Prediction of flange wrinkling in deep drawing process using bifurcation criterion

Surface distortions in the form of wrinkles are often observed in sheet metals during stamping and other forming operations. Because of the trend in recent years towards thinner, higher-strength sheet metals, wrinkling is increasingly becoming a more common and troublesome mode of failure in sheet metal forming. The prediction and prevention of wrinkling during a sheet forming process are important issues for the design of part geometry and processing parameters. This paper treats the phenomenon of flange wrinkling as a bifurcated solution of the equations governing the deep drawing problem when the flat position of the flange becomes unstable. Hill’s bifurcation criterion is used to predict the onset of flange wrinkling in circular and square cup drawing. In particular, the maximum cup height that can be drawn without the onset of flange wrinkling is predicted for the given set of process parameters. A parametric study of the maximum cup height is also carried out with respect to various geometric, material and process parameters. Finite element formulation, based on the updated Lagrangian approach, is employed for the analysis. The incremental logarithmic strain measure, which allows the use of a large incremental deformation, is used. The stresses are updated in a material frame. The material is assumed to be elastic–plastic, strain hardening, yielding according to an anisotropic yield criterion of Barlat et al. (2005) [23] (named as Yld2004-18p). Isotropic power law hardening is assumed. Inertia forces are neglected due to small accelerations. Modified Newton–Raphson iterative technique is used to solve the nonlinear incremental equations.     

板料成形模拟中压边力处理的研究

针对弹性压边建立了一个力学模型 ,用于板料成形的动力显式有限元模拟。以NU MISHEET’93方盒拉深标准考题为例 ,模拟了方盒在不同压边力情况下的起皱情况。数值计算结果表明该模型能有效地模拟弹性压边力 ,并预测工件法兰部位的起皱状况。     

Modeling and validation of deformation process for incremental sheet forming

Incremental Sheet Forming (ISF) is an emerging sheet metal prototyping technology where a part is formed as one or more stylus tools are moving in a pre-determined path and deforming the sheet metal locally while the sheet blank is clamped along its periphery. A deformation analysis of incremental forming process is presented in this paper. The analysis includes the development of an analytical model for strain distributions based on part geometry and tool paths, numerical simulations of the forming process with LS-DYNA, and experimental validation of strain predictions using Digital Image Correlation (DIC) techniques. Three kinds of parts include hyperbolic cone, skew cone and elliptical cone are constructed and used as examples for the study. Analytical, numerical and experimental results are compared, and excellent correlations are found. It is demonstrated that the analytical model developed in this paper is reliable and efficient in the prediction of strain distributions for incremental forming process.     

The development of a microscale strain measurement system applied to sheet bulge hydroforming

Micro and multiscale sheet metal forming processes represent new and attractive solutions to many manufacturing problems. However, evaluating the strains in these products is a difficult endeavor. Larger organizations are utilizing commercially available microscale digital image correlation systems to measure the strains in microscale parts or on macroscale parts with critical microscale features. The cost of these strain measurement systems is preventing smaller research and development organizations from entering this challenging area or they are forgoing the ability to determine strains. The present paper describes the development of a method for creating microscale grids and measuring strains on microscale parts or microscale locations on larger parts. The method developed was able to measure true strains up to 0.618 for square grids that are 127 μm measured from center-to-center. Microscale strains resulting from sheet bulge hydroforming experiments using 11 mm, 5 mm, and 1 mm diameter dies were evaluated and material properties of the sheet metal were estimated based upon the strains measured in conjunction with FEA simulations and compared to analytical solutions and microscale tension tests. The material properties determined using the strains and FEM approach were consistent with the other methods.     

金属板材成形的静力隐式有限元分析与程序设计

阐述了用于板材成形过程静力隐式数值模拟的弹塑性大变形有限元方法 ,基于给出的方法编制了板材成形过程数值模拟软件 ,并对矩形板的液压胀形进行了有限元分析 ,计算结果与典型的实验结果吻合很好。对球形模具拉伸成形过程进行了数值模拟 ,给出了计算结果     

YBT试验与板材抗皱性

方板对角拉伸试验(统称为YBT—Yoshida Buckling Test)是研究复杂钣金件成形中非均匀拉伸起皱的一种模拟试验。此文测量和分析了起皱过程,在试验的基础上探讨了板材性能、板厚和材料方向性、加载条件等因素对板材抗皱性的影响。     

Warm forming die design,part I: Experimental validation of a novel thermal finite element modeling code

Finite element analysis (FEA) has become an invaluable tool in the design of sheet metal stamping dies and processes. FEA has gained widespread acceptance as the best method of optimizing dies for conventional stamping processes. More recently, FEA has been shown to be an effective method of designing tooling for sheet forming processes. In this work, an FEA based approach is applied to the warm stamping (warm forming) process. This work introduces a new thermal finite element analysis software called PASSAGE®/Forming (PASSAGE) that enables the up-front design of the thermal management of warm forming dies. This thermal finite element analysis software is designed to specifically handle the forming and optimization scenarios related to the heating of a stamping die while minimizing user interface time. In this work, PASSAGE has been applied to a simple block of steel embedded with cartridge heaters to validate the prediction capability of this software under two different heating conditions. The results show that PASSAGE is capable of predicting the actual steady-state temperature distribution within the block with an acceptable level of accuracy while yielding notable information to the user with respect to specifying power requirements. A finite element software package like PASSAGE is a valuable tool that will aid greatly in the implementation of warm forming as a manufacturing process beyond the scope of the laboratory and into production.     

采用薄膜模型模拟三维板料成形过程

此文基于有限变形理论建立了三维金属板料成形过程的弹塑性有限元数学模型。数学模型采用物质坐标系中的Total Lagrange描述、J_2型本构方程、等向强化假设,考虑了板料的厚向异性,对于金属板料与模具的摩擦采用近似的库仑摩擦定律以改善计算的收敛性。为简化计算采用薄膜单元。采用根据此模型编制的程序模拟了机油收集器基本件的成形过程,并与实验结果进行了对比。     

LDR and hydroforming limit for deep drawing of AA5754 aluminum sheet

The goal of the research was to determine the limits and conditions in which the sheet hydroforming process provides a significant advantage over stamping in deep drawing of AA5754 aluminum sheets. Specifically, the maximum draw depth achievable by stamping, warm stamping (WF), sheet hydroforming (SHF), and sheet thermo-hydroforming (THF) of AA5754 aluminum alloy were quantified through experimental and computational modeling. A limited number of forming experiments were conducted with AA5754 aluminum sheets using a cylindrical punch and counteracting fluid at different temperatures and pressures. Several parameters, such as force–displacement, hydroforming pressure and temperature, and the maximum draw depth prior to wrinkling or tearing were measured during the forming process to make comparisons with simulations. The computational study included the simulation of stamping, WF, SHF and THF of AA5754 aluminum sheet with the LS-Dyna code, and the Barlat 2000-2d yield function with temperature-dependent coefficients. To predict the onset of wrinkling and tearing, the numerically generated, temperature-dependent forming limit diagrams (FLDs) based on the Barlat 2000-2d yield function were used. It was found that compared with stamping, SHF and THF can achieve more than 100% deeper draw depths with AA5754 aluminum sheet. The stamping simulations were used also to calculate the optimum blank size and die corner radii for the limiting draw ratio (LDR). The LDR was found to be very sensitive to the punch and die corner radii used in the experiments, which represent the curvature of character lines in an actual part. The LDR for AA5754 aluminum sheet was found to be 1.33 and 2.21 for sharp and round die corner radii, respectively. Overall, it was concluded that SHF is most ideal for deep drawing of aluminum sheets with sharp radii features. With the additional drawability provided by SHF, the automotive industry would be able to make difficult-to-form aluminum parts that cannot be stamped without product concessions such as increasing the die radii.     

Analytical prediction of stepped feature generation in multi-pass single point incremental forming

Single point incremental forming (SPIF) is a new sheet metal forming process characterized by higher formability, product independent tooling and greater process flexibility. The inability of conventional single pass SPIF to form vertical walls without failure is overcome by forming multiple intermediate shapes before forming the final component, i.e., multi-pass single point incremental forming (MSPIF). A major issue with MSPIF is significant geometric inaccuracy of the formed component, due to the generation of stepped features on the base. This work proposes analytical formulations that are shown to accurately and quantitatively predict the stepped feature formation in MSPIF. Additionally, a relationship is derived among the material constants used in these analytical equations, the yield stress and thickness of the blank material, such that the computational effort required for the calibration of these constants can be minimized. Finally, the physical effects of yield stress and sheet thickness on the rigid body translation are further discussed.     

多点成形--金属板材柔性成形的新技术

多点成形是一种板材三维曲面柔性成形的新技术.简述了多点成形的基本原理与系统构成并介绍了几种成形方式.在多点成形中,厚板曲面件可直接成形,薄板曲面件则采用柔性压边技术进行成形.利用多点成形技术的柔性特点还能实现分段成形、反复成形、多道成形以及闭环成形等新的板材成形工艺.     

材料结构参数对薄板冲压成形仿真结果的影响

采用不同材料屈服模型及参数对典型冲压过程进行了仿真 ,对仿真结果进行了分析。分析表明材料模型及参数对仿真结果影响至关重要 ,板料的厚度分布及成形极限图对结构参数n和r值很敏感。这对更好的了解不同材料在冲压时的力学行为和进行冲压仿真时怎样合理选择材料模型及参数提供了帮助     

半球形件粘性介质压力成形的数值模拟研究

粘性压力成形技术是金属板材成形领域里新近出现的一种先进的加工工艺。对双面施放粘性介质的板材胀形进行了模拟研究 ,在双面施放介质的板材粘性介质胀形时 ,排放孔位置分布对毛坯成形过程有明显的影响。排放孔位置分布的不同造成模腔内介质的速度场不同 ,进而导致板材不同部位的流动速度的方向大小发生变化 ,最终使成形件的厚度分布不同     

板料冲压成形中坯料优化设计方法研究

较为系统地介绍了冲压件坯料的优化设计方法 ,并比较分析了其各自的优缺点。相比于其他方法 ,一步模拟方法效率高、适应面广、功能较强 ,可以在产品的设计之初提供较为丰富的信息。因此 ,对其作了较为详细的论述。实例证明 ,一步模拟法可用于板料成形工艺方案的快速评价及参数优化。     

Bending of Titanium Sheet Using Laser Forming

Laser forming, a novel manufacturing method for bending sheet metal first reported in 1985, has been investigated as an alternative to hot brake forming (industry standard) of titanium sheet parts for the aircraft industry. Laser forming involves scanning a focused or partially defocused laser beam over the surface of a titanium workpiece to cause localized heating along the bend line and angular deflection toward the beam. The main advantage that laser forming has over conventional brake forming is increased process flexibility. An experimental investigation of this process (primarily designed experiments) met the following objectives: identified the response variables related to change in geometry (bend angle) and material microstructure; characterized the influence of process variables (scanning speed, beam diameter, laser power) on these response variables; determined the degree of controllability over the process variables; and evaluated the suitability of laser forming for the aircraft industry (most important), all with respect to titanium sheet. It has been determined that laser forming with an Nd:YAG laser is a controllable, flexible manufacturing process for titanium sheet bending. Unfortunately, these advantages over traditional hot brake forming are overshadowed by the fact that, with regard to forming with titanium, laser forming is significantly slower and more labor and energy intensive, and results in unacceptable material properties at the bend line according to aircraft industry standards. These findings cast doubt over the assertions of some researchers that laser forming may be a viable manufacturing process for parts made in small batches. Instead, it appears that it may be best suited for rapid prototyping of sheet metal parts.     

有反向压力粘性介质胀形铝、钛合金板实验研究

粘性介质压力成形是一种新发展起来的板金软模成形工艺,其对板料成形性能的影响可以通过胀形实验来检测和评价.文中采用一种具有应变速率敏感性的半固态粘性物质作为传力介质,采用胀形实验研究了在有、无施加反向压力的情况下,铝和钛合金板料的成形形状特征与应变分布.结果表明,粘性介质压力成形,尤其是存在反向压力时可提高板料的成形性能.     

多通管塑挤胀形关键技术的研究

多通管是工业上应用量大面广的一种基础件,但其传统的制造工艺目前还只限于铸造、焊接和机加工等,从经济或技术的角度讲都显得比较落后.此文讨论了利用聚氨酯橡胶棒为胀形介质,采用塑挤胀形的复合工艺把无缝管坯一次成形为成品的加工方法,分析了多通管塑挤胀形的过程,讨论了冲头作用于管坯的压力、聚氨酯橡胶棒产生的内压和平衡缸应提供的背压三者间应具备的合理的关系.以使变形区处于最有利于变形的强烈的三向压应力和合适的偏应力状态,获得尽可能长的支管.     

板材成形新技术及其发展趋势(Ⅰ)

近些年来,面对全球化的竞争,越来越需要小批量、多样化、周期短的新的成形技术.薄板成形技术在成形工艺中占有很重要的地位,其多样化趋势已经变得越来越明显,出现了多种加工方法,它对将来的工业结构和产品的生产技术将是一场革命.文中介绍了变压边力技术、成对液压成形技术、粘介质成形技术、无模分层成形技术等几种柔性化程度高的板材成形技术及其发展趋势.     

研究单位压边力值对板料成形性能的影响

主要介绍板料在拉深成形过程中单位压边力值对板料成形性能的影响。研究方法采用仿真技术 ,指出板料在成形过程中如按最初设定的单位压边力值进行成形 ,那么 ,板料成形过程中单位压边力值将逐渐增大 ,严重影响板料成形极限。如果采用控制板料在成形过程中压边圈下坯料的单位压边力值 ,将提高板料成形极限。其结论为生产现场提高板料成形性能和表面质量提供了一种可实施的方法。     

薄板拉延成形的固定压边间隙控制方式研究

分析了薄板拉延过程中应用固定压边间隙控制方式的成形特点，指出选择合理的压边间隙量是保证冲压件成形质量的关键。应用有限元数值仿真技术，以NUMISHEET＇99圆柱杯深拉延——反向深拉延标准考题(Benchmark-C)为例，模拟了其成形过程，在仿真结果与KimHyungjong的实验结果较好地吻合的基础上，通过对大量仿真计算结果的分析．提出了薄板多工序拉延成形中采用压边间隙控制方式时合理选择间隙量的方法。