Development of tooling concepts to increase geometrical accuracy in high speed incremental hole flanging

Incremental Sheet Forming (ISF) has been developed as a flexible manufacturing technology for small batch production and prototyping. ISF can also be used to form additional features or stiffening elements such as hole flanges. Incremental Hole Flanging (IHF) operations seem to be a promising alternative to conventional hole flanging. If it was possible to exploit the extended formability of ISF while achieving accuracy and process times of conventional hole flanging, IHF could substitute conventional flanging operations in many cases. However, the long process times and limited geometrical accuracy hinder industrial take-up. In this work, two different tooling concepts which allow incremental hole flanging operations at high speeds are investigated. The first tool is designed as a single forming tool that offers high flexibility and a comparison to conventional Incremental Hole Flanging. The second tool consists of four forming tools to improve the geometrical accuracy of hole flanges. In order to achieve high speeds, the experimental setup is installed on a turning machine. Compared to hole flanging with a conventional CNC machine, the forming time to expand a hole from 50 mm to 100 mm could be reduced from 1680 s to 15.7 s. The geometrical accuracy of the parts formed with the second tool concept could be improved significantly (up to 3 times regarding to the mean surface deviation to at maximum speed). Furthermore, it is shown that forming at high speeds has no significant influence on the characteristics of sheet thickness, strain, forces or geometrical accuracy.     

Fast simulation of incremental sheet metal forming by adaptive remeshing and subcycling

In order to use incremental sheet forming (ISF) in an industrial context, it is necessary to provide fast and accurate simulation methods for virtual process design. Without reliable process simulations, first-time right production seams infeasible and the process loses its advantage of offering a short lead time. Previous work indicates that implicit finite element (FE) methods are at present not efficient enough to allow for the simulation of AISF for industrially relevant parts, mostly due to the fact that the moving contact requires a very small time step. Finite element methods based on explicit time integration can be sped up using mass or time scaling to enable the simulation of large-scale sheet metal forming problems. However, AISF still requires dedicated adaptive meshing methods to further reduce the calculation times. In this paper, an adaptive remeshing strategy based on a multi-mesh method is developed and applied to the simulation of AISF. It is combined with subcycling to further reduce the calculation times. For the forming of a cone shape, it is shown that savings in CPU time of up to 80 % are possible with acceptable loss of accuracy, and that the simulation time scales more moderately when the part size is increased, so that larger, industrially relevant parts become feasible.     

板料渐进成形数值模拟与实验研究

为提高渐进成形的成形效率和成形质量,了解板料渐进成形的变形规律及工艺参数对成形的影响,采用有限元方法对板料渐进成形过程进行了数值模拟研究,分析了斜壁盒形件渐进成形过程应力分布和厚度变化趋势,通过对不同进给量和不同成形路径进行数值模拟,分析了工艺参数对成形的影响.结果表明,斜壁盒形件最大应力和最大厚度减薄发生在底面拐角处;成形过程中工具头运动轨迹应尽量采用走螺旋线的方式,可以提高成形件的成形能力和成形质量.渐进成形实验表明,数值模拟结果与实验结果基本吻合.     

大幅面板材渐进折弯成形的本构建模及模拟

为了提高大幅面板材成形的模拟精度,在板材折弯平面应变假设条件下,推导出基于Hill各向异性屈服准则的弹塑性本构方程.借助ABAQUS有限元软件本构模块用户子程序接口,通过编程将上述推导的应力-应变本构关系显示表达式嵌入ABAQUS分析平台.以超长大开口半椭圆形工件成形为例,建立了大幅面钢板渐进折弯的三维弹塑性有限元模型,并数值模拟了多道次渐进折弯成形及回弹全过程.模拟效果和工程应用结果表明,与传统的基于平面应力假设的本构关系模型相比,采用平面应变假设的本构关系模型的模拟结果更接近实验值.     

Capability of iterative learning control and influence of the material properties on the improvement of the geometrical accuracy in incremental sheet forming process

Incremental Sheet Forming (ISF) is a flexible technology that allows the deformation of blank sheets without the need of complex and high cost tools or equipments. One of the main lacks of ISF is the geometrical accuracy which is not comparable with the one achieved by using traditional sheet stamping processes. There are several approaches proposed to enhance this aspect and among them the Authors have developed a method based on an Iterative Learning Control (ILC). ILC consists of a cyclic and progressive error compensation method that improves the quality of the manufactured parts. ILC has been proved to be capable for optimising the production of parts with tight tolerances when dealing with ductile materials (aluminium and steel alloys) and small deformations. In this paper, the method was tested to investigate its capability in a virtual environment. The suggested compensations were checked with Finite Element Method (FEM) so to reduce the number of parts to be manufactured saving time and costs. Moreover, the algorithm was tested considering both a difficult to form material (titanium alloy) and high deformation conditions. The results demonstrated how the precision of ISF processes depends on the blank material properties. Moreover, the capabilities of ILC are shown and discussed.     

Efficient implicit simulation of incremental sheet forming

In single point incremental forming (SPIF), the sheet is incrementally deformed by a small spherical tool following a lengthy tool path. The simulation by the finite element method of SPIF requires extremely long computing times that limit the application to simple academic cases. The main challenge is to perform thousands of load increments modelling the lengthy tool path with elements that are small enough to model the small contact area. Because of the localised deformation in the process, a strong nonlinearity is observed in the vicinity of the tool. The rest of the sheet experiences an elastic deformation that introduces only a weak nonlinearity because of the change of shape. The standard use of the implicit time integration scheme is inefficient because it applies an iterative update (Newton–Raphson) strategy for the entire system of equations. The iterative update is recommended for the strong nonlinearity that is active in a small domain but is not required for the large part with only weak nonlinearities. It is proposed in this paper to split the finite element mesh into two domains. The first domain models the plastically deforming zone that experiences the strong nonlinearity. It applies a full nonlinear update for the internal force vector and the stiffness matrix every iteration. The second domain models the large elastically deforming zone of the sheet. It applies a pseudolinear update strategy based on a linearization at the beginning of each increment. Within the increment, it reuses the stiffness matrix and linearly updates the internal force vector. The partly linearized update strategy is cheaper than the full nonlinear update strategy, resulting in a reduction of the overall computing. Furthermore, in this paper, adaptive refinement is combined with the two domain method. It results in accelerating the standard SPIF implicit simulation of 3200 shell elements by a factor of 3.6. Copyright © 2011 John Wiley & Sons, Ltd.     

基于金属板材渐进折弯成形的闭环控制系统性能分析

为了解决板材成形回弹问题,将自动控制理论中反馈控制的思想引入到板材渐进折弯成形过程.利用先进的控制技术来解决板材工件的精确成形.然而,闭环控制的稳定性、准确性和快速性会直接影响到系统的正常工作.针对闭环系统稳定性对工件成形品质的影响,建立了渐进折弯成形闭环控制系统模型.通过理论推导,利用小线性化处理及模型简化方法求解闭环系统传递函数及特征方程.采用离散根轨迹法并进行了Matlab/Simulink仿真试验,其结果表明,系统总增益等于一时能确保成形工件形状稳定、快速、准确收敛到目标形状.     

Thermal modeling in electricity assisted incremental sheet forming

A thermal model was built to account for the effects of geometrical parameters of sheet specimen, process parameters and material parameters on the temperature increase of the sheet specimen in Electricity-Assisted Incremental Sheet Forming (EISF). In the EISF, the local area of sheet specimen contacting with a forming tool is heated by direct current, which flows through the forming tool to the sheet specimen. EISF experiments of two high strength steel sheets were carried out to validate the thermal model. The thermal model can be integrated into the control program of EISF system to achieve more accurate temperature control.     

Titanium based cranial reconstruction using incremental sheet forming

In this paper, we report recent work in cranial plate manufacturing using incremental sheet forming (ISF) process. With a typical cranial shape, the ISF process was used to manufacture the titanium cranial shape by using different ISF tooling solutions with and without backing plates. Detailed evaluation of the ISF process including material deformation and thinning, geometric accuracy and surface finish was conducted by using a combination of experimental testing and Finite Element (FE) simulation. The results show that satisfactory cranial shape can be achieved with sufficient accuracy and surface finish by using a feature based tool path generation method and new ISF tooling design. The results also demonstrate that the ISF based cranial reconstruction has the potential to achieve considerable lead time reduction as compared to conventional methods for cranial plate manufacturing. This outcome indicates that there is a potential for the ISF process to achieve technological advances and economic benefits as well as improvement to quality of life.     

Experimental investigations and optimization of forming force in incremental sheet forming

Incremental sheet forming process has been proved to be quiet suitable and economical for job and batch type production, which exempts expensive and complex tooling for sheet forming. Investigation of forming forces becomes important for selecting the appropriate hardware and optimal process parameters in order to assure perfection and precision of process. Moreover, lack of available knowledge regarding the process parameters makes the process limited for industrial applications. This research paper aims at finding out effects of different input factors on forming forces in single-point incremental forming (SPIF) process. For operation sustainability and hardware safety, it becomes critical to optimize forming forces for a given set of factors to form a particular shape. In this study, optimization of input factors has been performed to produce conical frustums with helical tool path using Taguchi analysis as design of experiment (DOE) and analysis of variance (ANOVA). The optimal experimental conditions for forming forces have been calculated as sheet thickness (0.8 mm), step size (0.2 mm), tool diameter (7.52 mm), tool shape (hemispherical), spindle speed (1000 rpm), feed rate (1000 mm/min) and wall angle (50^o). Effects of tool shape and viscosity of lubricants have also been investigated. An intensive understanding of the mechanism of forming forces has been presented, which shows that force trend after peak values depends upon instant input factors that can be categorized as a safe, severe and crucial set of parameters.     

Shape accuracy analysis of multi-point forming process for sheet metal under normal full constrained conditions

In order to study the shape accuracy of multi-point forming (MPF) process for sheet metal under normal full constrained conditions, the in-depth analysis of shape accuracy of workpieces in multi-point forming with individually controlled force-displacement (MPF-ICFD) process is conducted in this paper by combining experiment, theoretical analysis and numerical simulation. The influences of normal force, material thickness and material properties on the shape accuracy of the feature surface are studied, and the shape accuracy characteristics of the sheet under different parameters are obtained. Afterwards, the stress and strain characteristics of sheets are obtained by numerical simulation. Finally, the effect of normal force on shape accuracy was revealed by establishing a mechanical model of the sheet metal under normal full constrained conditions. Moreover, the amount of springback reduction in MPF-ICFD is defined quantitatively. Compared with the normal unconstrained conditions, the shape accuracy of sheet metal is improved significantly under normal full constrained conditions. The stress and plastic deformation are more uniform and the amount of springback is smaller. For Q 295 steel plate with thickness of 2.0 mm, the difference between the maximum value and the minimum value of the reaction force of punch decreases from 4515.9 N to 1475 N when the forming force is 2500 N. Besides, the bending moment of the sheet on the unit width decreases from 357.9 N???mm to 328.1 N???mm. The average shape error E _rr and the amount of springback Δk decreases by 60.05% and 16.03%, respectively.     

汽车板精益成形技术

建立了面向零件成形特征的合理选材方法,发明了高效的拉延筋阻力混合优化设计方法,构建了变压边力控制实验平台。从降低成形质量对材料和工艺过程波动敏感性的角度出发,研究面向材料和工艺参数随机波动的成形质量的稳健控制。形成了基于“合理选材、工艺优化、稳健设计”思想的汽车板精益成形技术体系。通过在宝钢股份和多家汽车厂10多年的成功应用,支持宝钢汽车板的市场占有率稳步达到50％,有效地推动了国产汽车板使用技术的发展。     

V型伸长翻边解析与数值研究

为了预示曲面翻边成形性能,采用有限元仿真、解析计算与人工神经网络的方法对V型零件翻边成形进行了分析.通过建立有限元模型研究了工艺参数对成形性能的影响;基于全量塑性理论及膜应变假定,推导了轴对称情况的解析计算模型;以数值模拟结果作为训练样本,建立了V型翻边成形性能预测的BP神经网络模型.研究结果表明:工艺条件对翻边成形有较大影响,其中以张角的影响最为显著;解析模型计算简便,但是只适用于零件张角较小以及相对翻边高度较小的情况;有限元仿真与人工智能相结合的BP人工神经网络模型可以快速有效地预测翻边成形性.     

Tolerance transfer in sheet metal forming

A literature review of sheet metal forming errors as well as geometrical dimensions and tolerances (GD&T) shows that the theoretical means for the allocation of process tolerances with respect to GD&T are insufficient. In order to judge the influence of geometrical process errors (e.g., angular errors of bends), two typical sheet metal designs with parallelism and a position tolerance are studied. These case studies comprise a detailed analysis of tolerance chains including angular errors of bends and their positions. The resulting errors are compared with those resulting from length dimensional process errors and conclusions are drawn.     

POD surrogates for real-time multi-parametric sheet metal forming problems

Our approach aims at coupling the ever increasing off-line computing power of mainframe computers with the interactive on-line possibilities of ubiquitous low computing power devices at the early design stages in order to provide insight into the design problems and to search for candidate optimal design points. In the off-line phase, the method under investigation relies on combining an optimized space-filling sampling plan on the design parameter space with extensive finite elements (FE) simulations yielding a learning set of displacement fields. The objective of this paper is the on-line phase. We provide a rigorous mathematical presentation of a family of non-intrusive, bi-level surrogates. We focus on displacement field approximation by Proper Orthogonal Decomposition (POD) combined with kriging interpolation of coefficients. The method is illustrated with two simple, easily reproduced numerical examples of quality assessment of deep-drawing process of a cylindrical cup by on-the-fly plotting forming limit diagrams (FLDs) and related quantities enabling thus to spot improved design points.     

渐进成形A3003铝板减薄带分析及数值模拟研究

研究渐进成形过程中板料减薄带的变化,可以提供合理的加工参数,提高板料的成形性能和加工利用率,减少零件破裂失效.基于渐进成形过程中金属板料轮廓的变化与理想情况下轮廓的区别,对渐进成形初始成形阶段A3003铝板减薄带的产生原因和剪切力的变化过程进行了理论分析,并通过有限元模拟分别从未变形区金属板料的长度和强度两个角度对板料渐进成形过程中未变形区下沉的影响,以及成形角度和杨氏模量对变形区回弹的影响两个方面,对减薄带的产生原因进行研究.结果表明:板料未变形区的下沉和变形区的回弹使板料在初始加工阶段形成一段平缓区域,工具头在平缓区域的变形性质发生了变化,平缓区域发生剪切变形导致了板料在初始加工阶段形成了减薄带;渐进成形时减小板料未变形区的长度,增大板料与垂直方向的角度可以一定程度上阻碍减薄带的产生,模拟结果与理论分析相符合.     

Negative and positive incremental forming: Comparison by geometrical,experimental, and FEM considerations

This study compares negative incremental forming (NIF) and positive incremental forming (PIF) processes by geometrical considerations, finite element method (FEM) analyses, and experimental evaluations. Conical frusta were manufactured starting from AA5052H19 aluminum alloy sheets using both techniques. The processes were also simulated with LS-DYNA software and a close correlation between the experimental and numerical results was observed. The analysis of forming forces, forming limit diagrams (FLDs), and sheets thinning highlights that the PIF technique allows one to reach higher formability and geometrical accuracy. Finally, the differences in terms of surface quality were also discussed.