The aluminothermic (AT) welding process, also known as Thermit welding, is an essential process for joining and repairing rails due to its simplicity, robustness, portability and economic usage. This book presents a multi-fluid, multiphase numerical model to predict the thermal flow field within the mould during the pouring and solidification stages of the AT welding process, developed using the finite volume method-based open-source CFD software OpenFOAM. The numerical model is rigorously validated not only against well-documented cases in the literature but also through an in-house, low-cost experiment designed to investigate the temporal and spatial evolution of the solid-liquid interface front during the melting of paraffin wax in a rectangular enclosure in the presence of a gas phase. The simulation results show good agreement with the validation cases. Moreover, insights into the thermal pouring stage with solid-liquid phase change phenomena are provided. For this purpose, the available k − ε turbulence model is also extended to incorporate the solid-liquid phase change phenomena. Finally, the temporal and spatial evolution of the solid-liquid interface front during the solidification stage is examined, and the influence of initial molten metal temperature and thermophysical properties on the final weld profiles is systematically studied.