For reducing the inferior castings of aluminum
alloys caused by porosity it is necessary to clarify,
the shrinkage morphology of poured melt and the
factors affecting the change in shrinkage morphology.
In this study, the change in the amount of porosity
was investigated for JIS AC2B secondary alloy
ingots from the viewpoint of hydrogen absorption
by using a conical mold developed for evaluating
the shrinkage morphology. The amount of porosity
changes, mainly depending on the holding time
and temperature of the molten metal as well as
the humidity in the holding atmosphere. The change
in the amount of porosity differs according to
raw ingots, because the quantity of the hydrogen
absorbed in the molten metal is different. The
hydrogen absorption is closely related with the
oxidation of the molten metal. Particularly, in
the case of the ingot containing both Mg and Ca,
remarkable hydrogen absorption occurred even during
a short holding time, and the oxide formed on
the melt surface was sunk. It is considered that
the oxide is sunk because Ca improves the wettability
between the oxide and the molten metal. This suggests
that the oxidation of the melt is promoted, causing
remarkable hydrogen absorption. It is found effective
for minimizing the change in the amount of porosity
to retard the melt oxidation by Be addition and
to remove Ca by SF6 gas bubbling.
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