Publications
409 results found
Lv J, Zheng J, Yardley VA, et al., 2020, A review of microstructural evolution and modelling of aluminium alloys under hot forming conditions, Metals, Vol: 10, ISSN: 2075-4701
Microstructural evolution during hot forming of aluminium alloys plays a critical role in both the material flow behaviour during the deformation and the post-form mechanical properties in service. This paper presents a comprehensive review on the recrystallisation mechanisms, the interrelations between microstructures and macroscopic responses, and the associated modelling methods for aluminium alloys under hot forming conditions. Particular attention is focused on dynamic recrystallisation (DRX), which occurs during hot forming. The mechanisms, key features, and conditions of occurrence (forming temperature, strain rates, etc.) during hot forming for each type of DRX type are classified. The relationships between microstructures and macroscopic responses, including the flow behaviour, the post-form strength and ductility, are summarised based on existing experimental results. Most importantly, the associated modelling work, describing the recrystallisation and the viscoplastic behaviour under hot forming conditions, is grouped into four types, to enable a clear and concise understanding of the existing quantitative micro–macro interactions, which are particularly valuable for the future development of advanced physically based multi-scale modelling work for hot-forming processes in aluminium alloys.
Li Y, Gu B, Jiang S, et al., 2020, A CDRX-based material model for hot deformation of aluminium alloys, International Journal of Plasticity, Vol: 134, Pages: 1-17, ISSN: 0749-6419
During hot deformation, continuous dynamic recrystallisation (CDRX) is believed to occur, and even dominates microstructural evolution in many metallic materials with high stacking fault energy, such as aluminium alloys. A unique material model for hot deformation of aluminium alloys is proposed in this paper, based on consideration of two processes: (i) increase of dislocation density, induced by plastic deformation, leading to generation, rotation and migration of low angle grain boundaries (LABs) and their transformation into high angle grain boundaries (HABs); (ii) migration of HABs leading to annihilation of both LABs and HABs. At large strain, the above counteracting processes, guided by different mechanisms, lead to saturation of HABs fraction. The model is applied to hot deformation of AA5052 and AA7050 alloys under various temperatures and strain rates, and calculated flow stress, HABs fraction and grain size evolution for both alloys agree well with the corresponding experimental data. The capability of predicting saturation of HABs fraction and average subgrain misorientation angle of both alloys under large strains demonstrate the potential applicability of the model to a wide range of hot forming process conditions.
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.
Zhou W, Afshan S, Lin J, 2020, An investigation of damage healing in high temperature compressive forming process, International Conference on Metal Forming
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.
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.
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.
Zinong T, Bing Z, Jun J, et al., 2020, A study on the hot roll bonding of aluminum alloys, Procedia Manufacturing, Vol: 50, Pages: 56-62, ISSN: 2351-9789
Joining of aluminum alloys through plastic deformation and short-time diffusion bonding has significant applications in a wide range of engineering sectors. It is also important in the elimination of voids and defects in casting ingots through hot rolling processes. An aluminum alloy, AA1060, was selected for the roll-bonding experimental research at vacuum conditions. Parameters including grain size and orientation, dislocation distribution, oxide film breakage, interface welding ratio were studied and analyzed at different hot rolling conditions. Analytical techniques were also developed to analyze the deformation, stress and strain states of the material at hot rolling conditions. The results show that under the condition of 580℃ and 60% deformation, the welding interfaces disappears, and the alloy matrix structure is not overburned.
Pruncu CI, Hopper C, Hooper PA, et al., 2020, Study of the effects of hot forging on the additively manufactured stainless steel preforms, Journal of Manufacturing Processes, Vol: 57, Pages: 668-676, ISSN: 1526-6125
The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.
Ganapathy M, Li N, Lin J, et al., 2020, A feasibility study on warm forming of an as-quenched 22MnB5 boron steel, International Journal of Lightweight Materials and Manufacture, Vol: 3, Pages: 277-283, ISSN: 2588-8404
In this paper, the feasibility of a newly proposed forming method, being the warm forming of as-quenched 22MnB5 boron steels, was studied through a series of proof of concept experiments. To assess the material thermo-mechanical behaviours under the proposed forming conditions, first, the as-received 22MnB5 boron steel was austenized and quenched to below the martensite transformation finish temperature to obtain a martensitic microstructure; second, uniaxial tensile tests of the as-quenched steel were conducted under proposed warm forming conditions on a Gleeble 3800 materials simulator. To evaluate the material post-form properties, first, tempering treatments on the as-quenched steel samples were performed to simulate the heat-treating conditions in the proposed warm forming process; second, the mechanical properties (hardness, strength, and ductility) of as-tempered samples were measured and a microstructure analysis was conducted. From the experimental results, it was found that, under the proposed warm-forming process conditions (420 °C–620 °C), the material showed significant strain softening, which would increase the tendency of necking during stamping and adversely affect its drawability. In addition, it was found that the heating of martensite in a 22MnB5 boron steel to higher temperatures (>400 °C) adversely affected its post-form strength and ductility due to the tempering effect. Therefore, according to the results obtained in this study, the warm forming of as-quenched 22MnB5 boron steel may reduce the strength of formed parts by more than 50% in comparison to the possible strength the material could achieve under the investigated process.
Zhang R, Shao Z, Lin J, et al., 2020, Measurement and analysis of heterogeneous strain fields in uniaxial tensile tests for boron steel under hot stamping conditions, Experimental Mechanics, Vol: 60, Pages: 1289-1300, ISSN: 0014-4851
BackgroundA significant amount of uniaxial tensile tests has been carried out using Gleeble systems to investigate the viscoplastic deformation of boron steel (22MnB5) under hot stamping conditions. However, due to heat loss through the end clamps, a temperature gradient in the reduced parallel section of dog-bone shaped specimens is inevitable.ObjectiveIn the work reported in this paper, the effect of temperature gradient on measured outcomes is examined.MethodsUniaxial tensile tests on 1.5 mm thick boron steel specimens are carried out, under hot stamping conditions and strain fields are quantified using the digital image correlation (DIC) technique. The effect of gauge length on the properties of boron steel, as calculated from observed test results, is determined.ResultsCompared with the test at room temperature, a bell-shaped strain distribution occurs within the gauge length even before the appearance of the maximum load. Also, average strain within the gauge length, especially in the later stages, changes with gauge length within the investigated range, and thus, different engineering stress-strain curves and fracture strains are determined. In addition, normalized strain rate is significantly dependent on gauge length, which results in over 16% difference among the computed flow stresses by using a unified constitutive model.ConclusionsThe characterized properties of the material are dependent on gauge length and thus, a testing standard for measuring thermal-mechanical data of materials by using a Gleeble need to be defined.
Zhou W, Yu J, Lin J, et al., 2020, Effects of die land length and geometry on curvature and effective strain of profiles produced by a novel sideways extrusion process, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 282, ISSN: 0924-0136
Yasmeen T, Zhao B, Zheng J-H, et al., 2020, The study of flow behavior and governing mechanisms of a titanium alloy during superplastic forming, Materials Science and Engineering: A, Vol: 788, Pages: 1-19, ISSN: 0921-5093
TA15 (Ti–6Al–2Zr–1Mo–1V) is a near-α titanium alloy and has wide applications in the aerospace industry because of its high strength to mass ratio, good weldability, and superior creep resistance at high temperatures up to 550 °C, compared to other titanium alloys. This study investigates the flow behavior and microstructural evolution as functions of temperatures and strain rates during deformations under the superplastic conditions at 880 °C/0.01s−1, 900 °C/0.01s−1, 880 °C/0.001s−1, and 920 °C/0.0005s−1. Results showed that this alloy exhibit excellent superplastic behavior for all selected temperatures and strain rates. The maximum tensile elongation of 1450% is achieved at 880 °C with a strain rate of 0.001s−1. Flow softening is observed under deformation conditions of 880 °C/0.01s−1 and 900 °C/0.01s−1, while strain hardening is observed at deformation conditions of 880 °C/0.001s−1 and 920 °C/0.0005s−1. These complex flow behaviors are rationalized by characterizing the underlying microstructures on the interrupted tensile samples using electron backscatter diffraction (EBSD) and backscattered electrons (BSE). The geometrically necessary dislocations (GNDs) density, which is caused by lattice rotation and misorientations and plays a vital role in the plastic constitutive behaviors, was for the first time, systematically revealed. Together with other key microstructures, i.e. grain sizes, texture, phase fractions, the results show that the dominant deformation mode changes at initial, intermediate, and final stages of the deformation. The probable deformation mechanisms, such as grain boundary sliding (GBS) under different deformation conditions, are discussed in terms of grain morphology, GNDs, and texture evolution. Also, it is observed that the β-phase transformation is accelerated during deformation and contributes to the enhancemen
Ren X, Ren H, Shang Y, et al., 2020, Microstructure evolution and mechanical properties of Ti2AlNb/TiAl brazed joint using newly-developed Ti-Ni-Nb-Zr filler alloy, PROGRESS IN NATURAL SCIENCE-MATERIALS INTERNATIONAL, Vol: 30, Pages: 410-416, ISSN: 1002-0071
Zhou X, Shao Z, Pruncu CI, et al., 2020, A study on central crack formation in cross wedge rolling, Journal of Materials Processing Technology, Vol: 279, ISSN: 0924-0136
Cross wedge rolling (CWR) is an innovative roll forming process, used widely in the transportation industry. It has high production efficiency, consistent quality and efficient material usage. However, the continual occurrence of crack formation in the centre of the workpiece is a critical problem excluding the CWR technique from more safety-critical applications, in particular, aerospace components. The mechanisms of central fracture formation are still unclear because of a combination of complicated stress and strain states at various stages of CWR. Thus, the aim of this study is to understand the stress/strain distribution and evolution during the CWR process and identify the key variables which determine central crack formation. A comprehensive investigation was then conducted to simulate 27 experimental cases. The stress and strain distributions in the workpiece were evaluated by finite element analysis. Various damage models from literature were applied and compared. A new fracture criterion was proposed, which was able to successfully determine the central crack formation in all 27 experimental cases. This criterion can be applied in CWR tool and process design, and the enhanced understanding may enable the adoption of CWR by the aerospace industry.
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.
Shao Z, Lee J, Wang J, et al., 2020, A study of various heating effects on the microstructure and mechanical properties of AA6082 using EBSD and CPFE, Journal of Alloys and Compounds, Vol: 5 nov 2019, Pages: 1-13, ISSN: 0925-8388
The solution heat treatment (SHT) process resolving hardening precipitates in high strength aluminium alloys is a critical step for high-efficient forming processes, such as Hot Form Quench (HFQ®). SHT largely determines the overall cycle time of a forming process. However, effects of heating process parameters, such as the heating rate and soaking time, on the microstructure and the associated mechanical properties of aluminium alloy 6082, one of the most commonly used aluminium alloys, for HFQ applications have not been systematically investigated. The aim of this study is to explore and understand the relationships among heat treatment conditions, grain microstructure and associated mechanical properties for AA6082. A series of uniaxial tensile tests conducted under various SHT conditions revealed significant variation on mechanical behaviour characterised by stress-strain curves. To correlate these stress-strain relationship with underlying microstructure, the grain and orientation distribution of each heat-treated sample were characterised by the electron backscatter diffraction (EBSD) technique. Due to the presence of a large number of microscopic variables, such as grain size, morphology, texture, grain boundary and etc., the crystal plasticity finite element (CPFE) modelling was employed to identify the key microscopic factors which determine the differences in the observed strength and ductility for all samples. A new CPFE model integrated with local strain criterion was proposed and validated to correlate the ductility and the strength with the material microstructure. This rigorous investigation provides more insights on how microstructure (grain size and texture) affects the mechanical behaviour for AA6082, which enables to enlarge the capability of HFQ for industrial applications.
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.
Shao Z, Jiang J, Lin J, 2020, Damage in advanced processing technologies, Mechanics of Materials in Modern Manufacturing Methods and Processing Techniques, Pages: 143-172, ISBN: 9780128182338
This chapter introduces the recent development for damage mechanics applied in advanced processing technologies of hot stamping and hot forming used for manufacturing complex-shaped components. Mechanical behavior and formability of materials are able to be predicted by using constitutive equations in continuum damage mechanics–based material models, which are calibrated by experimental data obtained from uniaxial and multiaxial tensile tests.
Zheng J-H, Pan R, Lin J, et al., 2020, FE Simulation of the residual stress reduction in industrial-sized T-section component during a newly proposed manufacturing process, 18th International Conference on Metal Forming, Publisher: ELSEVIER SCIENCE BV, Pages: 492-497, ISSN: 2351-9789
Yu J, Lin J, Dean TA, 2020, Development of novel differential velocity sideway extrusion techniques to fabricate lightweight curved structural components, International conference of metal forming, Publisher: Elsevier BV, Pages: 125-128, ISSN: 2351-9789
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.
Yasmeen T, Shao Z, Zhao L, et al., 2019, Constitutive modelling for the simulation of the superplastic forming of TA15 titanium alloy, International Journal of Mechanical Sciences, Vol: 164, ISSN: 0020-7403
Titanium alloy, TA15, has a high strength-to-weight ratio, high weldability, and superior creep resistance at high temperatures up to 550°C. TA15 is difficult to deform, especially for forming complex-shaped large-scale web–rib components, due to its low plasticity, large inhomogeneous deformation and narrow processing window. The objective of this research is to model the superplastic mechanisms in TA15 alloy with equiaxed, fine grain structure, and applying the proposed constitutive model to investigate the maximum grid aspect ratio, that can be achieved in superplastic forming (SPF), for a TA15 sheet with an initial thickness of 1.2 mm. Thermo-mechanical tensile tests are conducted first to characterize the superplastic behaviour of the material in the temperature range of 880– 940°C and the strain-rate of 0.0005 – 0.01s−1. A set of mechanism-based unified visco-plastic constitutive equations has been proposed and calibrated based on the results of stress-strain data. A gradient-based optimization method is applied for the calibration of constitutive equations. The constitutive model is incorporated into FEA code through creep subroutine to check the validity of the proposed material model against the experimental SPF test of a multi-box die. Predicted sheet thickness and thinning in a die entry radius region at the end of forming are examined in detail. Preliminary results show a good agreement between the computational and experimental results.
Zheng J-H, Dong Y, Zheng K, et al., 2019, Experimental investigation of novel fast-ageing treatments for AA6082 in supersaturated solid solution state, Journal of Alloys and Compounds, Vol: 810, ISSN: 0925-8388
Developing a fast-ageing treatment can significantly reduce the current processing time (180 °C × 9 h) of high strength AA6082 automotive components. In this study, a fast ageing treatment in supersaturated solid solution state was developed, such that the mechanical properties can be rapidly achieved after the paint bake (PB) treatment through introducing a pre-ageing (PA) treatment. The determined fast ageing method considered effects of temperature & time, heating rate and subsequent PB on the ageing response. Tensile tests and TEM observations of typical conditions were undertaken to examine evolved strength and precipitate distribution. Results showed that 210 °C was the optimum pre-ageing temperature as uniformly sized and distributed small precipitates were obtained. The final strength of about 280 MPa, that is 95% of the nominal strength for T6 temper, can be obtained within 15 min soaking for fast heating, and nearly this value for slow heating. More prolific nucleation occurred during slow heating, resulting in more finely distributed precipitates and a higher strengthening. It was observed that PB further increased the strength of over-aged alloy pre-aged at a high temperature of 240 °C. The subsequent PB enabled further nucleation of small clusters and growth of the pre-ageing-induced precipitates which were smaller than 20 nm. This resulted in an improvement in the material strength potentially to satisfy the safety requirements in automotive industry.
Ganapathy M, Li N, Lin J, et al., 2019, Experimental investigation of a new low-temperature hot stamping process for boron steels, International Journal of Advanced Manufacturing Technology, Vol: 105, Pages: 669-682, ISSN: 0178-0026
This paper demonstrates the promise of a new low-temperature hot stamping process with pre-cooling for 22MnB5 boron steels. It is the first time for the new process being successfully implemented for producing an automotive demonstrator component assisted with thorough experimental studies. The studies mainly include hot forming experiments carried out on an industrial prototyping line, post-form examinations, and in-die quenching tests. Automotive B-Pillar components with two designed drawing depths (50 and 64 mm) were hot stamped at a wide range of temperatures and forming speeds, through both the conventional hot stamping processes and the new processes with pre-cooling applied. For the as-formed B-Pillars, 3D shape scanning was conducted to investigate the thickness distribution of the components; uniaxial tensile testing, hardness testing, and scanning electron microscopes (SEM) observation were conducted to assess the final mechanical properties and microstructures. To understand the benefit of the low-temperature hot stamping in reducing cycle time, a separate set of in-die quenching experiments were designed and carried out, with combinations of three different process parameters: workpiece start quenching temperature, initial tool temperature, and die-workpiece contact pressure. The results of this work confirmed that low-temperature hot stamping could be performed successfully in producing complex-shaped components, such as automotive B-Pillars, with much reduced cycle time.
Zheng K, Zhu L, Lin J, et al., 2019, An experimental investigation of the drawability of AA6082 sheet under different elevated temperature forming processes, Journal of Materials Processing Technology, Vol: 273, ISSN: 0924-0136
The performed research has, for the first time, investigated and compared the drawability of AA6082 at a comparable temperature range between two elevated temperature forming processes: termed (i)Low Temperature Hot Form and Quench (LT-HFQ®)or pre-cooled HFQ®, patented by Adam et al. (2015)and (ii)Direct Heating Aluminium Forming (DHAF)which represents a refinement of conventional warm forming targeting a higher temperature range. A series of uniaxial tensile and cylindrical deep drawing experiments were conducted. According to uniaxial tensile test results, the most obvious work-hardening and reasonable ductility was observed under LT-HFQ® at a deformation temperature of 350 °C and strain rate of 1 s−1, which can enhance drawability. For deep drawing experiments, it was found that preheating conditions of each process prior to forming significantly affected forming characteristics and post-formed hardness of the alloy; both the achieved maximum Draw ratio (DR)and limit Blank Holding Force (BHF)at some specific process parameters were increased under LT-HFQ®. Forming speed and temperature had significant effects on alloy deformation and thus drawability for both processes. In addition, by evaluating the post-formed hardness, process drawability and microstructural evolutions under different processes were simultaneously analyzed.
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.
Zheng K, Dong Y, Zheng J-H, et al., 2019, The effect of hot form quench (HFQ (R)) conditions on precipitation and mechanical properties of aluminium alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 761, ISSN: 0921-5093
Zheng J-H, Pan R, Wimpory RC, et al., 2019, A novel manufacturing process and validated predictive model for high-strength and low-residual stresses in extra-large 7xxx panels, Materials and Design, Vol: 173, ISSN: 0264-1275
A novel manufacturing process, enabling the production of high quality (i.e. with low and controllable residual stress distributions and good mechanical properties) T-section 7xxx panels, has been established. This process provides a solution to residual stress induced distortion problems, which greatly concerns a range of industries and especially the aircraft industry. This process consists of three sequential steps — water quenching (WQ), cold rolling (CR) and constrained ageing (CA). The effectiveness of this process was experimentally verified, through applying this process to laboratory sized 7050 T-section panels. The RS was measured by neutron diffraction and X-ray techniques, in addition to deflections and hardness at each processing stage. An integrated Finite Element (FE) model, including all three steps, was developed to simulate this manufacturing process and predict both the RS and the final strength distributions. It has been concluded that this novel process can effectively reduce the residual stresses from ±300 MPa to within ±100 MPa and produce T-section panels with required mechanical properties (i.e. hardness: ~159 HV10). A cold rolling level of 1.5% was found most appropriate. The residual stress and yield strength distributions were accurately predicted by FE, providing a valuable prediction tool to process optimization for industrial applications.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.