Forge: Aluminum Casting

I have experimented with aluminum casting with the 12 quart forge twice now.  Both times I ran into problems.

This forge is suitable for melting aluminum, perhaps more suitable than it is for forging iron.  Even with regular charcoal fuel (not pellets), it gets hot enough to melt aluminum fairly quickly.  The problem is containing the molten metal.  Many sources online suggest cast iron, which works very well.  The problem is that it is difficult to find cast iron in any form other than skillets.  So, the first time I did this, I used a small cast iron skillet.  Unfortunately, it gets too hot to hold the handle.  So, I put a piece of rebar through the hold in the handle, and used that to hold it.  This was not very stable, and in the end, it spilled around half a cup of liquid aluminum in the forge.  As expected, this ruined the forge and required severe damage to the refractory to remove.  I eventually repaired the forge.  The next try, I used a stainless steel cup from a thrift store.  I put holes through the top rim and put bolts in it, to support the cup over the forge and to give my tongs a place to grab the cup.  Unfortunately, the stainless steel corroded through after around 30 minutes.  As before, molten metal spilled in the forge and now it is broken again.  A nearby hardware store is selling a cast iron pot that is around 8 inches in diameter.  Once I have enough money, I plan on buying one and using it.  That should handle the heat better than the cup and be more stable than the skillet.

Besides the two disasters, I learned some other things about casting aluminum.  First, a mini muffin tin makes a really good ingot mold for aluminum.  I now have somewhere around 12 mini muffin shaped aluminum ingots.  The second time around, I also tried to make 3 castings.  These mostly failed.  I learned some things.  First, you cannot cast aluminum in open top molds.  This allows it to cool to quickly, which causes warping due to shrinkage of the metal as it cools.  Instead, the mold should have sprues (tubes) that extend up from the casting.  The metal is poured into these sprues until both the casting mold and the sprues are filled.  If the sprues are large enough, then they will provide additional metal to the casting as it shrinks, eliminating warping.  The molten metal in the sprues will also help the casting cool more slowly, which will result in a higher quality casting.  These are well known metal casting techniques (which I was already familiar with).  Laziness in metal casting does not pay.

I will post more on this once I have time to repair the forge again.  I am considering building a dedicated smelter.  These can be designed to be spill tolerant.  If I do this, I will be using propane to fuel it, instead of charcoal.

Lord Rybec