Aluminum is one of the most abundant elements on the earth and is widely found in many minerals. In fact, it is a very commonly used metal in today's world and we find it in soda cans, aluminum foil, vehicles, aircraft, windows, doors etc. However, throughout most of mankind's history, people did not know how to extract the aluminum metal out of ores. The first successes were discovered in the mid 1850s or so, but the yield was small and slow. In fact, before the mid 1880s, pure aluminum was more expensive than even gold! It is no coincidence that the top of the Washington Monument was topped off by an aluminum cap stone when it was first built in 1884. Napoleon III of France held banquets where honored guests were supplied with aluminum utensils, whereas less honored guests were given gold utensils! Think of that the next time you throw a soda can into the trash.
Once the process of extracting aluminum from ores via electrolysis was discovered in 1886 and factories using this method started to open shortly after, the price of aluminum began to drop. The invention of airplanes made the demand for the metal even more and after World War II, aluminum prices dropped even more. In modern times, there are many factories around the world producing aluminum.
The main advantage of aluminum is that it is pretty strong compared to its weight. It can be easily shaped and offers pretty good corrosion resistance. Aluminum is usually never used in its pure form, but usually in the form of an alloy, with other elements such as copper, zinc, magnesium, silicon etc. added. These other elements improve the mechanical properties of the aluminum alloy. When aluminum is exposed to air, it forms a thin layer of aluminum oxide, which prevents further oxidation of the inner layers and gives it corrosion resistance.
The two common aluminum alloys used in firearms are 6061 and 7075 aluminum. 6061 aluminum is about 95.8-98.5% aluminum and contains magnesium and silicon as its major alloying elements. It exhibits ease of machining and welding. It also exhibits good corrosion resistance and is used for cans, boats, scuba tanks etc. 7075 aluminum is an alloy that contains about 5.6-6.1% of zinc as its major alloying element. It is much stronger than 6061 aluminum, but is harder to machine and weld than 6061 aluminum. It is also more expensive.
Aluminum is used to construct the frames and receivers of some pistols and rifles, most notably the M16 family. It is also used for magazines, sight rings, scope bodies etc.
In Vietnam, the original M16 used aluminum receivers made of 6061 aluminum originally, but later switched to 7075 aluminum. The reason given was that when the receiver was forged from 6061 aluminum, the forging process made them prone to intergranular exfoliation in environments of high temperature and humidity, such as that found in the jungles of Vietnam, especially when combined with human sweat. Upon a suggestion by Eugene Stoner, the receivers were changed to use 7075 aluminum instead.
Of course, after the machining process, the aluminum has to be hardened to withstand stress forces. This is done by a process called anodizing. The parts to be anodized is connected to a positive electrode (the anode) and dropped into a tank containing an acid solution. Direct current is applied to the anode and cathode and a layer of aluminum oxide forms on the piece. The coating forms a thick layer pretty quickly, much quicker and thicker than if the aluminum was to be exposed to air directly. The layer is very hard, but it contains pores, which could let air or water go through into the inner layers of the piece. Therefore, a sealant is applied after anodizing to seal off the pores.
US military specifications for aluminum alloys used in firearms talk about 7075-T6. The T6 at the end specifies the treatments applied to the aluminum at the end (heat treated and artifically aged). US military spec MIL-A-8625 specifies how the anodization of aluminum should be done.
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