Void growth in ductile materials with actual porous microstructures

Abstract

In this paper, we have investigated void growth in ductile materials with actual porous microstructures. For that purpose, we have performed calculations of cubic unit-cells subjected to periodic boundary conditions and containing porosity distributions representative of three additively manufactured materials, namely, aluminium alloy AlSi10Mg,9 stainless steel 316L and Inconel 718. The initial void volume fraction in the calculations varies between 0.00564% and 1.75%, the number of voids between 14 and 5715, and the pores size from 2.3 μm to 110 μm. Several realizations with different void sizes and positions have been generated for each of the porous microstructures considered. The simulations have been carried out with random spatial distributions of voids and with clusters of different sizes. The matrix material is modeled using isotropic linear elasticity and von Mises plasticity with an associated flow rule and isotropic hardening, being the flow stress dependent on strain and strain rate. The macroscopic stress state in the unit-cell is controlled by prescribing constant triaxiality (T) and Lode parameter (L) throughout the loading. We have performed calculations with stress states resulting from a combination of three different triaxiality and Lode parameter values, i.e., T = 1, 2, 3 and L = −1, 0, 1. To the authors’ knowledge, this is the first and the most comprehensive study that performs 3D unit-cell calculations with actual representation of porous microstructures, and analyzes the effects of size and spatial distribution of voids on the macroscopic response of the porous aggregate and on the collective behavior of individual pores. The results obtained with the actual porous microstructures have been compared with unit-cell calculations having an equivalent single central pore, and with calculations in which the material behaviour is modeled with Gurson plasticity. It has been shown that both initial void volume fraction and distribution of void sizes affect the macroscopic response of the porous aggregate and the void volume fraction evolution. Moreover, the calculations with random spatial distribution of voids have brought out that different realizations of the same microstructure carry significant variations to the effective behaviour of the porous aggregate, and that the interaction between neighboring pores dictates the volume evolution of individual voids, especially at higher macroscopic triaxiality. The calculations with clusters have shown that pores clustering promotes coalescence localization due to increased interaction between the voids, which results in an increased growth rate of voids in clusters with large number of pores.