| Summary: | 碩士 === 國立東華大學 === 材料科學與工程學系 === 91 ===
During alloy solidification, segregation results from the redistribution of solutes. It is the important defect of castings. Depending on the scale, there are two types of segregation, one is the microsegregation, the other is
macrosegregation. It is well understood that the macrosegregation can occur by a variety of mechanisms, including thermally and solutally driven natural convection of the melt in the mushy zone, residual flow after mold filling, flow induced by solidification contraction and the sedimentation of free equiaxed crystals. In this study, a comprehensively mathematical model and numerical technology had been developed to simulate the phenomena of mold filling, solidification and macrosegregation. A mathematical model based on the
continuum formulation and one-phase model was used. The model can simultaneously solve the macroscopic mass, momentum, energy and species conservation equations and the latent heat of phase change. The SOLA algorithm is applied to solve the velocity-pressure coupling momentum
equations. The VOF and Donor-Acceptor technology is used to deal the phenomena of free surface moving. Two different casting patterns are designed, including unidirectional solidification casting and metal mold
casting with bottom pouring, top pouring and side pouring of Al-4.1wt%Cu alloy, to evaluate the influence of filling processes on macrosegregation. Based on the results of simulation, it was found that there was a strong
relationship between flow pattern and solute distribution. If the residual flow after mold filling could not be canceled by the flow of natural convection due to temperature and/or composition gradients, it will dominate the formation of macrosegregation. Comparing the calculated results of different pouring velocities, it was also found that the patterns of macrosegregation after
solidification were the same if the flow patterns were the same even in different filling conditions.
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