aluminum

machine#construction: casting#materials: aluminum

Aluminum

Like frozen cream cheese.

When talking about casting, "aluminum" generally refers to an aluminum alloy designed specifically for casting.

Common casting alloys & description:

from http://www.mse.mtu.edu/classes/my4130/databank/labdata.html
As you can see a lot of casting alloys contain a good portion of Silicon to add hardness and reduce shrinkage.
(Verify this) You can increase the silicon content of your metal by adding silica sand. This will generate a lot of dross since the aluminum will react with the sand to form aluminum oxide.

Heat Treating Aluminum

Aluminum can be heat treated. Generally it is solution heat treated and then artificially age hardened. Solution heat treating for 6061 T6 consists of holding it at around 900 degrees for several hours followed by artificial age hardening done by holding it at 350 degrees for eight hours. Not all alloys will respond to heat treatment. If you are trying to increase the wearing qualities you can also look at hard anodizing, which is different than the cosmentic anodizing you typically see. Finally there is another wear treatment that uses a high silica aluminum mix that after casting is acid etched to expose the silica (there may be carbide in the mix as well) Finally, there are a number of coatings that have been tried in the engine industry to try to improve wearing characteristics for engine cylinders made of alumium, inlcuding nikasil, which is a nickel/silicon carbide coating. None of these processes is particularily homeshop friendly. *1

Anodizing will only add another .001 or so if you're doing it on a hobby basis. Any thicker requires refrigeration and higher voltages, i think.

If you want to increase the hardness of the aluminum, you could add some copper to the mix. One or two percent should be enough. You're dissolving the copper into the aluminum, not melting it, so it should be finely divided. Copper wire should work for this.

Cooling your castings slowly will also increase the hardness right out of the sand. Aluminum hardens when cooled slowly, the opposite of steel. I noticed that filing was much easier using lost foam with dry sand vs lost foam with greensand, since the metal doesn't cool rapidly when all the water boils off. *2

Porosity and its cause &aname(porosity)

Porosity is caused by hydrogen evolving from the liquid aluminum as it cools. When aluminum heats up, it reacts with water in the air, forming aluminum oxide and dissolved hydrogen. Fortunately, hydrogen doesn't like to be dissolved in aluminum, so it is easy to wash it out with some other substance.

Any gas that does not contain water or hydrogen and will not react with the aluminum alloy or your crucible can be used, but most modern foundries use dry nitrogen gas. They blow the gas through a steel tube while the crucible is spun. The nitrogen abrsorbs the dissolved hydrogen and bubbles up out of the crucible. If you cannot get a dry inert gas and a regulator, you could use
chlorine gas, which is readily available in the form of crystalline pool shock. Fill a small piece of aluminum foil with 1/4 tsp (1 gram) of pool shock, stick it in a tube with one end smashed shut and welded to the end of a steel rod, and immerse in your liquid aluminum just before pouring. Be careful of the chlorine fumes, they will form HCl that's hydrochloric acid and it will burn your eyes and lungs. Don't do this indoors obviously, and wear a respirator rated for acid fumes.

Pigging Out

Of course, it is a lot easier to just make sure the hydrogen doesn't dissolve into the metal in the first place. Usually, when melting down a lot of scrap metal, it becomes overheated in the pot and absorbs hydrogen from the air. Instead of melting down scrap and casting it, you should melt down scrap, scrape off the dross, pour it into ingots, and then cast your parts using the ingots. The hydrogen picked up during the first melt will bubble out when the ingots solidify. The second time you melt you will have more control over temperature since you aren't fooling around with scrap metal. There will be much less dross since it will have less time to react with the air, and less dirt is introduced via dirty scrap. This whole process of "pigging out" is actually easier and more efficient since you can break up all your scrap at once and melt it all down with one heat, saving time and fuel. You will also have a nice neat pile of ingots instead of a monstrous pile of engine blocks and transmissions. I label my ingots by what it was originally and the number of times it's been melted.

Why pigging out? In the "good old days" when they reduced iron oxide to iron in a cupola, they would pour it down a trough made in the sand in the floor. The trough was called the "sow" and the little ingots sticking off of it were called the "pigs" like baby pigs suckling on their mother.