastic Theories, Jixie Gongcheng Xuebao/Journal of Mechanical Engineering, Vol: 58, Pages: 42-51, ISSN: 0577-6686

Aiming at the inconsistent of mechanisms and insufficient prediction models for stress relaxation of aluiminium alloys in elastic and plastic regions, a method that utilises thermally activated plastic deformation theories for mechanism characterisation and modelling during stress relaxation ageing (SRA) process in both elastic and plastic regions is proposed. The apparent activation volumes obtained from the theories for AA7B04 and AA6082 alloys indicate a dislocation-obstacles interaction mechanism for stress relation in both elastic and plastic regions. The stress relaxation behaviour in elastic region is found to be mainly affected by the effective stress, while the more significant stress relaxation in plastic region has been attributed to the combined effect of increasing effective stresses and decreasing apparent activation energies, which have been quantified by the theories in this study. Based on these theoretical analysis results, a simple constitutive model considering the changing apparent energies has been proposed, providing an effective way to successfully predict the different stress relaxation behaviour of aluminium alloys under various stress levels in the elastic-plastic deformation range for potential CAF process applications. The proposed method overcomes limitations of the conventional creep stress exponent analysis method for SRA behaviour analysis, and provides abundant information (i.e. stress components, apparent activation volumes and energies) to support the characterization of deformation mechanisms in SRA and enable accurate prediction of different SRA behaviour in elastic and plastic regions.

Journal article

Wang X, Rong Q, Shi Z, Li Y, Cao J, Chen B, Lin Jet al., 2022, Investigation of stress effect on creep, precipitation and dislocation evolution of Al–Li alloy during creep age forming, Materials Science and Engineering: A, Vol: 836, ISSN: 0921-5093

The 3rd generation Al–Li alloy is a good candidate for large panel fabrication adopting creep age forming (CAF) because of its balanced synergy of high strength, good toughness, corrosion resistance and light weight. In this study, the creep-ageing behaviour of a 2xxx series 3rd generation Al–Li alloy in T8 state has been experimentally investigated under different stresses ranging from 300 to 430 MPa at 143 °C for up to 20 h. The corresponding evolution of precipitate size, its distribution and dislocation density during CAF has been analysed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) tests. With the increase of the applied stress, the dominant deformation mechanism transformed from diffusion creep to dislocation creep at around 400 MPa, with corresponding stress exponent n = 2.9 in the former and n = 8.4 in the latter. A strong stress dependent characteristic has been observed in creep deformation and evolutions of dislocation density and precipitate size during creep-ageing. The creep strain increased dramatically above 400 MPa and the accumulated creep strain at 430 MPa was 7 times higher than that at 400 MPa after 20 h creep-ageing. Much higher dislocation density was observed at the applied stress above 400 MPa according to XRD measurements, which accelerates the precipitation kinetics. It was also found that the dominance of T1 over θ′ precipitates increased with applied stress and ageing time during creep-ageing at 143 °C.

Journal article

Zhou W, Shi Z, Rong Q, Bai X, Zeng Y, Lin Jet al., 2022, Experimental and numerical investigations on buckling behaviour of stiffened panel during creep age forming, Thin-Walled Structures, Vol: 172, ISSN: 0263-8231

Experimental and numerical studies on the buckling behaviour of stiffened panels under four point bending during creep age forming (CAF) have been carried out in this study for the first time. The experimental programme comprised buckling tests of five different sizes of stiffened panels of aluminium alloy 7050 at room temperature and buckling tests with different loading degrees in CAF including the loading, heating and creep-ageing stages. The buckling mode, the strain distribution and the strain evolution of the stiffened panel were obtained from the experiments, using digital image correlation (DIC) for the room temperature tests and strain gauges for the CAF tests. The effect of stiffener height and stiffener thickness and the effect of the heating and creep-ageing stages in CAF on the buckling behaviour have been investigated and discussed. It was found that buckling mode varied from one half-wave cosine mode in the elastic loading to three half-wave cosine mode with the increase of buckling stress from elastic to plastic region, and during CAF buckling mainly occurred and grew in the heating process. The corresponding non-linear finite element (FE) simulations of stiffened panels at room temperature and ageing temperature (160 °C) have also been carried out, and the FE results of buckling strain and buckling mode shape show a good agreement with experimental results. The non-linear FE method can provide accurate results of buckling strain and formability limits for cold forming and CAF processes, which can be used to guide the structural design of stiffened panels.

Journal article

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), Publisher: Springer, 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), Publisher: Springer, 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

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