基于有限元仿真的左半边成形极限图建立方法

有限元仿真中,凸模与板料接触力出现峰值的时刻,板料上变薄量最大单元的邻近单元应变即是材料的缩颈极限应变,通过改变试件形状及尺寸就可以得到在不同应变状态下的缩颈极限应变。把这些极限应变点在二维应变空间里连成曲线,即为用有限元仿真方法得到的材料成形极限图(forruing limits diagram,简称为FLD)。用该方法得到的FLD与按照GB／T 15825．8—1995金属薄板成形性能与试验方法：成形极限图(FLD)试验利用半球形凸模拉伸试件测得的材料FLD符合程度良好。     

Al-Mg-Si基合金车身板材成形极限及数值应用

采用成形极限图（FLD）试验,运用网格应变自动测试分析系统ASAME,对Al-Mg-Si基合金车身板的成形极限进行了检测,同时,将FLD应用于有限元分析作为成形时出现破裂缺陷的判据,并分析车门外板在冲压成形过程中破裂危险部位主应变在FLD上分布。结果表明：Al-Mg-Si基合金板材的成形极限高于目前常用的AA6111铝合金车身板;拉延筋的形状和位置对成形过程的影响很大,采用双曲面法设计车门可以改善车门腹部拉深不充分引起的刚度问题。     

差厚激光拼焊板的成形极限

为了进一步了解不同厚度钢板拼焊在一起后组成的板料(差厚激光拼焊板)在各种应变路径下的成形极限,通过试验检测了0.8,1.2,1.5 mm三种厚度的ST16板料在不同组合激光拼焊下组成的板料以及其各自母材的成形极限,并研究了不同加载情况下焊缝附近的应变分布.结果表明:差厚激光拼焊板焊缝附近的成形极限低于母材的成形极限,主要取决于较薄一侧母材的成形极限;厚度差越大焊缝两侧的应变梯度越大,成形性能越差.     

基于GTN细观损伤模型的激光拼焊板成形极限预测

建立了从细观损伤角度预测拼焊板成形极限的Gurson-Tvergaard-Needleman(GTN)损伤模型,用有限元逆向法确定了损伤模型中的各损伤参数。采用有限元软件ABAQUS耦合基于Mises屈服准则的弹塑性GTN损伤模型,对拼焊板半球凸模胀形过程进行了数值模拟。设计了拼焊板半球凸模胀形物理试验,试验过程中通过改变试件的宽度得到了不同应变状态下完整的拼焊板成形极限图,并与GTN细观损伤模型预测到的拼焊板成形极限图进行对比分析,验证了GTN细观损伤模型预测拼焊板成形极限图的准确性。     

数字图像相关法测量板料成形应变

针对板料成形极限试验中传统应变测量方法的不足,提出并实现一种基于数字图像相关法的板料成形极限应变测量方法。对其中涉及的关键技术进行深入研究。针对板料成形中大变形测量的难题,根据系列变形图像相邻状态变形的连续性,提出一种大变形分步匹配算法,并介绍三维坐标重建和三维应变求解的方法。根据测得的试件表面应变分布,提出一种通过构建应变场截面线来拟合成形极限曲线的方法。基于Visual Studio 2010开发环境,开发用于板料成形极限应变测量的计算软件。为了验证方法的可行性,结合自研的图像采集装置和改进的板料成形试验机搭建成形极限测量试验平台。对SPCC钢板试件的测量结果显示所提出的方法能够快速、有效、直观地测量试件在整个成形过程中的表面应变分布,为建立成形极限图提供一种有效手段,且与传统的坐标网格等方法相比优势明显。     

直缝激光焊管液压成形极限图试验研究

在管件液压成形中,准确地评价管材缩颈或破裂现象是工程上备受关注的问题。成形极限图作为评价成形性能的一种有效手段,得到广泛应用。基于椭圆胀形试验方法,通过改变椭圆比大小实现拉-拉应变区管材极限应变的测量。联合已有的自由胀形试验和椭圆胀形试验,实现管材液压成形极限试验评价。采用镶嵌式模具结构设计,建立一种柔性化的管材液压成形极限试验装置,完成多种规格激光焊管的液压成形极限试验。激光焊管成形极限试验结果表明,所建立的成形极限试验方法是可行的;同种牌号的激光焊管,厚度和直径不仅影响着平面应变点的成形极限,而且也影响着成形极限曲线的形状;由于受起皱失稳的影响,直径厚度比越小的管材,拉-压应变区的极限应变比值也越小。     

板料成形极限预测新判据

从控制塑性变形能的角度出发,基于总塑性功的积分形式,建立板料的成形极限预测判据.这种板料成形极限预测判据考虑应变路径变化、材料的硬化指数、各向异性系数及材料的初始厚度等对成形极限的影响.判据中的参数可由常用的单向拉伸极限应变试验确定.由此通过数值模拟可以预测板料在各种不同应变路径下的成形极限,适用于成形极限图的拉-压和拉-拉应变区,从而建立完整的成形极限曲线,可大大减少试验工作量.试验验证表明,这种成形极限预测判据对于钢板和铝合金板的成形极限可做出较为准确的预测.实现了只进行单拉试验即可预测板料在不同应变路径下的成形极限.     

基于对向液压拉深的高精度拉深件的应用研究

对向液压拉深是在凹模兼液压室的型腔内充满液体,利用凸横带动板料进入凹模后建立的反向液压而使板料成形的方法。对向液压拉深方法可提高拉深时的成形极限、抑制侧壁起皱．使零件具有很高的尺寸、形状精度及表面质量。可实现零件的一体化成形,在精度要求高的拉深件中可得到广泛应用。     

盘形件增量拉深成形过程的数值模拟

增量拉深成形由于旋转的模具与坯料接触区域的不断变化而一直是数值模拟的难点,为此用Deform-3D有限元软件对为A1100铝合金的盘形件增量拉深过程进行了数值模拟.结果表明:增量拉深成形凸模的受力与传统拉深工艺相比大大降低,据此得到了凸模受力图和板料应变云图,每一步凸模受力峰值和坯料等效应变值与已有的盘形件工艺试验研究结果一致.     

基于成形应力极限的管材液压成形缺陷预测

基于塑性应力应变关系及Hill79屈服准则,推导出极限应力与极限应变间转化关系,进而建立2008T4铝合金的成形应力极限图(Forming limit stress diagram,FLSD)。采用LS-DYNA软件对三通管液压胀形过程进行模拟,应用FLSD预测胀形过程中破裂的发生及成形压力极限,并与传统成形极限图(Forming limit diagram,FLD)结果进行了对比。研究表明,FLD与FLSD预测结果中破裂缺陷位置相同,但极限内压力值存在很大差别,而FLSD预测结果与物理试验结果较吻合。考虑到FLD受应变路径影响显著的因素,将FLSD作为管材液压成形等复杂应变路径下的成形极限的判据更加方便可靠。     

Determination of forming limit diagrams of sheet materials with a hybrid experimental-numerical approach

A new methodology is proposed to obtain the forming limit diagram (FLD) of sheet materials by utilizing routinely obtained experimental punch load versus displacement traces from hemispherical punch stretching experiments and by analyzing strain history of the test samples from finite element simulations of the experiments. The simulations based characteristic points of diffuse and localized necking are utilized to obtain the limit strains. The proposed method for FLD determination considers out-of-plane displacement, punch-sheet contact and friction, and avoids using experimental strain measurement in the vicinity of the neck on the dome specimens and the rather arbitrary inhomogeneity factor to trigger localization such as in the commonly used Marciniak-Kuczynski method. A criterion of maximum in the ‘pseudo’ major strain acceleration for the onset of localized necking, proposed earlier by the present authors, is utilized to determine the limit strain in FE simulations as well as in FLD verification experiments. The proposed overall approach for obtaining FLD is rapid and accurate and could be implemented for routine FLD generation in a laboratory setting with significant reduction in cost, effort and subjectivity.     

A theoretical and experimental investigation of limit strains in sheet metal forming

The paper deals with an analytical technique for predicting FLDs based on linear, bilinear and trilinear straining paths—although any general curvilinear strain path can be handled by the method. The analytical procedure was based on the work of Marciniak and Kuczynski (known as the M-K model). The influence of material properties, such as anisotropy, strain hardening and strain rate sensitivity, as well as the effect of strain path, on the shape and level of the FLD were investigated. Material property data were determined from tests on Interstitial Free (IF) steel sheet.An experimental FLD was constructed for the IF steel sheet in the as-received condition. Additional FLDs were also produced following pre-straining of the as-received sheet. Comparison between the experimentally determined FLDs and those predicted from the theoretical model was favourable.Conventional FLDs are constructed in strain space with the principal surface strains as coordinate axes. However, they can be plotted in principal stress space, and some investigators have claimed this is a better representation. By knowing the strain path the stress state at the limit strain can be determined, and these limit stresses were plotted in principal stress space in order to construct a Forming Limit Stress Diagram (FLSD). It turned out that regardless of the shape of the FLD and the type of pre-strain imposed, all the FLSDs were almost identical. In contrast when plotted in strain space the FLD was very sensitive to the type of straining path.     

Impact of die wear and punch surface textures on aluminium can wall

Aluminium can manufacturing uses a wide range of punch sleeve surface roughness and textures. The ironing die and the punch tooling both can vary in the roughness from 0.04 μm to 0.4 μm Ra during the can forming process. This, together with the roughness of the incoming can body sheet (from 0.3 μm to 0.6 μm Ra) creates a wide range of tool/metal interface coefficients of friction. Ironing dies become rougher and have to be replaced frequently once they lose their shape. Punches maintain a consistent roughness for periods of a week to a month and any surface wear is compensated for with die changes. The initial die and punch surface finish adopted by a manufacturing unit determines the long time plant productivity and punch life. A higher friction on the punch side, compared to the die side, is the preferred manufacturing operating condition. Departures from the preferred condition with ground, polished, cross-hatch and media textured punches are examined. Plants that prefer polished carbide punches over cross-hatched must have their lubrication and coolant parameters controlled within a very narrow operating window. A larger operating window and better performance is achieved with the cross-hatch and micro-textured punches having a Ra less than half that of the can body sheet. Above all, a random isotropic texture is identified as the ideal punch sleeve surface texture and the best performer for aluminium can manufacture.     

Experimental evaluation of coating delamination in vinyl-coated metal forming

In this paper, a new evaluation and prediction method for coating delamination during sheet metal forming is presented. On the basis of the forming limit diagram (FLD), the current study evaluates the delamination of PET coating by using a cross-cut specimen, dome test, and rectangular-cup drawing test. Dome test specimens were subjected to biaxial, plane strain, and uniaxial deformation modes. Rectangular cup-drawing test specimens were subjected to the deep-drawing deformation mode, and compression deformation mode. A vinyl-coated metal (VCM) sheet consists of three layers of polymer on the sheet metals: a protective film, a PET layer and a PVC layer. The areas with coating delamination were identified, and the results of the evaluation were plotted according to major and minor strain values, depicting coating delamination. The constructed delamination limit diagram (DLD) can be used to determine the forming limit of VCM during the complex press-forming process. ARGUS (GOM) was employed to identify the strain value and deformation mode of the delaminated surface after the press forming. After identifying the areas of delamination, the DLD of the PET coating can be constructed in a format similar to that of the FLD. The forming limit of the VCM sheet can be evaluated using the superimposition of the delamination limit strain of the coating onto the FLD of VCM sheet. The experimental results showed that the proposed test method will support the sheet metal forming process design for VCM sheets. The assessment method presented in this study can be used to determine the delamination limit strain under plastic deformation of other polymer coated metals. The experimental results suggested that the proposed testing method is effective in evaluating delamination for specific applications.     

Influences of material properties of components on formability of two-layer metallic sheets

Formability of two-layer metallic sheet is constrained by plastic instability and localized necking. Forming limit diagram (FLD) is an accepted measure of sheet metal formability. The formability of two-layer sheets depends on the material properties of their components such as strain hardening exponent, strain rate sensitivity coefficient, stiffness coefficient, and grain size. In this paper, the effects of the mentioned parameters on the FLD of two-layer sheets are investigated with a theoretical model which has been verified with an experimental approach. The results show that the forming limit of two-layer sheet lies between the forming limits of its components depends on their material properties.     

On thin rings and short tubes subjected to centrally opposed concentrated loads

The deformation behaviour of mild steel rings and short tubes of various diameters, thicknesses and lengths loaded centrally by opposed conically-headed cylindrical punches is examined. The test equipment and procedure are briefly described and typical histories of deformation, punch load—displacement diagrams and the increase in the horizontal ring or tube radius for increasing punch travel are presented. A comparison is made between the latter and those predicted by use of a simple expression based on the assumption of rigid-plastic material deformation. Good correlation is obtained with rings of different diameter to thickness ratio.     

The design of a semi-additive manufacturing shape using metal 3D printing for a partially strengthened mold based on a high-alloy tool steel powder

In this paper, describe the fabrication of high strength punch molds that can be applied to ultra-high strength sheet materials after processing. A method for improving the strength of the punching die by additive manufacturing (AM) of a high strength powder material using a metal 3D printer was proposed. Furthermore, a semi-additive technique was proposed to increase the punch strength through partial AM of specific parts of the punch that require high strength. A preprocessing process for predicting the semi-additive shape for the punch function portion is proposed for application of the AM technology of a metal 3D printer to this semi-additive technique. The preprocessing for determining the semi-additive shape consists of the predicting step of the punch strength based on the shear process of the sheet material, analyzing step the stress distribution of the punch, defining step the semi-additive range, designing step the semi-additive shape, and verifying step the additive interface strength. Based on this simulation, the range of shapes for the semi-additive was 1.21 mm and 2.62 mm for sheet material CP1180 and 1.3 mm and 3.2 mm for sheet material 22MnB5. The shape and range determined in the simulation process defines a semi-additive area (volume) for the 3D printing AM technique using a high-strength powder material, and a semi-additive punch was manufactured according to the defined area. The semi-additive punch (HWS powder material) fabricated in this study was performed a durability test for validity verification in the piercing process of high-strength sheet material (CR980). This validation test compared the state of the punch after 1000 piercing processes with a typical cold piercing punch (SKD11 solid material). From this test, the feasibility of the semi-additive punch was confirmed by showing a similar state of scratches and abrasion from the two punches. The simulation analysis processor for the additive shape and the additive range prediction for the semi-additive punch manufacturing presented in this paper can be useful for the additive manufacture of cutting and trimming punch mold.

     

环保型礼花弹壳的结构改进与模具制造

根据环保型礼花弹壳的生产工艺特点和应力应变有限元分析结果,对弹壳结构进行改进,并将热压成型模具设计成分体结构,采用CAD和CAM技术,时形状复杂的凸模采用与模座装配后一次装夹、在数控机床上整体加工的方法,保证了凸模上各突起部分的形状以及各凸模之间的位置精度.弹壳结构的改进,可减少资源消耗,产生良好的经济效益和社会效益.     

加载路径对AA5083管件热态非金属颗粒介质成形性能的影响

将颗粒介质作为传力介质,应用于铝合金管件内高压热成形工艺。通过热单向拉伸试验建立AA5083板材的本构模型。通过管材热态颗粒介质胀形数值模拟,结合AA5083理论成形极限图的分析,研究了不同加载路径对管件壁厚分布、管端缩料量和主应变曲线的影响规律,并进行了相应的工艺试验验证。研究结果表明,合理匹配初始压头力和管端进给量参数,使预成形管坯在胀形区形成有益皱纹,可为胀形区管坯变形提供聚料作用,从而提高管件成形质量和胀形极限。