02Kissell Page 1 Wednesday, May 23, 2001 9:52 AM Materia: Ciencia de los Materiales Fuente: Handbook of Materials for Product Design – Charles Harper, 3° Ed. 2 Aluminum and Its Alloys J. Randolph Kissell TGB Partnership Hillsborough, North Carolina 2.1 Introduction This chapter describes aluminum and its alloys and their mechanical, physical, and corrosion resistance properties. Information is also pro- vided on aluminum product forms and their fabrication, joining, and finishing. A glossary of terms used in this chapter is given in Section 2.10, and useful references on aluminum are listed at the end of the chapter. 2.1.1 History When a six-pound aluminum cap was placed at the top of the Wash- ington Monument upon its completion in 1884, aluminum was so rare that it was considered a precious metal and a novelty. In less than 100 years, however, aluminum became the most widely used metal after iron. This meteoric rise to prominence is a result of the qualities of the metal and its alloys as well as its economic advantages. In nature, aluminum is found tightly combined with other elements, mainly oxygen and silicon, in reddish, clay-like deposits of bauxite near the Earth’s surface. Of the 92 elements that occur naturally in the Earth’s crust, aluminum is the third most abundant at 8%, sur- passed only by oxygen (47%) and silicon (28%). Because it is so diffi- cult to extract pure aluminum from its natural state, however, it wasn’t until 1807 that it was identified by Sir Humphry Davy of En- gland, who named it aluminum after alumine, the name the Romans gave the metal they believed was present in clay. Davy successfully produced small, relatively pure amounts of potassium but failed to iso- late aluminum. In 1825, Hans Oersted of Denmark finally produced a small lump of aluminum by heating potassium amalgam with aluminum chloride. Aluminio y sus Aleaciones Material preparado por: Ing. Diego F. Zalcman 1 2.1de 92 02Kissell Page 2 Wednesday, May 23, 2001 9:52 AM Materia: Ciencia de los Materiales Fuente: Handbook of Materials for Product Design – Charles Harper, 3° Ed. 2.2 Chapter 2 Napoleon III of France, intrigued with possible military applications of the metal, promoted research leading to Sainte-Claire Deville’s im- proved 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 U.S.A. and Can- ada 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. Héroult in Paris, working inde- pendently, discovered virtually simultaneously the electrolytic process now used for the commercial production of aluminum. The Hall- Héroult process begins with aluminum oxide (Al2O3), a fine white ma- terial known as alumina, produced by chemically refining bauxite. The 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 large bars, called in- gots. Hall went on to patent this process and to 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 discov- ery of Germany’s Karl Joseph Bayer about this time of a practical pro- cess that bears his name for refining bauxite into alumina. 2.1.2 Attributes Aluminum is a silvery metallic chemical element with the symbol Al, atomic number 13, atomic weight 26.98 based on 12C, and valence +3. There are eight isotopes of aluminum, but by far the most common is aluminum-27, a stable isotope with 13 protons and 14 neutrons in its nucleus. Aluminum, in the solid state, has a face-centered crystal structure. Although aluminum is the most abundant metal in the Earth’s crust, it costs more than some less plentiful metals because of the cost to extract the metal from natural deposits. Its widespread use is due to the unique characteristics of aluminum and its alloys. The most sig- nificant of these properties are: 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 Aluminio y sus Aleaciones Material preparado por: Ing. Diego F. Zalcman 2 de 92 02Kissell Page 3 Wednesday, May 23, 2001 9:52 AM Materia: Ciencia de los Materiales Fuente: Handbook of Materials for Product Design – Charles Harper, 3° Ed. Aluminum and Its Alloys 2.3 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, trucks, and, increasingly, automobiles, as well as portable structures such as lad- ders, scaffolding, and gangways. Ready fabrication. Aluminum is one of the easiest metals to form and fabricate, including operations such as extruding, bending, roll-form- ing, drawing, forging, casting, spinning, and machining. 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 espe- cially 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 ox- ide film tightly bonds to the aluminum surface and quickly reforms when damaged. High electrical conductivity. Aluminum conducts twice as much elec- tricity as an equal weight of copper, making it ideal for use in electri- cal transmission cables. High thermal conductivity. Aluminum conducts heat three times as well as iron, benefitting both heating and cooling applications, includ- ing automobile radiators, refrigerator evaporator coils, heat exchang- ers, 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. Aluminio y sus Aleaciones Material preparado por: Ing. Diego F. Zalcman 3 de 92 02Kissell Page 4 Wednesday, May 23, 2001 9:52 AM Materia: Ciencia de los Materiales Fuente: Handbook of Materials for Product Design – Charles Harper, 3° Ed. 2.4 Chapter 2 Non-toxicity. Because aluminum is non-toxic, it is widely used in the packaging industry for food and beverages, as well as cooking utensils and piping and vessels used in food processing. Recyclability. Aluminum is readily recycled; about 30% of U.S. alumi- num production is from recycled material. Aluminum made from recy- cled 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 easily be roll- formed with portable equipment on site, and it is so resistant to corro- sion from exposure to the elements. Another is beverage cans, which benefit from aluminum’s light weight for shipping purposes, and its recyclability. 2.1.3 Applications In the U.S.A., about 21 billion pounds of aluminum worth $30 billion was produced in 1995, about 23% of the world’s production. (To put this in perspective, about $62 billion of steel is shipped each year). Of this, about 25% is consumed in transportation applications, 25% in packaging, 15% in the building and construction market, and 13% in electrical products. Other markets include consumer durables such as appliances and furniture; machinery and equipment for use in petro- chemical, textile, mining, and tool industries; reflectors; and powders and pastes used for paint, explosives, and other products. The current markets for aluminum have developed over the rela- tively brief history of industrial production of the metal. Commercial production became practical with the invention of the Hall-Héroult process in 1886 and the birth of the electric power industry, a requisite because of the energy required by this smelting process. The first uses of aluminum were for cooking utensils in the 1890s, followed by elec- trical cable shortly thereafter. Shortly after 1900, methods to make aluminum stronger by alloying it with other elements (such as copper) and by heat treatment were discovered, opening new possibilities. Al- though the Wright brothers used aluminum in their airplane engines, it wasn’t until the second world war that dramatic growth in alumi- num use occurred, driven largely by the use of aluminum in aircraft. Following the war, building and construction applications of alumi- num boomed due to growth in demand and the commercial advent of the extrusion process, an extremely versatile way to fabricate pris- matic members.