Subject: RE: Founderies

From: [email-masked] (BrnzCaster)
Date: 4 Jan 1995 07:48:13 -0500

Kevin Christiansen wrote:

>Subject: Founderies
>From: [email-masked] (Kevin Christiansen)
>Date: 03 Jan 1995 23:23:10 GMT
>Message-ID: <[email-masked]>

>I have been casing Aluminum for some time now, and
>have had trouble with gassy and brittle castings.
>Would any of the casting experts out there have any
>ideas on how to solve these problems?  Perhaps an
>easily-available, relatively cheap flux?...

The gas inclusion is probably absorbed hydrogen.
If you are using propane, you're running the mix too rich.
Cut back to a neutral or slightly oxidizing flame.
On very humid days, you'll get hydrogen from the water vapor
in the air; I never cast aluminum on rainy days.
I use chlorine gas to degas the melt just before pouring; this
is NOT for the hobby caster. The chloride-fluoride flux offered
by POWER MODEL SUPPLY and others will help minimize hydrogen
absorption, but is used mainly for when melting dirty scrap.

The brittle castings are probably due to large (coarse) grain
structure; many things can cause this:
1. Too much superheat; never take aluminum over about 1350F.
Don't "soak" the melt.
2. Wrong alloy; if you are using extrusion scrap as feed stock,
this is a very poor casting alloy, since it is almost 100% Al.
The Al-Si and Al-Cu alloys are what you want for casting. The
hobby caster can take extruded scrap and dope it with 8% Cu to
get a good casting alloy; first melt the Al, then added the 8% Cu
as fine clippings or cleaned wire and superheat another minute or
so with a little stirring (the molten Al actually dissolves the Cu).
As a general rule, "Use a casting to make a casting".
The Al-Si alloys will flow better than the Al-Cu alloys for thin
pattern work.
3. By using iron chills in the mold, you can control grain growth.
You'll have to experiment with placement since each pattern is
different.

Winston