Re: Casting aluminum


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Posted by Jim Clary on October 11, 19100 at 13:52:32:

In Reply to: Casting aluminum posted by Tom H. on September 22, 19100 at 21:41:09:

Hi Tom,

The pits can be from 2 sources, loose sand pits which will only appear on the surface and gas (H2) porosity, which will appear throughout the casting. We will eliminate porosity due to shrink (improper feeding) for this discussion. I�m going to assume that you are dealing with gas porosity because it is most common. Liquid aluminum has a great affinity for nascent hydrogen molecules (single molecules of hydrogen, H, hydrogen gas, H2, is insoluble in aluminum as we will see. Aluminum, especially at temperatures above around 1290F begins to �dissolve� H molecules like salt in hot water. When the aluminum begins to freeze it becomes a super-saturated solution of H molecules, which precipitate out, find other H molecules and quickly form H2, hydrogen gas that then follows all the physical laws governing gases. That means that the H2 expands to form little voids in the aluminum as it freezes. That�s why, when you try to plate such a casting, it looks like a chrome job with freckles. Not a pretty sight. Of course it cannot be polished out. The deeper you go into the casting, sometimes it seems, the worse it gets.

The problem can be solved by not letting the melt get above 1290-1300F in the first place. Best procedure is to get the molds ready first then bring the melt up to pouring temp and pour right then. The longer it sets at elevated temparatures the more H it will take in. Another way is to purge the melt with nitrogen gas. The gas should be infused into the melt so as to cause as many very small bubbles as possible. Submerging a lance with a porous carbon block to diffuse the N2 works in lieu of more expensive equipment. Just bubbling the N2 into the melt wastes a lot of N2 and just annoys the aluminum. Another way is to buy some hexachloroethane from your local foundry supply house. It is a white tablet that is submerged in the melt. It will also remove some of the magnesium along with the H2 but you are probably not heat treating your castings anyway as they are more of a decorative nature than structural. The alloy you are using is 356 the standard alloy used in making permanent mold wheel castings.

Good luck with your projects!

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