There are two types of aluminum alloy, extrusion and casting.
Extruded alloys are made to be pulled and squeezed into their final shape.
Examples: door frames, plate, al. ladders, window frames, sheet, pop cans.......
Extrusions are alloys not made for sand casting, but are OK for most home applications. They have a 4 digit number (6061) for identification.
Aluminum by itself is too soft for castings, elements are
added to the aluminum to give it strength. Casting alloys have a 3 digit number identifying the alloy
100's- Aluminum 99%+
300's- Silicon+ copper and/or magnesium
Note: Silicon based aluminum alloys will not anodize!
500's- Magnesium (fantastic polishing alloy!)
600's- Presently Unused, future alloys
Sometimes there is a number after the decimal. If the number after the decimal is a "0", it is a casting. If the number is a "1", it refers to ingot. Example: 356.0/casting, 514.1/ingot. The most popular casting alloys are "356" and "319". Anodized alloys are typically 214 or 514.
Can I melt pop cans and pour castings?
No, not to make castings.
1.The material used to make the cans is coated in vinyl to protect it from contents.
2. Surface area. When melting aluminum the outside layer of aluminum is highly oxidized and full of gas, which turns to dross. Dross is like the foam on beer, worthless. Pop cans are all surface area.
Pistons are the best aluminum "scrap" to melt for casting. Look for previously cast items to insure a casting alloy. Wheels and transmission housings are also good for casting.
( alloy chart from US Navy Foundry manual)