Abstract
Metal loss is an unavoidable consequence of the high production melting of aluminium and its alloys. Losses must be minimised, for reasons of economy and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting result primarily from the formation of dross, a mixture of oxide and melt. Results are presented from an initial study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the initial stages in oxide scale growth, a study of growth kinetics, and observations of the subsequent breakdown of these initial scales to form dross. It was found using TEM that a thin surface oxide of γ-alumina forms rapidly on melts of commercially pure aluminium at 750°C providing a highly effective barrier confining the molten aluminium. This thin surface oxide reduces scale growth to a low level before the onset of breakaway oxidation. Localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. In parallel studies of the characteristics of wetting of aluminium to alumina it is shown that the reported non-wetting is a result of the presence of this thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and it is shown that this accounts for the exudation of aluminium through the surface oxide and hence dross formation.
MST/990
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