Experimental results are summarized of the formation of spheroidal grain structures in an Al-4.5 wt pct Cu alloy by (1) isothermally holding very fine equiaxed dendrites in the liquid-solid region and (2) direct spheroidal solidification from the melt. The fine-grained dendritic structure was obtained by very rapid solidification in a thin-section (0.8 mm) permanent-mold plate casting under conditions such that solidification took place during the filling process and, hence, during rapid fluid flow. Grains of approximately 30 m in diameter (representing a grain density of about 3 ϫ 10 4 mm Ϫ3 ) were obtained; these ripened to a spheroidal morphology in the liquid-solid region in less than 5 seconds. A similarly high grain count was then obtained in the bulk melt by vigorous agitation and rapid cooling near the liquidus temperature; the melt was thereafter cooled more slowly. Under these conditions, a spherical morphology formed early in solidification and grew in that form. The evolution of particle size with time in the liquid-solid region is shown to be essentially identical in the initially dendritic and spherical-growth experiments.

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A model for diffusion-controlled spherical particle growth is presented and solved numerically, showing how, on cooling at sufficient rate from a given fraction solid, growth velocity first increases, and then decreases rapidly when solute fields of adjacent particles overlap. An approximate analytical solution for the spherical particle growth velocity is then developed and shown to be valid until the solute fields begin to overlap. A particle stability model is next presented, building on the above analytic solution. This model permits prediction of the maximum cooling rate at which a semi-solid slurry or reheated semi-solid billet can be cooled while still retaining the spherical growth morphology. The model shows that particle stability is favored by high particle density, high fraction solid and low cooling rate. The predictions of the stability model are found to be in good quantitative agreement with experimental data collected for Al-4.5wt%Cu alloy. Engineering applications of the results obtained are discussed.

Polyaromatic hydrocarbons (PAH) content from seven commercial waterpipe charcoals were determined during the smoking process to estimate how much PAHs would not be trapped by the water trap and could reach the lungs of the smokers. Naphthalene, 2-methylnaphthalene, acenaphthylene, acenaphthene, phenanthrene and fluoranthene were the most abundant PAH compounds produced during smoking. Naphthalene was the highest in all of the smoke contents and levels of 5 to 405 μg/15 minutes could be inhaled by the smoker. The amounts of PAHs produced during the smoking events in absence of tobacco varied greatly among different brands of charcoal. The amount and composition of the emitted PAH were not related to the amount and composition of the original chemicals in the charcoal prior to burning. Our findings suggest that public health agencies should regulate smoked charcoal products alongside tobacco.

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