Publications
135 results found
Du L, Shi Z, Han Y, et al., 2020, Development of constitutive equations for hot working of titanium matrix composites, The 14th World Conference on Titanium (Ti 2019), Publisher: EDP Sciences, ISSN: 2261-236X
This research is devoted to modelling the viscoplastic deformation behaviour and microstructure evolution of particle reinforced titanium matrix composites (TMCs) at hot working conditions. A series of Gleeble hot compression tests were conducted to obtain the stress-strain curves. According to the dominant mechanisms of TMCs during deformation, a set of mechanism-based constitutive equations was developed and fitted based on the experiment data. Lamellar alpha globularisation, dynamic recrystallization and damage were considered and incorporated into the constitutive equations to describe the viscoplastic flow behaviour.
Zhang R, Shao Z, Shi Z, et al., 2020, A study on ratio and linearity of strain path in in-plane biaxial tensile test for formability evaluation, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 584-588, ISSN: 2351-9789
In-plane biaxial tensile test is an alternative to determine the forming limit diagram (FLD) for evaluating the formability of metal sheets, in which cruciform specimens are deformed under the plane stress condition. Given that strong dependence of an FLD on both the strain state and the strain path, it is critical to realise the deformations under various proportional strain paths in the in-plane biaxial tensile test. In this study, three different stretching modes in a previously developed planar biaxial tensile rig, called stretching model-I, stretching model-II and stretching model-III, were applied to deform one type of cruciform specimen for AA5754 under an expected strain state of the equi-biaxial tension, the plane-strain tension and the uniaxial tension, respectively. The digital image correlation (DIC) technique was adopted for strain field measurement. By analysing the ratio and the linearity of the strain paths in the different zones within the gauge area of the cruciform specimens, it was found that, by using the stretching mode-I, the equi-biaxial strain state was obtained only in the central zone, and the corresponding strain path is linear. The plane-strain states were not achieved in any zones within the gauge area by using the stretching mode-II, and the corresponding strain paths are nonlinear. By using the stretching mode-III, the fracture occurred in a zone within the gauge area where the strain state is uniaxial and the corresponding strain path is linear, while the strain state in the central zone is close to the pure shear and the strain path is nonlinear.
Lu X, Yu J, Lin J, et al., 2020, Investigation of material flow behaviour and microstructure during differential velocity sideway extrusion, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 226-230, ISSN: 2351-9789
Differential velocity sideway extrusion (DVSE) process is a cutting-edge technology to manufacture curved profiles with solid or hollow cross-sections. Extrusion welding is inevitable to form hollow cross-section profiles during DVSE. In the present work, two billets (AA1070) were welded into a bar in the chamber of an extrusion die through DVSE. The material flow behaviour, grain structure and its development in the extruded bar were studied. Based on material flow behaviour, the flow plane of material in the chamber of extrusion die can be divided into metal dead zone (MDZ), shearing intensive zone (SIZ), and metal flow zone (MFZ, including the welding zone). A sound weld without any bonding interface can be obtained by DVSE welding at 500 ℃ and 0.1 mm/s. Before material arrives at the exit of extrusion die, banded structures form along the metal flow direction with the increase of deformation and the mean grain size continually decreases due to dynamic recrystallisation (DRX). Grains significantly grow after exiting from the extrusion die.
Zhou W, Rong Q, Shi Z, et al., 2020, Non-linear finite element investigation of formability limit by buckling in creep age forming of stiffened panels, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 625-629, ISSN: 2351-9789
This paper focuses on the prediction of the formability limit of the stiffened panels during the loading stage and heating stage of the creep age forming (CAF) process. A set of non-linear finite element (FE) simulations has been carried out to investigate the elastic-plastic buckling behaviour of stiffened panels subjected to pure bending at the room temperature and the ageing temperature (155 ℃) for AA2050. From the non-linear FE simulations, the effects of main geometric parameters (stiffener thickness and stiffener height) on the critical buckling stress and the critical radius of the stiffened panel have been investigated at both temperatures. Increasing the stiffener thickness or decreasing the stiffener height leads to an increase of the critical buckling stress and an increase of formability limit. The critical buckling stress at the ageing temperature is smaller than that at the room temperature when the stiffened panel enters the plastic region, however, the formability increases slightly at the ageing temperature.
Tong C, Li Y, Shi Z, 2020, Investigation of anisotropic creep-ageing behaviour of Al-Cu-Li Alloy AA2050, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 241-247, ISSN: 2351-9789
Creep age forming (CAF) of an Al-Cu-Li alloy (AA2050-T34) considering its anisotropic creep-ageing behaviour has been experimentally and numerically investigated in this study. A series of uniaxial creep-ageing tests of AA2050-T34 has been carried out in both tension and compression conditions. The creep-ageing results in longitudinal and transverse samples show some anisotropic behaviour in both the room temperature yield strength and the creep strain, in which more strengthening occurs at the intermediate stage and less creep strain is produced in the transverse samples than the longitudinal samples under the same loading condition. It is also found that compressive loading results in much higher anisotropic behaviour than tensile loading. A material model has been adopted for the creep-ageing of AA2050 and shows a good fit to experimental data. Based on the model, a four-point bending virtual test has been developed in PAM-STAMP for simulating the CAF process of AA2050. The results indicate that the anisotropic behaviour contributes an apparent effect on both creep-aged properties and springback of the CAF process and need be considered for CAF manufacture of AA2050 components.
Gu B, Jiang S, Shi Z, et al., 2020, On the lower limit of misorientation of grain boundaries in hot forging of AA7050, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 744-748, ISSN: 2351-9789
The goal of this work is to find a reasonable norm to measure the area of low angle grain boundaries (LABs) using EBSD data, which is useful for the metal forming community involved in developing appropriate constitutive models for dynamic recovery. Typically, LABs are defined as the grain boundaries with misorientation between 2° and 10°. However, it is found in this study that reasonable variations of area per volume of LABs (Slab) with strain level (ε), strain rate (έ) and temperature is not be observed for AA7050 with this definition. There is a competition among different processes, including sub-grain formation and coarsening, rotation of sub-grain/cell boundary and migration of LABs and high angle grain boundaries (HABs) during recovery and continuous dynamic recrystallisation. By analysing the evolution of Kernal Angle Misorientation (KAM), it is shown that recovery has more effect on the lower angle part (< 1°) of local misorientation. If the lower limit of LABs is shift to 0.5°, the evolution of Slab with ε and έ shows clear pattern and increasing trend of Slab with temperature is consistent with temperature effect on sub-grain formation process. With such definition, we can have a better understanding of the microstructure evolution in the dynamic restoration of AA7050.
Shi Z, Liu S, Lin J, et al., 2020, Reinforcement learning in free-form stamping of sheet-metals, 18th International Conference on Metal Forming 2020 (Virtual), Publisher: Elsevier, Pages: 444-449, ISSN: 2351-9789
Sheet-metal free-form stamping technology deforms sheet-metals with simple and low costs universal tools on a working bench, which is normally an anvil. This traditional forming method is praised for its high forming flexibility but complained due to its reliance on individual experience thus low repeatability. In this paper, a python-based overall learning algorithm, which incorporates a reinforcement learning (RL) algorithm, for a designed sheet-metal free-form stamping case is developed. A neural network system, known as deep Q-network (DQN), was used to approximate the action-value function (Q function) in the Deep Q-learning algorithm. The DQN was trained using mini-batch training method, with the computational experiment data provided through Finite Element (FE) simulations. The overall learning algorithm was instantiated and evaluated by training the RL model to convergence, which is able to predict the optimal forming route to achieve the desired shape. This algorithm achieves the intellectualisation of the traditional free-form sheet-metal stamping process for the first time, without prior expertise for guidance.
Li S, Han Y, Shi Z, et al., 2020, Synergistic strengthening behavior and microstructural optimization of hybrid reinforced titanium matrix composites during thermomechanical processing, Materials Characterization, Vol: 168, Pages: 1-12, ISSN: 1044-5803
In this study, titanium matrix composites (TMCs) reinforced with hybrid TiB, TiC and RexOy (rare earth oxides) were successfully fabricated by vacuum arc melting technique. Subsequently thermomechanical processing was carried out to optimize the microstructure and investigate the synergistic strengthening behavior. It is found that the optimized microstructure mainly contained two typical regions: Region 1, reinforcement-lean region with coarse lamellar grains. Region 2, reinforcement-rich region containing fine equiaxed α grains comparing with reinforcement-lean region, all hybrid reinforcements distributed homogeneously at their grain boundaries and TiB fibers are perpendicular to the forging direction. It is shown that the reinforcement can stimulate the dynamic/static recrystallization during the thermomechanical processing. The tensile strength was significantly enhanced by the ternary reinforcements and the thermomechanical processing. A well-matched relationship between microstructure and mechanical properties is obtained. When the reinforcement content is 2.5 vol%, the tensile strength at room temperature and high temperature (700 °C) increased to 1214 MPa and 552 MPa, while the TMCs maintained a good elongation of 5.1% and 58% respectively. The strengthening mechanism could be attributed to the refinement of the matrix grain, the solid solution strengthening of C element and the load-bearing capability of TiB and ternary oxide clusters.
Wang L, 2020, Review on additive manufacturing of tooling for hot stamping, International Journal of Advanced Manufacturing Technology, Vol: 109, Pages: 87-107, ISSN: 0178-0026
Sustainability is a key factor in an automotive OEMs’ business strategy. Vehicle electrification in particular has received increased attention, and major manufacturers have already undertaken significant investments in this area. However, in order to fully confront the sustainability challenge in the automotive industry, lightweight design in additional to alternative propulsion technologies is also required. Vehicle weight is closely correlated with fuel consumption and range for internal combustion and electrified vehicles, respectively, and therefore, weight reduction is a primary objective. Over the past decades, advanced steel and aluminium-forming technologies have seen considerable development, resulting in significant weight reduction of vehicle components. Hot stamping is one of the most established processes for advanced steel and aluminium alloys. The process offers low-forming loads and high formability as well as parts with high strength and minimal springback. However, the high temperatures of the formed materials over numerous cycles and the significant cooling required to ensure desirable component properties necessitate advanced tooling designs. Traditionally, casting and machining are used to manufacture tools; although in recent years, additive manufacturing has gained significant interest due to the design freedom offered. In this paper, a comprehensive review is performed for the state-of-the-art hot-forming tooling designs in addition to identifying the future direction of Additive Manufactured (AM) tools. Specifically, material properties of widely used tooling materials are first reviewed and selection criteria are proposed which can be used for the transition to AM tools. Moreover, key variables affecting the success of hot stamping, for example cooling rate of the component, are reviewed with the various approaches analysed by analytical and numerical techniques. Finally, a number of future directions for adopting additive manufactur
Chavoshi S, Tagarielli V, Shi Z, et al., 2020, Predictions of the mechanical response of sintered FGH96 powder compacts, Journal of Engineering Materials and Technology, Vol: 142, ISSN: 0094-4289
This paper presents predictions of the response of sintered FGH96 Ni-based superalloy powder compacts at high temperature, obtained by analysis of 3D representative volume elements generated by both X-ray tomography and a virtual technique. The response ofthe material to a multiaxial state of stress/strain for porosities as large as 0.3 is explored, obtaining the yield surfaces and their evolution as well as scaling laws for both elastic and plastic properties. The two modelling approaches are found in good agreement. The sensitivity of the predictions to particle size, inter-particle friction, applied strain rate,and boundary conditions is also examined.
Jin X, Gong Y, Han X, et al., 2020, A review of current state and prospect of the manufacturing and application of advanced hot stamping automobile steels, Acta Metallurgica Sinica (English Letters), Vol: 56, Pages: 411-428, ISSN: 1006-7191
Ultrahigh strength steels are highly competitive materials for vehicles to concurrently meet the increasing demand of the weight reduction and passenger safety. Hot stamping is the key forming technology to manufacture automobile components with high strength. Hot stamping steel and its manufacturing technology experienced a fast development in the past decade. This paper reviewed the state of the art of the manufacturing and applications of hot stamping steels/components in the following aspects: (1) hot stamping steels (from traditional MnB steels to recently newly developed hot stamping steels); (2) forming technologies (from traditional hot stamping process to industry 4.0 intelligent production); (3) novel hot stamping + quenching & partitioning (Q&P) process and fundamentals of deformation assisted heat treatments; (4) simulation techniques for hot stamping process (modeling of the temperature-stress field, microstructure field and simulation of the manufacturing process); (5) the assessments of in-service performance of hot stamped components. Finally, the trends of the development of hot stamping steels and related forming technologies in the future will be discussed.
Lyu F, Li Y, Shi Z, et al., 2020, Stress and temperature dependence of stress relaxation ageing behaviour of an Al–Zn–Mg alloy, Materials Science and Engineering: A, Vol: 773, Pages: 1-10, ISSN: 0921-5093
The stress (from elastic to plastic) and temperature dependence of stress relaxation ageing (SRA) behaviour of an Al–Zn–Mg alloy, AA7B04–P, has been experimentally investigated in this study. A series of SRA tests have been carried out under various initial stress levels in both elastic and plastic regions and at different temperatures. Corresponding microstructural evolution during SRA has been characterised using transmission electron microscopy (TEM). It is found that increasing the initial stress and/or temperature enhance the stress relaxation in both elastic and plastic regions. The dislocation creep mechanism plays the dominant role at the investigated temperatures during SRA, with the stress exponent n ranging from 3 to 8, decreasing with increasing temperature. External stresses accelerate the coarsening of GP zones and η’ precipitates and, when loaded to the plastic region, promote the formation of large rod-shaped η precipitates within 2 h of SRA tests, due to the high energy sites provided by dislocations from plastic loading. Yield strength shows a much higher sensitivity to the temperature than creep strain has during SRA tests. A temperature below 165 °C is suggested for SRA of AA7B04–P, so that a high stress relaxation level with less than 15% strength loss can be obtained after 16 h forming.
Li Y, Shi Z, Lin J, 2019, Experimental investigation and modelling of yield strength and work hardening behaviour of artificially aged Al-Cu-Li alloy, Materials and Design, Vol: 183, Pages: 1-15, ISSN: 0264-1275
The yield strength and work hardening properties of an Al-Cu-Li alloy AA2050 after artificial ageing have been experimentally investigated and modelled in this study. Uniaxial tensile stress-strain curves of the alloy artificially aged for up to 500 h have been acquired and evolutions of main precipitates during ageing have been summarised to elucidate the underlying mechanisms of the observed mechanical properties, such as yield strength and work hardening behaviour. Work hardening analysis with Kocks-Mecking plots has been performed to analyse the shearing-to-bypassing transition progress of the aged alloy and it has been found that the transition does not occur at the peak-ageing state. A new mechanism-based unified constitutive model, comprising three sub-models, has been developed to simultaneously predict the evolutions of microstructures, yield strength and work hardening properties of the artificially aged AA2050. It is the first unified model covering a wide range of artificial ageing conditions from under-ageing to over-ageing, providing an effective tool for performance prediction of the aged alloys for industrial applications. The model has the generic feature and could be applied to artificial ageing of other 2xxx series aluminium alloys with dominant T1 precipitates.
Zhou W, Li Y, Shi Z, et al., 2019, An analytical solution for elastic buckling analysis of stiffened panel subjected to pure bending, International Journal of Mechanical Sciences, Vol: 161-162, ISSN: 0020-7403
In this study, an analytical solution has been developed for the elastic buckling analysis of stiffened panels subjected to pure bending, and the effect of main geometric parameters of the stiffened panels on buckling strength has been investigated. A simplified model of stiffened panels has been proposed for buckling analysis, where an elastically built-in boundary condition replaces the skin's effect on buckling of the stiffened panels. The equilibrium method with a conventional rigid skin assumption and a new flexible skin assumption is developed for the simplified model to analytically capture the buckling behaviour of the stiffened panels. To consider the non-rigid rotation effect of flexible skin on buckling of stiffened panels, a new parameter, the effective width of stiffened panels, has been introduced, and a finite element (FE) assisted method has been employed to obtain its value for different stiffened panels. The results show that the flexible skin assumption significantly enhances the accuracy of buckling strength prediction compared with the conventional rigid skin assumption, and the maximum difference between analytical results and corresponding FE simulations is decreased from 12.2% with rigid skin assumption to only 3.9%. Based on the proposed analytical solution, effects of main geometric parameters of the stiffened panels (the stiffened panel length and width, the stiffener height, and the ratio of the skin thickness to the stiffener thickness) on their buckling coefficients have been discussed. Increasing stiffened panel length and/or reciprocal of stiffener height leads to an initial abrupt decrease of the buckling coefficient until reaching a stable level. When the stiffened panel width increases, the buckling coefficient first increases and then remains stable, whereas increasing thickness ratio leads to the increase of the buckling coefficient.
Chung TF, Yang YL, Shiojiri M, et al., 2019, An atomic scale structural investigation of nanometre-sized η precipitates in the 7050 aluminium alloy, Acta Materialia, Vol: 174, Pages: 351-368, ISSN: 1359-6454
Using high-angle-annular-dark-field (HAADF) scanning-transmission-electron microscopy (STEM), we have investigated η-precipitates in the Al-Zn-Mg-Cu (AA7050) aluminium alloy. The HAADF STEM images taken along the zone axes of [101¯0]η, [12¯10]η, and [0001]η illustrated the projected atomic-scale configurations of η-MgZn2 crystal. The precipitates developed in layer-by-layer growth, supplied with precursors such as Zn, Cu, and Mg, which were solute atoms segregated around the η/Al interfaces due to the higher lattice strain energy. Stacking faults and defect layers composed of flattened hexagons were frequently observed along the zone axes of [12¯10]η and [101¯0]η, respectively, and their formation was elucidated, similarly taking into account the layer-by-layer growth. Occasional coalescence between two precipitates yielded a complicated boundary or a twin-like boundary. Based on the differences in orientation relationships between η-types and the Al matrix reported to date, two new types of η precipitates have been recognized and named η4' and η12.
Li Y, Yang Y-L, Rong Q, et al., 2019, Effect of initial tempers on mechanical properties of creep-aged AA2050, Manufacturing Review, Vol: 6, ISSN: 2265-4224
The evolution of mechanical properties of a third-generation Al–Cu–Li alloy, AA2050, with different initial tempers (as-quenched WQ, naturally aged T34 and peak-aged T84) during creep-ageing has been investigated in this study. A set of creep-ageing tests was carried out under 150 MPa at 155 °C with different durations for all initial temper conditions and tensile tests were performed subsequently to acquire the main mechanical properties of the creep-aged alloys, including the yield strength, ultimate tensile strength and uniform elongation. The evolution of these mechanical properties during creep-ageing has been discussed in association with precipitation behaviour of AA2050 alloys with different initial tempers. The results indicate that the T34 alloy is the best choice for creep age forming (CAF) applications among these initial tempers, as it provides better yield strength and uniform elongation concurrently after creep-ageing. In addition, a work hardening rate analysis has been carried out for all the creep-aged alloys, helping to understand the detailed dislocation/precipitate interaction mechanisms during plastic deformation in the creep-aged AA2050 alloys with WQ, T34 and T84 initial tempers.
Li Y, Shi Z, Rong Q, et al., 2019, Effect of pin arrangement on formed shape with sparse multi-point flexible tool for creep age forming, International Journal of Machine Tools and Manufacture, Vol: 140, Pages: 48-61, ISSN: 0890-6955
The effect of forming pin arrangement on formed shape accuracy with sparse multi-point flexible (SMPF) tool has been experimentally and numerically investigated for creep age forming (CAF) process. An analytical method has been introduced to predict shape, stress and strain distributions of blanks loaded by SMPF tool with different pin configurations. Experiments and FE simulations of loading and CAF processes by SMPF tool with various pin number/interval conditions have been performed and the formed shapes after loading and CAF have been quantitatively analysed. The results show that increasing pin numbers in SMPF tool decreases shape errors and stress variations in the loaded blank, leading to lower deflections of the formed blank after CAF. With increasing pin numbers, the formed shape approaches the shape formed with corresponding surface tool. The shape error percentage in loaded blanks is significantly enlarged after CAF with SMPF tool, from 3% to more than 20% for singly-curved tool shapes with aluminium alloy 6082, and detailed value varies with tool shapes. Stresses in loaded blanks directly affect CAFed shapes and it has been found for the first time that there is a same stress discrepancy level between loaded blanks with SMPF tool and corresponding surface tool to achieve a particular shape accuracy after CAF with different tool shapes. It is proposed that the stress discrepancy parameter in loaded blanks can be used as a new and more efficient design criterion for pin arrangement in SMPF tool for CAF process. In addition, an asymmetric pin pattern, which reduces half of pins in SMPF tool and increases efficiency, has been proposed and its effectiveness for CAF process has been tested and discussed.
Li Y, Rong Q, Shi Z, et al., 2019, An accelerated springback compensation method for creep age forming, International Journal of Advanced Manufacturing Technology, Vol: 102, Pages: 121-134, ISSN: 0268-3768
Springback compensation is essential for tool design in creep age forming (CAF) process. In this study, a new accelerated springback compensation method integrating springback mechanism of a plate with creep-ageing behaviour of materials has been developed for CAF tool design to manufacture both singly and varyingly curved products. Springback compensation curves that relate the objective shapes and springback compensated shapes by their curvature, stress and strain states have been established, based on the numerical solution of springback behaviour of CAF process. For singly curved products, a one-step springback compensation method is proposed with reference to the springback compensation curves, and its effectiveness has been demonstrated by CAF test with a peak-aged aluminium alloy AA6082-T6. For products with varying curvatures, an accelerated method is developed for CAF tool design by integrating springback compensation curves with finite element (FE) assisted displacement adjustment techniques. The new accelerated method can significantly improve the tool design efficiency for CAF process when compared with conventional displacement adjustment techniques and has been verified by CAF manufacture of a varyingly curved product with AA6082-T6 material. The new accelerated springback compensation method developed in this study can be used for efficient tool design for CAF process of various products.
Li Y, Shao Z, Rong Q, et al., 2019, Development of similarity-based scaling criteria for creep age forming of large/extra-large panels, The International Journal of Advanced Manufacturing Technology, Vol: 101, Pages: 1537-1551, ISSN: 0268-3768
A scaling method is developed for the creep age forming (CAF) process to downscale manufacturing of large/extra-large panels to lab-scale experimental trials for industrial application. Similarity theory is applied to identify both the geometrical and physical (non-geometrical) similarities between large-size prototypes and scaled-down models in all process stages of CAF, including loading, stress-relaxation and unloading (springback). A constitutive model is incorporated into the theory in order to identify the similarity in the highly non-linear stress-relaxation behaviour for aluminium alloy plates during CAF, and to obtain the effective scaling criteria for the CAFed plates after springback. The method was demonstrated by scaling down CAF manufacturing of both singly curved and doubly curved large plates under both proportional and non-proportional geometrical scaling conditions. The analytical results of the scaling method and numerical results obtained by CAF FE modelling were found to be in good agreement. Scaling diagrams linking the key deformation (springback) and structural (flexural rigidity) variables to scaling ratios under both proportional and non-proportional conditions were generated, and the developed scaling diagrams have been validated by corresponding CAF experiments. The scaling method developed in this study provides guidance on the design of scaled-down CAF experimental trials and will be used in the practical CAF process of large/extra-large panels.
Rong Q, Li Y, Shi Z, et al., 2019, Experimental investigations of stress-relaxation ageing behaviour of AA6082, Materials Science and Engineering: A, Vol: 750, Pages: 108-116, ISSN: 0921-5093
Stress-relaxation ageing behaviour of peak-aged aluminium alloy 6082 (AA6082-T6) has been experimentally investigated with initial loading in both elastic and plastic regions in this study. The stress-relaxation ageing tests with various initial strain levels were carried out at 160 °C for up to 12 h and room temperature tensile tests were performed subsequently. Selected samples have been examined using transmission electron microscopy (TEM) to study microstructural evolution during the process. The results show that the total stress relaxed after 12 h test increases significantly with increasing initial strain levels. The initial strain levels contribute little effect on yield strength evolution during stress-relaxation ageing, as softening from the coarsening of precipitates is balanced by hardening from dislocations. Stress-relaxation mechanisms of the material have been analysed on the basis of creep stress exponent. In the elastic region, a creep stress exponent of 3 with a threshold stress of 70.1 MPa has been obtained, indicating a dislocation glide controlled creep mechanism; while in the plastic region, a decreasing threshold stress with increasing initial strain levels has been proposed to achieve a positive creep stress exponent.
Li W, Liu Y, Jiang S, et al., 2019, A study of thermomechanical behaviour and grain size evolution of AA7050 under hot forging conditions, International Journal of Lightweight Materials and Manufacture, Vol: 2, Pages: 31-39, ISSN: 2588-8404
A series of compression tests have been carried out using Gleeble (3800) thermomechanical simulator to investigate the viscoplastic behaviour of AA7050 as well as its microstructural characteristics under hot forging conditions. The thermomechanical tests were conducted at the temperatures of 350–450 °C with strain rates of 0.0005–0.5 s−1, which covers the range of hot forging conditions for AA7050. Interrupted tests have also been carried out to track the microstructural evolution under various thermomechanical deformation conditions. Particular attention has been focused on the criteria of dynamic recrystallisation in AA7050 during the uniaxial compression at high temperatures, which could help to obtain regular homogeneous grain structures for hot forged components. It was found that AA7050 has a strong viscoplastic behaviour and flow stress is more than halved from 350 to 450 °C. A suitably low strain rate and high temperature are required for significant dynamic recrystallisation (0.05 s−1 and 400 °C in current deformation level). The average grain size decreases with increasing strain and deformation temperature due to dynamic recrystallisation, but the relationship between average grain size and strain rate is not monotonic. Present findings provide a guideline for the selection of hot forging parameters so that quality components could be achieved with low forging force.
Lyu F, Li Y, Huang X, et al., 2019, An investigation of creep age forming of AA7B04 stiffened plates: Experiment and FE modelling, Journal of Manufacturing Processes, Vol: 37, Pages: 232-241, ISSN: 1526-6125
Creep age forming (CAF) of aluminium alloy 7B04 (AA7B04) stiffened plates has been experimentally and numerically investigated in this study. Creep-ageing experiments of AA7B04-T651 were conducted under different tensile stress levels at 140 °C for up to 20 h, and a set of unified constitutive equations was calibrated based on the experimental results of the evolutions of creep strain, yield strength and precipitate size, which was implemented into ABAQUS for CAF process modelling. CAF experiments and corresponding simulations of AA7B04 stiffened plates were then carried out and the effect of stiffener height and die radius on springback and yield strength was studied. It was found that the springback percentage decreases with increasing stiffener height and decreasing forming die radius, and the yield strength is slightly lower in the stiffener than in the skin of the CAFed stiffened plates due to stress effect on ageing progression. A good agreement has been achieved between experimental and corresponding FE results, with maximum deviations of 6.7% and 3.3% respectively for springback and yield strength.
Rong Q, Li Y, Sun X, et al., 2018, Experimental studies of the efficient use of flexible tool in creep age forming, 5th International Conference on New Forming Technology - ICNFT 2018, Publisher: EDP Sciences, ISSN: 2261-236X
Application of a newly developed flexible forming tool to creep age forming (CAF) process has been investigated in this study. The flexible tool mainly consists of sparsely distributed forming pins, splines and elastomeric sheet. The effect of key factors related to the forming tool on the shape of the formed parts has been studied through various CAF experiments. The key factors investigated in this study include: the interval between forming pins, the arrangement of pins, the accuracy requirement of pin height and the material of splines. It has been found that reducing the interval between pins can efficiently smooth the shape of CAFed plates. The feasibility of asymmetric arrangement of pins has been proven, which can decrease the number of used pins, reduce tool weight, and increase efficiency. The forming results are very sensitive to the pin height, thus the experimental set-up error should be carefully controlled. Additionally, compared with mild steel, spring steel is more suitable as the spline material.
Li Y, Yang Y-L, Rong Q, et al., 2018, Effect of initial temper on mechanical properties of creep-aged Al-Cu-Li alloy AA2050, 5th International Conference on New Forming Technology - ICNFT 2018, Publisher: EDP Sciences, ISSN: 2261-236X
The evolution of mechanical properties of a third generation Al-Cu-Li alloy, AA2050, with different initial tempers (T34 and as-quenched (WQ)) during creep-ageing has been investigated and analysed in this study. A set of creep-ageing tests under 150 MPa at 155 °C for up to 24 h was carried out for both initial temper conditions and tensile tests were performed subsequently to acquire the main mechanical properties of the creep-aged alloys, including the yield strength, ultimate tensile strength (UTS) and uniform elongation. Precipitation behaviour of the T34 and WQ AA2050 alloys has been summarised and successfully explains the detailed evolutions of the obtained mechanical properties of the alloy with these two initial tempers during creep-ageing. The results indicate that the T34 alloy can be a better choice for creep age forming (CAF) process compared with WQ alloy, as it provides better yield strength and uniform elongation properties concurrently after creep-ageing. In addition, a work hardening rate analysis has been carried out for all the creep-aged alloys, helping to reveal the detailed dislocation/precipitates interaction mechanisms during plastic deformation in the creep-aged T34 and WQ AA2050 alloys.
Li Y, Shi Z, Lin J, et al., 2018, Effect of machining-induced residual stress on springback of creep age formed AA2050 plates with asymmetric creep-ageing behaviour, International Journal of Machine Tools and Manufacture, Vol: 132, Pages: 113-122, ISSN: 0890-6955
Effect of machining-induced residual stresses on springback of creep age formed (CAFed) AA2050 plates with asymmetric creep-ageing behaviour has been investigated experimentally and numerically in this study. A finite element (FE) model integrating unified asymmetric creep-ageing constitutive model for CAF of AA2050 has been developed through a commercial FE software-PAM-STAMP and a simplified machining-induced residual stress field for aluminium alloy plates has been proposed and implemented into the FE model. Different levels of residual stresses have been considered in the FE model and numerical results have been compared with corresponding CAF experiments and surface residual stress measurements. Results show that the springback prediction of the FE models has been significantly improved when the residual stress effect was considered. The best prediction can be achieved with a certain residual stress level for each plate thickness and the level increases with decreasing thickness of the plates produced by machining, which agrees well with the surface residual stress measurement results. For CAF of AA2050 with asymmetric creep-ageing behaviour, different springback behaviour has been observed with different residual stress levels: introduction of residual stress fields can either decrease or increase formed deflections of CAFed AA2050 plates, depending on the relative levels of residual stresses and the stresses caused by applied external forces. When the residual stress is large, subsurface of the plate may exceed the elastic limit after applying external force, leading to a significant decrease of springback in the CAFed plates.
Chung T-F, Yang Y-L, Hsiao C-N, et al., 2018, Morphological evolution of GP zones and nanometer-sized precipitates in the AA2050 aluminium alloy, International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 142-156, ISSN: 2588-8404
Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM) was employed to examine the phases in Al-Cu-Li alloy (AA2050), including GP(T1), GP(θꞌꞌ) and GPB zones with their subsequent nanometer-sized products, T1 (Al2CuLi), θꞌ (Al2Cu), and S (Al2CuMg) precipitates, respectively. Under the peak-aging condition, some solute-atom enriched clusters could still be found, and the newly-formed nucleus of GP(θꞌꞌ) with a mono-layer {100} plane of Cu atoms occurred at the adjacent area of the joint between θꞌ and S precipitates or the edge of an individual S precipitate. The transition of a single Cu-layer GP(θꞌꞌ) → θꞌ was presumed to be transformation via in-situ nucleation. The developing GP(θꞌꞌ) zones and θꞌ precipitates were easily subjected to soft impingement. However, hard impingement between two variants of θꞌ presumably occurred, wherein one θꞌ variant precipitate was blocked out by the other θꞌ variant. As for the creep-ageing forming (CAF) treated sample, some precipitates of T1 and θꞌ were found to have the cutting characteristic on specific ledges.
Pan R, Shi Z, Davies CM, et al., 2018, An integrated model to predict residual stress reduction by multiple cold forging operations in extra-large AA7050 T-section panels, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol: 232, Pages: 1319-1330, ISSN: 0954-4054
A finite element (FE) model has been developed to determine the effectiveness of acold compression technique to reduce the large residual stresses generated fromquenching solution heat treated T-Section components of aluminium alloy AA7050. Tocompress long components, a multi-step process is required with some amount ofoverlap. A parametric study has been performed to determine the effect of thecompression ratio, friction coefficient, length of overlap and length of the T-sectioncomponent on the residual stress distribution post quenching and after subsequent coldcompression. Generally a percentage reduction in the peak residual stress of over 90%was found. The optimal parameters for residual stress relief by cold compression havebeen suggested from the cases considered.
Li Y, Shi Z, Lin J, et al., 2018, FE simulation of asymmetric creep-ageing behaviour of AA2050 and its application to creep age forming, International Journal of Mechanical Sciences, Vol: 140, Pages: 228-240, ISSN: 0020-7403
A finite element (FE) model has been developed and validated in this study for the first-time to simulate the asymmetric creep-ageing behaviour of an Al-Cu-Li alloy (AA2050) for creep age forming (CAF) applications. An implicit integration algorithm integrated with the Secant method was proposed to efficiently solve the creep-ageing constitutive model of AA2050 and a “maximum principal stress” technique was employed to characterise the asymmetric tension and compression creep behaviour for CAF FE simulation. The proposed algorithm has been implemented into the FE solver in PAM-STAMP via a user-defined subroutine and an implicit FE model has been developed for CAF of AA2050. The effectiveness of the developed FE model has been validated by four-point-bending creep-ageing experiments of AA2050 plates with different thicknesses. The springback behaviour of AA2050 plates after 18 h CAF with a doubly curved tool was then predicted using the validated FE model. The results show that springback levels of the CAFed plates decreased with increasing thickness. Significant springback was observed in all the CAFed AA2050 plates within elastic loading, for example, the 8 mm plate which was initially loaded to near yielding had a springback value of 87.1% after 18 h CAF at 155 °C. The implicit algorithm and the maximum principal stress technique can be employed for constitutive models for other alloys with asymmetric creep-ageing behaviour.
Chung T-F, Yang Y-L, Huang B-M, et al., 2018, Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy, Acta Materialia, Vol: 149, Pages: 377-387, ISSN: 1359-6454
High resolution transmission electron microscopy (HRTEM) with nanometer-scaled energy-dispersive X-ray (EDX) was employed to investigate the transformation mechanisms of the GP zone → η′ → η precipitation sequence of AA7050, an Al-Zn-Mg-Cu alloy. Serial in-situ HRTEM frames revealed that separated nucleation of an η′ precipitate occurred elsewhere as the adjacent GPII zone dissolved. Evidence from HRTEM coupled with EDX showed that in-situ nucleation of a new η2 precipitate (one form of η) took place, wherein it gradually developed from the original η′ precipitate via a similar hexagonal structure with different compositions. The in-situ transition product was composed of two distinctive regions; one was identified as η′, and the other, as η.
Zhou W, Shi Z, Lin J, 2018, Upper bound analysis of differential velocity sideways extrusion process for curved profiles using a fan-shaped flow line model, International Journal of Lightweight Materials and Manufacture, Vol: 1, Pages: 21-32
An analytical model for predicting the shapes of rectangular bars with variable curvatures along their lengths through a novel forming method, differential velocity sideways extrusion (DVSE), previously proposed by the authors, has been developed on the basis of the upper bound method. A new flow line function was presented to describe its deformation field. The plastic deformation zone (PDZ) was assumed to be fan-shaped, where the trajectory of the material flow within the PDZ had an elliptic shape. The proposed continuous flow line function was validated using finite element simulations. The flow patterns, extrusion pressure, curvature, and effective strain predicted by the analytical solutions agreed well with modelling results. Compared to the classical discontinuous simple shear model of channel angular extrusion (CAE) with a 90° die, the new approach was shown to predict the effective strain more closely.
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