Rule
No. 1:
"NO POURING!"
The best quality and most reproducible castings are those
produced by those few foundries designed to avoid the pouring
of metal. Hard to believe? Not when you think about what happens
to metal when it is poured, and how that affects it after
solidification.
Death by Oxides -- Nearly all metals, particularly
aluminum alloys, contain oxides that act as cracks. This can
be death to the mechanical properties, particularly elongation
and fatigue. The biggest source of these troublesome oxides
comes when the metal is exposed to the oxygen in open air
when the metal is in its molten state -- particularly where
agitation is involved.
Click here to view images
of aluminum damaged by oxide trails.
Step One: Cleanup -- Liquid aluminum alloys need treatment
to reduce hydrogen and oxides. Some foundries de-gas with
tablets, or with a simple open-ended lance, but this is not
enough. Current best practice is rotary degassing and
should be specified when the order is placed.
The most important outcome of this treatment is the removal
of oxides. If oxides are successfully reduced to a low level,
hydrogen porosity will not be a problem no matter what the
hydrogen level is!
Step Two: No Pouring! -- The subsequent handling of
the melt requires great care, so as to avoid the unnecessary
re-introduction of oxides. Thus the melt should not be poured
at all if possible, since pouring folds in oxides.
If transfer to another crucible or furnace is necessary,
any pouring height should be reduced to a minimum -- definitely
less than four inches (100 mm), and preferably less than
two inches (50 mm)!
Bottom Line: Ultimately, the best quality and most
reproducible castings are those produced by those few foundries
designed to avoid the pouring of metal, and in which the oxides
suspended in the melt are allowed to sink or float, and the
melt transferred into the mould cavity without any turbulence
whatever.
About John Campbell
Forward to Rule No. 2 . . .
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