Crystal morphology and gas evolution during solidification processes
Abstract
A theoretical and experimental study of the solidification process has been performed to obtain fundamental understanding relevant to metal casting, solidification of alloys, freezing of biological materials and other areas. The emphasis is on the effect of the morphology of the crystals on the solidification of binary systems and the role of dissolved gas evolution on gas porosity formation. Of specific interest is the effect of gas bubble nucleation and of crystal morphology on the effective thermal conductivity of the solidifying system. An analytical and a semi-analytical method are used to calculate the gas species redistribution due to the movement of the solid-liquid interface during the freezing processes. The gas segregation at the interface strongly depends on the solidification rate (i.e., the interface velocity). The results are important to predict the formation of gas voids in castings and, most importantly, to avoid them. It is found that for a constant solidification rate, bubble nucleation always occurs at the interface despite the magnitude of the interface velocity. On the other hand, when the solidification rate is inversely proportional to the square root of time bubble nucleation can be avoided by ensuring that the initial gas concentration is smaller than a ratio involving the gas solubilities in the liquid and in the solid. An experimental apparatus is designed and constructed to study solidification on a microscopic scale. The temperature gradient and the solidification rate are controlled and aqueous solutions of ammonium chloride of different initial concentrations are frozen in a controlled manner in order to measure the microscopic characteristic lengths of the crystals grown from ammonium chloride solutions of low initial concentrations. Air-saturated water is also solidified and the dissolved gas bubble nucleation observed. Microscopic geometric lengths of the crystal that form the mushy zone are correlated with the velocity of the interface and the temperature gradient. Qualitative observations of the bubble formation process are made and reported. An experimental apparatus is designed, built and instrumented for controlling the concentration of dissolved gas by imposing a vacuum on the system. Macroscopic experiments are performed in order to determine the effect of the crystal morphology and of the dissolved gas evolution on the solidification process. Mathematical models for diffusion controlled solidification of binary systems are considered. The effective thermal conductivity of the solidifying system is calculated using different models and the results compared with each other. These models are introduced and the numerical results are validated by comparing the predictions with the experimental data. Depending on the initial solute concentration, different models for the effective thermal conductivity of the mushy zone are recommended.
Degree
Ph.D.
Advisors
Viskanta, Purdue University.
Subject Area
Mechanical engineering|Materials science|Metallurgy
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