Aluminum is a ubiquitous material in modern society. Back in 1956 when Gene Stoner and a few others designed that first AR-15 rifle around aluminum receivers, they literally changed the landscape. In the 1980s, everybody in the combat handgun world was churning out high-capacity, aluminum-framed pistols. Nowadays, we discard or recycle aluminum beverage cans by the zillions.
One of the neat things about aluminum is the way it sort of heals itself. Pure aluminum is highly reactive when exposed to air. However, the resulting aluminum oxide is exceptionally stable. This results in a natural microscopic protective coating on exposed surfaces. In applications like window frames, mechanical trauma from repetitive use results in tiny scratches that instantly oxidize, ensuring a robust material that resists environmental degradation.
Aluminum is relatively soft and easy to both extrude and machine. There are dozens of recognized aluminum alloys. Most AR parts are formed from 6061, which includes trace amounts of silicon, magnesium, copper and chromium. The 7075 alloy includes zinc in place of the silicon.
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