Convective transport phenomena during solidification of binary metal alloys and the effects of magnetic fields
Abstract
A numerical and experimental study of convection during solidification of a Pb-Sn alloy is performed, and the effects of both steady and time-harmonic magnetic fields are considered. Experiments are performed in an axisymmetric annular mold, cooled along its outer wall. Temperature and heat transfer measurements are made during experiments, and ingots are analyzed chemically and metallographically to determine macrosegregation patterns. Numerical simulations are based on a continuum formulation which applies in fully melted, mushy, and fully solidified zones. Conditions with opposing thermal and solutal buoyancy forces are considered, and predicted macrosegregation is related to thermosolutal convection patterns and channeling of interdendritic fluid during early stages of solidification and to solutal convection during intermediate stages of solidification. Experimentally measured cooling curves reveal trends in convection patterns which agree with predictions, and reasonable agreement between predicted and measured macrosegregation is achieved. However, the distribution of solute in experimental ingots is three-dimensional, while axial symmetry is assumed in numerical calculations. Moreover, undercooling and solid particle transport in the melt are evident in experimental data but are not considered in the simulations. The numerical model is modified to account for such effects, and their influences are assessed with additional calculations. The effects of magnetic damping through the application of a steady magnetic field is investigated numerically. During early stages of solidification, the magnetic field significantly affects thermally driven flow in the melt, as well as interactions between thermally and solutally driven flows. However, interdendritic flows and macrosegregation patterns are not significantly altered by moderate magnetic fields. The effects of axial electromagnetic stirring (EMS) are studied both experimentally and numerically. Depending on whether an upward or downward traveling magnetic field is applied, EMS forces either augment or oppose those of solutal buoyancy, effecting increased and decreased macrosegregation, respectively. Measurements reveal that, when EMS opposes solutal buoyancy, three-dimensional effects are reduced by increasing magnetic field strength, and effects of turbulence, magnetic field strength, and slurry conditions are investigated numerically with a continuum phase change model, extended to account for electromagnetically induced body forces, nonequilibrium effects, solid particle transport, and turbulence.
Degree
Ph.D.
Advisors
Gaskell, Purdue University.
Subject Area
Mechanical engineering|Metallurgy
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