Aluminum Breakthrough
My idea this time was to make a mold for three Venuses and a basin where the aluminum would melt, so it would melt and flow in all at once. (And I had one more separate Venus as well.) I made the mold with plaster and sand in a 1:3 ratio, but it could be even higher next time.
The three-in-one Venus (a 'Threenus'?) was a good idea but it remained in the realm of ideas, supplanted primarily because of a chance conversation with my sculptor neighbour Alex (who has joined in with us before). I was walking my dog Laika and ran into him in the lane behind his house, and I asked him about the knee-less Venus ('Kneelus'?) I had cast before. Did I really need to vent such a small figure?
He asked me if perhaps the metal had cooled too much in the tin-can crucible before this pour, and also suggested we wrap some ceramic fibre around the crucible to keep the material hotter longer.
He also told me that plaster molds aren't typically heated up very much before metal is cast into them. So I thought that maybe going up to 670° C to melt the aluminum in place was asking too much of the plaster (it had cracked before), and wasn't necessary.
And lastly, he suggested that I try tipping the molds because maybe that would allow for enough venting of any hot air.
So as per usual I heated up the molds for about 2 hours to burn out the residual wax.
And then took them out and got to melting the aluminum. When I went to pour, the aluminum fell out of the crucible as one big blob into the pouring basin. Ugh. I didn't think that was going to work very well.
I knew this single Venus would have a seam since the mold had split during the wax burnout
but at least there were no bubbles or bad knees!
The next night I decided to cast a bit more, since I had a few empty molds (of dubious quality) to try out. I took the metal higher this time, to a pyrometer reading of 880°, when normally 830° is sufficient. The wrapped crucible worked great.
I tried out an old Venus mold I found, from about a year ago.
that seemed to have had some dirt in it at the bottom - note the smushed head. But no bad knees!
And another Venus mold, with a tail-vent:
that worked beautifully! Look at that surface!
So: tipping the molds helps, as does hotter metal. And finally, I discovered that the reason aluminum pours out like a blob is because that is the nature of aluminum: it has a very high surface tension. And since we're using metal from a heat sink, it's exceptionally pure. That surface tension can be slightly overcome by a hotter temperature but it's best overcome by adding in other elements.
Ta-dah... alloys! How exciting to encounter the significance of alloys in this way! From experience and need.
So next up we'll try the pop can aluminum that we scavenged, which is an alloy containing magnesium and manganese (either 3104 or 3004). Wikipedia says it is, "a general-purpose alloy with moderate strength, good workability, and good corrosion resistance. It is commonly rolled and extruded, but typically not forged. As a wrought alloy, it is not used in casting." [emphasis mine]
It looks like silicon would add fluidity, allowing the Al to flow longer before freezing up because in essence it lowers the melting point. (This nice presentation from FoundryJoe says that a Si-Al alloy melts at 577° - about 80° lower than pure Al!)
Now it would be very cool to try to make our own homemade silicon-Al alloy that pours better, don'cha think? (You can't say I don't know how to have fun.)
But first, we'll try the soda pop Al and see how it goes.
The three-in-one Venus (a 'Threenus'?) was a good idea but it remained in the realm of ideas, supplanted primarily because of a chance conversation with my sculptor neighbour Alex (who has joined in with us before). I was walking my dog Laika and ran into him in the lane behind his house, and I asked him about the knee-less Venus ('Kneelus'?) I had cast before. Did I really need to vent such a small figure?
He asked me if perhaps the metal had cooled too much in the tin-can crucible before this pour, and also suggested we wrap some ceramic fibre around the crucible to keep the material hotter longer.
 |
| old style |
 |
| new style |
He also told me that plaster molds aren't typically heated up very much before metal is cast into them. So I thought that maybe going up to 670° C to melt the aluminum in place was asking too much of the plaster (it had cracked before), and wasn't necessary.
And lastly, he suggested that I try tipping the molds because maybe that would allow for enough venting of any hot air.
 |
| tipped mold |
So as per usual I heated up the molds for about 2 hours to burn out the residual wax.
And then took them out and got to melting the aluminum. When I went to pour, the aluminum fell out of the crucible as one big blob into the pouring basin. Ugh. I didn't think that was going to work very well.
And, it didn't.
I knew this single Venus would have a seam since the mold had split during the wax burnout
The next night I decided to cast a bit more, since I had a few empty molds (of dubious quality) to try out. I took the metal higher this time, to a pyrometer reading of 880°, when normally 830° is sufficient. The wrapped crucible worked great.
 |
| [ red hot for a good while ] |
I tried out an old Venus mold I found, from about a year ago.
that seemed to have had some dirt in it at the bottom - note the smushed head. But no bad knees!
And another Venus mold, with a tail-vent:
that worked beautifully! Look at that surface!
So: tipping the molds helps, as does hotter metal. And finally, I discovered that the reason aluminum pours out like a blob is because that is the nature of aluminum: it has a very high surface tension. And since we're using metal from a heat sink, it's exceptionally pure. That surface tension can be slightly overcome by a hotter temperature but it's best overcome by adding in other elements.
Ta-dah... alloys! How exciting to encounter the significance of alloys in this way! From experience and need.
So next up we'll try the pop can aluminum that we scavenged, which is an alloy containing magnesium and manganese (either 3104 or 3004). Wikipedia says it is, "a general-purpose alloy with moderate strength, good workability, and good corrosion resistance. It is commonly rolled and extruded, but typically not forged. As a wrought alloy, it is not used in casting." [emphasis mine]
It looks like silicon would add fluidity, allowing the Al to flow longer before freezing up because in essence it lowers the melting point. (This nice presentation from FoundryJoe says that a Si-Al alloy melts at 577° - about 80° lower than pure Al!)
Now it would be very cool to try to make our own homemade silicon-Al alloy that pours better, don'cha think? (You can't say I don't know how to have fun.)
But first, we'll try the soda pop Al and see how it goes.
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