MODELING OF SUPERPLASTIC FORMING PROCESS FOR ALUMINUM ALLOYS WITH STRAIN HARDENING EFFECT

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Failure analysis in superplastic materials

Superplastic alloys and metals possess the ability to undergo large uniform strains prior to failure. Isothermal superplasticity of sheet metal is a phenomenon due to both peculiar process condition and material intrinsic characteristics. The material must have a grain size of less than 10 μm, the forming temperature of around half the absolute melting point and a very low strain rate (in the order of 10⁻⁵–10⁻³ s⁻¹).The instability of superplastic flow under uniaxial stress state has been the subject of different studies. In this paper, under biaxial stress conditions, instability analysis of superplastic PbSn60 alloy using the finite element method is investigated. An original model has been implemented successfully in commercial finite element code in order to predict the imminent failure of material during superplastic forming processes.     

Modeling of Transformation Superplastic Forming of Ti Alloys

Transformation superplastic forming is an attractive alternative forming technique to microstructural superplastic forming, since it requires no special microstructures and, therefore, eliminates the limitation of superplastic forming capability to only expensive materials with stable high-temperature fine grains. Transformation superplasticity occurs through biasing the internal stress produced from an allotropic phase transformation by a small external stress. In this work, finite element modeling was implemented to study the transformation superplastic forming of domes from flat circular thin plate samples. The evolution and distribution of stress, strain, and dome thickness was analyzed in detail. The thickness distributions in the formed domes were compared with the theoretical predictions of two models, which assume different stress states in the domes. The appropriate stress state was identified through this comparison. Different gas pressure amplitudes were applied during forming to investigate the effect on the formed-dome apex height, when the forming time was fixed. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

细晶AZ31薄板超塑气胀成形有限元分析

为研究细晶AZ31镁合金薄板超塑气胀成形的最佳工艺参数，利用有限元软件对超塑胀形过程进行了有限元数值模拟分析。通过选取不同的应变速率敏感性指数和成形压力，来分析应变速率敏感性指数和成形压力对厚度分布均匀性的影响，同时对成形过程中的应变速率变化进行了模拟分析。模拟结果表明．应变速率敏感性指数和成形压力对厚度分布均匀性影响很大；应变速率分布情况良好，在所要求的应变速率范围之内，说明应变速率敏感性指数，压力等参数的设计合理。     

Superplastic process modeling of plane strain components with complex shapes

Computational process models using membrane element method are developed in this paper for the superplastic forming of plane strain boxes with complex cross-sectional details. Many practical superplastic components manufactured in industry have sloping sidewalk with die bottoms either corrugated and/or at angles to the sides. The new method is used to develop process models for such configurations and the resulting software can be used interactively in a computer. The method is useful to a designer in the parametric study of die geometry, die wall friction, initial thickness, and material property, or to determine if a specific geometry is suitable for superplastic forming. The kinematics of deformation are illustrated, and the numerical results of the model are compared with continuum finite element solutions and also with experimental data.     

AZ31镁合金400℃本构律之有限元验证分析

本文以曲线拟合方法,分析AZ31B-H24镁合金的单轴拉伸试验,针对材料在400℃温度下,应变率ε＆=10-5-10-2 s-1范围之应力-应变关系曲线,找出一个以应变、应变率为函数的应力流方程式之本构模型,并将此模型掺入有限元（FEM）建构一合理的数值分析模式,仿真该单轴拉伸试验,以验证其可靠性。有限元分析（FEA）时以固体力学的弹-塑性理论来运算材料塑性流演化行为的应力增量-应变增量之关系。分析结果显示,FEA与单轴拉伸试验的应力-应变关系曲线,在各变形阶段上,二者皆具有相当不错的吻合性;且实验与FEA在极限应变状态下之杆件的变形形状,二者结果亦相当接近;本文并以此FEM分析模式预测固定速率之单轴拉伸案例,对该材料的吹制成型试验进行仿真,结果亦验证了本文所提出的本构模型拥有超塑性成型力学分析的实用性。本文对AZ31镁合金之超塑性力学分析提供了一个数值分析模式之参考。     

A review of the numerical analysis of superplastic forming

This paper reviews the literature on the numerical simulation of superplastic forming (SPF). After introducing the phenomena of superplasticity and superplastic constitutive equations, non finite element analyses are reviewed. The finite element method of solution to SPF simulation is then examined within the context of the standard flow formulation. However non steady state SPF is not ideally suited to a standard flow formulation and an alternative, incremental flow formulation is discussed.     

Superplastic forming by decomposition of (CaCO3 + C) and MgCO3

An innovative method has been developed that replaces argon as the pressure source for superplastic forming. In this new process, several solid materials are placed in a closed system to generate pressure and are capable of forming superplastic alloy plates at specific temperatures. In the present study, the total pressures for the decomposition of ( CaCO₃+ C) and MgCO₃ have been theoretically calculated from thermodynamics. The results show that a pressure range of 40 to 396 psi can be obtained for the ( CaCO₃ + C) system between 850 and 1000 °, which is suitable for the superplastic forming of Ti-6Al-4V and Superdux 64 ( Nippon Yakin Kogy Co., Ltd., Sanei Bridge, Kyobasi 1-5-8, Chyuoku, Tokyo 104, Japan) stainless steel. The pressure for MgCO₃ system between 480 and 515 ° ranges from 78 to 160 psi, which is suitable for the superplastic forming of 8090 Al-Li and 7475 Al-Zn-Mg alloys. The calculated temperature dependence of pressure is consistent with the experimentally measured results. Furthermore, the forming rates, wall thickness distributions, tensile properties, and microstructures of the four alloys after forming have been shown to be very similar to those of conventional superplastic forming by argon pressurization.     

Finite element modeling and optimization of superplastic forming using variable strain rate approach

Detailed finite element simulations were carried out to model and optimize the superplastic blow forming process using a microstructure-based constitutive model and a multiscale deformation stability criterion that accounts for both geometrical instabilities and microstructural features. Optimum strain rate forming paths were derived from the multiscale stability analysis and used to develop a variable strain rate forming control scheme. It is shown that the proposed optimization approach captures the characteristics of deformation and failure during superplastic forming and is capable of significantly reducing the forming time without compromising the uniformity of deformation. In addition, the effects of grain evolution and cavitation on the superplastic forming process were investigated, and the results clearly highlight the importance of accounting for these features to prevent premature failure. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

Numerical simulation and experimental study on cavity growth in uniaxial tension of superplastic magnesium alloy

1　INTRODUCTIONMagnesiumisthelightestmetalinallconven tionalcommercialstructuralalloys .Itshighspecificstrengthandrigidity ,gooddampingcapacitiesareat tractiveforvariousstructuralapplications[14 ] .Be causeofitshexagonalclosepacked (HCP )crystalstructurewithalimitednumberofoperativeslipsys temsatroomtemperature ,magnesiumalloysaremuchmoreworkableatelevatedtemperaturesasaddi tionalslipsystemsareavailable[57] .Applicationsofthesuperplasticformingtechnologyofmagnesiumal loyshaverecentlyattr…     

变速变载条件下Inconel带材的成形性能

Inconel铟镍合金是核电设备中燃料组件格架条带的常见原材料。为了提高核燃料组件格架条带产品的冲制质量,对铟镍合金板料冲压成形性能进行了研究。针对该材料价格昂贵和各向异性明显的特点,采用有限元方法进行分析。首先通过试验方法得到材料力学性能参数,再利用有限元模拟分析在变速变载条件下铟镍合金的成形性能。研究了冲压成形速度、速度模式、压边力大小和载荷模式对铟镍合金成形性能的影响。最后通过模具冲制试验结果分析验证了有限元模拟分析的正确性。结果表明:通过有限元数值模拟分析更加了解了铟镍合金板料冲制性能,并为格架条带冲制提供了工艺参数。     

Using the finite element method in metal-forming processes

In recent years, finite element methods have been used extensively in the metal-forming industry. In the rigid-viscoplastic domain, the finite element method is being utilized as a forming process simulation tool, and in elastic and thermal domains, the method is being applied in forming tooling design. Such applications of the finite element method have enabled a more scientific approach to forming process development and trouble shooting. The capabilities and applications of the finite element methods in optimizing and developing metal-forming processes are presented with an emphasis on steel forging processes for automotive parts.     

Study on superplastic blow-forming of 8090 Al–Li sheets in an ellip-cylindrical closed-die

The purpose of this paper is to explore the plastic deformation behavior of the sheet during blow-forming of a superplastic sheet into an ellip-cylindrical closed-die by the finite element method. A finite element commercial code “DEFORM” is used to carry out the simulations and calculate the pressurization profile and sheet thickness distribution during the blow-forming process. A pressure control algorithm is proposed to keep the maximum strain rate in the deformation zone of the sheet equal to the target value, which corresponds to the highest m value of the material being superplastically formed. The effects of various forming conditions, such as the friction coefficient between the sheet and die and the aspect ratio of the die, on the forming pressure and thickness distribution of the product are discussed. Experiments using 8090 Al–Li sheets on superplastic blow-forming in an ellip-cylindrical closed-die are also carried out. The theoretical predictions of thickness distribution of the product are compared with experimental results.     

Combined Mechanics-Materials Based Optimization of Superplastic Forming of Magnesium AZ31 Alloy

A new optimization approach for superplastic forming of Mg AZ31 alloy is presented and experimentally validated. The proposed new optimization approach is based on a multiscale failure criterion that takes into account both geometrical necking and microstructural evolution, yielding a variable strain rate forming path instead of the commonly used constant strain rate approach. Uniaxial tensile tests and free bulge forming experiments, in conjunction with finite element analysis, are used to evaluate the proposed optimization approach. Significant reduction in forming time is achieved when following the proposed optimization approach, without compromising the uniformity of deformation.     

Superplastic Response of Continuously Cast AZ31B Magnesium Sheet Alloys

Magnesium sheet is typically produced for commercial applications with the traditional DC-ingot casting method. As a result of the hexagonal close-packed crystallographic structure in magnesium, multiple rolling passes and annealing steps are required to reduce the thickness of the ingots. Thus, high fabrication costs characterize the creation of magnesium sheet suitable for common forming operations. Recently, continuous casting (CC) technology, where molten metal is solidified directly into sheet form, has been applied to magnesium alloys; this method has shown the potential to significantly reduce the cost of fabricating magnesium sheet alloys. In order to understand the viability of the CC process, a study was conducted to investigate the superplastic potential of alloys produced by this method. This study focused on AZ31B Mg that was continuously-cast on twin-roll casters from three different suppliers. These three materials were compared with a production DC-cast AZ31B alloy in terms of microstructure, elevated-temperature tensile properties, and superplastic forming response. The data from this study found that microstructural features such as grain size and segregation can significantly affect the forming response. Additionally, the CC alloys can have equivalent or superior SPF response compared to DC-cast alloys, as demonstrated in both elevated temperature tensile tests and superplastic forming trials using a rectangular pan die.     

快速超塑性的研究与应用

综述了高应变速率超塑材料种类、变形机理和应用技术的最新进展。高应变速率超塑材料主要是铝基复合材料及铝合金,最近,对镁合金、纳米材料、钛合金高应变速率超塑性能的研究也已开始。高应变速率超塑性在工业中的应用已经起步,例如快速超塑成形技术、一模多件技术等,可以实现中等批量、甚至大批量生产,但是主要集中在铝合金上。未来激光辅助超塑成形技术、电塑性辅助超塑成形技术值得期待。     

Elastic-plastic finite element analysis of automotive body panel stamping processes using dynamic explicit time integration scheme

In this article, the elastic-plastic finite element formulations using dynamic explicit time-integration schemes are proposed for numerical analysis of automotive body panel stamping processes. A general formulation of finite element simulation for complex sheet forming processes with arbitrarily shaped tools is briefly introduced. In finite element simulation of automotive body panel stamping processes, the robustness and stability of computation are important requirements since the computation time and convergency become major points of consideration besides the solution accuracy due to the complexity of geometry and boundary conditions. For analyses of more complex cases with larger and more refined meshes, the explicit method is more time effective than the implicit method, and it has no convergency problem and has the robust nature of contact and friction algorithms, although the implicit method is widely used because of excellent accuracy and reliability. The elastic-plastic scheme is more reliable and rigorous, while the rigid-plastic scheme requires short computation time. The performance of the dynamic explicit algorithms is investigated by comparing the simulation results of forming of complex-shaped automotive body parts, such as a fuel tank and a rear hinge, with the experimental results. It has been shown that dynamic explicit schemes provide quite similar results to the experimental results. It is thus shown that the proposed dynamic explicit elastic-plastic finite element method enables an effective computation for complicated automotive body panel stamping processes.     

一种耦合晶粒长大的超塑性本构及数值模拟

本文基于弹粘塑性有限元技术 ,将经过修正的晶粒长大模型耦合到材料粘塑模型中 ,得到一个考虑晶粒行为的超塑材料本构模型。利用所建立的考虑晶粒效应的本构模型和经典的Backofen本构模型 ,分别对Ti 6Al 4V圆杯进行超塑性成形模拟 ,其厚度分布结果表明 ,新本构能很好地描述晶粒敏感型材料的超塑性能。     

An Eulerian finite element model for the steady state forming of porous materials

This paper is concerned with an Eulerian finite element analysis for the steady state forming of porous materials, such as nano-grained material manufactured via cryogenic milling. The constitutive relation for such porous materials is different from that for a fully dense matrix. The general form of the constitutive equation for a porous material is derived from the yield functions for the plastic deformation of a porous material, as proposed by Shima, Green, Doraivelu, Gurson, Kuhn, Park, and Lee. Then, that general form is utilized in the Eulerian finite element formulation for the strain hardening, dilatant, and viscoplastic deformation. Initial estimation of the porosity distribution in an Eulerian mesh is obtained from the velocity and scaled pressure fields computed by the Consistent Penalty finite element method for the incompressible viscoplastic deformation of the matrix. Applications of the proposed method to rolling and extrusion are given. The change of the porosity is predicted by integrating its evolution equation along a particle path constructed in an Eulerian domain. Comparisons of the predicted distributions of porosity to those by a Lagrangian finite element method and to those by experiments reported in the literature validate the proposed method.     

纳米陶瓷超塑加工成形的研究进展

在对纳米陶瓷超塑性拉伸研究进行总结的基础上 ,综述了陶瓷超塑成形的研究进展情况。指出 :陶瓷材料由于其超塑特性 ,所以能够用各种现有的金属塑性成形方法来成形。最佳成形温度范围大约在 12 0 0℃～ 15 0 0℃之间 ,最佳晶粒平均直径为 10 0nm～ 2 0 0nm。