A geometrical model of the kinematics of incremental sheet forming for the prediction of membrane strains and sheet thickness

The sine law is a simple geometrical model for incremental sheet metal forming (ISF). It is based on the assumption that the deformation is a projection of the undeformed sheet onto the surface of the final part. The sine law provides approximations of sheet thinning for shear spinning and ISF at negligible computational cost, but as a plane strain model it can be applied only when plane strain deformation prevails.     

A review of spinning, shear forming and flow forming processes

In the last two decades or so, spinning and flow forming have gradually matured as metal forming processes for the production of engineering components in small to medium batch quantities. Combined spinning and flow forming techniques are being utilised increasingly due to the great flexibility provided for producing complicated parts nearer to net shape, enabling customers to optimise designs and reduce weight and cost, all of which are vital, especially in automotive industries.In this paper, process details of spinning and flow forming are introduced. The state of the art is described and developments in terms of research and industrial applications are reviewed. Also, the direction of research and development for future industrial applications are indicated.     

锥形件剪旋变形分析

根据剪旋过程的实际情况进行了应力应变分析，对剪旋变形机理提出了新的见解，将剪旋变形区分为拉弯区和辗压变形区。前者的受力情况类似于胀形，后者为母线方向拉应力作用下的辗压变形，该区起主要减薄壁厚的作用。此外，本文利用Z形图对剪旋的各种状态进行了应力应变分析，并确定出了剪旋时最佳应力应变状态的区域。     

Multi-pass deformation design for incremental sheet forming: Analytical modeling,finite element analysis and experimental validation

Incremental sheet forming (ISF) is a promising rapid prototyping technology with high potential to shape complex three-dimensional parts. However, a common technical problem encountered in ISF is the non-uniform thickness distribution of formed parts; particularly excessive thinning on severely sloped regions. This may lead to fracture and limit the process formability. Design of multi-stage deformation passes (intermediate shapes or preforms) before the final part, is a desirable and practical way to control the material flow in order to obtain a more uniform thickness distribution and avoid forming failure. In the present paper, a systematic methodology for designing multi-stage deformation passes considering the predicted thickness strains given the design shape is proposed based on the shear deformation and the strain compensation mechanism. In this methodology, two analytical models (M1 and M2) are developed by taking into account; the global average thickness strain and only the material in the final part region used in the forming (M1), and the local weighted average thickness strain and the additional material around the final part region used in the forming (M2), respectively. The feasibility of the proposed design methodology is validated by finite element analysis (FEA) and experimental tests using an Amino ISF machine. The results show that a more uniform thickness strain distribution can be derived using M2. The incurrence of the highest strains can be delayed in the intermediate stages and the flow of material is allowed into the deformed region, thereby allowing a compressive stress state to develop and enabling steeper shapes to be formed. Therefore, the process formability can be enhanced via the optimized design of deformation passes.     

A novel method to test the thinning limits of sheet metals in negative incremental forming

Single Point Incremental Forming (SPIF) is an emerging sheet-metal-forming technology, capable of manufacturing complex parts at low cost for small to medium-batch production. The present paper is focused on presenting an innovative and viable method to test the thinning limits of sheet metals in Negative Incremental Forming along with verification of the Cosine's law of thickness distribution. The Cosine's law was verified by comparing the experimentally measured thicknesses of incrementally formed parts with those predicted by the law. To test the thinning limit of a sheet metal, the idea is based on the forming of an axi-symmetric part varying its slope with depth corresponding to varying thinning. An arc of a circle was selected as generatrix to model such an axi-symmetric part. Based on the Cosine's law, mathematical expressions were derived to predict the thickness distribution along the depth of the part and the thinning limit of the sheet-metal. The Aluminum sheet metal was used as an experimental material. In order to test its thinning limit, the axi-symmetric part, modeled with a generatrix arc, was formed incrementally until it cracked. Thickness of the fractured part was measured at various points along its depth and compared with that predicted by the Cosine's law. The maximum thinning at a point, at which thickness followed the Cosine's law, was called the thinning limit of the sheet metal. In order to obtain accurate results, four such parts having the same generatrix design were formed. Based on these results, several axi-symmetric and asymmetric parts were formed at fixed slopes. It was found that the thinning limits obtained from the parts formed at fixed slopes were a little lower than those obtained from the parts modeled with the same generatrix design. As conclusion, a strategy to test the lowest possible thinning limits of sheet metals has been proposed. The proposed method is capable to test the thinning limits of sheet metals at reduced processing time and cost.     

Forecast of shear spinning force and surface roughness of spun cones by employing regression analysis

A series of shear spinning experiments has been performed to produce axi-symmetric cones from blank sheet. The experiments investigated influences of roller nose radius, mandrel revolution and roller feed on the spinning force and the inside/outside surface roughness of spun cone. Statistical analysis was adopted to construct the regression equations governing these parameters. Independent experiments verified the findings. It was discovered that the established regression equations possess a significant degree of reliability.     

A study of effects of roller path profiles on tool forces and part wall thickness variation in conventional metal spinning

Sheet metal spinning has seen significant developments in recent years. However, in the present industrial practice, the process design still highly relies on undocumented expertise; trial-and-error approach is commonly used to design the roller path and passes. In this paper, four different roller path profiles, i.e. combined concave and convex, convex, linear, and concave, have been designed and used to carry out spinning experiments and to conduct finite element (FE) analysis. The effects of these roller paths on tool forces, part wall thickness and stress variations in conventional metal spinning have been analysed numerically. The results show that the concave path produces highest tool forces among these four roller path profiles. Using the concave roller path tends to cause higher reductions of wall thickness of the spun part and using the convex roller path helps to maintain the original wall thickness unchanged. A greater curvature of the concave path would result in more thinning in wall thickness of the spun part.     

A new incremental in-plane bending of thin sheet metals for micro machine components by using a tiltable punch

This paper proposes an innovative incremental in-plane bending of thin metal sheets for manufacturing microscopic machine components. The unique feature of the process is that a tiltable punch having a beating face with trapezoidal profile is used. The beating face enables the punch to bend thin metal sheets in in-plane manner. Working conditions, including indentation and feeding pitch, can easily and flexibly control the bending radius and even the bending direction. The in-plane bent thin sheet products are expected to be used as springs, conical cylinders, bushes and other components of micro machines such as medical instruments.     

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.     

碟形件新型旋压成形工艺实验研究

介绍了基于离散点逐个调形原理的新型高效旋压成形工艺的原理,在研制出的旋压成形装置上进行了碟形件的成形实验,研究了成形辊加载方式对板材成形的影响,分析了蝶形件成形过程中起皱和减薄原因。结果表明,两种加载顺序蝶形件都有减薄现象出现,但是由中心向边缘的成形顺序的减薄率略低于由边缘向中心成形实验件的减薄率,着重分析了厚度为2 mm板材的旋压成形件减薄规律,为较厚板材蝶形件的加工奠定基础。     

大型复杂薄壁壳体多道次旋压过程中的壁厚变化

基于ABAQUS/Explicit和Standard建立的包含回弹与退火的大型复杂薄壁壳体多道次旋压全过程模拟模型,分析了该过程中壁厚的分布与变化及工艺参数对壁厚的影响规律。结果表明,壁厚减薄经历了剪切减薄和拉薄两个阶段,壁厚剧烈减薄部位位于旋轮后方的环带并向工件口部移动,而且其值逐渐减小;壁厚沿工件母线方向分布不均匀,沿周向分布较均匀;回弹对壁厚的分布影响不大。摩擦系数在一定范围内的增大,可以有效地抑制第一道次旋压过程中壁厚过度减薄的发生,使壁厚分布更均匀;而旋轮进给比对工件壁厚的影响与摩擦系数的作用相反。在后续道次旋压过程中,工件壁厚差随着摩擦系数的增大先减小后增大,随着旋轮进给比的增大逐渐减小。这些结果可为大型复杂薄壁壳体多道次旋压成形参数的确定和优化设计提供理论依据。     

Validation of a modified material model for use with shell elements to improve the predictive accuracy of the thickness distribution in superplastic forming of sheet metals

The automotive industry has recently begun using the superplastic forming (SPF) process to fabricate complex aluminum and magnesium alloy panels that cannot be formed at room temperature due to insufficient formability. One of the manufacturing problems encountered during SPF is excessive thinning in the form of a localized neck; which can lead to fracture. Localized necking can be predicted with the use of continuum elements in finite element analysis (FEA); however, the use of these elements in simulating SPF of large automotive panels is computationally intensive and often computationally prohibitive due to convergence issues. This paper examines the use of a modified material model (developed by engineers at Livermore Software Technology Corporation (LSTC) that can be used with conventional Belytschko-Tsay shell elements. This model considers normal stresses during SPF, which is needed to predict necking locations. The paper reports the results on investigating means for improving computational efficiency with this new formulation (i.e. element size, mass scaling, and adaptive meshing) and compares the performance of the normal stress element formulation with that of Belytschko-Tsay shell element in simulating the SPF process. The findings indicate that the newly developed formulation can be used for predicting localized thinning under SPF conditions.     

Fundamentals for controlling thickness and surface quality during dieless necking-in of tubes by spinning

Dieless necking-in by spinning is a highly flexible process to manufacture tubular parts with variable cross-sections and nearly arbitrary contours. However, the thickness distribution of such products is influenced primarily by the toolpath. Based on analytical models this study introduces the fundamentals to control the tube thickness. Two principal tool movements are identified causing different deformation modes: shear-necking, which leads to thickening, and stretch-necking, which leads to thinning. Based on an additional model for the surface quality general criteria are derived to setup basic process parameters. The developed approach is validated by various experiments.     

TA15钛合金热剪旋件的几何精度与显微组织分析(英文)

为了揭示偏离率和加热温度对TA15钛合金加热剪切旋压件的影响机制,研究了不同偏离率下热剪旋件的几何精度和微观组织。结果表明:在负偏离下工件会发生径缩,且温度的突然降低会使径缩更严重。在正偏离条件下且加热温度缓慢降低时,沿工件底部到口部有明显扩径。沿热剪旋工件厚向的不均匀变形使工件晶粒细化且近外层晶粒比近内层晶粒细小。在近外层可能形成纤维组织,该纤维组织的扭曲程度随着偏离率的减小而增加。近零偏离和保持加热温度在合理范围时有利于获得几何精度高和微观组织均匀的工件。     

多楔带轮旋压增厚成形阶段金属流动规律分析

钣制旋压带轮以其重量轻、生产效率高、节能、节材及动平衡性好等优点,在众多领域逐步取代以铸、锻等传统方法加工的带轮,得到广泛应用.本文深入分析了多楔带轮旋压成形特点,将其变形过程划分为增厚及旋齿两个变形阶段;基于增厚成形过程的重要性,采用有限元分析软件MSC,Marc对该过程预成形、腰鼓成形及增厚成形3个工步进行了数值模拟.结果表明,预成形时最大径向拉应变出现在缩径区,该区易产生成形缺陷;腰鼓成形以胀形为主要变形方式;增厚成形时径向应变以正应变为主、轴向及切向应变以负应变为主,体现为径向压缩、侧壁增厚的成形效果.在数值模拟结果的基础上,进行了工艺试验验证,结果吻合良好,验证了数值模拟分析的有效性.     

On-Line Control of Single Point Incremental Forming Operations through Punch Force Monitoring

Among the innovative sheet metal forming processes, Single Point Incremental Forming (SPIF) represents the simplest and the cheapest one. Despite its relevant advantages, up to now no specific CAE tools for SPIF were developed and the tool trajectory is generally defined utilizing CAD/CAM software developed for machining applications. In the paper an innovative monitoring and control approach, aimed to define and in-process update the most relevant process parameters during an industrial SPIF operation, is proposed. The strategy utilizes as monitoring variable the punch force trend: a set of preliminary tests demonstrated, in fact, its suitability as “spy variable” of the process mechanics and, in particular, of excessive sheet thinning and material failure approaching.     

Forming of tailor blanks having local thickening for control of wall thickness of stamped products

A two-stage forming process of tailor blanks having local thickening for controlling the distribution of wall thickness of stamped parts was developed. In the 1st stage, the target portion of the sheet for the local thickening is drawn into the die cavity, and then the bulging ring is compressed with the flat die under the clamping of the flange portion in the 2nd stage. The effects of the punch stroke, the clearance between the punch projection and die, and the projection width on the amount of the location thickening were examined. A 12% increase in thickness of the blank was obtained in an experiment. The tailor-formed blank having local thickening was applied to increase the wall thickness at the inner corner of a wheel disk. An 11% increase in wall thickness at the inner corner was successfully obtained in the experiment. The tailor blanks having local thickening obtained by the two-stage forming process are effective in controlling the distribution of wall thickness of the stamped parts.     

铝合金超塑性气胀成形壁厚分布工艺研究

应用MARC软件对铝合金的超塑性胀形进行仿真,分析正反向和正向超塑胀形对成形件壁厚分布以及不同变形量对胀形结果的影响,比较了2种不同胀形方式对成形件壁厚分布和成形极限的影响。结果表明,采用合理的正反胀形可以很好地改善成形件的厚度不均匀性并大大提高成形极限,实验验证了仿真和实验结果相吻合。     

超塑成形薄壁构件壁厚分布的预测

壁厚分布对薄壁构件的结构性能有重要影响。本文研究超塑成形件壁厚分布的预测技术,实现了超塑成形过程的有限元数值模拟的成形件厚度分布曲线的自动预测,以半球壳和矩形盒成形为例,为自由胀形和约束胀表两种情形形件厚度变化进行了分析,预测结果与实验数据吻合。     

喷涂角度对HVOF WC-Co-Cr涂层分布的影响

在喷涂复杂形状部件时,喷涂焰流和基体表面几乎不可能固定一个角度。研究喷涂角度对涂层性质的影响是必要的。本文研究凹面曲率半径对沉积率的影响。实验使用团簇烧结的WC-10Co-4Cr粉末（粒径15~45μm）。实验时喷涂参数不变,喷涂半径为10,15,25mm的凹面。研究表明,喷涂角度严重影响涂层沉积率。喷涂角度减小导致沉积率减少。当喷涂角度少于30°.,涂层性质显著降低。涂层沉积分布和不同凹面半径的关系被推导。