390 results found
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.
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.
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.
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.
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.
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.
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.
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.
Zhou W, Lin J, Dean TA, 2022, Microstructure and mechanical properties of curved AZ31 magnesium alloy profiles produced by differential velocity sideways extrusion, Journal of Magnesium and Alloys, ISSN: 2213-9567
Lightweight curved profiles are widely utilised in the transportation industry considering the increasing need for improving aerodynamic efficiency, aesthetics and cutting emissions. In this paper, curved AZ31 Mg alloy profiles were manufactured in one operation by a novel process, differential velocity sideways extrusion (DVSE), in which two opposed rams were used. Effects of extrusion temperature and velocity (strain rate) on curvature, microstructure, and mechanical properties of the formed profiles were examined. Profile curvature was found to be more readily controlled by the velocity ratio of the bottom ram v2 to the top ram v1, whereas extrusion temperature (T = 250, 300, 350°C) and extrusion velocity (v1 = 0.1, 1 mm/s) slightly affect curvature for a given velocity ratio. A homogeneous microstructure with equiaxed grains (∼ 4.5 µm) resulted from dynamic recrystallisation (DRX), was observed after DVSE (v2/v1 = 1/2) at 300°C and v1 = 0.1 mm/s, where the initial billet had an average grain size of ∼25 um. Increasing extrusion temperature leads to grain growth (∼ 5 µm) at 350°C and v1 = 0.1 mm/s. DRX is incomplete at the relatively low temperature of 250 °C (v1 = 0.1 mm/s), and higher strain rate with v1 = 1 mm/s (T = 300 °C), resulting in inhomogeneous bi-modal necklace pattern grains ranging in size around 1-25 µm for the former and 2-20 µm for the latter. Grain refinement is attributed to DRX during the severe plastic deformation (SPD) arising in DVSE, and initiates at the prior boundaries of coarse grains in a necklace-like manner. Compared with the billet, micro-hardness and ultimate tensile strength of the profiles have been enhanced, which is compatible with grain refinement. Also, an obvious increase in tensile ductility was found. However, yield strength slightly decreases except for the complete DRXed case (300 °C, v1 = 0.1 mm/s), where a slightly higher value was found, indicating strengthen
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
Zhang R, Lin J, 2022, New Challenges on Developing Experimental Methods for Innovative Metal Forming Techniques, ICEM 2022, Publisher: MDPI
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: 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.
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
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.
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, 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.
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, 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.
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, Lin J, Balint D, et 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.
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.
Tong C, Zhu G, Rong Q, et 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.
Lu X, Yu J, Yardley VA, et 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.
Zhou W, Yu J, Lu X, et 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
Rong Q, Shi Z, Li Y, et 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.
Zhou W, Shi Z, Li Y, et 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.
Altıparmak SC, Yardley VA, Shi Z, et 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.
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