Michael J. Walker

Remelting of Solid and its Effect on Macrosegregation During Solidification

A macroscopic model for simulating local remelting during binary alloy solidification is presented. In order to model the remelting phenomenon, the modified liquid concentration during remelting is calculated by taking into account the previously frozen solid concentration profile. This procedure is integrated into an existing macroscopic solidification model, in which the complete set of volume-averaged equivalent single-phase governing equations are solved using a pressure-based finite-volume method. As case studies, simulations are performed for a binary solution of NH4Cl–69 wt%H2O and for Pb–15 wt%Sn alloy solidified in a side-cooled cavity. Predicted results with the present model are compared with experimental results available in the literature, and the agreement is found to be good.

Initiation of microporosity from pre-existing bubbles: a computational study

Representation of the pore nucleation phenomenon has been a weak link in models of microporosity formation in castings. Porosity models in the literature use different criteria for the stage at which a pore first appears. Pre-existence of microbubbles in the melt has been proposed by many as the reason for non-classical nucleation of pores at low supersaturations. However, nucleation of pores from pre-existing bubbles has not been explicitly modeled or included in models of microporosity. In this paper we present a model for initiation of hydrogen porosity by diffusion of hydrogen into pre-existing bubbles containing insoluble gas. We find that small pre-existing microbubbles have a quiescent stage of very slow growth until a critical supersaturation is built up, followed by a stage of rapid growth that exhausts most of the built-up supersaturation. After that the growth takes place at a small supersaturation until the end of solidification. The phenomenon is analogous to the undercooling and recalescence that occurs during nucleation and growth of solid grains.