On the Formability of Magnesium Alloy Sheets in Warm Conditions
On the Formability of Magnesium Alloy Sheets in Warm Conditions
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镁合金板在温暖条件下的成形性能
DOI：
10.4028/www.scientific.net/kem.344.55
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发表时间：
2007
期刊：
影响因子：
--
通讯作者：
F. Gabrielli
中科院分区：
文献类型：
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作者：
G. Ambrogio;C. Bruni;L. Filice;F. Gabrielli
在冲压工业中，为了开发新材料和新制造技术的应用，对成形性知识的不断提高是一个不断满足的需求。在本研究中，为了研究材料的成形性和起皱行为，通过两种实验研究了镁合金AZ31B板材在温暖条件下的成形性：半球形冲床的面外试验和楔形试验。正如预期的那样，在所调查的范围内，执行的测试突出显示了更高温度下更大的过程窗口。然后将得到的结果引入有限元求解器，并将其作为具体案例研究的设计工具。本文对基本证据和结果进行了准确的讨论。可持续制造是现代制造业的新目标。在这种情况下，使用轻质材料，如镁合金，成为一个新的优先事项，因为它们允许，例如，在汽车领域节省相关的燃料成本和减少污染。特别是，当必须获得薄壁结构部件时，首选钣金成形而不是压铸成型，但是，由于此类合金在室温下的成形性较差，它们必须在更高的温度下成形。在由意大利大学和研究部资助的研究中，研究了AZ31B镁合金在200°C至300°C温度范围内的成形行为。通常，可能会出现两种不同的现象，导致成形极限，即颈缩和起皱。后一种现象发生在压边压力不够高的情况下。换句话说，可以定义一个可成形性窗口，其中包括一组允许流程成功的可能条件。此外，用适当的规则来描述这些区域的特征是非常有趣的，以便在有限元分析中将它们用作边界条件。通过这种方式，有限元分析可以构成过程设计和过程验证步骤的战略工具。为此，已开发并设置了一种实验设备，以执行两种不同的测试，以确定材料成形行为的极限。更详细地说，进行了楔形试验和面外试验；前者用于确定起皱极限曲线（WLC），而后者基于半球形冲头的使用，用于确定成形极限曲线（FLC）。在这两种情况下，测试设备都使用专用炉进行适当加热，并通过放置在空气中、工具中和试样上的不同热电偶来控制温度。通常，根据理论，当温度升高时，可以观察到成形性的提高。所有的试验都是在厚度为1mm的AZ31镁合金板上进行的。最后，利用实验证据为简单几何的有限元模拟定义适当的材料行为规则，以突出其作为设计工具的潜力。关键工程材料在线：2007-07-15 ISSN: 1662-9795，卷344，pp 55-62 doi:10.4028/www.scientific.net/KEM.344.55©2007 Trans Tech Publications Ltd， Switzerland版权所有。未经Trans Tech Publications Ltd， www.scientific.net的书面许可，不得以任何形式或任何方式复制或传播本文的部分内容。（语义学者网站：12/03/20,15:11:19）楔形试验如上所述，板料成形过程中出现的问题之一是起皱。这一问题的发生是因为在材料的某一点上发生的较大的压缩应变定额没有得到非相邻方向上的拉伸应变的充分补偿。在这种情况下，达到局部不稳定，材料呈现典型的波浪形状。尽管材料在这些条件下不会破裂，但皱折在任何地方都被认为是一种缺陷，因此，使用适当的压边机或管理过程以避免其出现在邮票图案之外，即在要修剪的单张部分。然而，有可能有一个适当的工具，能够很好地预测皱纹的发生，可以代表一个合适的解决方案的过程设计者。当然，必须提供有关皱纹发生的可靠数据，以便为进一步的应用建立一个足够强大的知识基础。在这里的研究中，已经完成了这项任务，设计了一种能够在温暖条件下进行楔形试验的适当设备[4,5]。该设备（图1）允许绘制梯形试样（图2），其特征是小基座b0=30mm，绘制角α=15°，不同的主基座尺寸。图1所示。为了方便地确定得到的应变分布，在薄片试样上应用了线距为3mm的规则线网格。压边架的设计是为了允许恒定的间隙约为板材厚度的15%。所有的测试都是在一个适当的熔炉中进行的，该熔炉能够容纳所有设备并控制内部温度。56钣金2007
In the stamping industry, the knowledge enhancing on formability is a continuous need to be satisfied, in order to develop the application of new materials and manufacturing technologies. In the present study, the formability of Magnesium-alloy AZ31B sheets in warm conditions was investigated by conducting two kinds of experiments: an out-of-plane test using a hemispherical punch and the Wedge test, in order to investigate both the material formability and wrinkle behaviour. As expected, the executed tests highlighted a larger process window for higher temperatures, in the investigated range. The obtained results were then introduced into a Finite Element solver and applied as design tool for a specific case study. Basic evidences and results are accurately discussed in the paper. Introduction The new goal of the modern manufacturing is represented by the sustainable manufacturing. In this context, the use of lightweight materials, such as Magnesium alloys, becomes a new priority since they allow, for instance, a relevant fuel cost saving and pollution reduction in automotive field. In particular, when thin walled structural components have to be obtained, the sheet metal forming is preferred to die casting, but, due to the poor formability at room temperature of such alloys, they have to be formed at higher ones [1]. In the study here addressed, funded by Italian Ministry of University and Research, the forming behaviour of the AZ31B Magnesium alloy, in the temperature range varying between 200°C and 300°C, has been investigated. As usual, two different phenomena, leading to the forming limits, may occur, namely necking and wrinkling. The latter phenomenon occurs when blank-holder pressure is not sufficiently high. In other words, it is possible to define a formability window, which includes the set of the possible conditions that allow the process success. In addition, it is very interesting to characterise these regions by proper rules in order to use them as boundary conditions in a Finite Element Analysis. By thus way, FEA may constitute a strategic tool for both the process design and the process verify steps. For this purpose, an experimental equipment has been developed and set-up to execute two different tests in order to define the limits of the material forming behaviour. More in detail, the wedge test and an out-of-plane test have been performed; the former has been used to define the wrinkling limit curves (WLC), whilst the latter, based on the use of a hemispherical punch, to define the forming limit curves (FLC). In both the cases, the testing equipment has been properly heated using a dedicated furnace and the temperature controlled by means of different thermocouples placed in air, in the tools and on the specimen. In general, when temperature increases a formability increasing may be observed, according to theory. All the tests have been carried out on AZ31 Magnesium alloy sheets, characterised by a thickness of 1mm. Finally, the experimental evidences have been utilised in order to define proper material behaviour rules for the FE simulation of a simple geometry, in order to highlight its potentiality as design tool. Key Engineering Materials Online: 2007-07-15 ISSN: 1662-9795, Vol. 344, pp 55-62 doi:10.4028/www.scientific.net/KEM.344.55 © 2007 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-12/03/20,15:11:19) Experimental campaign Wedge test As above described, one of the problems that occurs in sheet metal forming is wrinkling [2]. This problem occurs because the greater quota of compressive strain occurring on a point of material is not adequately compensated by a tensile strain on a not adjacent direction. In this case, a local instability is reached and the material assumes the typical wavy shape. Despite material does not break in these conditions, wrinkles are everywhere regarded as a defect and, for this reason, their insurgence is avoided using a proper blank-holder or managing the process in order to allow its occurrence outside the stamp figure, i.e. in the sheet portion to be trimmed [3]. However, the possibility to have a proper tool able to well predict also the wrinkles occurrence can represent a suitable solution for the process designers. Of course, reliable data concerning wrinkles occurrence have to be supplied in order to build a base of knowledge sufficiently robust for the further applications. In the study here addressed, this task has been performed designing a proper equipment able to carry out the wedge test in warm conditions [4,5]. The equipment (Figure 1) allows the drawing of trapezoidal specimens (Figure 2) characterized by a minor base b0=30mm, a drawing angle α=15° and different major base dimension. Fig. 1. The wedge-test equipment A regular line grid with 3 mm in line distance has been applied to the sheet specimens in order to easily determine the resulting strain distribution. The blank-holder was designed in order to allow a constant clearance of about 15% of the sheet thickness. All the tests have been carried out, using a MTS810 testing machine, in a proper furnace able to contain all the equipment and to control the temperature inside. 56 Sheet Metal 2007