Effect of buoyancy and externally induced forces on the solidification of binary mixtures. Final report, August 1, 1987--July 31, 1997

Research performed under this contract originated with the premise that much could be done to improve existing techniques for modeling the effects of convection on solidification in mixtures by eliminating arbitrary characterizations of the mushy region and its coupling with the melt. It was therefore proposed that a set of continuum conservation equations be derived from the principles of classical mixture theory and that the model concurrently treat melt, mushy and solid regions as a single domain (a continuum). The conservation equations would accommodate all pertinent convection effects, and closure would be achieved by assuming local composition equilibrium at phase interfaces. The need for simplifying assumptions concerning the geometric regularity of the interfaces would be eliminated, along with the need for separately tracking the interfaces and using moving numerical grids and/or coordinate mapping procedures. Accordingly, specific objectives of the work have been to (i) develop models and procedures for simultaneously solving the coupled set of conservation equations which govern mass, momentum, energy and species transfer for solidification in a mixture, (ii) use the models to predict, as a function of time and over a representative range of operating conditions, velocity, temperature, and composition fields throughout solid, mushy and liquid regions of analog and metal alloys, (iii) validate model predictions by visualizing flows and performing temperature and concentration measurements under test cell conditions which simulate those of the computations, and (iv) delineate mechanisms responsible for macrosegregation and develop control strategies for its suppression. Studies were performed for the unidirectional solidification of NH{sub 4}Cl-H{sub 2}O and Pb-Sn and Pb-Sn-Sb alloys