Aluminum has come to mean a large family of aluminum alloys, not just a single metal. As first produced, aluminum is 99.5 to 99.76 percent pure. It is somewhat soft and not very strong. However, its strength can be greatly increased by adding small amounts of alloying elements, heat-treating, or cold working. A combination of the three techniques has produced aluminum alloys that, pound for pound, are stronger than structural steel. In addition to increasing strength, alloying elements can be selected that will improve welding characteristics, corrosion resistance, machinability, etc.
There are two main classes of aluminum alloys-wrought alloys and cast alloys. The shape of wrought alloys is changed by mechanically working them, forging, rolling, extruding, hammering, etc. Cast alloys are shaped by pouring molten metal into a mold and allowing it to solidify.
Aluminum alloys have a number of desirable qualities. They are lighter than most other commercially available metals. They do not rust or corrode under normal conditions. They can be shaped and formed easily, and are readily available in a large assortment of shapes, sizes, and alloys.
The alloys that are lower in resistance to corrosion are often clad with pure aluminum to eliminate this problem. This covering is only a few thousandths of an inch thick, and is bonded to both sides of the sheet when it is rolled to the desired thickness.