Multi-scale parallel finite element analyses of LDH sheet formability tests based on crystallographic homogenization method

A multi-scale parallel finite element (FE) procedure based on the crystallographic homogenization method was applied to the LDH sheet formability test analysis. For the multi-scale structure, two scales are considered. One is a microscopic polycrystal structure and the other is a macroscopic elastic plastic continuum. The analysis code can predict the formability of sheet metal in macro-scale, simultaneously the crystal texture and hardening evolutions in the micro-scale (Nakamachi E et al. Int J Plasticity 2007;23:450-8). Since huge computation time is required for the nonlinear dynamic multi-scale FE analysis, parallel computing technique based on domain partitioning of FE model for macro-continuum is introduced into the multi-scale code using the message passing interface (MPI) library and PC cluster (Kuramae H et al. In: Proceedings of the eighth international conference on computational plasticity, Part 1, 2005. p. 622-5). The explicit time stepping solution scheme in the nonlinear multi-scale FE dynamic problem is well-suited for parallel computing on distributed memory environment such as PC cluster because solving simultaneous equation is not required. We measured crystal morphologies of four automotive sheet metals, aluminum alloy sheet metals A6022-T43 and A5182-O, an asymmetrically rolled aluminum alloy sheet metal A6022-ASR, and mild steel HC220YD, by using the scanning electron microscope (SEM) with electron back scattered diffraction (EBSD) analyses, and defined a three-dimensional representative volume element (RVE) of micro polycrystal structure, which satisfy the periodicity condition of crystal orientation distribution. We evaluate not only macroscopic formability of the automotive sheet metals by the multi-scale LDH test analysis, but also microcrystalline texture evolution during plastic deformation. Furthermore, a relationship between the macroscopic formability and the microcrystal texture evolution was discussed through looking at multi-scale FE results. It is concluded that the mild steel HC220YD was the highest formability than the aluminum alloy sheet metals because of remaining and generating the γ-fiber texture, such as {1 1 1}〈1 1 0〉-{1 1 1}〈1 1 2〉 orientations, during plastic deformation.     

基于有限元逆算法的板料成形模拟拉深筋的灵敏度优化

基于有限元逆算法和灵敏度优化的BFGS （Broyden-Fletcher-Goldfarb-Shanno）算法，提出一种以成形极限曲线和起皱极限曲线作为目标函数，优化拉深成形中的拉深筋位置和大小的拉深筋灵敏度优化算法。优化算法综合考虑冲压件成形过程中起皱和破裂对成形性的影响因素，比只考虑板厚变化更能准确地反映板料的成形性，而且基于逆算法的灵敏度优化算法计算速度很快，可以应用于大型复杂冲压件的工艺参数优化过程模拟。通过两个典型的零件验证了该算法的优化结果符合实际工艺。     

Development of a hydroforming setup for deep drawing of square cups with variable blank holding force technique

Sheet hydroforming is a process that uses fluid pressure for deformation of a blank into a die cavity of desired shape. This process has high potential to manufacture complex auto body and other sheet metal parts. Successful production of parts using hydroforming mainly depends on design aspects of tooling as well as control of important process parameters such as closing force or blank holding force (BHF) and variation of fluid pressure with time. An experimental setup has been designed and developed for hydroforming of square cups from thin sheet materials. Square cups have been deep drawn using constant and variable BHF techniques. A methodology has been established to determine the variable BHF path for successful hydroforming of the cups with the assistance of programmable logic controller and data acquisition system. Finite element (FE) simulations have also been carried out to predict formability with both of these techniques. It has been found that it is possible to achieve better formability in terms of minimum corner radius and thinning in the case of variable BHF technique than in the case of constant BHF technique (constant force during forming and calibration). The results of FE analysis have been found to be in good agreement with experimental data.     

面向设计的板料成形快速仿真系统FASTAMP

FASTAMP是基于改进的有限元逆算法和动力显式算法的板料成形快速仿真软件。改进的逆算法求解器采用了考虑弯曲效应的DKQ四边形单元及方程组快速求解算法,真实考虑了摩擦、压边力、背压力和曲压料面等实际工艺条件,在模拟精度和速度上均有较大的提高。系统结合了两种算法的优势,将产品设计、选材和工艺设计三个独立的过程紧密结合起来,可快速分析产品设计中的潜在缺陷,为工艺设计人员提供有效的工艺设计参考和强有力的设计辅助分析工具。     

Optimization of drawbead restraining forces and drawbead design in sheet metal forming process

This paper presents an optimization procedure of drawbead restraining forces in order to improve the sheet metal formability in deep drawing process. A simplified finite element method called inverse approach (IA) has been developed for sheet forming analysis with the consideration of the drawbead restraining forces. This IA is combined with a mathematical programming algorithm to optimize the restraining forces and then to design the drawbeads. The obtained optimization procedure is very efficient due to the simplified assumptions of the IA and the analytical sensitivity analysis. The Square cup of Numisheet’93 and the Renault Twingo dashpot cup are presented to demonstrate the usefulness of the proposed optimization procedure for industrial applications. Verifications of the obtained results have been carried out using a precise incremental commercial code OPTRIS^TM based on explicit dynamic approach to show the effectiveness of our approach.     

有限元逆算法与板料成形工艺的评价

依据理想形变理论，研究开发了冲压成形过程模拟的有限元逆算法，根据变形体的整体塑性功取相对极值的条件，导出了塑算法有限元方程。提出了求逆算法初始解以及求解与给定形状的毛坯相对应的冲压件形状的迭代计算方法。采用有限元塑算法预测了与冲压件形状相对应的冲压件毛坯的展开形状，根据给定的板坯形状计算了冲压件最终构形及应变分布。分析计算实例表明，逆算法可用于对板料成形工艺方案进行快速评价，对冲压工艺参数进行优化。     

冲压模参数化NURBS曲面造型及有限元网格划分

针对冲压模具的特点 ,提出了模具型面参数化NURBS曲面分片 ,局部网格调整的思想 ,较好地解决了板料成形有限元模拟前处理中模具型面造型及网格划分的问题。基于NURBS方法 ,在VC + +平台上开发了一个NURBS曲面造型及网格生成系统 ,对实际的盒形件拉深凹模型面进行了描述 ,获得了满意的曲面形状 ;基于参数空间法对型面进行了四边形网格剖分 ,并自动实现四边形到三角形网格的转换     

Effect of elastic-plastic behavior of coating layer on drawability and frictional characteristic of galvannealed steel sheets

During a galvannealed sheet metal forming, the failures of coating layers (powdering, flaking and cracking) frequently affect the strain state of sheets and deteriorate the frictional characteristic between sheets and tools. Two FE-models in this study were suggested to investigate the effects of the mechanical behavior of coating layers on the formability and friction of the coated steel sheets in FE analysis; the first is one-layer model to express the coated sheet as one stress-strain curve and the second is a multiple-layer model which is composed of substrates and coating layers, separately. First, the frictional properties and the formability of the coated sheets were experimentally investigated using a cup deep-drawing trial. After, the drawing process was simulated by FE analysis of the two models. In the multiplelayer model, the mechanical behavior of the coating is defined as a stress-strain curve which was determined using the nanoindentation test of the coating, its FE analysis and artificial neural network method. The result showed that the multiple-layer model provides more accuracy predictions of drawing loads than the one-layer model in the FE analysis, compared to the actual cup drawing test.

     

基于有限元分析及非线性规划的金属成形过程优化设计

综述显式有限元分析和非线性规划算法的理论基础，进而将它们在金属成形分析和设计中的应用加以回顾。     

板料成形中悬空区起皱过程的数值模拟

以逐级更新Ｌａｇｒａｎｇｅ描述的有限变形虚功原理为理论基础，采用Ｍｉｎｄｌｉｎ退化板壳单元，开发板料冲压成形起皱过程的有限元数值软件ＷＲＳＩＭ，模拟了３种典型悬空区起皱问题，所得结果与实验观察基本一致，该软件可与其他成形过程数值模拟软件一起用于模具ＣＡＥ，以使在模具设计冲压件件的可成形性。     

晶体塑性模型在板材成形计算机模拟中的应用

阐述晶体塑性理论的基本原理，包括单晶体塑性本构关系，多晶体塑性模型的构造，介绍它在金属成形模拟中的应用概况。在速率相关的晶性模型的引入滑移系活动与否的判断准则，忽略不活动滑移系的贡献，从而在保持计算精度的时间时，大大减少了本构关系的计算时间。采用动力显式有限元方法和多晶体塑性模型模拟铝板的ＬＤＨ试验过程，考察板材的织构对其成形性能的影响。     

增材制造工艺匹配性评估的宏微观决策模型

增材制造存在悬空区域难制造、不同结构和不同性能要求零件的可成形性难预测等突出问题,提前评估工艺的匹配性和合理性具有重要意义。提出了一种宏微观一体化综合决策模型,从增材制造工艺角度评估零件的可成形性,根据打印零件的效果推演出最佳的成形策略。宏观决策阶段建立设计需求与制造工艺间的数学模型,从可成形性角度评判工艺的匹配程度;微观决策阶段建立表面粗糙度、支撑结构体积和打印时间的数学模型,从打印质量的角度优化打印工艺。最后,以拓扑优化的梁结构零件为例,利用建立的综合决策模型优化其打印工艺,结果表明,实际打印结果与模拟打印结果一致,验证了数学模型的有效性。     

Study on design of progressive dies for manufacture of automobile structural member using DP980 advanced high strength steel

Advanced high-strength steel (AHSS) is widely used in automobile manufacturing to reduce the weight of vehicles, thereby improving fuel efficiency. However, the high yield and tensile strength of AHSS leads to a serious springback problem in the cold sheet metal forming process. This phenomenon has delayed the implementation of AHSS in vehicle parts due to the resulting negative impact on part accuracy. In this study, parameter optimization and multi-stage die compensation were conducted with Finite element (FE) analysis to develop a progressive forming process for automobile structural members using DP980. The FE simulation used the Yoshida-Uemori model to predict the springback phenomenon accurately. The key parameters that significantly influence the springback behavior were optimized using FE simulation and the Taguchi method. The simulation results were used to determine the die and mold compensation. After the parameter optimization and multi-stage die compensation, the final part was obtained with acceptable dimensional accuracy.

     

Optimum blank design in sheet metal forming by the deformation path iteration method

Optimum blank design methods have been introduced by many researchers to reduce development cost and time in the sheet metal-forming process. Direct inverse design method such as Ideal Forming (Chang and Richmond, Int J Mech Sci 1992; 34(7) and (8): 575–91 and 617–33) [7, 8] for optimum blank shape could play an important role to give a basic idea to designer at the initial die design stage of the sheet metal-forming process. However, it is difficult to predict an exact optimum blank without fracture and wrinkling using only the design code because of the insufficient accuracy. Therefore, the combination of a design code and an analysis code enables the accurate blank design. In this paper, a new blank design method has been suggested as an effective tool combining the ideal forming theory with a deformation path iteration method based on FE analysis. The method consists of two stages: the initial blank design stage and the optimization stage of blank design. The first stage generated a trial blank from the ideal forming theory. Then, an optimum blank of the target shape is obtained with the aid of the deformation path iteration method which has been newly proposed to minimize the shape errors at the optimization stage. In order to verify the proposed method, a square cup example was investigated.     

铝合金阶梯形件粘性介质压力成形的研究

通过压缩试验得到了粘性介质的应力－－应变关系、压力衰减随冲头加载速率及时间变化的规律。采用有限元软件DEFORM分析了在不同压边力下分别用钢凸模和粘性介质成形铝合金阶梯形件的工艺过程。结果显示，采用VPF可以提高板料的成形性。根据模拟结果，采用VPF成功地制备了阶梯形件，这表明有限元法对VPF试验具有很好的指导意义。     

An approach to eliminate stepped features in multistage incremental sheet forming process: Experimental and FEA analysis

Incremental sheet forming (ISF) is a recently developed manufacturing technique. In ISF, forming is done by applying deformation force through the motion of Numerically controlled (NC) single point forming tool on the clamped sheet metal blank. Single Point Incremental sheet forming (SPISF) is also known as a die-less forming process because no die is required to fabricate any component by using this process. Now a day it is widely accepted for rapid manufacturing of sheet metal components. The formability of SPISF process improves by adding some intermediate stages into it, which is known as Multi-stage SPISF (MSPISF) process. However during forming in MSPISF process because of intermediate stages stepped features are generated. This paper investigates the generation of stepped features with simulation and experimental results. An effective MSPISF strategy is proposed to remove or eliminate this generated undesirable stepped features.     

The formability of annealed and pre-aged AA-2024 sheets in single-point incremental forming

The formability of AA-2024 sheets, an aerospace grade material, in the annealed and pre-aged conditions has been investigated in the single-point incremental forming (SPIF) process. The major operating parameters, namely step size, tool radius, and forming speed, of SPIF process were varied over wide ranges, and their effect on the formability was quantified through a response surface method called as central composite rotational design. It was found that the interaction of step size and tool radius is very significant on the formability. Moreover, a variation in the forming speed does not affect the formability of annealed AA-2024 sheet. However, the formability of pre-aged AA-2024 sheet decreases with the increase in the forming speed. Furthermore, the annealed sheet shows higher formability than the pre-aged sheet.     

基于动态序列响应面方法的钣金成形过程参数优化

为得到钣金成形最佳工艺方案，结合有限元方法将设计转化为特定目标和约束的待优化问题；针对成形模拟易受干扰和强非线性特点，提出采用逐次逼近模型，分解复杂设计函数为显式简单函数组合，进行递进全局寻优，避免出现局部最优解和过程发散现象；并利用动态多项式序列响应面方法构造目标和约束的简单近似响应面，用以去除噪声干扰及最大程度减少精确分析计算量。该方法无需求解复杂函数敏度，经实例验证具有很高的精度和效能。     

板材冲压成形的晶体塑性有限元模拟

将率相关晶体塑性本构理论引入Mindlin曲壳单元模型与动力显式有限元法,采用切线系数法计算剪切应变率,根据晶体取向正态分布规律在单元积分点处分配晶体取向,按各晶体取向体积分数的加权平均计算多晶体应力,开发晶体塑性动力显式有限元程序,实现板材冲压成形过程模拟和晶体取向演化预示.以主要初始织构为铜织构和S织构的轧制铝板为对象,对方盒件冲压成形过程及织构演化进行数值模拟,计算结果与试验结果呈现出较好的一致性.通过晶体弹塑性有限元法不仅可以预示板材宏观成形构形变化,而且能够预测板材织构的演化情况.模拟结果显示在方盒件冲压成形过程中,铜织构和S织构为不稳定取向,变形后逐渐转到其他取向.     

分布式钣金CAD/CAM应用系统研究

为在网络环境下解决钣金展开设计、优化排样和数控下料等问题，研究并开发了分布式钣金CAD／CAM应用系统。系统以浏览器／服务器（B／S）模式实现展开计算和绘图，以客户／月艮务器（C／S）模式实现钣金件优化排样和处理数控代码；为实现系统分布式服务，应用了基于．NET框架的Web服务和代理技术；通过分布式产品数据共享，采用遗传算法或其他启发式算法，将展开设计结果进行优化排样；同时，通过数控工艺规划、前后置参数设置和数控代码仿真，获得数控加工代码。最后，以实例说明了钣金设计的网络化应用。