Hemming of Aluminum Alloy Sheets Using Electromagnetic Forming

Hemming is usually the last stage of production for automotive closures, and therefore has a critical effect on the quality of the final assemblies. The insufficient formability of aluminum alloys creates a considerable problem in the hemming process. To address this issue, electromagnetic forming was utilized to hem aluminum alloy sheets. Electromagnetic forming is a high strain rate forming process that is currently being investigated by both academia and industry. Past studies have shown that the formability of metals can be significantly improved during electromagnetic forming, which benefits the hemming of aluminum alloys. This article presents the experimental results of hemming Al 6061-T6 sheets using electromagnetic forming. The effects of the parameters of electromagnetic hemming on the hem quality are discussed. In addition, the numerical simulation results of electromagnetic hemming are presented to enhance the understanding of the process and to determine the efficacy on an industrial scale.     

Expansion ratio analysis for initial die-face design of panel drawing dies

When the initial die-face of panel drawing dies is designed, the method of expansion ratio analysis (ERA) is usually employed to carry out formability analysis. Traditionally, ERA is performed with the help of geometry and experiential knowledge. Inevitably, design results differ from each other to the extent designers’ experiences vary. This is a rare work in which the authors research ERA by means of numerical simulations and experiments. Some useful rules are found, which can guide engineers in designing the initial die-face of panel drawing dies. ERA is less accurate than numerical simulation. As it locates upriver of panel dies development, its effect is significant. If the initial die-face is found to be acceptable after ERA, the efficiency and accuracy of numerical simulation can be improved greatly in designing the fine die-face and forming process.     

A better sheet-formability test

Instability predictions are important in sheet-metal forming processes, one such instability being splitting failures due to localized necking. The majority of such sheet-metal industrial splitting failures occur near to the plane-strain state. Therefore, sheet-metal industries have always been looking for an “ideal” formability test which allows them to evaluate sheets for their ability to resist splitting failures under near plane-strain conditions. Several formability tests have been developed in the past but none have been very successful. Presently, a new test has been designed, constructed and used to evaluate the formability of different sheet materials. The results from this new test are compared with standard limiting-dome-height (LDH) tests and finite-element simulations. The results show that the new test is more reproducible, more closely follows the desirable plane strain path and takes roughly 1/6 of the time to perform relative to the LDH test. Strain measurements and finite-element simulations have revealed that the improvements are a result of the new test geometry, which produces a more stable and repeatable plane-strain state near to the splitting failure locations. The failure heights in the new tests were predicted using a section analysis finite-element program, SHEET-S.     

Post-uniform deformation in uniaxial and equi-biaxial stretching of aluminium alloy sheets

Post-uniform deformation which affects the formability of sheet metals has been studied in the uniaxial and equi-biaxial stretching of aluminium alloy sheets. The materials used were AA5050, AA3105, and AA8014 aluminium alloys having negative, zero and positive strain-rate sensitivities, respectively. Post-uniform deformation has been characterized in terms of ‘neck breadth’ in uniaxial tension, and ‘proportional post-uniform increment in bulge height’ in biaxial tension. The results have been analyzed on the basis of the strain hardening and strain-rate hardening parameters of the materials.     

变压边力对矩形件成形性能的影响

起皱和断裂是板料成形过程的主要失效模式 ,合理控制成形过程中压边力 ,可以消除这些缺陷 ,提高成形性能。本文通过对随位置变化的变压边力作用下的矩形盒拉深过程进行数值模拟 ,研究各部位压边力变化对整体成形性能影响、及其影响范围 ,为分块压边圈的压边力的调整提供一定的依据。     

Analysis of plastic bending of adhesive-bonded sheet metals taking account of viscoplasticity of adhesive

The present authors proposed a new technique of plastic bending of adhesive-bonded sheet metals. In this process, large transverse shear deformation occurs in the adhesive layer, which in some cases would induce the geometrical imperfection (so-called ‘gull-wing bend’) and the delamination. Since the strength of the adhesive is highly rate-sensitive, the amount of shear deformation of the adhesive layer and, as a result, ‘gull-wing bend’, are strongly influenced by the forming speed. In the present work, the effect of forming speed on the deformation characteristics of adhesively bonded aluminium sheets was investigated by performing V-bending experiments with various punch speeds at room temperature. In order to discuss the effect quantitatively, the numerical simulations for the bending were also conducted using a rate-dependent constitutive model of plasticity for the adhesive. Consequently, it was found that the large shear deformation and ‘gull-wing bend’ are suppressed by high-speed forming since the deformation resistance becomes higher at high strain rate.     

基于金属板材数控渐进技术的汽车座椅成形工艺

金属板材数控渐进成形工艺不需要专用的模具,是一种柔性的成形工艺.在工艺规划中,通过零件的数字模型来调节成形工具的运行轨迹,达到渐进成形零件形状的目的.由于汽车座椅形状复杂,金属板材渐进成形较困难.本文对其数控渐进成形工艺进行了分析,提出由建模、工艺分析、轨迹生成、HrpDSF处理、安装定位、加工润滑、后处理等组成的金属板材数控渐进成形工艺规划方法,成功地应用于汽车座椅的数控渐进成形中.并对汽车座椅数控渐进成形加工中,通常出现的破裂、回弹、起皱等严重影响成形质量的缺陷原因进行分析,采取不同的处理防范措施,取得满意的效果.为今后完善和发展金属板材数控渐进成形技术起到重要作用.     

Evaluation of the dynamic explicit/elastoviscoplastic finite-element method in sheet-forming simulation

The application of the dynamic explicit time integration method has been proven to be efficient and robust for sheet-metal forming simulation. However, the sheet stamping operation in the automobile industry is usually a quasi-static process. Therefore, there remain some subjects to investigate, such as the estimation of dynamic effects and the evaluation of numerical schemes of so-called mass scaling, damping scaling and material viscosity scaling. In this paper, these problems are investigated by the approaches of the theoretical formulation and numerical verification. Firstly the principle of virtual power is proposed to interpret the quasi-static deformation process, adopting the Lagrange multiplier method to introduce quasi-static conditions. Next, a discussion is focused on the comparison between the real forming process and the virtual simulation process. The latter employs a high forming speed to reduce the consumptions of computation time. Finally the hemispherical-punch deep-drawing of a square sheet is analyzed using a new dynamic explicit/elasto viscoplastic finite-element program developed by the authors. The numerical results are compared with either the experiments or static explicit/elastoplastic analyses. The suitability of the newly developed program for the quasi-static deformation process is confirmed.     

航空发动机零件冲压过程模拟及回弹分析

应用ANSYS/LS_DYNA软件对某航空发动机钣金件的三维冲压成形过程进行模拟 ,并对成形后的钣金件回弹进行了分析 ,给出了分析结果并对结果进行了分析总结。分析结果可指导钣金件冲压模具设计 ,可根据分析结果对钣金件的冲压模具进行科学有效的修正 ,缩短模具制造周期 ,提高模具使用寿命 ,得到高质量的钣金件 ,使航空发动机产品的可靠性得到更大的保障     

Formability evaluation for hot-rolled HB780 steel sheet based on 3-D non-quadratic yield function

A common practice to evaluate formability in the typical sheet metal forming process is to measure hardening behavior and a forming limit diagram as separate material properties, and perform numerical forming simulations utilizing various yield functions. The measured forming limit diagram is applied as the failure criterion. However, the performance of material properties such as hardening behavior and yield functions in predicting strain localization in the simple tension and forming limit diagram tests is seldom validated before their application to forming simulation. In this study, a new numerical formability evaluation procedure was proposed, in which not only hardening behavior but also measured forming limit data were employed in characterizing the input data for the hardening behavior and the yield function. Besides, strain localization was directly monitored to determine failure without employing any forming limit criterion. The new procedure was applied for rather thick advanced high strength hot-rolled steel sheet so that 3-D continuum elements were utilized along with 3-D non-quadratic Hosford and quadratic Hill yield functions.     

Application of laser stereolithography in FE sheet-metal forming simulation

This paper describes a new method developed by authors, by which the results of FE simulation in sheet-metal forming operations can be output as 3-dimensional substantial objects, where previously it could only be done with computer graphics. The system employed in this method is a kind of laser stereolithography system added with interface software to FE data. This method is being used to confirm the design of dies and to analyze successfully problems that may arise during the forming process.     

XPS, TEM and SAD investigations of nanosized CoxByHz particles obtained by two different borohydride methods

The nanosized Co, B, H, particles synthesized by the ‘tea’ and ‘antigravity’ methods using a borohydride reduction process have been subjected to structure and composition studies by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area diffraction (SAD). The amounts of the elements Co, B, O₂, H₂ and C as mean volume values, and surface values for the as-prepared particles, as well as after Ar⁺ etching to a depth of about 15 nm and 30 nm from the initial particle surface, are determined. About 1.5 atoms of cobalt per atom of boron correspond to samples obtained by the ‘antigravity’ method. The binding energy (BE) of 1s electrons of boron atoms has only one value. These particles are angular and are in the typical nanocrystalline state. In the case of samples prepared by the ‘tea’ method, two atoms of cobalt per atom of boron are found. The presence of two kinds of BE (B¹ and B¹¹) of 1s electrons of boron atoms in the particles obtained by the ‘tea’ method is observed and almost equal amounts of these two states are established in the spectrum. The particles' shape and structure are typical of the amorphous state. The fact that there is one peak when the ‘antigravity’ method is applied, in contrast to the two peaks with the ‘tea’ method indicates the presence of a metal amorphous state in the latter case.     

基于数值模拟的级进模筒形拉深工步工艺分析

在带有拉深工步的级进模设计中，用数值模拟技术对其拉深工步进行工艺分析是非常重要的。由于级进模中的拉深工步要受到其他工步的牵制，其拉深成形比单个零件的拉深要困难一些，因此在对拉深工步的工艺性进行数值模拟时，所建立的毛坯几何模型、边界条件和压边条件等也都与单工序模有差别。结合某工厂的攻关项目，运用ANSYS对级进模中筒形拉深工步的成形性能和数值模拟的关键技术进行了研究，经生产实际验证，该分析过程合理、准确。     

Evaluation of effect of flow stress characteristics of tubular material on forming limit in tube hydroforming process

The material properties for the analytical and numerical simulation in sheet metal processes, especially in tube hydroforming process, are generally obtained from the uniaxial tensile test of raw sheet material. However, the validation of the formability and reliability of the numerical simulation for the tube hydroforming process arises from the fact that the material characteristics of tubes are different from those of the raw sheet materials. In order to determine the most suitable material property of the tubular material for the evaluation of forming limit on the THF process, the uniaxial tensile test for the specimens of the raw sheet metal and the roll-formed tube and the free bulge test for the roll-formed tubular material are carried out in this paper. The forming limit curves are also derived using plastic instability based on three kinds of necking criteria, which are Hill’s local necking criterion for sheet and Swift’s diffuse necking criteria for sheet and tube, to describe and explain the forming limits for the roll-formed tubular material in the THF process. In order to acquire the informative data on the forming limit curves in the THF process, the loading condition of the free bulge test is controlled. The proper band from nearly necking initiation to nearly bursting initiation has been defined for the roll-formed tubular material in the THF process. It can be concluded that the flow stress of the tubular material should be determined from the actual free bulge test to find the practically valuable forming limit curve for the THF process.     

Computer-Aided Forming for Sheet-Metal Parts

Computer-aided modeling techniques have been widely used to speed up the design process and improve the quality of sheet-metal parts. The numerical modeling technique enables the die designer to explore alternative die designs or evaluate trade-offs on the computer, before the manufacturing engineer performs the costly and time-consuming steps of fabricating the dies and process try outs.     

Logistic regression analysis for experimental determination of forming limit diagrams

The forming limit diagram (FLD) is probably the most common representation of sheet metal formability and can be defined as the locus of the principal planar strains where failure is most likely to occur. Experimental determination of the FLD consists in performing a set of formability tests on a sheet metal blank, where a regular grid has been previously etched. After each test, the deformation of the grid is measured and the relative strains computed. Strains observed closely at the fracture location are related to as ‘failed’ points, while strains observed on the sound areas of the specimens are labelled as ‘safe’ points. Starting from a set of experimental tests, the FLD should be empirically determined through a statistical analysis of collected data. In fact, statistical approaches (such as linear regression) are required to properly account for the internal randomness of failure occurrence. Linear regression, as well as most of the other empirical approaches in the scientific literature, takes into account only information related to the safe points.This paper proposes a different approach, the logistic regression, for the empirical determination of FLDs. Logistic regression allows to directly derive the probability of an event (e.g. the failure) as a function of different predictor variables (both the principal planar strains). Therefore, by using logistic regression, the process designer can directly associate the failure probability to the scrapping costs, in order to economically evaluate a new sheet metal forming operation.Logistic regression allows the determination of the FLD by including information concerning both safe and failed points.     

Evaluation of forming limit in viscous pressure forming of automotive aluminum alloy 6k21-T4 sheet

A ductile fracture criterion is introduced into numerical simulation to predict viscous pressure forming limit of the automotive body aluminum alloy 6k2 l-T4. The material constant in the ductile fracture criterion is determined by the combination of the viscous pressure bulging （VPB） test with numerical simulation. VPB tests of the aluminum alloy sheet are carried out by using various elliptical dies with different ratios of major axis to minor axis（β）, and the bugling processes are simulated by the aid of the finite element method software LS-DYNA3D. On the basis of the stress and strain calculated from numerical simulations, the forming limits of bulging specimens obtained are predicted by the ductile fracture criterion, and compared with experimental results. The fracture initiation site and the minimal thickness predicted by the ductile fracture criterion are in good agreement with the experimental results.     

Enhancement of bending formability of brittle sheet metal in multilayer metallic sheets

The formability of multilayer metallic sheets is evaluated by tensile, V-bending, hat bending and hemming tests. A monolithic type-420J2 stainless steel sheet cannot be formed because of poor elongation as small as 1.7%. Marked enhancement of the bending formability was observed in the bending of type-420J2 stainless steel sheets when they are layered by type-304 stainless steel sheets and composed into a multilayer metallic sheet. The mechanism of the enhancement of the formability of type-420J2 stainless steel in a multilayer metallic sheet is investigated analytically by focusing on the delay of the initiation of necking, and by performing stress analysis by finite element method (FEM).     

烟盒折边模设计

介绍了1副折边模,采用合页折边结构形式,将凸、凹模设计成铰链结构,一次冲压翻包折边成形,即可完成盒体和盒盖的弯曲、整形。解决了不锈钢礼品烟盒折边中表面会产生拉丝伤痕等表面质量问题,同时一次冲压完成盒体和盒盖的折边,提高了生产效率。     

Comparison of formability of steel strips, which are used for deep drawing of stampings

The paper concerns evaluation and comparison of formability of steel strips, which are used in Czech Republic for production of intricate deep stampings. The properties of sheet-metal which have the principal influence upon the success of deep drawing or strech-forming are described. Two methods used for determination of strain-hardening exponent are compared. It is concluded, that the method according to SN 42 0436 is suitable for approximation of stress-strain curve of given sheet-metal only from point of view of functional values. The values of strain-hardening exponent, calculated by the method using maximum uniform elongation, are more strongly correlated with pure stretchability than are the values of it, calculated by the method according to SN 42 0436.