Porosity is known to be one of the primary detrimental factors controlling fatigue life and total elongation of several cast alloy components. The two main aims of this work are to examine pore nucleation and growth effects for predicting gas microporosity and to study the physics of bifilm dynamics to gain understanding in the role of bifilms in producing defects and the mechanisms of defect creation. In the second chapter of this thesis, an innovative technique, based on the combination of a set of conservation equations that solves the transport phenomena during solidification at the macro-scale and the hydrogen diffusion into the pores at the micro-scale, was used to quantify the amount of gas microporosity in A356 alloy castings. The results were compared with published experimental data. In the reminder of this work, the Immersed Element-Free Galerkin method (IEFGM) is presented and it was used to study the physics of bifilm dynamics. The IEFGM is an extension of the Immersed Finite Element method (IFEM) developed by Zhang et al. [50] and it is an attractive technique for simulating FSI problems involving highly deformable bifilm-like solids.