A geometry-dependent model for void closure in hot metal forming

Abstract

International audienceDuring production of large metal workpieces, an internal presence of voids is usually observed. Such internal defects are generally closed up using hot metal forming processes, such as hot forging or hot rolling. Prediction models for void closure, associated with process simulation, are extremely powerful tools and might significantly support process design. However, there is at present a lack of accurate models being able to predict void closure according to industrial conditions, particularly in terms of void geometries. In this paper, an original model for void closure is presented, accounting for the void׳s geometry and orientation, as well as the mechanical state during deformation. The model is build and calibrated based on a wide campaign of finite element simulations at the scale of a representative volume element. Various void geometries are defined and several mechanical states are prescribed on a range that is representative of industrial loadings. The model׳s accuracy is verified using industrial data and was compared to several models from literature. Great advantages are obtained for non-spherical voids in terms of void volume evolution

Influence of Lode angle on modelling of void closure in hot metal forming processes

“…(14)). This work will not include material parameters eect, as it was shown by (Saby et al, 2015) they have a second order inuence on void closure for our material and thermomechanical conditions of interest.…”

“…In Figure 5a, where T x = −1, which is a favorable case for closure (Saby et al, 2015), the axisymmetric loading condition µ = 1 is also favorable for void closure.…”

“…V z (t) is then dened as −˙ zz D z (t) with D z (t) the current RVE height. It is worth mentioning that the equivalent strain rate has a second order inuence on void closure for our materials and thermomechanical conditions of interest (Saby et al, 2015), therefore, it will not be considered as a parameter of the new model. So a value˙ zz = 1s −1 is used for all the explicit simulations on RVE.…”

“…To develop a mean eld prediction model, (Saby et al, 2015) and (Zhang et al, 2009) used nonlinear regression to t models constants based on RVE FE simulations. This is the commonly used method due to its simplicity regarding parameters identication.…”

“…The model accounts for the void's shape to characterize the pores and assumes that the void's orientation is parallel to the principle stress, which is not in agreement with experimental observations. (Saby et al, 2015) used a mesoscale method to develop a model based on a large campaign of simulations on Representative Volume Elements (RVE). This is the only model, to the authors' knowledge, that accounts for the geometry of voids (orientation and form) in addition to mechanical loading parameters (stress triaxiality ratio T x and equivalent strain ).…”