Understanding and modeling of void closure mechanisms in hot metal forming processes

Abstract : 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.
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Michel Saby. Understanding and modeling of void closure mechanisms in hot metal forming processes. Other. Ecole Nationale Supérieure des Mines de Paris, 2013. English. ⟨NNT : 2013ENMP0075⟩. ⟨pastel-00979301⟩

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