91 Description

When a 6-lb aluminum cap was placed at the top of the Washington Monument upon its completion in 1884, aluminum was so rare it was considered a precious metal. In less than 100 years, however, aluminum became the most widely used metal after iron. While all aluminum alloys are recent discoveries compared to metals such as iron, copper, lead, and tin, the aluminum industry continues to develop new alloys and applications. Understanding of the new trends, though, is enhanced by an understanding of aluminum's history.

In nature, aluminum is found tightly combined with other elements, mainly oxygen and silicon, in red claylike bauxite deposits near the Earth's surface. Because it is so difficult to extract pure aluminum from its natural state, it wasn't until 1807 that it was identified by Sir Humphry Davy of England, who named it aluminum after alumine, the metal the Romans believed was present in clay. Davy had successfully produced small, relatively pure amounts of potassium but failed to isolate aluminum.

In 1825 Hans Oersted of Denmark finally produced a small lump of aluminum by heating potassium amalgam with aluminum chloride. Napoleon III of France, intrigued with possible military applications of the metal, promoted research leading to Sainte-Claire Deville's improved production method in 1854, which used less costly sodium in place of potassium. Deville named the aluminum-rich deposits near Les Baux in southern France "bauxite" and changed Davy's spelling to "aluminium." Probably because of the leading role played by France in the metal's early development, Deville's spelling was adopted around the world, including Davy's home country; only in the United States and Canada is the metal called "aluminum" today.

These chemical reaction recovery processes remained too expensive for widespread, practical application, however. In 1886, Charles Martin Hall of Oberlin, Ohio, and Paul L. T. Heroult in Paris, working independently, discovered virtually simultaneously the electrolytic process now used for the commercial production of aluminum. The Hall-Heroult process begins with aluminum oxide (Al2O3), a fine white material known as alumina and produced by chemically refining bauxite. Alumina is dissolved in a molten salt called cryolite in large, carbon-lined cells. A battery is set up by passing direct electrical current from the cell lining acting as the cathode and a carbon anode suspended at the center of the cell, separating the aluminum and oxygen. The molten aluminum produced is drawn off and cooled into a solid. Hall went on to patent this process and help found, in nearby Pittsburgh in 1888, what became the Aluminum Company of America, now called Alcoa. The success of this venture was aided by the discovery of Germany's Karl Joseph Bayer about this time of a practical process that bears his name for refining bauxite into alumina.

9.1.1 Attributes

Although aluminum is the most abundant metal in the Earth's crust, it costs more than some less plentiful metals because of the energy needed to extract the metal from ore. Its widespread use is due to the aluminum's qualities, which include:

High Strength-to-Weight Ratio Aluminum is the lightest metal other than magnesium, with a density about one-third that of steel. The strength of aluminum alloys, however, rivals that of mild carbon steel, and can approach 100 ksi (700 MPa). This combination of high strength and light weight makes aluminum especially well suited to transportation vehicles such as ships, rail cars, aircraft, rockets, trucks, and, increasingly, automobiles, as well as portable structures such as ladders, scaffolding, and gangways.

Ready Fabrication Aluminum is one of the easiest metals to form and fabricate, including operations such as extruding, bending, roll-forming, drawing, forging, casting, spinning, and machining [9]. In fact, all methods used to form other metals can be used to form aluminum. Aluminum is the metal most suited to extruding. This process (by which solid metal is pushed through an opening outlining the shape of the resulting part, like squeezing toothpaste from the tube) is especially useful since it can produce parts with complex cross sections in one operation. Examples include aluminum fenestration products such as window frames and door thresholds, and mullions and framing members used in curtainwalls, the outside envelope of many buildings.

Corrosion Resistance The aluminum cap placed at the top of the Washington Monument in 1884 is still there today. Aluminum reacts with oxygen very rapidly, but the formation of this tough oxide skin prevents further oxidation of the metal. This thin, hard, colorless oxide film tightly bonds to the aluminum surface and quickly reforms when damaged [24].

High Electrical Conductivity Aluminum conducts twice as much electricity as an equal weight of copper, making it ideal for use in electrical transmission cables.

High Thermal Conductivity Aluminum conducts heat three times as well as iron, benefiting both heating and cooling applications, including automobile radiators, refrigerator evaporator coils, heat exchangers, cooking utensils, and engine components.

High Toughness at Cryogenic Temperatures Aluminum is not prone to brittle fracture at low temperatures and has a higher strength and toughness at low temperatures, making it useful for cryogenic vessels.

Reflectivity Aluminum is an excellent reflector of radiant energy; hence its use for heat and lamp reflectors and in insulation.

Nontoxic Because aluminum is nontoxic, it is widely used in the packaging industry for food and beverages, as well as piping and vessels used in food processing and cooking utensils.

Recyclability Aluminum is readily recycled; about 30% of U.S. aluminum production is from recycled material. Aluminum made from recycled material requires only 5% of the energy needed to produce aluminum from bauxite.

Often a combination of the properties of aluminum plays a role in its selection for a given application. An example is gutters and other rain-carrying goods, made of aluminum because it can be easily roll-formed with portable equipment on site and it is so resistant to corrosion from exposure to the elements. Another is beverage cans, which benefit from aluminum's light weight for shipping purposes, and its recyclability.

9.1.2 The Aluminum Association

In the United States the Aluminum Association, founded in 1933, is composed of the primary American aluminum producers. The Aluminum Association is the main source of information, standards, and statistics concerning the U.S. aluminum industry. Contacts for the Aluminum Association are:

Mail: 900 19th Street, N.W., Suite 300, Washington, DC, 20006

Phone: 202-862-5100

Fax: 202-862-5164

Internet: www.aluminum.org

The Aluminum Association publishes standards on aluminum alloy and temper designations and tolerances for aluminum mill products. Publications offered by the association also provide information on applications of aluminum such as automotive body sheet and electrical conductors. Other parts of the world are served by similar organizations such as the European Aluminum Association in Brussels.

9.1.3 Alloy and Temper Designation System

Metals enjoy relatively little use in their pure state. The addition of one or more elements to a metal results in an alloy that often has significantly different properties than the unalloyed material. While the addition of alloying elements to aluminum sometimes degrades certain characteristics of the pure metal (such as corrosion resistance or electrical conductivity), this is acceptable for certain applications because other properties (such as strength) can be so markedly enhanced. About 15 alloying elements are used with aluminum, and, though they usually comprise less than 10% of the alloy by weight, they can dramatically affect material properties.

Aluminum alloys are divided into two categories: wrought alloys, those that are worked to shape, and cast alloys, those that are poured in a molten state into a mold that determines their shape. The Aluminum Association maintains a widely recognized designation system for each category, described in ANSI H35.1, Alloy and Temper Designations for Aluminum [15], discussed below.

9.1.3.1 Wrought Alloys

In the Aluminum Association's designation system for aluminum alloys, a four-digit number is assigned to each alloy registered with the association. The first number of the alloy designates the primary alloying element, which produces a group of alloys with similar properties. The last two digits are assigned sequentially by the association. The second digit denotes a modification of an alloy.

TABLE 9.1 Wrought Alloy Designation System and Characteristics
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