Aluminum and Aluminum Alloys

Aluminum, Al, is a silver-white metallic element in Group III of the Periodic Table. Aluminum exhibits a valence of in all compounds except for a few high temperature gaseous species in which the aluminum may be monovalent or divalent. Aluminum is the most abundant metallic element on the surfaces of the earth and moon, comprising 8.8% by weight (6.6 atomic %) of the earth's crust. However, it is rarely found free in nature. Nearly all rocks, particularly igneous rocks, contain aluminum as aluminosilicate minerals. Aluminum reflects radiant energy throughout the spectrum. It is odorless, tasteless, nontoxic, and nonmagnetic. Because of its many desirable physical, chemical, and metallurgical properties, aluminum is the most widely used nonferrous metal. The utility of the metal is enhanced by the formation of a stable adherent oxide surface that resists corrosion. Because of high electrical conductivity and lightness, aluminum is used extensively in electrical transmission lines. Its alloys, containing small amounts of other elements, have high strength-to-weight ratios. Alloys of aluminum are readily formable by many metalworking processes; they can be joined, cast, or machined and accept a wide variety of finishes. Aluminum, having a density about one-third that of ferrous alloys, is used in transportation and structural applications where weight-saving is important. The properties of aluminum vary significantly according to purity and alloying. Aluminum, the third most abundant element in the earth's crust, is usually combined with silicon and oxygen in rock. Bauxite is, with rare exceptions, the starting material for the production of aluminum. Metallurgical grade alumina, Al2O3, extracted from bauxite by the Bayer process, is generally referred to as the ore. Aluminum is obtained by electrolysis of this purified ore. Tremendous growth has been experienced in the aluminum industry, as compared to these of other nonferrous metals. The principal markets for aluminum in the United States are transportation containers, packaging, building and construction. Fluoride emission from aluminum smelting cells has long been an area of great concern. Treatment consists of highly (over 99%) efficient dry scrubbers that catch particulates and sorb HF on alumina that is subsequently fed to the cells. Important binary alloys are Al–Fe, Al–Cu, Al–Mg, and Al–Li. Almost all commercial alloys are of ternary or higher complexity. Alloy type is defined by the nature of the principal alloying additions, and phase reactions in several classes of alloys can be described by reference to ternary phase diagrams. Minor alloying additions may have a powerful influence on properties of the product because of the influence on the morphology and distribution of constituents, dispersoids, and precipitates. Important ternary alloys are Al–Fe–Si, Al–Mg–Si, Al–Mg–Mn, and Al–Mg–Zn. Commercially important alloys among the latter always contain more zinc than magnesium to provide attractive combinations of strength, extrudability, and weldability. Further additions to commercial aluminum alloys usually are made either to modify the metastable strengthening precipitates or to produce dispersoids. The highest strength aluminum alloy products are based on the Al–Cu–Mg–Zn system and all are strengthened by precursors to the η-phase. When combined with magnesium, silver, Ag, has found commercial use as an alloying element in several aluminum alloys for specialized applications. It is an essential component with the Weldalite series of alloys. The three elements commonly added to precipitation hardenable alloys to form dispersoids are manganese, chromium, and zirconium. Aluminum alloys are subjected during manufacture to a variety of thermal treatments that range from heating to assist fabrication, to heating for control of final properties. Aluminum alloys are commercially available in a wide variety of cast forms and in wrought mill products produced by rolling, extrusion, drawing, or forging. The mill products may be further shaped by a variety of metal-working and forming processes and assembled by conventional joining procedures into more complex components and structures. Aluminum and aluminum alloys are employed in many applications because of the ability to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. Wrought alloys of the Al, Al–Mn, Al–Mg, and Al–Mg–Si types have excellent corrosion resistance in most weathering exposures, including industrial and seacoast atmospheres. Transportation has replaced the building and construction industry as the largest consumer of aluminum in the United States because aluminum is impermeable to gas, resistant to corrosion, and recyclable. The most prominent has been the use of aluminum for beer and carbonated beverages. The largest market worldwide for aluminum products is in the building and construction industry. Because of aluminum's low density, the field of transportation is another large market for aluminum alloys. Aluminum is an excellent conductor of electricity, having a volume conductivity 62% of that of copper. It also has many applications in the chemical and petrochemical industries as piping and tanks. Keywords: aluminum; alloys; cryolite; hall-heroult; cell; recycling; transportation packaging; building materials