Formability limits by fracture in sheet metal forming

The aim of this paper is twofold: first, to revisit the forming limit diagram (FLD) in the light of fundamental concepts of plasticity, damage and ductile fracture mechanics and, second, to propose a new experimental methodology to determine the formability limits by fracture in sheet metal forming. The first objective makes use of the theory of plasticity applied to proportional strain loading paths, under plane stress conditions, to analyze the fracture forming limit line (FFL) and to introduce the shear fracture forming limit line (SFFL). The second objective makes use of single point incremental forming (SPIF), torsion and plane shear tests to determine the experimental values of the in-plane strains at the onset of fracture. Results show that the proposed methodology provides an easy and efficient procedure to characterize the formability limits by fracture in sheet metal forming. In particular, the paper shows that the FFL determined by means of tensile and conventional sheet formability tests is identical to that determined from SPIF tests on conical and pyramidal truncated specimens. The new proposed approach is expected to have impact in the established methodologies to outline the formability limits on the basis of the forming limit curves (FLC's) at the onset of necking.     

Prediction of shear-induced fracture in sheet metal forming

Necking has been the dominant failure mode in sheet metal forming industry and several analytical and numerical tools were developed to predict the onset of necking. However, the introduction of Advanced High Strength Steels (AHSS) with reduced ductility brought up an issue of a shear fracture which could not be predicted using the concept of Forming Limit Curve (FLC). The Modified Mohr-Coulomb fracture criterion (MMC) was recently shown to be applicable to problems involving ductile fracture of materials and sheets. In the limiting case of plane stress, the fracture locus consists of four branches when represented on the plane of the equivalent strain to fracture and the stress triaxiality. A transformation of above 2D fracture locus to the space of principal strains was performed which revealed the existence of two new branches not extensively studied before. The existence of those branches explains the formation of shear-induced fracture. As an illustration of this new approach, initiation and propagation of cracks is predicted and compared with series of deep-drawing punch tests of ThyssenKrupp AHSS (grade RA-K 40/70, standard HCT690T) performed at ThyssenKrupp. It was shown that the location of fracture as well as the magnitude of punch travel corresponding to first fracture was correctly predicted by MMC fracture criterion for both circular and square punch.     

板材单点渐进成形破裂模拟优化与实验验证

针对渐进成形工艺加工半锥角较小制件侧壁厚度减薄及破裂问题,基于ABAQUS建立渐进成形有限元分析模型,并进行数值模拟与结果分析。对于板材破裂及厚度分布不均现象提出优化方案,并完成数值模拟与实验验证,结果显示,实验加工与数值模拟结果基本一致。最后总结出渐进成形中板材发生破裂的原因、本质及解决方法。     

Fundamental studies on the incremental sheet metal forming technique

The idea of incremental forming technique has been investigated for production of sheet metal components. With this technique, the forming limit curve (FLC) appears in a different pattern, revealing an enhanced formability, compared to conventional forming techniques. In the present study, the formability of an aluminum sheet under various forming conditions was assessed and difficult-to-form shapes were produced with the technique. By utilizing knowledge and experience obtained during the present study, it became possible to produce some free surfaces.     

Influence of mechanical properties of the sheet material on formability in single point incremental forming

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them single point incremental forming operations, in which the final shape of the component is obtained by the relative movement of a simple and small punch with respect to the blank, appear quite promising.In the paper, material formability issues in incremental forming were studied. Some relevant correlations among material formability and other mechanical properties of the material were analysed. The FLD₀ value, i.e. the major strain at fracture in plane strain conditions, was determined for different materials and the influence of the main material parameters on formability was accurately investigated through a statistical analysis.     

Investigation of material deformation mechanism in double side incremental sheet forming

Double side incremental forming (DSIF) is an emerging technology in incremental sheet forming (ISF) in recent years. By employing two forming tools at each side of the sheet, the DSIF process can provide additional process flexibility, comparing to the conventional single point incremental forming (SPIF) process, therefore to produce complex geometries without the need of using a backing plate or supporting die. Although this process has been proposed for years, there is only limited research on this process and there are still many unanswered open questions about this process. Using a newly developed ISF machine, the DSIF process is investigated in this work. Focusing on the fundamental aspects of material deformation and fracture mechanism, this paper aims to improve the understanding of the DSIF process. Two key process parameters considered in this study include the supporting force and relative position between master and slave tools. The material deformation, the final thickness distribution as well as the formability under varying conditions of these two process variables are investigated. To obtain a better understanding from the experimental results, an analytical model has been developed to evaluate the stress state in the deformation zone. Using the developed model, an explicit relationship between the stress state and key process parameters can be established and a drop of stress triaxiality can be observed in the double contact zone, which explains the enhanced formability in the DSIF process. Based on the analytical and experimental investigation, the advancements and challenges of the DSIF process are discussed with a few conclusions drawn for future research.     

A new method of determining forming limit diagram for sheet materials by gas blow forming

A new methodology is proposed to obtain forming limit diagrams (FLDs) of sheet materials using gas blow forming process at elevated temperatures. Tension–tension side of the forming FLD is achieved by using circular as well as elliptical dies of different aspect ratios. To achieve tension–compression side of FLD un-bonded bi-layer specimen with slots are utilized. The widths between the slots are varied to achieve different strain paths. A correlation is established between the hemispherical punch-based tests and GBF tests of samples with slots to achieve different strain paths. FLDs for automotive AZ31 magnesium sheet at 300 °C and 400 °C in two different orientations are determined. Increase in forming limits of AZ31 with increase in temperature is observed.     

薄板无模分层成形技术

本文介绍了薄板无模分层成形技术的基本原理 ,优点以及工具运动轨迹的生成     

Fundamental investigations on the material flow at combined sheet and bulk metal forming processes

The class of combined sheet and bulk metal forming (SBMF) processes denotes the forming of sheet metals with typical bulk forming operations. As a consequence, simultaneous 2D and 3D stress and strain states are present. Furthermore the local shape elements formed out of the sheet plane are in the magnitude of the sheet thickness. This paper deals with process combinations upsetting/lateral extrusion and deep drawing/upsetting. It has been shown that an insufficient mould filling is nearly inevitable using conventional tooling systems. Hence, the prospect of newly developed forming approaches will be discussed.     

铝合金板温成形过程摩擦在线检测系统的研究

介绍并分析了对板料成形过程摩擦问题研究的发展与现状。设计出一种用于动态测量板料成形真实过程摩擦系数的探针传感器,并在此基础上开发出铝合金板料温成形过程摩擦在线检测系统。利用该系统进行了铝镁合金板(5182)筒形件温成形过程摩擦在线检测,得到了合理的实验结果。     

In-situ springback compensation in incremental sheet forming

Limited geometric accuracy in incremental sheet forming is a major obstacle for its wide adoption in industry. This paper develops a generic methodology, suitable for arbitrary part geometries and various ISF processes, for addressing one of the main causes of geometric inaccuracy, i.e., in-process springback. The methodology consists of three main elements: determination of key control points to treat geometric complexity, simplified simulation models to predict springback offline, and in-situ toolpath modification during forming. It is shown experimentally that the method provides an efficient and robust solution for various geometries with negligible setup cost.     

Experimental and numerical study of an AlMgSc sheet formed by an incremental process

A recently developed AlMgSc alloy is studied since this material, which is well adapted to the aeronautic domain, is poorly known.The first objective is to reach a better knowledge of this alloy to provide the missing useful information to the aeronautic industry and to help research institutes who want to simulate sheet forming processes by Finite Element (FE) simulations. A set of experimental tests has been performed on the as-received sheets, material laws have been chosen and the corresponding material parameters have been adjusted to correctly describe the material behaviour.The second objective is to study the applicability of the Single Point Incremental Forming process (SPIF) on this material. Truncated cones with different geometries were formed and the maximum forming angle was determined. A numerical model was developed and proved to be able to predict both the force evolution during the process and the final geometrical shape. Moreover, the model helps reaching a better understanding of the process.The characterisation method described in this research and applied on the AlMgSc alloy can be extended to other alloys. In addition, the numerical simplified model, able to accurately describe the SPIF process with a reduced computation time, can be used to study more complex geometries.     

板料单点渐进成形数值模拟研究

板料单点渐进成形过程变形复杂、影响因素多,工艺参数选择是提高成形质量和成形效率的关键。对单点渐进成形过程进行数值模拟,分析了成形件的应力分布和厚度变化以及成形路径、进给量等工艺参数对成形过程的影响。结果表明:成形件最大应力和最大厚度减薄发生在底面附近;采用螺旋进给方式可有效提高成形质量和成形效率。实验结果显示,实验结果与数值模拟结果基本吻合。     

板料增量成形的研究进展

板料增量成形是采用简单模具对板料进行逐次塑性加工的一种工艺,不需要专用的模具就可以成形较为复杂的零件,同时还具有成形力小、柔性高的特点,特别适合多品种小批量零件的生产方式,因此得到国内外学者的重视。本文重点从板料的增量压弯成形、增量拉深胀形、增量微成形3个方面对板料增量成形的发展进行综述,还对板料增量成形工艺的发展前景进行了展望,指出进行理论创新、开发新的模拟软件、探索新的成形方案、开发增量成形新设备是发展趋势。     

板料点压渐进成形的成形性能研究

板料渐进成形技术作为柔性制造技术,其研究已经取得相当进展。但是在进给轨迹连续的普通渐进成形中,材料在成形过程容易产生沿轨迹方向的推挤,而产生不必要的变形,影响加工能力和加工质量。通过圆弧沟槽实验,对比点压渐进成形和连续渐进成形,发现点压渐进成形可获得的沟槽长度为60.5 mm,大于连续渐进成形中48.9 mm的沟槽长度;破裂位置最大主应变分别为1.212和0.982,说明板料点压渐进成形时的成形性能比连续渐进成形好。通过改变点压渐进成形中正弦波的波长和振幅,说明两参数对成形性能的影响。采用成形性能更好的点压渐进成形,可减少多道次渐进成形的成形道次,拓展渐进成形所能成形的零件结构类型。     

工艺参数对板料成形性能的影响

传统上，人们对于工艺参数对板料成形性能的影响的认识只是定性的。利用板料成形的计算机数值模拟技术，可以定量地考察各种工艺参数对板料成形性能的影响。     

基于机器视觉的机床自寻位在无模渐进成形设备中的实现

在板料的无模渐进成形过程中,使用导正杆定位底支撑模型,由于配合误差和模型浮动等问题造成定位效率低、精度差.基于机器视觉的机床自寻位系统利用图像采集系统对定位孔进行图像捕捉与处理,获得底支撑模型的坐标系,然后在数控系统中对机床坐标系进行离线变换,实现成形轨迹位置到底支撑模型的自适应,完成了金属板料无模渐进成形中底支撑模型的非接触式高精度快速定位.     

板材液压成形实验研究

分析了液压胀形变形特点及成形机理,对板材液压成形进行了实验研究,分析了坯料厚度变化规律,确定了提高材料胀形成形极限的具体措施。     

Springback prediction for incremental sheet forming based on FEM-PSONN technology

In the incremental sheet forming (ISF) process, springback is a very important factor that affects the quality of parts. Predicting and controlling springback accurately is essential for the design of the toolpath for ISF. A three-dimensional elasto-plastic finite element model (FEM) was developed to simulate the process and the simulated results were compared with those from the experiment. The springback angle was found to be in accordance with the experimental result, proving the FEM to be effective. A coupled artificial neural networks (ANN) and finite element method technique was developed to simulate and predict springback responses to changes in the processing parameters. A particle swarm optimization (PSO) algorithm was used to optimize the weights and thresholds of the neural network model. The neural network was trained using available FEM simulation data. The results showed that a more accurate prediction of springback can be acquired using the FEM-PSONN model.     

TRIP590板料渐进成形失效研究11,2111

通过胀形试验和以沟槽法为基础获取普通板料渐进成形极限图的方法,分别获取TRIP590板料的传统成形极限图和渐进成形极限图。为了直观地比较TRIP590板料在不同成形方式中具有的成形性能以及验证获取的渐进成形极限图是否满足于TRIP590板料成形,设置了变角度圆锥台和变角度方盒进行验证。两组试验证明:TRIP590板料在渐进成形中的成形性能远强于传统成形,并且获取的线性方程为ε2=0.525 4ε1+0.989 1的渐进成形极限图也适用于TRIP590板料的成形。