Understanding and modeling of void closure mechanisms in hot metal forming processes
Understanding and modeling of void closure mechanisms in hot metal forming processes
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发表时间：
2013-12
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通讯作者：
M. Saby
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作者：
M. Saby
在大型金属工件的生产过程中，通常会观察到内部存在空隙。这种内部缺陷通常在热成形工艺的第一道次过程中被封闭。然而，目前缺乏关于空隙闭合机制的知识，并且没有可靠的模型可以准确地预测空隙闭合。因此，不可交付产品的数量相对较高。本工作的目的是更好地了解有关的材料，工艺和空洞的形态参数空洞闭合机制。这项工作得到了一个由六个伙伴组成的工业联合体的支持。工业问题，因此，特别是多样化的材料，工艺和初始void states.A广泛的研究，在文献中现有的模型。两个主要的方法进行了讨论：显式全场方法和微观分析方法。结果表明，这两种方法都不能准确预测空泡闭合，因此提出了一种新的中尺度方法。使用代表性体积元（RVE）的空隙闭合机制进行了研究。使用这种方法，可以在RVE尺度上获得空隙状态的准确三维描述。边界条件也可以施加，以准确地表示热机械条件从macroscale.Local机制的空洞闭合研究使用一个大型活动的三维有限元模拟在RVE规模。研究的参数为：材料参数、空洞的形态和空洞在热成形过程中可能经受的热机械载荷。研究表明，孔洞的形态和应力状态对孔洞闭合都有一级影响。材料参数表现出二阶的空洞闭合的影响。因此，一个新的可靠的预测模型，提出了关于一阶参数。空隙的形态定量研究方面的等效尺寸（三维纵横比），和取向（相对于主变形方向）。采用应力三轴度比表示应力状态，并在材料成形有限元软件中实现了该模型。验证案例使用工业过程，以突出这种新模式的优点和局限性，相对于现有的模型从文献中。还使用多孔样品的压缩测试进行了实验验证。在压缩之前和之后，使用X射线显微断层摄影术检查样品。
During production of large metal workpieces, an internal presence of voids is usually observed. Such internal defaults are generally closed up during the first passes of hot forming processes. Yet, there is at present a lack of knowledge regarding void closure mechanisms and there is no reliable model that can accurately predict void closure. The amount of non-deliverable products is consequently relatively high. The present work aims to better understand void closure mechanisms with respect to the involved materials, processes and voids' morphological parameters. This work was supported by an industrial consortium involving six partners. The industrial issues were thus particularly diversified in terms of materials, processes and initial void states.An extensive study regarding existing models in the literature is first presented. Two main approaches are discussed: the explicit full-field approach and the micro-analytical approach. It is shown that none of both approaches is sufficient to precisely predict void closure according to the industrial issues.A new approach is thus proposed at the mesoscale. Void closure mechanisms are studied using a representative volume element (RVE). Using this approach an accurate tridimensional description of the void state can be obtained at the RVE scale. Boundary conditions can also be imposed in order to accurately represent thermomechanical conditions from the macro-scale.Local mechanisms of void closure are studied using a large campaign of 3D finite element simulations at the RVE-scale. The studied parameters are: the materials parameters, the void's morphology and the thermomechanical loading that a void might undergo during hot forming processes. The study shows that both the void's morphology and the stress state exhibit a first-order influence on void closure. Materials parameters exhibit a second-order influence on void closure. A new reliable prediction model is thus proposed with respect to the first-order parameters. The void's morphology is quantitatively studied in terms of equivalent dimensions (tridimensional aspect ratios), and orientation (with respect to principal deformation direction). The stress state is expressed using the stress triaxiality ratio.The proposed model was finally implemented in a material forming finite element software. Validation cases are presented using industrial processes in order to highlight the benefits and limitations of this new model with respect to the existing models from the literature. An experimental validation was also performed using compression tests of porous samples. The samples were examined using X-ray micro-tomography before and after compression.