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It has been amply shown that porosity defects can degrade the mechanical properties of solidified alloys. In particular, the presence of large pores or bubbles in the melt during solidification can cause fluid flow along the bubble-melt interface due to surface tension gradients. This flow can distort the direction of dendrite growth and produce defects such as microsegregation between grain boundaries. In addition, the flow is responsible for the fragmentation of dendrite side-arms and their rotation, creating nuclei for spurious grain formation. Therefore, understanding the consequences of the presence of bubbles on microstructural evolution during alloy solidification can provide helpful information to enhance the microstructure and properties of solidified materials.In this dissertation, computational models are developed to study various phenomena during the solidification of binary alloys under microgravity conditions focusing on Marangoni convection. The problem is tackled by solving continuity, fluid flow, energy, and solute transport equations, and implementing models to simulate the interface between solid and liquid and gas. The cellular automaton (CA) and Allen-Cahn Phase Field (PF) models were utilized to track the solid/liquid interface while lattice Boltzmann (LB) and finite difference (FD) models were applied for solute transport equation. FD method and frozen temperature approximation were implemented for energy equation. Different types of the lattice Boltzmann multiphase flow models were tested to recover continuity and fluid flow equations. The models include Shan-Chen based, and Cahn–Hilliard (CH)-PF based LB. To increase the computational speed of these models for large-scale simulations, Message Passing Interface (MPI), CUDA GPU (graphics processing unit), as well as parallel computing algorithms were utilized. The results extracted from the microgravity experiments conducted on the International Space Station were employed to validate several aspects of the models. |
