Three-dimensional analysis of real void closure at the meso-scale during hot metal forming processes

In the metal forming industry, elimination of internal defects is a prerequisite to avoid catastrophic failure of workpieces. The two different approaches used in the literature to study void closure, respectively at the process-scale and void-scale, present a certain number of limitations. In this paper, an alternative method is presented at a meso-scale. Simulations are performed using a Representative Volume Element (RVE) with real void geometries generated from 3D images obtained using computed microtomography. Complex mechanical loadings are imposed using advanced boundary conditions coming from process numerical modeling. The simulations provide very accurate results to model void closure in real industrial conditions. Equivalent spherical and ellipsoidal voids are also generated and void volume evolution is compared to the case of real geometries. It is shown that the global tendency of void closure is well respected, even though the use of a sphere underestimates void closure, whereas an ellipsoid overestimates void closure, on the final stage. A sensitivity study to mechanical parameters shows that strain-rate has no major influence, while stress triaxiality ratio exhibits a strong influence on the closure of real void. Non-uniform boundary conditions also applied to demonstrate that most existing predictive models are unable to predict void closure in non-uniform mechanical loadings.