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Bronze casting porosity


#1

I am having a real issue with porosity. I am casting in bronze. Here
is my process. I use a vacuum casting machine to de-bubble my flasks
and for the casting as well. I use a electro melt furnace to melt my
metal.

Burnout cycle:
2 hours at 300
2 hours at 700
3 hours at 1350 
1 hour at casting temp( I have varied this from
700-800-900 degrees)

melting temp of the metal is 950 degrees, I am casting it at just
under 1000 C. I am using 100% new metal.

I usually give it about 15 minutes and I quench the flask in water.

Several of the parts I cast look like the surface has a type of
crystalisation. That is when I look at the surface under a magifying
glass it appears like many lines that have spaces between them
overlapping each other. Kind of like fiberglass. I hope I am clear
what I am trying to explain. And the porosity is strongest in the
center of the object. I have a link to a couple photos below so you
can see what I am trying to describe:

These pics are worst case scenarios. But I still get it to some
degree in all my casts. But not alway’s this bad.

Sorry for the poor pics but I can only get so close to the object
with my digital camera before it losses focus.


#2

Carlos: I looked at your casting.

  1. If you put a round ball between the part and the button , about
    half way between. This will help out on the Porisity problem/ .

  2. Make sure that the button has at least 1 1/2 to 2 1/2 times the
    weight of the part.

  3. Before you cast the part prepare a container that has at least 2
    to 3"" space from the part to the outside of the container andatleast
    4’’ from the bottom and 4’’ from the part to the top os the dand that
    has been throughly dried.

  4. Before the part is cast prepare the container by pouring silica
    sand such as play pen sand that Home Depo sells.

  5. Pour sand in to about 3 or 4’’ deep .

  6. Before the part is cast have a gas torch ready to light by the
    sand container.

  7. When the part is cast put in the container on top of the sand
    and fill with additional sand to the top of the button.

  8. Light the torch and play the flame onto the button until it is
    slight red and then cover with the ramaning sand to at least 4 to 5’'
    above the button.

  9. Let it cool down for a couple houra or more. this ahould help
    out on the porisity problen.

  10. The button has to be the last to cool last.

Yours :
Billy S. Bates
royalminiatures.com


#3

Carlos, From your picture and description< I don’t think that you
have a “porosity” problem, per se. It seems that what you have is
very a large grain structure, which might be polishable.

Grains growth occurs as the casting cools. The higher the flask
temperature or the hotter the molten metal, the larger the grains
will grow after casting. Dental gold alloys are often marketed as
"grain refined", that is, engineered to keep the grains small and
well integrated with each other.

Large grain structure yields brittle castings. Think of a pebble
beach as opposed to a white sand beach. It is easy to trace a
boundary-- or break-- between the large pebbles of a pebble beach.
The stones are not packed tightly together. A white sand beach has
grains that are packed very tightly together. It is quite dense and
it would be very difficult to draw a clean line or crack between the
grains.

I would lower either the flask temperature or the casting temp. of
the metal. I would also consider quenching the cast flask after the
red has gone from the button.

I would also try polishing one of these castings to see how bad they
are.

Good luck, Andy


#4
    I am having a real issue with porosity. I am casting in
bronze. Here is my process. I use a vacuum casting machine to
de-bubble my flasks and for the casting as well. I use a electro
melt furnace to melt my metal. 
Burnout cycle:
2 hours at 300
2 hours at 700
3 hours at 1350
1 hour at casting temp( I have varied this from
700-800-900 degrees)

It seems like that final burnout temperature is a tad high. (This is
Farenheit, right?) I’ve never needed to go above 1200F. When I’ve
inadvertantly let the temperature get to 1350 or so, I’ve noticed
some investment breakdown. You might also try a cooler flask
temperature. If the mold is relatively cool, the metal will chill
against the surface instead of staying liquid.

melting temp of the metal is 950 degrees, I am casting it at just
under 1000 C. I am using 100% new metal.

What kind of bronze are you using? Most people around here use
silicon bronze (hercaloy or everdur) because it melts cleanly without
flux and welds well too.

    I usually give it about 15 minutes and I quench the flask in
water. 

That might be a little quick- try giving them 1/2 an hour.

Several of the parts I cast look like the surface has a type of
crystalisation. That is when I look at the surface under a magifying
glass it appears like many lines that have spaces between them
overlapping each other. Kind of like fiberglass. I hope I am clear
what I am trying to explain. And the porosity is strongest in the
center of the object. I have a link to a couple photos below so you
can see what I am trying to describe:

These pics are worst case scenarios. But I still get it to some
degree in all my casts. But not alway’s this bad.

Sorry for the poor pics but I can only get so close to the object
with my digital camera before it losses focus.

Most porosity is due to inadequate gating: when there isn’t enough
liquid metal to feed the piece as it shrinks, then the piece will
draw metal from its own semisolid areas toward the surface, resulting
in a shriveled surface with sub-surface porosity. From the look of
the pieces you show, this seems to be at least part of the problem.
Is that long skinny gate part of the piece? If it’s necessary, point
that toward the top of the mold, and use a shorter and thicker gate
from the button to the thickest part of the piece. And just after
casting, put a piece of soft firebrick on top of the mold to keep the
heat in the button, which encourages that reservoir of melted metal
to stay liquid longer.

Andrew Werby
www.unitedartworks.com


#5

G’Day Carlos. What you have described is classic shrinkage porosity.

One question firstly, What type of bronze are using?

I will assume that it is a type of Silicon Bronze, and the response
will be for that type of material. The burnout cycle is ok and the
flask temperature should be 490 Deg C.

Now here’s your problem your sprue is way too long, what is actually
happening in simple terms is that the metal in the sprue is
solidifying before the object has had time to solidify. As your
casting is solidifying it requires a supply of liquid metal (from
the button) if this supply is not there then what happens is the
highest melting point metal starts to solidify and forms the basic
crystal structure which in essence under the Microscope looks like a
Christmas tree, any way when that element has solidified then the
lower melting point alloy will fill the spaces in between the
branches but if that supply is cut off then the result will be just
as you described. So by keeping the sprue short and large you are
ensuring that as the casting is cooling it has an uninterrupted
supply of metal. Cut your sprue length to 1/4 to 1/3 of its present
length.

If it is silicon Bronze you may quench after the red colour has gone
out of the button.

I have tried to simplify the answer as I could spend a week
describing the Solidification Process of various alloys.

Regards Michael W Kohlleppel
Art Tech Castings Australia
Investmentcast@aol.com


#6

I took everyones advice and cast with a shorter and larger sprue with
a resivior and I switched to silver as I don’t know for sure the
melting point of this bronze. It cast really well so I think I have
the issue fixed. I am doing a 10 kt gold cast tonight I hope to have
good results.

I got a pyrometer also and found that pretty much every machine I had
was un calibrated to some degree one was off by 100 degrees which
prolly doesn’t make things better. It’s a digital pyrometer for 70
dollars I’ll not spam the link cause I dunno the rules about that if
you e-mail me i’ll send it to you. Thanks for all the help.


#7

Carlos, Porosity, if it is shrinkage is caused by improper metal
flow. It can also be worsened by casting at too high a system (
flask + metal) t emperature. From what you describe the grain
structure is dendritic. Try low ering your flask temperature until
the piece won’t fill. Then raise back to the last temperature that
you gotr complete fill. This should be the coolest you can cast it.
Better to raise the metal temp slightly and go with a lowe r flask
temperature than the other way around. If your going to have porosit
y, you want to drive it to the center of the piece where it won’t be
visible If possible try gating with a shepards hook to the middle of
the piece. Th e more you improve your metal flow, the lower a system
temperature you can c ast at, the better your casting quality. I’m
casting sterling silver pieces at almost 200F flask temperature and
the pieces come out great. Don’t be afr aid to get nonfill, that
condition gives you a better idea of where your sys tem temperature
is at.

Tino Volpe
Metallurgist, Technical Manager
Tiffany & Co.
300 Maple Ridge Drive
Cumberland, RI 02864-8707
401-288-0124