Characteristics

Aluminum cans are about 98% aluminum, 1% magnesium, and 1% other metals. Above, we have the aluminum-magnesium phase diagram as it serves as a rough approximation of our alloy. Since the cans have a very high concentration of aluminum, we will mostly be looking at the right side of the diagram. This means the melting point we are dealing with is very near that of 660°C. Even then, the melting point of magnesium is very near that 660°C mark, so at temperatures above that, all metals will be in a liquid state. This works well given that the charcoal foundries we built can reach 800-900°C whereas the electric foundry can well exceed 1000°C.

The next characteristic we wanted to look at was hardness, for which there are many different methods and scales. One of the more common, yet crude, measurements is that of Vickers. Aluminum alloys have a range of hardness values ranging from around 160 to 350 Vickers. On the Vickers scale, this is a relatively low range. This makes sense given that aluminum is a ductile and relatively malleable metal. It is much easier to scratch, dent, and bend compared to other metals like iron or chromium, which have hardness values of about 600 and 1000 respectively. That means that, once we are to metal our aluminum down and reform it, we should be able to show that aluminum isn't very hard.

There are, however, ways to control hardness in casting, and they have everything to do with the cooling process. Generally, the higher the cooling rate, the harder, and more brittle, a metal becomes. More specifically, the cooling rate just before solidification onset is what effects the hardness.

Casting Defects

Defects in casted parts can degrade a part's structural integrity, causing premature failure. Casting defects can fall into several categories that are outlined below.

Aluminum Mining

Aluminum mining is incredibly energy intensive, and the scale of the industry is enormous. It takes 17 mWh of electricity to produce one ton of aluminum. Yearly, to produce about 6,000 tons of aluminum, the industry uses close to the equivalent of 100Twh of electricity, gas, and other fuels for mining and subsequent processing.

Aluminum is highly reactive with oxygen, so it is never found in its elemental state. In the earth's crust, it generally appears as bauxite, which supplies 99% of the world's metallic aluminum. 90% of the world's bauxite resources are found in tropical areas like Brazil, Guinea and Australia. It occurs mostly near the surface, under a meter or two of top soil and vegetation, where it is generally surface-mined using an open-pit method. This removal, if the ore cannot be dug up easily, involves drilling, placing of explosives and/or ripping with bulldozers.

The ore is crushed, sorted, washed, then refined. Refining is a two step process. The bauxite, which is a mixture of various aluminum compounds, is dissolved in hot sodium hydroxide to produce alumina (Al2O3). Most of alumina then undergoes the Hall-Heroult electrolytic process to be transformed into aluminium. This involves dissolving the alumina once again in a salt bath, then electrolyzing it, bringing it to temperatures of close to 1000 degrees Celsius. Electrolysis is a technique which uses current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is a step where much of aluminum production's electricity use occurs.

Overall, the process of producing fresh aluminum, like all metal production, is costly in energy resources, damages the landscape of its mining sites, and can have health effects in the residential zones near mining sites due to pollution.

Aluminum Recycling

Aluminum recycling, as compared to aluminum mining from bauxite, is highly efficient. It takes less than 1mWh of electricity to produce a ton of recycled aluminum. Today, an estimated half of all products manufactured with aluminium are sourced from recycled aluminium material, and 75% of all aluminum produced in history is still in use. Aluminum recycling is cost effective, coming out on top of mining even when collection and separation are taken into account, and of course it has the added benefit of keeping material out of landfills.

The process of recycling aluminum cans, the item we mainly melted in our foundries, is as such:

• Cans are separated out using an eddy current separator, a machine which uses magnets to separate metals.

• Cans are cleaned and loaded into a furnace at about 750 degrees Celsius.

• Pure metals are added to alter the composition of the molten metal if necessary.

• The molten metal is cast into ingots or otherwise stored for later use.

This process requires much less energy because the aluminum coming in is already mostly pure. It bypasses the need for ripping up the earth, for as much transportation of partially-processed materials, and for electricity-intensive electrolysis.

Of course, if you make your own foundry, you're going even further to take the recycling of aluminum into your own hands.