Characteristics

Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull grey, depending on the surface roughness. Aluminium is nonmagnetic and nonsparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.^[2] Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, and extruded.

Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.^[2] This corrosion resistance is also often greatly reduced when many aqueous salts are present however, particularly in the presence of dissimilar metals.

Aluminium atoms are arranged in a face-centered cubic (FCC) structure, which may explain its high melting point. Aluminium has a high stacking-fault energy of approximately 200 mJ/m².^[3]

Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3000–10000 nm (far IR) regions, while in the 400–700 nm visible range it is slightly outdone by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.Aluminium is a good thermal and electrical conductor, by weight better than copper. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.

Production and refinement

Although aluminium is the most abundant metallic element in the Earth's crust (believed to be 7.5 to 8.1 percent), it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold. The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was more expensive than silver, gold, or platinum. Aluminium has been produced in commercial quantities for just over 100 years.

 Applications

General use

A piece of aluminium metal.

Aluminium is the most widely used non-ferrous metal.^[17] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes).^[18] Relatively pure aluminium is encountered only when corrosion resistance and/or workability is more important than strength or hardness. A thin layer of aluminium can be deposited onto a flat surface by physical vapor deposition or (very infrequently) chemical vapor deposition or other chemical means to form optical coatings and mirrors. When so deposited, a fresh, pure aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared.

Pure aluminium has a low tensile strength, but when combined with thermo-mechanical processing, aluminium alloys display a marked improvement in mechanical properties, especially when tempered. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength-to-weight ratio. Aluminium readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon (e.g., duralumin). Today, almost all bulk metal materials that are referred to loosely as "aluminium," are actually alloys. For example, the common aluminium foils are alloys of 92% to 99% aluminium.^[19]

Some of the many uses for aluminium metal are in:

household aluminium foil

• Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles etc.)
• Packaging (cans, foil, etc.)
• Water treatment
• Treatment against fish parasites such as Gyrodactylus salaris.
• Construction (windows, doors, siding, building wire, etc.)
• Cooking utensils
• Street Lighting
• Apple MacBooks are now all made of aluminium
• Electrical transmission lines for power distribution
• MKM steel and Alnico magnets
• Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
• Heat sinks for electronic appliances such as transistors and CPUs.
• Substrate material of metal-core copper clad laminates used in high brightness LED lighting.
• Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels and thermite.
• In the blades of prop swords and knives used in stage combat.
• Aluminium is widely used in watch production as it provides durability and resists tarnishing and corrosion.^[20]

 Aluminium compounds

• Aluminium ammonium sulfate ([Al(NH₄)](SO₄)₂), ammonium alum is used as a mordant, in water purification and sewage treatment, in paper production, as a food additive, and in leather tanning.

• Aluminium acetate is a salt used in solution as an astringent.

• Aluminium borate (Al₂O₃ B₂O₃) is used in the production of and ceramic.

• Aluminium borohydride (Al(BH₄)₃) is used as an additive to jet fuel.
• Aluminium bronze (CuAl₅)
• Aluminium chloride (AlCl₃) is used: in paint manufacturing, in antiperspirants, in petroleum refining and in the production of synthetic rubber.

• Aluminium chlorohydride is used as an antiperspirant and in the treatment of hyperhidrosis.

• Aluminium fluorosilicate (Al₂(SiF₆)₃) is used in the production of synthetic gemstones, glass and ceramic.

• Aluminium hydroxide (Al(OH)₃) is used: as an antacid, as a mordant, in water purification, in the manufacture of glass and ceramic and in the waterproofing of fabrics.

• Aluminium oxide (Al₂O₃), alumina, is found naturally as corundum (rubies and sapphires), emery, and is used in glass making. Synthetic ruby and sapphire are used in lasers for the production of coherent light.

• Aluminium phosphate (AlPO₄) is used in the manufacture: of glass and ceramic, pulp and paper products, cosmetics, paints and varnishes and in making dental cement.

• Aluminium sulfate (Al₂(SO₄)₃) is used: in the manufacture of paper, as a mordant, in a fire extinguisher, in water purification and sewage treatment, as a food additive, in fireproofing, and in leather tanning.

• In many vaccines, certain aluminium salts serve as an immune adjuvant (immune response booster) to allow the protein in the vaccine to achieve sufficient potency as an immune stimulant.

Aluminium alloys in structural applications

Aluminium foam

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).

The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation. (See main article)

One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure will eventually occur under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life.

Another important property of aluminium alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminium, unlike steel, will melt without first glowing red. Forming operations where a blow torch is used therefore requires some expertise, since no visual signs reveal how close the material is to melting. Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by heat-treating the parts in an oven, followed by gradual cooling -- in effect annealing the stresses.

The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region.

Household wiring

Compared to copper, aluminium has about 65% of the by volume, although 200% by weight. Traditionally copper is used as household wiring material. In the 1960s aluminium was considerably cheaper than copper, and so was introduced for household electrical wiring in the United States, even though many fixtures had not been designed to accept aluminium wire. However, in some cases the greater of aluminium causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection. Also, pure aluminium has a tendency to creep under steady sustained pressure (to a greater degree as the temperature rises), again loosening the connection. Finally, Galvanic corrosion from the dissimilar metals increased the electrical resistance of the connection.

All of this resulted in overheated and loose connections, and this in turn resulted in fires. Builders then became wary of using the wire, and many jurisdictions outlawed its use in very small sizes in new construction. Eventually, newer fixtures were introduced with connections designed to avoid loosening and overheating. The first generation fixtures were marked "Al/Cu" and were ultimately found suitable only for copper-clad aluminium wire, but the second generation fixtures, which bear a "CO/ALR" coding, are rated for unclad aluminium wire. To adapt older assemblies, workers forestall the heating problem using a properly-done crimp of the aluminium wire to a short "pigtail" of copper wire. Today, new alloys, designs, and methods are used for aluminium wiring in combination with aluminium terminations.

Ancient Greeks and Romans used aluminium salts as dyeing mordants and as astringents for dressing wounds; alum is still used as a styptic. In 1761 Guyton de Morveau suggested calling the base alum alumine. In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first termed alumium and later aluminum (see Etymology section, below).

The metal was first produced in 1825 (in an impure form) by Danish physicist and chemist Hans Christian Orsted. He reacted anhydrous aluminium chloride with potassium amalgam and yielded