aluminum processing

Introduction

      preparation of the ore for use in various products.

      Aluminum, or aluminium (Al), is a silvery white metal with a melting point of 660° C (1,220° F) and a density of 2.7 grams per cubic centimetre. The most abundant metallic element, it constitutes 8.1 percent of the Earth's crust. In nature it occurs chemically combined with oxygen and other elements. In the pure state it is soft and ductile, but it can be alloyed with many other elements to increase strength and provide a number of useful properties. Alloys of aluminum are light, strong, and formable by almost all known metalworking processes. They can be cast, joined by many techniques, and machined easily, and they accept a wide variety of finishes.

      In addition to its low density, many of the applications of aluminum and its alloys are based on its high electrical and thermal conductivity, high reflectivity, and resistance to corrosion. It owes its corrosion resistance to a continuous film of aluminum oxide that grows rapidly on a nascent aluminum surface exposed to air.

History

Early use and extraction

      Before 5000 BC people in Mesopotamia were making fine pottery from a clay that consisted largely of an aluminum (alum) compound, and almost 4,000 years ago Egyptians and Babylonians used aluminum compounds in various chemicals and medicines. Pliny refers to alumen, known now as alum, a compound of aluminum widely employed in the ancient and medieval world to fix dyes in textiles. By the 18th century, the earthy base alumina was recognized as the potential source of a metal.

      The English chemist Humphry Davy (Davy, Sir Humphry, Baronet) in 1807 attempted to extract the metal. Though unsuccessful, he satisfied himself that alumina had a metallic base, which he named “alumium” and later changed to “aluminum.” The name has been retained in the United States but modified to “aluminium” in many other countries.

      A Danish physicist and chemist, Hans Christian Ørsted (Ørsted, Hans Christian), in 1825 finally produced aluminum. “It forms,” Ørsted reported, “a lump of metal which in color and luster somewhat resembles tin.”

      A few years later Friedrich Wöhler (Wöhler, Friedrich), a German chemist at the University of Göttingen, made metallic aluminum in particles as large as pinheads and first determined the following properties of aluminum: specific gravity, ductility, colour, and stability in air.

Deville process

      Aluminum remained a laboratory curiosity until a French scientist, Henri Sainte-Claire Deville (Sainte-Claire Deville, Henri-Étienne), announced a major improvement in Wöhler's method, which permitted Wöhler's “pinheads” to coalesce into lumps the size of marbles. Deville's process became the foundation of the aluminum industry. Bars of aluminum, made at Javel Chemical Works and exhibited in 1855 at the Paris Exposition Universelle, introduced the new metal to the public.

      Although enough was then known about the properties of aluminum to indicate a promising future, the cost of the chemical process for producing the metal was too high to permit widespread use. But important improvements presently brought breakthroughs on two fronts: first, the Deville process was improved; and, second, the development of the dynamo made available a large power source for electrolysis, which proved highly successful in separating the metal from its compounds.

The work of Hall (Hall, Charles Martin) and Héroult

      The modern electrolytic method of producing aluminum was discovered almost simultaneously, and completely independently, by Charles M. Hall of the United States and Paul-Louis-Toussaint Héroult (Héroult, Paul-Louis-Toussaint) of France in 1886. (By an odd coincidence, both men were born in 1863 and both died in 1914.) The essentials of the Hall-Héroult processes were identical and remain the basis for today's aluminum industry. Purified alumina is dissolved in molten cryolite and electrolyzed with direct current. Under the influence of the current, the oxygen of the alumina is deposited on the carbon anode and is released as carbon dioxide, while free molten aluminum—which is heavier than the electrolyte—is deposited on the carbon lining at the bottom of the cell.

      Hall immediately recognized the value of his discovery. He applied July 9, 1886, for a U.S. patent and worked energetically at developing the process. Héroult, on the other hand, although he applied several months earlier for patents, apparently failed to grasp the significance of the process. He continued work on a second successful process that produced an aluminum-copper alloy. Conveniently, in 1888, an Austrian chemist, Karl Joseph Bayer, discovered an improved method for making pure alumina from low-silica bauxite ores.

      Hall and a group of businessmen established the Pittsburgh Reduction Company in 1888 in Pittsburgh. The first ingot was poured in November that year. Demand for aluminum grew, and a larger reduction plant was built at New Kensington, Penn., using steam-generated electricity to produce one ton of aluminum per day by 1894. The need for cheap, plentiful hydroelectric power led the young company to Niagara Falls, where in 1895 it became the first customer for the new Niagara Falls power development.

      In a short time, the demand for aluminum exceeded Hall's most optimistic expectations. In 1907 the company changed its name to Aluminum Company of America (Alcoa). Until World War II it remained the sole U.S. producer of primary aluminum, but within a half-century there were 15 primary producers in the United States.

European industry

      Neuhausen, Switz., is the “nursery” of the European aluminum industry. There, to take advantage of waterpower available from the falls of the Rhine, Héroult built his first aluminum-bronze production facility, which later became the Aluminium-Industrie-Aktien-Gesellschaft. The British Aluminium Company Limited, organized in 1894, soon recognized the wealth of cheap electric power available in Norway and became instrumental in building aluminum works at Stongfjorden in 1907 and later at Vigeland. In France the Société Électrométallurgique Française, also based on Héroult's patent, was started near Grenoble about 1888. An aluminum smelter was started up in Lend, Austria, in 1899. Little aluminum was produced in Germany before 1914, but World War I brought an urgent demand, and several smelters went into production employing electricity generated by steam power. Later the U.S.S.R. began producing substantial amounts of aluminum in the Ural industrial complex, and by 1990 primary metal was produced in 41 nations throughout the world. The largest aluminum smelter in the world (capacity one million metric tons per year) is located in the Siberian city of Bratsk.

Ores

      Aluminum is the third most abundant element on the Earth's surface. Only oxygen and silicon are more common. The Earth's crust to a depth of 16 kilometres (10 miles) contains 8 percent aluminum. Aluminum has a strong tendency to combine with other common elements and so rarely occurs in nature in the metallic form. Its compounds, however, are an important constituent of virtually all common rocks. It is found in clay, shale, slate, schist, granite, syenite, and anorthosite.

      The most important aluminum ore, an iron-containing rock consisting of about 52 percent aluminum oxide (alumina), was discovered in 1821 near Les Baux in southern France. The material was later named bauxite. Bauxite is best defined as an aluminum ore of varying degrees of purity in which aluminum in the form of aluminum hydroxide or aluminum oxide is the largest single constituent. The impurities are largely iron oxide, silica, and titania.

      Bauxite varies greatly in physical appearance, depending on its composition and impurities. It ranges in colour from yellowish white to gray or from pink to dark red or brown if high in iron oxides. It may be earthy, or it may range in form from clay to rock. Bauxite has been found in all the world's continents except Antarctica. The richest deposits generally lie in areas that during formation were in tropical and subtropical climates, providing optimal conditions of heavy rainfall, constant warm temperatures, and good drainage.

      Large deposits are found in the Caribbean islands, northern South America, Australia, India, Indonesia, Malaysia, China, Russia, Kazakhstan, western Africa, Greece, Croatia, Bosnia and Herzegovina, Montenegro, Hungary, Italy, and France.

      Not all bauxites are economical for aluminum production. Only earth with an aluminum oxide content of 30 percent or more is considered practical. Only those ores containing significant concentrations of the minerals gibbsite and boehmite, which contain 65 and 85 percent alumina, respectively, are generally considered economical to be processed. Gibbsite is found largely in tropical areas on either side of the Equator, while boehmite is found largely north of the subtropical belt in Russia, Kazakhstan, Turkey, China, and Greece.

      Known deposits of bauxite can supply the world with aluminum for hundreds of years at present production levels. When high-grade bauxite deposits are depleted, substantial reserves of secondary ores will remain to be exploited: laterite deposits in the northwestern United States and Australia, anorthosite in the western United States, apatite and alunite in Europe, kaolinite in the southeastern United States. Other nonbauxite sources of alumina are also available: alumina clays, dawsonite, aluminous shales (shale), igneous rocks (igneous rock), and saprolite and sillimanite minerals. In Russia, alumina is refined from nonbauxitic ores—namely nepheline syenite and alunite. Vast bauxite developments in Australia, Guinea, and Indonesia have tended to postpone interest in secondary ores elsewhere.

Mining

      By far the greatest quantity of commercially exploited bauxite lies at or near the Earth's surface. Consequently, it is mined in open pits requiring only a minimal removal of overburden. Bauxite beds are blasted loose and dug up with power shovel or dragline, and the ore is transported by truck, rail, or conveyor belt to a processing plant, where it is crushed for easier handling. Refining plants are located near mine sites, if possible, since transportation is a major item in bauxite costs.

      Approximately 90 percent of all bauxite mined is refined into alumina, which is ultimately smelted into aluminum. The remaining 10 percent is used in other applications, such as abrasives, refractories, and proppants in the recovery of crude oil. Approximately four tons of high-grade bauxite yield two tons of alumina, from which one ton of aluminum is produced.

Extraction and refining

      The production of aluminum from bauxite is a two-step process: refining bauxite to obtain alumina and smelting alumina to produce aluminum. Bauxite contains a number of impurities, including iron oxide, silica, and titania. If these impurities are not removed during refining, they will alloy with and contaminate the metal during the smelting process. The ore, therefore, must be treated to eliminate these impurities. Purified alumina usually contains 0.5 to 1 percent water, 0.3 to 0.5 percent soda, and less than 0.1 percent other oxides. The Bayer process, with various modifications, is the most widely used method for the production of alumina, and all aluminum is produced from alumina using the Hall-Héroult electrolytic process.

Refining the ore

      There are a number of alkaline, acid, and thermal methods of refining bauxite, clay, or other ores to obtain alumina. Acid and electrothermal processes generally are either too expensive or do not produce alumina of sufficient purity for commercial use. A process that involves treatment of ore with lime and soda is used in China and Russia.

      The Bayer process involves four steps: digestion, clarification, precipitation, and calcination.

      In the first step, bauxite is ground, slurried with a solution of caustic soda (sodium hydroxide), and pumped into large pressure tanks called digesters, where the ore is subjected to steam heat and pressure. The sodium hydroxide reacts with the aluminous minerals of bauxite to form a saturated solution of sodium aluminate; insoluble impurities, called red mud, remain in suspension and are separated in the clarification step.

      Following digestion, the mixture is passed through a series of pressure-reducing tanks (called blow-off tanks), where the solution is flashed to atmospheric pressure. (The steam generated in flashing is used to heat the caustic solution returning to digestion.) The next step in the process is to separate the insoluble red mud from the sodium aluminate solution. Coarse material (e.g., beach sand) is removed in crude cyclones called sand traps. Finer residue is settled in raking thickeners with the addition of synthetic flocculants, and solids in the thickener overflow are removed by cloth filters. These residues are then washed, combined, and discarded. The clarified solution is further cooled in heat exchangers, enhancing the degree of supersaturation of the dissolved alumina, and pumped into tall, silolike precipitators.

      Sizable amounts of aluminum hydroxide crystals are added to the solution in the precipitators as seeding to hasten crystal separation. The seed crystals attract other crystals and form agglomerates; these are classified into larger product-sized material and finer material that is recycled as seed. The product-sized agglomerates of aluminum hydroxide crystals are filtered, washed to remove entrained caustic or solution, and calcined in rotary kilns or stationary fluidized-bed flash calciners at temperatures in excess of 960° C (1,750° F). Free water and water that is chemically combined are driven off, leaving commercially pure alumina—or aluminum oxide—a dry, fine, white powder similar to sugar in appearance and consistency. It is half aluminum and half oxygen by weight, bonded so firmly that neither chemicals nor heat alone can separate them.

      During World War II the Alcoa combination process was developed for processing lower-grade ores containing relatively high percentages of silica. Very briefly, this process reclaims the alumina that has combined with silica during the digestion process and has been filtered out with the red mud. The red mud is not discarded but is heated with limestone (calcium carbonate) and soda ash (sodium carbonate) to produce a sintered product containing leachable sodium aluminate. This product is digested or leached in a manner similar to that for bauxite to extract the sodium aluminate from the insoluble iron, calcium, and silicon materials. The slurry then proceeds through the remaining steps of the Bayer process. The waste residue is called brown mud.

      Alumina produced by the Bayer process is quite pure, containing only a few hundredths of 1 percent of iron and silicon. The major impurity, residual soda, is present at levels of 0.2 to 0.6 percent. In addition to being the primary raw material for producing metallic aluminum, alumina itself is an important chemical. It is used widely in the chemical, refractories, ceramic, and petroleum industries (see below Chemical compounds (aluminum processing)).

      Refining four tons of bauxite yields about two tons of alumina. A typical alumina plant, using the Bayer process, can produce 4,000 tons of alumina per day. The cost of alumina can vary widely, depending on the plant size and efficiency, on labour costs and overhead, and on the cost of bauxite.

smelting

      Although there are several methods of producing aluminum, only one is used commercially. The Deville process, which involves direct reaction of metallic sodium with aluminum chloride, was the basis of aluminum production in the late 19th century, but it has been abandoned in favour of the more economical electrolytic process. A carbothermic approach, the classical method for reducing (removing oxygen from) metallic oxides, has been for years the subject of intense research. This involves heating the oxide together with carbon to produce carbon monoxide and aluminum. The great attraction of carbothermic smelting is the possibility of bypassing alumina refining and of starting with lower-grade ores than bauxite and lower-grade carbon than petroleum coke. Despite many years of intensive research, however, no economic competitor has been found for the Bayer-Hall-Héroult approach.

      Although unchanged in principle, the Hall-Héroult smelting process of today differs greatly in scale and detail from the original process. Modern technology has produced substantial improvements in equipment and materials, and it has lowered final costs.

      In a modern smelter, alumina is dissolved in reduction pots—deep, rectangular steel shells lined with carbon—that are filled with a molten electrolyte consisting mostly of a compound of sodium, aluminum, and fluorine called cryolite.

      By means of carbon anodes, direct current is passed through the electrolyte to a carbon cathode lining at the bottom of the cell. A crust forms on the surface of the molten bath. Alumina is added on top of this crust, where it is preheated by the heat from the cell (about 950° C [1,750° F]) and its adsorbed moisture driven off. Periodically the crust is broken, and the alumina is fed into the bath. In newer cells, the alumina is fed directly into the molten bath by means of automated feeders.

      The results of electrolysis are the deposition of molten aluminum on the bottom of the cell and the evolution of carbon dioxide on the carbon anode. About 450 grams (1 pound) of carbon are consumed for every kilogram (2.2 pounds) of aluminum produced. About 2 kilograms of alumina are consumed for each kilogram of aluminum produced.

      The smelting process is continuous. Additional alumina is added to the bath periodically to replace that consumed by reduction. Heat generated by the electric current maintains the bath in a molten condition so that fresh alumina dissolves. Periodically, molten aluminum is siphoned off.

      Because some fluoride from the cryolite electrolyte is lost in the process, aluminum fluoride is added, as needed, to restore the chemical composition of the bath. A bath with an excess of aluminum fluoride provides maximum efficiency.

      In actual practice, long rows of reduction pots, called potlines, are electrically connected in series. Normal voltages for pots range from four to six volts, and current loads range from 30,000 to 300,000 amperes. From 50 to 250 pots may form a single potline with a total line voltage of more than 1,000 volts. Power is one of the most costly ingredients of aluminum. Since 1900, aluminum producers have searched for sources of cheap hydroelectric power but have also had to construct many facilities that use energy from fossil fuels. Technological advances have reduced the amount of electrical energy necessary to produce one kilogram of aluminum. In 1940 that figure was 19 kilowatt-hours. By 1990 the amount of electrical energy consumed for each kilogram of aluminum produced had declined to about 13 kilowatt-hours for the most efficient cells.

      Molten aluminum is siphoned from the cells into large crucibles. From there the metal may be poured directly into molds to produce foundry ingot, it may be transferred to holding furnaces for further refining or for alloying with other metals, or both, to form fabricating ingot. As it comes from the cell, primary aluminum is about 99.8 percent pure.

      Automation and computer control have had a marked effect on smelter operations. The most modern reduction facilities use fully mechanized carbon plants and computer control for monitoring and automating potline operations.

recycling

      Because the remelting of aluminum scrap consumes only 5 percent of the energy required to make primary aluminum from bauxite, “in-process” scrap metal from fabricating sheet, forgings, and extrusions has found its way back to the melting furnace ever since production began. In addition, shortly before World War I, “new” scrap produced during the fabrication of commercial and domestic products from aluminum was collected by entrepreneurs who began what is known as the secondary aluminum industry. The chemical composition of new scrap is usually well defined; consequently, it is often sold back to the primary aluminum producers to be remade into the same alloy. “New” scrap is now greatly supplemented by “old” scrap, which is generated by the recycling of discarded consumer products such as automobiles or lawn chairs. Because old scrap is often dirty and a mixture of many alloys, it usually ends up in casting alloys, which have higher levels of alloying elements.

      Used aluminum beverage containers constitute a unique type of old scrap. Although the bodies and lids of these cans are made from different aluminum alloys, both contain magnesium and manganese. Consequently, recycled beverage containers can be used to remake stock for either product. The energy required to produce a beverage can from scrap is about 30 percent of the energy needed to produce the can from primary metal. For this reason, the recycling of used beverage containers represents an increasing source of metal for primary metal producers.

The metal and its alloys (alloy)

      A ductile, silvery white metal usually with dull lustre owing to a surface film of aluminum oxide, aluminum is light, weighing approximately one-third as much as an equal volume of copper or steel. It is corrosion-resistant, is an excellent conductor of heat and electricity, reflects both light and radiant heat, is nonmagnetic, does not readily absorb neutrons, can be safely used with foods and medicines, and can be formed by all known metalworking processes.

      Aluminum can be joined by welding, brazing, soldering, adhesive bonding, riveting, stitching, or stapling and by means of a number of mechanical assemblies such as nuts and bolts, screws, and nails. It can be given a wide variety of mechanical finishes by grinding, polishing, buffing, abrasive blasting, and burnishing. A variety of chemical finishes can be used, such as alkaline or acid etches, bright dips (these give an extremely shiny finish to metal), chemical milling, and immersion plating. It is suited to an electrochemical process called anodizing. Or it can be electroplated with other metals or given organic coatings such as paint, lacquer, and plastic films. Aluminum can be finished by porcelain enameling or metallizing.

      High-purity aluminum (99.9 percent) is relatively soft and has a fairly low tensile strength of about 50 megapascals (500 kilograms per square centimetre, or 7,000 pounds per square inch) in the annealed condition. (Annealing involves heating and then cooling slowly to make the metal less brittle.) By alloying and proper thermal and mechanical treatment, however, it can be made much harder and stronger, with tensile strengths as high as 700 megapascals. Unlike some other metals, the strength and ductility of aluminum increase at very low temperatures. Upon melting, the solid metal expands about 7 percent in volume, the solidification shrinkage being 6.6 percent of the liquid volume. Hydrogen is the only gas known to be appreciably soluble in molten aluminum; its solubility increases with temperature but becomes nearly zero when the metal freezes.

      Aluminum may act as a base to form salts with acids or as a weak acid to form salts with strong alkalies. It is stable in air because of a thin, transparent oxide film that forms on exposure to air, protecting the aluminum from further oxidation and reaction. Growth of this natural oxide film is self-limiting—that is, when a thin layer is formed, further growth is halted. Molten aluminum is protected in air by a thicker oxide coating, which also deters further oxidation. Finely divided atomized or flake aluminum mixed with air and ignited will explode violently. Aluminum reacts rapidly with boiling water to liberate hydrogen and form aluminum hydroxide.

      In its superpure condition (99.99 percent), aluminum lacks strength and hardness but is formable, weldable, corrosion-resistant, and an excellent conductor of electricity. Superpure aluminum has many applications: in chemical equipment, in reflectors, as a catalyst in making gasoline, in fine jewelry, and in electronic components. Most aluminum used today, however, is alloyed with other elements to increase strength.

      The most common alloying elements are manganese (Mn), magnesium (Mg), copper (Cu), zinc (Zn), and silicon (Si). (Lithium [Li] is added to some of the newest alloys for the aerospace industry.) Smaller amounts of chromium (Cr), zirconium (Zr), vanadium (V), titanium (Ti), boron (B), tin (Sn), bismuth (Bi), and lead (Pb) may be added for particular purposes. Iron is present as an impurity.

      Aluminum alloy products may be cast in a foundry into their final shape through sand-casting, permanent-mold-casting, or die-casting, or they may be cast into cylinders or rectangular blocks that are worked, or wrought, into products such as sheet, plate, forgings, or extrusions.

Foundry alloys

       Designation of aluminum foundry alloysThe Aluminum Association of the United States has established systems for classifying foundry and wrought aluminum alloys. Foundry alloys are identified by four-digit numbers, with the first numeral indicating the major alloying element or group of elements. (Table (Designation of aluminum foundry alloys); sometimes a letter precedes the four digits to identify a variant of the original composition.)

       Nominal compositions of aluminum foundry alloysCompositions of the major foundry alloys are listed in the Table (Nominal compositions of aluminum foundry alloys). In addition to the major elements, foundry alloys may contain a small amount of titanium to refine the size of the crystallites or grains that make up the casting, as well as small amounts of manganese, chromium, or nickel for increased strength. The metallurgical structures and properties of the castings are also affected by the rate of cooling, which in turn is strongly affected by the casting method.

      The 3XX.X alloys are used in the highest volume. Both copper and magnesium increase strength in the as-cast temper, and strength is increased by subsequent precipitation treatments at mildly elevated temperatures to produce fine intermetallic particles such as Mg₂Si or Al₂Cu. Even higher strength and ductility are obtained by a high-temperature solution treatment followed by rapid cooling and precipitation treatment. When the silicon (Si) content exceeds 12 percent, silicon crystals in the castings enhance wear resistance as well. In the automotive industry, 3XX.X castings have replaced cast iron in transmission cases, intake manifolds, engine blocks, and cylinder heads because the reduced weight improves fuel economy.

      The 2XX.X alloys develop the highest strengths. Good design and foundry techniques must be followed to produce acceptable products, and heat treatment must be applied to develop high strength and to ensure high resistance to stress- and corrosion-induced cracking. Because they have lower general corrosion resistance than other aluminum alloy castings, aluminum-copper castings are usually coated for critical applications.

      The 5XX.X alloy castings are specified when high resistance to corrosion in marine and other severe environments is demanded. These alloys are also used where the finish is of paramount importance and in the food-processing industry.

      The 7XX.X alloys exhibit good finishing characteristics, are resistant to corrosion, and are capable of developing high strength by precipitation at room temperature.

      The 8XX.X alloys are used for sleeve bearings and bushings because the tin prevents seizing and galling.

      The 4XX.X alloys are used when moderate strength along with high ductility and impact resistance are required. They are also used when stability after exposure to elevated temperatures is important.

Wrought alloys

       Designation of aluminum wrought alloysWrought alloys are identified by a four-digit system. Again, the first numeral indicates the major alloying element or group of elements (Table (Designation of aluminum wrought alloys)).

       Nominal Compositions of Aluminum Wrought Alloys, TableThe compositions of the major wrought alloys are given in the Table (Nominal Compositions of Aluminum Wrought Alloys, Table), but properties of wrought alloy products depend on temper as well as composition. For example, when the highest formability is desired, the products are softened by exposing them to an elevated temperature and cooling them slowly. The 3XXX and 5XXX products are strengthened by working them at room temperature to induce strain hardening, while the 2XXX, 6XXX, and 7XXX products achieve their highest strengths by heat treatment to promote precipitation of the major alloying elements.

      Aluminum- manganese alloys are the oldest yet most widely used because of their combination of strength, formability, and corrosion resistance. The bodies of aluminum beverage containers are made from alloy 3004. Alloy 3003 is used for flexible packaging such as frozen food trays, and, along with 3004 and 3105, it is used for residential siding and industrial and farm roofing. Cooking utensils, gutters, and downspouts also are made from 3XXX alloys.

      Aluminum-magnesium alloys provide higher strength than the 3XXX alloys and are also formable, corrosion-resistant, and weldable. Alloy 5182 is used for the lids of beverage cans. Alloys 5005 and 5083 and varieties of 5052, 5056, and 5086 are used in appliances, utensils, sheet-metal work, pressure vessels, television towers, welded structures, boats, and chemical-storage tanks. Screens, nails, and other fasteners are usually made from 5XXX alloys.

      Aluminum-magnesium- silicon alloys develop strength through thermal treatments that precipitate fine Mg₂Si particles. The most widely used 6XXX alloy products are 6063 extrusions and 6061 sheet, plate, forgings, and extrusions. The 6063 extrusions are widely used for storm doors, window frames, furniture tubing, and miscellaneous architectural uses. Alloy 6061 products are employed in the transportation industry in trucks, boats, and railroad cars, as well as for furniture, pipelines, and heavy-duty structures requiring good corrosion resistance. Highly polished and precipitation-strengthened 6061 truck wheels save fuel because they weigh less than steel wheels. Alloy 6201 wire has proved suitable for electrical conductor cable. One of the newest 6XXX alloys, 6013, has applications in aircraft construction because of its attractive combination of density, strength, formability, and corrosion resistance. Another pair of 6XXX alloys, 6009 and 6010, are used for hoods and deck lids of automobiles because they save fuel by reducing structural weight.

      Aluminum- copper alloys are capable of developing higher strength than either 3XXX, 5XXX, or 6XXX alloys, but their corrosion resistance is generally lower. Alloy 2014 forgings find wide application in the transportation industry, and 2024 sheet, plate, and extrusions are used extensively for the fuselages and lower portion of the wings of civilian and military transport aircraft. (The 2024 sheet used on the fuselages of most commercial aircraft is clad with a thin layer of essentially pure aluminum to provide improved corrosion resistance.) New aluminum-copper alloys containing lithium are beginning to be specified for military and commercial aircraft because of their lower density. The magnesium-free alloy 2219 is used for the fuel and oxidizer tanks of space vehicles because it is weldable and develops high strength at cryogenic as well as elevated temperatures. Alloys 2036 and 2008 are used in the automotive industry for hang-on components such as hoods, deck lids, and doors.

      Aluminum-zinc-magnesium (zinc) alloys develop the highest strength. The copper-free alloy 7005, being weldable and showing good corrosion resistance, is used in the ground transportation industry. The highest strength 7XXX alloys contain copper and are not weldable; they find use mainly in the aircraft industry because of their high ratio of strength to density. (The joints in aircraft construction are riveted, so that weldability is not a concern.) Alloy 7075 has been the workhorse of high-strength aluminum alloys since the 1950s. New tempers were developed for this alloy in the 1970s to provide improved resistance to stress and corrosion cracking and to exfoliation corrosion, and variants were developed for more attractive combinations of strength and fracture toughness. Alloy 7050 was developed in the 1970s to provide high strength combined with high resistance to stress and corrosion cracking in bulkheads and other components machined from thick products for military aircraft. A higher-strength variant, 7150, was developed in the early 1980s for use on the upper wing skin of commercial aircraft, and a new temper of this variant was introduced in the late 1980s to provide high resistance to corrosion at the highest strength level.

      Aluminum-silicon alloys are used for welding wire and brazing material, because large amounts of silicon impart great fluidity to molten aluminum.

Chemical compounds

Aluminum oxide (alumina)

      Aluminum oxide exists in several different crystallographic forms, of which corundum is most common. Corundum is characterized by a high specific gravity (4.0), a high melting point (about 2,050° C, or 3,700° F), great insolubility, and hardness.

      Aluminum oxide is the major ingredient in the commercial chemicals known as aluminas. Of the pure, inorganic chemicals, aluminas are among the largest volume produced in the world today. Rubies (ruby) and sapphires (sapphire) are crystalline, nearly pure varieties of alumina, coloured by small amounts of impurities. Synthetic rubies and sapphires are made commercially by fusing a mixture of high-purity aluminum oxide with colouring agents in an oxyhydrogen blowpipe flame. Most are cut and drilled to form tiny “jewel” bearings in watches and various precision measuring instruments.

      Activated alumina is a porous form of aluminum oxide from which much of the chemically combined water has been driven off at temperatures low enough to avoid sintering. It is chemically inert to most gases, nontoxic, and will not soften, swell, or disintegrate in water. It has the ability to adsorb and hold moisture without change in form or properties, and it has high resistance to shock and abrasion. Activated alumina is used in oil, chemical, and petrochemical industries as a dehydration agent and purifier in the manufacture of gasoline, petrochemicals, natural gas, and hydrogen peroxide.

      Calcined alumina is aluminum oxide that has been heated at temperatures in excess of 1,050° C (1,900° F) to drive off nearly all chemically combined water. In this form, alumina has great chemical purity, extreme hardness (9 on the Mohs hardness scale, on which diamond is 10), high density, and a high melting point (slightly above 2,050° C [3,700° F]). It possesses good thermal conductivity, heat and shock resistance, and high electrical resistivity at elevated temperatures. This combination of properties makes calcined alumina useful in abrasives, glass, porcelains, spark plugs, and electrical insulators, but the greatest quantity of calcined alumina is used to obtain aluminum.

      Tabular alumina is aluminum oxide that has been heated to temperatures above 1,650° C (3,000° F). Composed of tabletlike crystals, it has high heat capacity and thermal conductivity as well as exceptional strength and volume stability at high temperatures. For these reasons, a major use of tabular alumina is in the production of high-quality refractories (refractory), the materials used for lining industrial furnaces. High-alumina refractories are used in the metal and glass industries in boiler installations, in large furnaces and kilns for smelting metals and firing glass, pottery and porcelain, and in the manufacture of building bricks.

      Most refractories are produced in the form of brick, bonded and fired in furnaces. Some castable refractories are made in the form of mortars, usually tabular alumina with calcium aluminate cement as a binder. These mortars, called grog, are sprayed under pressure to form the linings of the steel industry's electric and basic oxygen furnaces, ladles, and coke ovens and for steam boilers, rotary kilns, and many other high-temperature applications.

      Fused aluminas are used in special refractories for the glass industry. Fused alumina is calcined alumina that is melted in electric-arc furnaces, cooled, crushed, and recast into desired shapes. In another application, industrial processes requiring hot gases use a unique heat-transfer device called a pebble heater. Gases to be heated are passed through a bed of tabular alumina balls that have been heated to extreme temperatures. In still another application, an aluminous insulating material is formed by melting alumina and silica in an electric furnace and subjecting the molten mixture to high-velocity gases to produce fine white fibres.

Other compounds

      The hydrous forms of alumina, called aluminum hydroxide, may contain either one or three molecules of water. Each may exist in two different crystalline phases, known as alpha and beta. In both forms, the alpha variety is more common. The alpha trihydroxide (gibbsite) and alpha oxide hydroxide (boehmite) occur in bauxite.

      Aluminum trihydroxide is used extensively in the production of aluminum chemicals, such as aluminum sulfide, sodium aluminate, aluminum fluoride, and aluminum chloride hexahydrate. It is a raw material in the manufacture of petroleum catalysts, plastic and rubber goods, paper, glass and vitreous enamel, adhesives, varnishes, and toothpastes.

      Aluminum sulfate is employed in water purification. Aluminum chloride in various forms is used as a catalyst in organic chemistry and in the cosmetic industry as a deodorant. Aluminum fluoride is used widely in the production of aluminum.

Kent R. Van Horn Peter R. Bridenbaugh James T. Staley

Additional Reading

Comprehensive and up-to-date information on many aspects of metallurgy, individual metals, and alloys can be found in convenient reference-form arrangement in the following works: Metals Handbook, 9th ed., 17 vol. (1978–89), a massive and detailed source prepared under the direction of the American Society for Metals, with a 10th edition that began publication in 1990; Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978–84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; and its European counterpart, the first English-language edition of a monumental German work, Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985– ). Ed. John E. Hatch (ed.), Aluminum: Properties and Physical Metallurgy (1984), is a reference work that treats aluminum and its alloys in their many product forms, covering properties of pure aluminum, physical metallurgy, metallography and characteristics of aluminum alloys, and nominal properties of many commercial alloys. A.K. Vasudevan and R.D. Doherty (eds.), Aluminum Alloys—Contemporary Research and Applications (1989), contains articles ranging from the history of wrought aluminum alloy development to the physical metallurgy of highly sophisticated aluminum alloy products using rapid solidification processing. Aluminum Standards and Data (biennial) is a publication of the Aluminum Association that includes ranges as well as nominal chemical compositions of all aluminum alloys, along with physical and mechanical properties and definitions of tempers. The development of aluminum alloys for the aerospace industry, including alloys for use at elevated temperatures, is discussed in Edgar A. Starke, Jr., “Alloying of Aluminum: Development of New Aluminum Alloys,” in John L. Walter, Melvin R. Jackson, and Chester T. Sims (eds.), Alloying (1988), pp. 165–197. Rhea Berk et al., Aluminum, Profile of the Industry, 1982 (1982), examines the world aluminum industry broadly, in nontechnical language, at the same time providing worthwhile information for professionals. Peter R. Bridenbaugh James T. Staley

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