Abstract:
Metal loss is an unavoidable consequence of the large scale
melting of aluminium and its alloys. The objective of such
processing must be to minimise losses, both from an economic
viewpoint and to ensure optimum quality of cast and wrought
products. Aluminium losses during melting and casting are
primarily due to the formation of dross, a mixture of oxide and
melt. Many of the commercially important aluminium alloys contain
appreciable levels of magnesium (up to 5%) which can result in
enhanced oxidation rates that give rise to particular problems
in recycling.
Results are presented from a 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 early stages in oxide
scale growth, a study of growth kinetics and subsequent breakdown
of these initial scales to form dross.
In humid atmospheres, the amorphous oxide covering both
aluminium and aluminium-magnesium at 750°C provides a highly
effective barrier protecting the molten metal. In the absence of
water vapour, oxide crystal development in aluminium-magnesium
alloys is dominated by magnesium, and is extremely rapid in
comparison with pure aluminium. Despite the different oxides
formed, the manner of crystal formation at the 'amorphous'
oxide-melt interface at 750°C on both aluminium and
aluminium-magnesium is comparable.
Nucleation and growth of crystals in the 'amorphous' film
generates high stresses which result in failure of the surface
oxide. Scanning electron microscopy has shown that the 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.
Parallel studies of the wetting characteristics of aluminium
to alumina have shown that the reported non-wetting is due to the
presence of the thin alumina film on the melt surface. Once
broken, wetting of the alumina takes place and accounts for the
exudation of molten metal through the surface oxide and hence dross
formation.
