Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close‐packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi‐static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson‐Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.     

热轧AZ31镁合金薄板的室温成形性

针对AZ31镁合金板材室温冲压成形较差的特点,采用不同轧制温度获得镁合金板材,使用半球形凸模胀形,绘制镁合金室温成形极限图并分析轧制温度对镁合金板材组织和室温成形能力的影响.发现AZ31镁合金板材的成形性能不仅与晶粒尺寸有关,还与晶粒取向有关.基面织构的减弱可明显提高板材的胀形性能,在基面织构强度相似的情况下,晶粒尺寸对板材的成形性能起决定性影响.      

Ductile Fracture Prediction in Rotational Incremental Forming for Magnesium Alloy Sheets Using Combined Kinematic/Isotropic Hardening Model

To predict the ductile fracture of a magnesium alloy sheet when using rotational incremental forming, a combined kinematic and isotropic hardening law is implemented and evaluated from the histories of the ductile fracture value (I) using a finite element analysis. Here, the criterion for a ductile fracture, as developed by Oyane (J. Mech. Work. Technol., 1980, vol. 4, pp. 65–81), is applied via a user material based on a finite element analysis. To simulate the effect of the large amount of heat generation at elements in the contact area due to the friction energy of the rotational tool-specimen interface on the equivalent stress-strain evolution in incremental forming, the Johnson–Cook (JC) model was applied and the results compared with equivalent stress-strain curves obtained from tensile tests at elevated temperatures. The finite element (FE) simulation results for a ductile fracture were compared with the experimental results for a (80 mm × 80 mm × 25 mm) square shape with a 45 and 60 deg wall angle, respectively, and a (80 mm × 80 mm × 20 mm) square shape with a 70 deg wall angle. The trends of the FE simulation results agreed quite well with the experimental results. Finally, the effects of the process parameters, i.e., the tool down-step and tool radius, on the ductile fracture value and FLC at fracture (FLCF) were also investigated using the FE simulation results.     

Liquid Film Migration in Warm Formed Aluminum Brazing Sheet

Warm forming has previously proven to be a promising manufacturing route to improve formability of Al brazing sheets used in automotive heat exchanger production; however, the impact of warm forming on subsequent brazing has not previously been studied. In particular, the interaction between liquid clad and solid core alloys during brazing through the process of liquid film migration (LFM) requires further understanding. Al brazing sheet comprised of an AA3003 core and AA4045 clad alloy, supplied in O and H24 tempers, was stretched between 0 and 12 pct strain, at room temperature and 523K (250 °C), to simulate warm forming. Brazeability was predicted through thermal and microstructure analysis. The rate of solid–liquid interactions was quantified using thermal analysis, while microstructure analysis was used to investigate the opposing processes of LFM and core alloy recrystallization during brazing. In general, liquid clad was consumed relatively rapidly and LFM occurred in forming conditions where the core alloy did not recrystallize during brazing. The results showed that warm forming could potentially impair brazeability of O temper sheet by extending the regime over which LFM occurs during brazing. No change in microstructure or thermal data was found for H24 sheet when the forming temperature was increased, and thus warm forming was not predicted to adversely affect the brazing performance of H24 sheet.

     

Warm Forming of Mg Sheets: From Incremental to Electromagnetic Forming

Magnesium alloys are generating interest in the automotive and aeronautic industries due to their low density and potential to reduce gross vehicular weight. However, the formability of these alloys is poor and they are very difficult to be formed at room temperature due to their strong basal texture in rolled form. In this paper, the potential of magnesium alloy sheets to be processed at warm conditions is studied for four different forming technologies: incremental forming (IF), deep drawing (DD), hydroforming (HF), and electromagnetic forming (EMF). Forming mechanisms and process window are experimentally characterized by monitoring different process parameters. Special focus is made on the influence of the forming temperature and the strain rate. Thus, experiments at temperatures from room to 523 K (250 °C) and a wide range of strain rates, between 10^?3 up to 10³ s^?1 according to each process nature and scope, are conducted. It is observed that, even the inherent forming rate range of each process vary considerably, increasing forming temperature increases formability for all of these forming processes. In the other hand, an opposing effect of the strain rate is observed between the quasi-static processes (IF, DD, and HF) and the high speed process (EMF). Thus, a detrimental effect on formability is observed when increasing strain rate for quasi-static processes, while a mild increase is observed for EMF.     

Gas‐pressure Forming of an AlMg‐Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project.     

Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Criteria

An important concern in metal forming is whether the desired deformation can be accomplished without any failure of the material, even at elevated temperatures. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element (FE) method for predicting the onset of fracture in warm metal working processes of magnesium alloy sheets. The uniaxial tensile tests of AZ31 alloy sheets with a thickness of 3 mm and FE simulations were performed to calculate the critical damage values for three kinds of ductile fracture criteria. The critical damage values for each criterion were expressed as the function of strain rate at various temperatures. In order to find out the best criterion for failure prediction, Erichsen cupping tests under isothermal conditions were carried out at various temperatures and punch velocities. Based on the plastic deformation histories obtained from FE analysis of the Erichsen cupping tests and the critical damage value curves, the initiation time and location of fracture were predicted under bi-axial tensile conditions. As a result, Cockcroft–Latham’s criterion showed good agreement with the experiments.     

再结晶退火对TC4合金板材性能的影响

研究了在不同温度与不同时间下TC4合金板材的再结晶退火对其显微组织和室温力学性能的影响。利用再结晶退火消除加工硬化和调节显微组织,使其达到较好的室温力学性能,总结出TC4合金板材较合理的再结晶退火制度。     

Experimental Studies on the Superplastic Forming of Square Shaped Components and Diffusion Bonding Characteristics of Ti–6Al–4V Alloy

Superplasticity is the ability of a polycrystalline material to exhibit, in a relatively isotropic manner, large elongations when deformed in tension. This property is exploited during superplastic forming in the fabrication of complex-shaped components which are otherwise technically difficult or economically costly to form by conventional methods. The ability of some titanium alloys to undergo superplastic deformation coupled with their diffusion bonding capability provides excellent opportunities to fabricate intricate parts resulting in significant cost and weight savings, particularly in the manufacture of aerospace structures. In the present work, experimental studies on the superplastic forming of square shaped components from titanium alloy Ti–6Al–4V sheets of 2 mm thickness that are commonly used in aerospace structural applications are reported. Superplastic forming of suitably sized blanks was carried out at temperatures of 1,148 K (875 °C), 1,173 K (900 °C) and 1,200 K (927 °C) using constant argon gas pressures of 1, 1.4 and 1.8 MPa. The formed components were characterized for their thickness distribution, mechanical and metallurgical properties. Diffusion bonding characteristics of the alloy sheet of 1 mm thickness were investigated for varying time durations at different temperatures and 4 MPa stress under an argon atmosphere and lap shear strength values of the joints are reported. Efforts were then made to carry out diffusion bonding concurrent with superplastic forming (SPF/DB). For these experiments, two sheets of 1 mm thickness each were superplastically formed into square components of size 80 mm square and 60 mm deep with an initial forming cycle followed by a diffusion bonding cycle by subjecting the component to a static pressure (higher than the forming pressure) for a specified period of time, which ensured good bonding between the two sheets. The components formed by the SPF/DB process were compared with those formed from the monolithic 2 mm sheet and the results are presented.     

Development of a Robot‐Based Sheet Metal Forming Process

This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.     

Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

1060 Al Electric Hot Incremental Sheet Forming Process: Analysis of Dimensional Accuracy and Temperature

The sheet with 1060 Al often is used to fabricate aluminum parts at room temperature in the incremental sheet forming process due to the fact that the metal has a high ductility in the normal temperature. However, the material can cause a large quantity of springback for parts formed to influence dimensional accuracy. In this paper, the use of the electric hot incremental forming process (EHIF) can effectively improve the dimensional accuracy of parts compared to single-stage forming and double-stage forming at room temperature. The effect of main process parameters, such as tool diameter, feed rate, step size, and current, on temperature is studied in detail using the EHIF. Some target values, namely, the maximum temperature, the average temperature, and the maximum temperature difference, are measured with a cone using 1060 Al. Moreover, the response surface methodology and Box–Behnken design have been employed to analyze results in detail and to establish respectively corresponding models to predict target values. Finally, the evaluating model of temperature uniformity is proposed and verified according to the previous models.     

Simulation of the Forming Behaviour of a Boron Modified P91 Ferritic Steel

The forming behaviour at high temperature of a modified 9%Cr‐1%Mo (P91) ferritic steel containing B and Ti for elevated temperature service was investigated. The microstructure of the as‐received material is mainly martensite at room temperature, but special etching revealed prior austenite grains of about 25 μm in size. Torsion tests were conducted at temperatures in the range 850 to 1250 °C to simulate the hot rolling process under comparable conditions of temperature, strain rate and strain. The deformation data obtained from these tests were correlated with the Garofalo equation with a stress exponent of 4.6 and an activation energy of 315 kJ/mol. This equation was used to predict the formability behaviour for the rolling process and also to determine the maximum forming efficiency and stability of the steel. A temperature of 1200 °C is recommended to conduct the forming process.     

基于M-K理论的6016铝合金成形极限曲线预测

近年来,为实现汽车车身轻量化,大量的铝合金材料被用于汽车车身制造,由于6016铝合金具有良好的烘烤性能,被大量使用.但是传统的冷成形技术并不能成形复杂零件,因此热冲压-冷模具淬火成形技术被用到铝合金的成形过程中,板材成形领域中一个重要的性能指标是成形极限.本论文使用理论预测和试验两种方法对6016铝合金成形极限曲线进行了研究.首先,建立了考虑应变强化和应变速率强化的Fields-Bachofen本构方程,并将此本构方程引入到成形极限理论推导过程中;然后,基于M-K凹槽理论,对6016铝合金成形极限曲线进行了理论预测,并且采用Nakazima试验方法对预测结果进行了验证.结果显示,随着初始厚度不均度的增加,预测曲线向纵坐标的正方向移动;通过实验值和预测值的对比发现M-K凹槽理论对成形极限曲线的预测是可行的、准确的.      

Hydro‐mechanical Deep Drawing of Rolled Magnesium Sheets

Magnesium sheets offer high specific properties which make them very attractive in modern light weight constructions. The main obstacles for a wider usage are their high production costs, the poor corrosion properties and the limited ductility. Until today, forming processes have to be conducted at temperatures well above T = 220 °C. In the first place, this is a cost factor. Moreover, technical aspects, such as grain growth or the limited use of lubrication speak against high temperatures. The first aim of the presented research work is to increase the ductility at lower temperatures by alloy modification and by an adapted rolling technology. The key factor to reach isotropic mechanical properties and increased limit drawing ratios in deep drawing tools, is to achieve fine, homogeneous microstructures. This can be done by cross rolling at moderate temperatures. The heat treatment has to be adapted accordingly. In a second stage, hydro‐mechanical deep drawing experiments were carried out at elevated temperature. The results show that the forming behaviour of the tested Mg‐alloys is considerably improved compared to conventional deep drawing.     

Material Flow Control and Optimization in High‐Pressure Sheet Metal Forming by Active‐Elastic Tools

High‐pressure forming of metal sheets is an innovative forming technology for the production of complex components and offers high potentials to improve the properties and qualities of sheet metal parts. This report describes investigations of a newly developed active‐elastic tool system referred to as ACTEC system. Unlike the use of a comparable semi‐rigid tool system, the ACTEC system shows improvements with respect to the material flow in the flange area and reduced sheet thinning in critical corner regions of the workpiece. In addition, the clamping forces respectively sealing forces necessary to avoid leakage in the tool system during the forming process can be reduced. Moreover, the specific design of the ACTEC‐system as well as current experimental examinations are presented and discussed.     

Punch-stretching behavior of a 2014 aluminum alloy in the temperature range 250° to 500 °C

The influence of forming temperatures in the range 250° to 500 °C on the performance of a 2014 aluminum alloy in punch stretching has been investigated. In tests at moderate strain rates within the 250° to 450 °C range, the biaxial stretching limits of annealed sheets were greatly superior to the room-temperature values. Limiting depths of pressing increased with increasing temperature in the range 250° to 400 °C as a result of improvement in both limit strains and strain distribution, but increasing deformation temperature above ~400 °C caused the limit strains to decrease as a result of cavitation at high strains. Under comparable conditions of temperature and extension rate, such cavity growth was more rapid in equibiaxial stretching than in uniaxial or plane-strain stretching. At 500 °C with a punch speed of 0.083 mm s^-1, the thickness strain which could be applied in biaxial stretching without significant cavitation damage was less than 0.4. Thus, susceptibility to cavitation imposes an important restriction on opportunities for combining solution treatment with a hot stretch-forming operation when using an alloy based on Al-4 pct Cu.     

双线性应变路径下St14-T冷轧薄板成形极限研究

在多轴加载试验机上开展了St14-T冷轧薄板单拉和双拉预变形试验，在此基础上采用CAMSYS自动网络分析仪测量了板料的极限应变，分析了预变形对板料成形极限的影响，试验结果表明：加载应变路径的变化会蚊蝇乱形极限曲线的高低。单拉预应变路径将显著提高正应变比区的成形极限，而对负应变比区的成形极限影响不大。等双拉预应变将明显降低正应变比区的成形极限，对负应变比区的成形极限有所提高，随着等双拉预应变量的增加，整条成形极限曲线将降低。     

An electron microprobe analysis of solute segregation near grain boundaries in an Al-Zn-Mg alloy

The concentration of zinc and magnesium across grain boundaries was measured by means of an electron probe microanalyzer for an Ag-Zn-Mg alloy after different quenching (brine, water, oil, and air) and aging heat treatments (room temperature, 165°, and 200°C). Significant solute segregation was detected in quenched specimens and also in specimens that were aged at room temperature. While no solute enrichment was measured in specimens that were aged at elevated temperatures, solute depletion was observed in a considerable proportion of the examined boundaries. It was concluded that solute segregation to grain boundaries occurred during quenching and was relieved during aging. C. R. SHASTRY, Formerly Graduate Research Assistant at Rens-selaer Polytechnic Institute, Troy, N. Y.