Re: ATM - Casting Aluminum

["PETER JOHN SMITH" <pjifl@bigpond.com>]

Joseph O'Neil wrote

>       This was at a large steel foundy.  His job was introducing new ignots
into
> the forge for melting, but he had to pre-heat them first.  He forgot to
> pre-heat one of them past room temperature ("room temperature" in this
case
> being in the 80 to 90F range, due to the heat of the forge), and he said
> when he put the ignot in, the difference in heat caused it to explode
like
> a stick of dynamite.  He says he was lucky to escape alive.

Preheating is primarily a way to ensure no droplets of water or layer of
condensation is present.  In large sizes it also reduces thermal gradient
stress.

If any water drops get in the molten metal its very spectacular and
dangerous.

> casting, only that it is quite the art, and not just a simple process.
> joe

Yes.

There are a lot of areas of casting where experience and black art are
involved.

One is how much moisture may be present in casting sand.  Sone can be
allowed if the sand is porous enough and vented sufficiently and little
dissolved gas is present in the metal. Too much and its dangerous.

For a newcomer it is better to use well baked dry molds or even preheated
metal molds in very small sizes. Baked clay is OK if little dissolved gas
is present in the metal.  A slightly porous casting will probably not
matter for ATM work.

Another area is strategies allowing for contraction of metal during
cooling.

Rather than go into detail on this list where this is probably a bit off
topic I would suggest reading some of the literature.

If you are interested, go ahead.  Fabricated Al parts from standard bar,
pipe, and plate (MIG or TIG) and a local foundry are other options which
may make more sense.  

My experience is that Foundry charges are stratospheric for one off jobs.

Have fun but use a lot of common sense.

Peter Smith.

(PS - my understanding is that most Mags - wheel rims - have a low content
of Mg.  Improves properties a lot and is more machineable. More an
advertising stunt for common mags - sounds good.  Having said that, some
specialist ones may be nearly 100 %.  One of my childhood entertainments
was to burn WW2 fighter plane wheel rims.  What a waste of good metal !!!! 
 )



Have fun but use a lot of common sense.