Publications
126 results found
Wang X, Shi Z, Lin J, 2023, Experimental study and modelling of anisotropic behaviour of aluminium-lithium alloys in creep age forming, International Journal of Mechanical Sciences, Vol: 260, ISSN: 0020-7403
The anisotropic deformation behaviour of the 3rd generation aluminium-lithium (Al-Li) alloys can significantly affect the accuracy of prediction and fabrication, especially for fabricating complex shaped components. To address this challenge and shed light on improving the prediction accuracy with the existence of material anisotropy, this study systemically investigated the anisotropic behaviour during deformation and creep-ageing, and proposed a comprehensive anisotropic material model and its implementation method in the numerical simulation, which was verified with four-point bending creep age forming (CAF) tests. The anisotropic behaviours of the material including yielding, hardening, and creep deformation were investigated with uniaxial tensile and creep-ageing tests adopting specimens cutting along 0°, 15°, 30°, 45°, 70° and 90° to the rolling direction. The yield strengths varied in the range of 378 to 456 MPa for different directions, with the highest and lowest values appearing at 0° and 70°. The creep deformation also showed strong anisotropic behaviour and the accumulated creep strain for 90° (0.52%) was four times larger than the value for 0° (0.11%) under 415 MPa at 143 °C for 5 h An anisotropic material model was established based on the modified non-associated quadratic Hill48 yield function and can adequately capture the observed anisotropic behaviour. An implementation strategy using the bisection method with the map returning scheme was proposed for incorporating the proposed model in numerical simulation. A good agreement was achieved between the predicted and experimental results for four-point bending CAF tests, demonstrating the validity of the proposed anisotropic material model and the implementation method.
Zhang Z, Tong C, Zhou W, et al., 2023, Investigation of the mechanisms on the abnormal features observed in thermal-mechanical testing of AA6061 under extrusion conditions, Materials Science and Engineering: A, Vol: 884, ISSN: 0921-5093
Hot extrusion is the most common forming technology for aluminium alloy AA6061 due to its good extrudability, and thus it is important to study its high-temperature deformation characteristics. In this study, three abnormal features are observed in thermal-mechanical testing under extrusion conditions of AA6061 specimens from one billet: 1) Two types of specimens with grey-coloured surface or silver-coloured surface appear after solution heat treatment (SHT); 2) The silver-coloured specimens show orange peel surface after hot compression tests; 3) The silver-coloured specimens have lower flow stresses than the grey-coloured specimens. This paper investigates the mechanisms behind the above abnormal features. A laser scanning confocal microscope is employed to examine the surface roughening, and electron back scatter diffraction is used to characterise microstructural changes. It is found that the main causes of the above behaviour are due to different initial grain morphologies and the evolution of dislocation density after SHT. The silver-coloured specimens initially have smaller columnar grains which undergo recrystallisation and extensive growth during SHT, and the dislocation density decreases significantly, leading to orange peel defect and low flow stress during compression tests, respectively. The grey-coloured specimens have larger columnar grains. After SHT, some grains undergo recrystallisation, but others still maintain the shape of the large columnar grains, and the dislocation density does not change significantly, resulting in surface oxidation with smooth surface after thermal-mechanical testing and 10–25 MPa (30–50%) higher flow stress compared to the silver-coloured specimens in compression tests.
Cheng Q, Zhang R, Shi Z, et al., 2023, Review of Common Hydrogen Storage Tanks and Current Manufacturing Methods for Aluminium Alloy Tank Liners, International Journal of Lightweight Materials and Manufacture, ISSN: 2588-8404
Tai C-L, Chen M-C, Chung T-F, et al., 2023, The nano-structural characterization of Ω and S phases in Al-5.1Cu-1.0 Mg-(0.4Ag) AA2024 aluminum alloys, Materials Science and Engineering: A, Vol: 881, Pages: 1-16, ISSN: 0921-5093
The effects of a minor addition (0.4 wt%) of Ag to Al-5.1Cu-1.0 Mg aluminum alloy on the microstructural evolution and mechanical properties have been investigated. The under-ageing, peak ageing, over-ageing, and prolonged ageing at 185 °C were studied for the alloys with and without Ag additions to further correlate the microstructure with mechanical properties. The faster increment of hardness during the early stage (1–5 h) and much higher ultimate tensile strength (530 Mpa) at peak ageing condition in Ag-containing specimen were attributed to the formation of the Ω phase triggered by the Ag addition. In over-aged specimens, the S phase in the Ag-free specimen coarsened significantly, whereas the S phase in the Ag-containing specimen remained nearly the same size as that in its peak-ageing treated specimen. The formation of the Ω phase in the Ag-containing specimen presumably consumed the same solute atoms as the S phase, i.e., Cu and Mg, leading to the depleted zones of Cu–Mg in the aluminum matrix and thus suppressing the coarsening of the S phase during the ageing time from 11 h (peak ageing) to 18 h (over ageing). After ageing for 100 h (prolonged ageing), the S phase coalesced in both alloys. However, the Ω phase in the Ag-containing specimen still maintained a thickness of ∼3 nm with a high level of volume percent (2.74 vol %), which contributed significantly to the UTS (449 MPa).
Zhang Z, Zhou W, Shi Z, et al., 2023, Investigation of die designs on welding quality and billet material utilisation for multi-container extrusion of wide stiffened aluminium panels, The International Journal of Advanced Manufacturing Technology, Vol: 127, Pages: 4149-4162, ISSN: 0268-3768
Wide stiffened aluminium panels are extensively used in aerospace, marine, and civil industries due to their light-weight structure and high stiffness. In this paper, a wide stiffened aluminium panel was manufactured using the principle of the multi-container extrusion, and a comparative study was conducted using two different die designs at the same extrusion condition, in which metal flow behaviour, extrusion force, welding quality, and billet material utilisation have been investigated numerically. Additionally, the effect of extrusion speed on the extrusion process was evaluated with the modified design. It was shown that, compared with the initial design, better metal flow behaviour can be obtained in the modified design. Multi-container extrusion greatly reduces the extrusion force, and the modified design results in a more uniform extrusion force for each extrusion container. The total extrusion force for the modified design is slightly higher compared with the initial die design, due to the increased friction in the upper die channels and the second-step welding chamber. Besides, the modified design of the multi-container extrusion can obtain better welding quality evaluated by different welding criteria, and the extrusion speed has a minor effect on the welding quality. The most notable feature is that the modified design greatly improves the material utilisation, which could save 39.5% material compared to the initial design.
Lv J, Shi Z, Yu J, et al., 2023, Analysis of solid-state welding in extruding wide aluminium hollow profiles using a new three-container extrusion system, Journal of Manufacturing Processes, Vol: 94, Pages: 146-158, ISSN: 1526-6125
A novel multi-container extrusion has recently been proposed to extrude multiple billets simultaneously for manufacturing thin-walled wide engineering components with greatly reduced force. The metal flow behaviour and solid-state welding in the process are crucial factors that influence the final product quality and industrial application. In this study, thin-walled wide AA6063 hollow profiles were extruded by three-container extrusion technology under different combinations of extrusion temperature and speed. The bonding quality of the welds that formed between the adjacent billets during the extrusion process was experimentally evaluated via tensile tests of specimens at different positions of the extrudates. A three-dimensional numerical model was established and validated to characterise the metal flow behaviour and the solid-state welding. Ten points in the first 140 mm of the weld of the extrudate were traced to investigate the flow paths and the related physical information such as pressure, effective stress, velocity and strain rate to reveal the weld formation mechanism. The simulated results showed that the whole three-container extrusion process includes five stages, i.e. separation stage, smooth flow stage, welding chamber filling stage, die bearing breakthrough stage and steady extrusion stage. By evaluating the simulated results and the tensile testing results, the pressure-time (Q) criterion was found able to evaluate the welding quality individually for each extrusion condition, while the pressure-time-flow (K) criterion appeared suitable to predict the welding quality for various extrusion conditions within a small range of critical limits.
Wang X, Rong Q, Shi Z, et al., 2023, An efficient closed-form solution for springback prediction and compensation in elastic–plastic creep age forming, The International Journal of Advanced Manufacturing Technology, Vol: 125, Pages: 1115-1133, ISSN: 0268-3768
Accurately predicting the amount of springback has always been a prior focus in metal forming industry, particularly for creep age forming (CAF), for its significant effect on tool cost and forming accuracy. In this study, a closed-form solution for CAF springback prediction covering deformation from elastic to plastic loadings was developed by combining the beam theory and Winkler’s theory, based on which an efficient springback compensation method for CAF was proposed. This developed solution extends the application area beyond the traditional beam theory-based springback prediction methods, maintaining its validity with large loading deflection in plastic range. Finite element (FE) simulation and four-point bending CAF tests adopting a 3rd generation Al-Li alloy were conducted in both elastic and plastic forming regions and the results showed close agreement with the closed-form springback predictions. For the proposed compensation method, an adjustment factor was introduced for complex flexible tool CAF to consider its deviation from the uniform stress loading and can be obtained using the closed-form solution. The flexible tool CAF tests using the Al-Li alloy demonstrated the applicability of the proposed compensation method to obtain the target shape within reasonable iterations, which can be further reduced by combining FE simulation.
Zhou W, Shi Z, Lin J, 2023, Semi-analytical study of buckling response for grid-stiffened panels during creep age forming, Thin-Walled Structures, Vol: 182, ISSN: 0263-8231
The elastic–plastic buckling response of panels with grid stiffeners during creep age forming (CAF) has been studied using a semi-analytical method in this research. A simplified model for panels with multiple grid stiffeners under bending has been established and the effective width has been applied to consider the flexible skin effect and multiple stiffener effect. In the semi-analytical method, the equilibrium equation for the simplified model has been solved via differential quadrature method to acquire the buckling stress of grid-stiffened panels, using deformation theory (DT) and incremental theory (IT) of plasticity. The results with IT show good agreements with both the published experimental and non-linear FE results, demonstrating its effectiveness. Based on the method, the effects of geometric dimensions of grid-stiffened panels, multiple stiffeners and temperature on buckling response in the elastic and plastic regions have been studied. It is found that the transverse stiffener provides a 13.9% higher buckling stress of grid-stiffened panels than blade-stiffened panels, and buckling may occur in the heating stage during CAF. The proposed semi-analytical method based on IT provides accurate predictions of buckling stress during either CAF or cold forming, which can guide the parameter optimisation of grid-stiffened panels in design.
Li C, Viswanathan-Chettiar S, Sun F, et al., 2023, Effect of CFRP surface topography on the adhesion and strength of composite-composite and composite-metal joints, Composites Part A: Applied Science and Manufacturing, Vol: 164, Pages: 1-10, ISSN: 1359-835X
Manufacturing carbon-fibre reinforced polymer (CFRP) composites via different techniques often leads to contrasting surface topographies. Such differences can affect any subsequent surface pre-treatments that are performed and these can ultimately affect joint strength. In the present work, CFRP adherends made using compression moulding or autoclaving were investigated. Pre-treatment techniques of acetone cleaning, plasma treatment, and grit blasting were studied. It was found that the patterned surfaces which had resulted from the vacuum autoclave moulding resulted in improved joint performance when CFRP substrates were bonded together (homogeneous bonding) compared to joints formed with flatter surfaces following compression moulding. However, when CFRP substrates were bonded to flat aluminium alloy substrates (hybrid bonding) the patterned surfaces resulted in inferior joint performance compared to the flatter CFRP substrates. It is proposed that the dissimilarity of surface topographies on the metal and composite substrates negatively influences the strength of the joint.
Lv J, Yu J, Shi Z, et al., 2023, Feasibility study of a novel multi-container extrusion method for manufacturing wide aluminium profiles with low force, Journal of Manufacturing Processes, Vol: 85, Pages: 584-593, ISSN: 1526-6125
Extrusion profiles are extensively used in industries and any improvement in the process could potentially have a large impact on energy and cost savings. In this study, a novel extrusion method, namely multi-container extrusion, was proposed for producing thin-walled wide aluminium components with low force. Its basic principle is to enable multiple billets to be welded and forced through die orifice simultaneously. To demonstrate its feasibility, a series of studies have been conducted including the experimental design and set-up of multi-container extrusion, microstructural and mechanical characterisations of the extruded components, and the comparison with the existing extrusion method. A three-container extrusion tooling was designed and manufactured to produce wide hollow profiles with plasticine and aluminium alloy AA6063. Optical microscopic observations and tensile tests were carried out for different positions of AA6063 extrudates. For the specimens near the extrudate front (8 mm and 23 mm away), tensile fractures occurred exactly along the distinct welds which were formed when individual billets met inside the die during extrusion. Further away from the extrudate front (83 mm, 98 mm and 113 mm), the welds were hardly observed, and the tensile fractures occurred outside the welds, indicating that good welding quality can be obtained in the multi-container extrusion process. Compared with the conventional porthole extrusion method, three-container extrusion could significantly reduce the required force to about 15 % for extruding the same profile. This study demonstrates that the proposed multi-container extrusion method can form wide profiles and greatly reduce the extrusion force requirement.
Li J, Cheng Q, Zhang R, et al., 2022, New developments of formability evaluation methods for hot stamping, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, Pages: 1-9, ISSN: 1757-8981
Formability is an essential material property that needs to be considered when selecting materials for hot stamping applications. Due to the difficulties of achieving rapid cooling before deformation and the failure of lubricant systems, however, it is challenging to use conventional Nakajima and Marciniak tests to evaluate the formability of materials under hot stamping conditions. Recently, biaxial test methods have shown great potential to overcome this challenge. In this paper, recent developments of the biaxial test methods for formability evaluation are reviewed, including testing machines, specimen designs, specimen heating methods, testing procedures, and limit strain determination methods. Compared to the Nakajima or the Marciniak tests, the biaxial test method can provide better simulation for hot stamping conditions and it can be a promising method for evaluating the formability of sheet metals under hot stamping conditions. However, more developments such as the standardisation of the specimen designs and the limit strain determination methods, are still needed for the wide use of the biaxial test method.
Alfawzan Y, Shi Z, Lin J, 2022, Development and perspectives of rolling of rib stiffened plate, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, ISSN: 1757-8981
Thin-wide aluminium rib stiffened plate structure reduces the weight and provides high stiffness, which makes it in high demand in transportation vehicles, marine, and structural applications for energy saving and CO2 emission reduction. In this study, the applicability of rolling in manufacturing an integral rib stiffened plate is reviewed through analysis of different rolling processes. The effect of rolling parameters and rib geometry on the formation and quality of ribs are analysed. The analysis provides an important basis to advance the understanding of metal flow, viscoplastic deformation and roll shape design during rolling, significantly helping the design of an innovative rolling process for rib stiffened plate. In addition, geometry and rolling parameters, plate curvature, and roll pass design (RPD), that highly affect the rolling process design are discussed. This paper presents the limitations and improvements of specific shape rolling processes and preliminarily concludes that rolling could be a potential method to produce high quality thin-wide panels with high stiffeners.
Dear J, Shi Z, Lin J, 2022, An efficient numerical integration system for stiff unified constitutive equations for metal forming applications, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, ISSN: 1757-8981
Unified constitutive equations have been developed in recent years to predict viscoplastic flow and microstructural evolution of metal alloys for metal forming applications. These equations can be implemented into commercial FE code, such as ABAQUS and PAMSTAMP, to predict mechanical and physical properties of materials in a wide range of metal forming processes. These equations are normally stiff and need significant computer CPU time to solve. In this research, a series of numerical analyses are performed to investigate the difficulties within MATLAB of solving these stiff unified constitutive equations. A metric is introduced to allow evaluation of the numerical stiffness to assess the most appropriate numerical integration method. This metric is based on the ratio of maximum to minimum eigenvalue. This metric allows for an appropriate numerical method to be chosen giving more effective modelling of deformation and plasticity processes. Based on the theoretical work described above, a user-friendly system, based on MATLAB, is then developed for numerically integrating these types of stiff constitutive equations. This is particularly useful for metal forming engineers and researchers who need an effective computational tool to determine constitutive properties well based on numerical integration theories.
Zhang Z, Zhou W, Shi Z, et al., 2022, Advances on manufacture methods for wide lightweight aluminium stiffened panels, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, Pages: 1-9, ISSN: 1757-8981
Wide aluminium panels with stiffeners are extensively applied in aviation, bridge and marine structures due to their high stiffness and light-weighting. Many efforts and contributions have been made to overcome the manufacturing difficulties of wide aluminium stiffened panels over years. The aim of this study is to analyse and compare current methods for producing wide stiffened panels for different applications. The manufacturing techniques for stiffened panels such as riveting, welding, machining, additive manufacturing, extrusion, are discussed. Thereinto, extrusion is a very promising technology in the production of wide aluminium stiffened panels as it can efficiently obtain extruded products with high material utilisation, good mechanical properties and structure integrity. Therefore, the methods of widening the stiffened panels in extrusion technology such as spread extrusion and postproduction flattening after extrusion are analysed emphatically. The present study is an attempt to analyse these efforts in order to guide future work in the area of producing much wider aluminium stiffened panels.
Li W, Yu J, Lv J, et al., 2022, Development of a multi-container extrusion method for extruding lightweight wide plates and sheets, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, ISSN: 1757-8981
Extrusion of wide plates and sheets of light alloys has been studied over a long period of time, yet the extrudable width of the material is still limited due to high extrusion force requirement. To overcome this drawback, a new multi-container extrusion process is proposed in the research, which allows the production of lightweight plates and sheets with less force compared to that of existing extrusion methods. A lab scale feasibility study system with three containers has been designed and built and tested for AA1060 billets. Experimental work has been carried out with the extrusion temperature of 450°C and extrusion speed of 0.5 mm/s. Optical microscopy observation and tensile tests have been performed for the extruded materials at different positions to investigate the extrusion welding quality between the three extrusion billets. The test results show that the welding quality improves as extrusion progresses and the overall welding quality is stable. This study demonstrates the feasibility of the new multi-container extrusion method.
Lu X, Yu J, Yardley V, et al., 2022, Experimental investigation of longitudinal and transverse welds during sideways extrusion, The 19th International Conference on Metal Forming (MF 2022), Publisher: IOP Publishing, Pages: 1-7, ISSN: 1757-8981
Differential velocity sideways extrusion (DVSE) is a novel process for fabricating curved profiles; welding quality during extrusion is an important issue for its industrial application. In this study, solid bars of aluminium alloy AA1070 were extruded from multiple billets at 500 °C using the novel process and the microstructure and mechanical properties of the extrudate were investigated. The weld formed between the billets includes longitudinal and transverse solid-state weld regions formed as the metal was extruded. The longitudinal welds have better mechanical properties than the transverse welds. Dynamic recrystallisation (DRX) occurred in areas with high dislocation density during the extrusion process and as a result, grains across the bonding interfaces of longitudinal welds have been formed, which improves the weld quality. In the areas with the transverse welds, macro weld defects can be observed at the weld front. With further progress of sideways extrusion, the defect density reduced and new grains formed at the bonding surface.
Wang X, Rong Q, Shi Z, et al., 2022, Improved creep behaviour for a high strength Al-Li alloy in creep age forming: Experimental studies and constitutive modelling, International Journal of Plasticity, Vol: 159, Pages: 1-21, ISSN: 0749-6419
The creep deformation of 3rd generation 2xxx Al-Li alloys in creep age forming (CAF) is unsatisfactorily low due to its high strength and required low ageing temperature, especially for the commonly used T8 temper. Promoting creep deformation in such process will significantly reduce springback in CAF, making easier the springback compensation in tool design. In this study, two possible process routes of inducing increased creep deformation were explored, one by employing high applied stress to introduce small plastic strain in loading stage; and the other by conducting pre-deformation prior to creep-ageing. The creep behaviour with and without pre-deformation was investigated through creep-ageing tests under stresses ranging from 300 to 430 MPa at the ageing temperature of 143°C. Due to the decrease of threshold stress with introduced plastic strain in the loading stage and/or pre-stretching, a faster increase of creep strain with applied stress was observed for both methods when the applied stress surpassed 400 MPa. The strain promotion of the former was higher than the latter when above 415 MPa. A mechanism-based constitutive model was proposed, with additional work hardening equations to describe the relationship between threshold stress and dislocation density induced by plastic strain in the loading stage and pre-stretching, to model the microstructural evolution and reflect the nonlinear increase of creep strain with stress in both methods. The adequacy of this proposed unified model was demonstrated with good agreement with experimental data utilising both methods. The adaptability of the model in multiaxial case was verified using four-point bending CAF tests with the same Al-Li alloy.
Altıparmak SC, Yardley VA, Shi Z, et al., 2022, Extrusion-based additive manufacturing technologies: State of the art and future perspectives, Journal of Manufacturing Processes, Vol: 83, Pages: 607-636, ISSN: 1526-6125
Extrusion-based additive manufacturing (AM) has recently become widespread for the layer-by-layer fabrication of three-dimensional prototypes and components even with highly complex shapes. This technology involves extrusion through a nozzle by means of a plunger-, filament- or screw-based mechanism; where necessary, this is preceded by heating of the feedstock material to reduce its viscosity sufficiently to facilitate extrusion. Extrusion-based AM offers greater design freedom, larger building volumes and more cost-efficient production than liquid- and powder-based AM processes. Although this technology was originally developed for polymeric filament materials, it is now increasingly applied to a wide variety of material classes, including metallic, edible and construction materials. This is in part thanks to the recent development of AM-specific feedstock materials (AM materials), in which materials that are not intrinsically suited to extrusion, for example because of high melting points or brittleness, are combined with other, usually polymeric materials that can be more readily extruded. This paper comprehensively and systematically reviews the state of the art in the field of extrusion-based AM, including the techniques applied and the individual challenges and developments in each materials class for which the technology is being developed. The paper includes material- and process-centred suitability analysis of extrusion-based AM, and a comparison of this technology with liquid- and powder-based AM processes. Prospective applications of this technology are also briefly discussed.
Tong C, Yardley VA, Shi Z, et 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.
Zhang R, Shi Z, Yardley VA, et 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
Tong C, Rong Q, Yardley VA, et 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: 117623-117623, ISSN: 0924-0136
Jiang S, Yardley VA, Li N, et al., 2022, Revealing the Geometrically Necessary Dislocation Density Evolution during Hot Compression of AA7050, 7th International Conference on Materials Science & Smart Materials “MSSM2021”, Publisher: Trans Tech Publications, Ltd., Pages: 109-116
<jats:p>Dislocations play a critical role in metal forming processes, and accurate values of dislocation density are important in modelling these processes. However, direct determination of the dislocation density is challenging. In this study, electron backscatter diffraction is used to estimate the evolution of geometrically necessary dislocation density as a function of plastic strain, strain rate and temperature in hot compression of AA7050 alloy. The geometrically necessary dislocation density was found to increase at a higher strain rate and lower temperature; the higher dislocation density in these samples promoted continuous dynamic recrystallisation leading to grain refinement. At lower strain rates and higher temperatures, the dislocation densities were lower and dislocations formed into walls, channels and cells. These observations agree with accepted theories of dislocation evolution and demonstrate the capability of electron backscatter diffraction to provide representative dislocation density values as well as comprehensive information linking plastic flow with microstructural evolution.Keywords: electron backscatter diffraction (EBSD), geometrically necessary dislocations (GNDs), hot deformation, AA7050</jats:p>
Zhou W, Shi Z, Lin J, et 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
Liu S, Xia Y, Liu Y, et 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
Zhang R, Shi Z, Yardley VA, et 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.
Li Y, Shi Z, 2022, A New Method to Characterize and Model Stress-Relaxation Aging Behavior of Aluminum Alloys Under Age Forming Conditions, Metallurgical and Materials Transactions A, Vol: 53, Pages: 1345-1360, ISSN: 1073-5623
Li Y, Shi Z, Lü F, et 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.
Wang X, Rong Q, Shi Z, et 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.
Zhou W, Shi Z, Rong Q, et al., 2022, Experimental and numerical investigations on buckling behaviour of stiffened panel during creep age forming, Thin-Walled Structures, Vol: 172, Pages: 108940-108940, ISSN: 0263-8231
Zhang R, Shi Z, Shao Z, et 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.
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