More foundry adventures
Kurt Bjorn
I have been having quite a bit of fun with my new Pyramid foundry setup for
sandcasting. Like welding, casting has really broadened my shop
capabilities to where I can now cast, weld, or machine just about anything.
Without becoming repetitive, I'll mention briefly what I've discovered
relative to sand casting and melting aluminum from a newbie perspective.
First, books: there is a US Navy training manual on foundry work which has
been the best source to date on sand-casting info. I got it from a friend
of a friend, but inside the cover, it says "copies may be obtained from:
Book: Patternmaker/Molder
NAVEDTRA 12207 Aug 1993 0502-LP-477-3300
Superintendent of Documents
Government Printing Office
Wash. DC 20402-0001
Anyway, it's fascinating that each warship has a small foundry for the
on-board casting of parts, and they need to be pretty much self-sufficient.
The book goes into great detail on pattern making and sand casting.
On to the casting!
RAMMING: I am using Pyramid supplied PetroBond sand. This is an oil based
sand with a high green strength and fine grain size. This stuff is sticky
and interesting to work. It rams well. I have been using an air chisel
fitted with some different sized "feet" to ram the sand over the pattern,
and I have yet to ram too hard... the harder I ram, the better the finish I
have been obtaining. Apparently, the volume of gas generated is very low
and has no trouble finding its way out even from a densely packed mold.
MELT TEMP: I have a thermocouple rig to measure temp, but rarely use it
anymore. I don't regret its purchase because I will be able to use it with
other applications as well, but don't think you need one for successful
castings. In general, slightly elevated temps work better, maybe 1400 when
you kill the gas to the furnace. By the time you degas, flux, skim and
pour, the melt will probably rise to 1450 or more from latent heat, then
drop to 1350 or so for the pour. This is with a half-full #8 silicon
carbide crucible.
DEGASSING AL: Aluminum gathers moisture from the air, strips the O2 to form
Al oxide and brings the Hydrogen into solution while melted. As the metal
cools, the H2 expands to form gas voids. Real foundries use Nitrogen gas to
degas their melts, and now you can too BECAUSE IT WORKS GREAT! I must admit
I never used chemical degassers, but here is what I have observed... the
first few castings I poured without degassing. After machining, they
exhibited small gas voids, which were measured with a comparator at .003 to
.010 and having a density of perhaps 10-15 occlusions per square inch. I
then began to degas for 30 - 60 seconds with a stainless wand connected to
dry nitrogen, available at any welding shop. Stainless is not optimum,
graphite would be a better choice. To degas, first PURGE the degas line
with a strong N2 flow, then dial the regulator to zero flow. Immerse the
wand and just barely crack the regulator. Too much N2 too fast will toss
little molten blobs of aluminum several feet, NOT fun. Make the surface of
the melt roll for a minute, then skim the oxides and pour.
Yes Jens, the latest pours have exhibited near ZERO gas voids when using dry
N2 gas. :-) Whooo Hooo!
FLUX: For aluminum, it is a must.
HEAT TREATMENT: True T6 heat treatment of castings is possible but requires
a computerized electric furnace. I did some experiments with heat treating
Al and there is no doubt it renders it harder, stronger, and better
machining. But I have also machined some raw castings and these are
acceptable to machine as well. So unless you need the strength, heat
treatment is probably not necessary.
Oh well, thats enough babbling. I hope more people get into foundry work,
it is FUN!
--
Kurt Bjorn
Ft. Worth
Don Foreman
How much flux do you use per lb of aluminum?
Would argon work for degassing?
Kurt Bjorn
Hi Don,
The flux I use is a commercial Al flux and is used at 1 tsp for about 4 lb
of aluminum, more or less to suit. As for the Argon, I haven't tried it
yet, but my gut feel is that it will work great, and that the degas action
is mechanical. Nitrogen is used at big foundries because it is very cheap
relative to Argon. Whatever gas you use must be inert and absolutely dry.
A good welding gas already is dry. Be sure to purge the line prior to
degas.
--
Kurt Bjorn
Ft. Worth
Don Foreman wrote in message
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