documento de inglés

aluminium
I. ASM International. Handbook Committee.TA459.M43 1990 620.1'6 90-115ISBN 0-87170-378-5 (v. 2)SAN 204-7586Printed in the United States of AmericaIntroduction to Aluminum and Aluminum AlloysElwin L. Rooy, Aluminum Company of AmericaIntroductionALUMINUM, the second most plentiful metallic element on earth, became an economic competitor in engineering applications as recently as the end of the 19th century. It was to become a metal for its time. The emergence of three important industrial developments would, by demanding material characteristics consistent with the unique qualities of aluminum and its alloys, greatly benefit growth in the production and use of the new metal.When the electrolytic reduction of alumina (Al2O3) dissolved in molten cryolite was independently developed by Charles Hall in Ohio and Paul Heroult in France in 1886, the first internal-combustion-engine-powered vehicles were appearing, and aluminum would play a role as an automotive material of increasing engineering value. Electrification would require immense quantities of light-weight conductive metal for long-distance transmission and for construction of the towers needed to support the overhead network of cables which deliver electrical energy from sites of power generation. Within a few decades the Wright brothers gave birth to an entirely new industry which grew in partnership with the aluminium industry development of structurally reliable, strong, and fracture-resistant parts for airframes, engines, and ultimately, formissile bodies, fuel cells, and satellite components.The aluminum industry's growth was not limited to these developments. The first commercial applications of aluminium were novelty items such as mirror frames, house numbers, and serving trays. Cooking utensils, were also a major early market. In time, aluminum grew in diversity of applications to the extent that virtually every aspect of modern life would be directly or indirectly affected by its use.Properties. Among the most striking characteristics of aluminum is its versatility. The range of physical and mechanical properties that can be developed--from refined high-purity aluminum (see the article "Properties of Pure Metals" in this Volume) to the most complex alloys--is remarkable. More than three hundred alloy compositions are commonly recognized, and many additional variations have been developed internationally and in supplier/consumer relationships. Compositions for both wrought and cast aluminum alloys are provided in the article "Alloy and Temper Designation Systems for Aluminum and Aluminum Alloys" that immediately follows.The properties of aluminum that make this metal and its alloys the most economical and attractive for a wide variety of uses are appearance, light weight, fabricability, physical properties, mechanical properties, and corrosion resistance.Aluminum has a density of only 2.7 g/cm3, approximately one-third as much as steel (7.83 g/cm3), copper (8.93 g/cm3), or brass (8.53 g/cm3). It can display excellent corrosion resistance in most environments, including atmosphere, water (including salt water), petrochemicals, and many chemical systems. The corrosion characteristics of aluminum are examined in detail in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook.Aluminum surfaces can be highly reflective. Radiant energy, visible light, radiant heat, and electromagnetic waves are efficiently reflected, while anodized and dark anodized surfaces can be reflective or absorbent. The reflectance of polished aluminum, over a broad range of wave lengths, leads to its selection for a variety of decorative and functional uses.Aluminum typically displays excellent electrical and thermal conductivity, but specific alloys have been developed with high degrees of electrical resistivity. These alloys are useful, for example, in high-torque electric motors. Aluminum is often selected for its electrical conductivity, which is nearly twice that of copper on an equivalent weight basis. The requirements of high conductivity and mechanical strength can be met by use of long-line, high-voltage, aluminum steelcoredreinforced transmission cable. The thermal conductivity of aluminum alloys, about 50 to 60% that of copper, is advantageous in heat exchangers, evaporators, electrically heated appliances and utensils, and automotive cylinder heads and radiators.Aluminum is nonferromagnetic, a property of importance in the electrical and electronics industries. It is nonpyrophoric, which is important in applications involving inflammable or explosive-materials handling or exposure. Aluminum is also nontoxic and is routinely used in containers for foods and beverages. It has an attractive appearance in its natural finish, which can be soft and lustrous or bright and shiny. It can be virtually any color or texture.Some aluminum alloys exceed structural steel in strength. However, pure aluminum and certain aluminum alloys are noted for extremely low strength and hardness.Aluminum ProductionAll aluminum production is based on the Hall-Heroult process. Alumina refined from bauxite is dissolved in a cryolite bath with various fluoride salt additions made to control bath temperature, density, resistivity, and alumina solubility. An electrical current is then passed through the bath to electrolyze the dissolved alumina with oxygen forming at and reacting with the carbon anode, and aluminum collecting as a metal pad at the cathode. The separated metal is periodically removed by siphon or vacuum methods into crucibles, which are then transferred to casting facilities where remelt or fabricating ingots are produced.The major impurities of smelted aluminum are iron and silicon, but zinc, gallium, titanium, and vanadium are typically present as minor contaminants. Internationally, minimum aluminum purity is the primary criterion for defining composition and value. In the United States, a convention for considering the relative concentrations of iron and silicon as the more important criteria has evolved. Reference to grades of unalloyed metal may therefore be by purity alone, for example, 99.70% aluminum, or by the method sanctioned by the Aluminum Association in which standardized Pxxx grades have been established. In the latter case, the digits following the letter P refer to the maximum decimal percentages of silicon and iron, respectively. For example, P1020 is unalloyed smelter-produced metal containing no more than 0.10% Si and no more than 0.20% Fe. P0506 is a grade which contains no more than 0.05% Si and no more than 0.06% Fe. Common P grades range from P0202 to P1535, each of which incorporates additional impurity limits for control purposes.Refining steps are available to attain much higher levels of purity. Purities of 99.99% are achieved through fractional crystallization or Hoopes cell operation. The latter process is a three-layer electrolytic process which employs molten salt of greater density than pure molten aluminum. Combinations of these purification techniques result in 99.999% purity for highly specialized applications.Production Statistics. World production of primary aluminum totaled 17,304 thousand metric tonnes (17.304 × 106 Mg) in 1988 (Fig. 1). From 1978 to 1988, world production increased 22.5%, an annual growth rate of 1.6%. As shown in Fig. 2, the United States accounted for 22.8% of the world's production in 1988, while Europe accounted for 21.7%. The remaining 55.5% was produced by Asia (5.6%), Canada (8.9%), Latin/South America (8.8%), Oceania (7.8%), Africa (3.1%), and others (21.3%). The total U.S. supply in 1988 was 7,533,749 Mg in 1988, with primary productionrepresenting 54% of total supply, imports accounting for 20%, and secondary recovery representing 26% (Fig. 3). The source of secondary production is scrap in all forms, as well as the product of skim and dross processing. Primary and secondary production of aluminum are integrally related and complementary. Many wrought and cast compositions are constructed to reflect the impact of controlled element contamination that may accompany scrap consumption. A recent trend has been increased use of scrap in primary and integrated secondary fabricating facilities for various wroughtproducts, including can sheet.Aluminum AlloysIt is convenient to divide aluminum alloys into two major categories: casting compositions and wrought compositions. A further differentiation for each category is based on the primary mechanism of property development. Many alloys respond to thermal treatment based on phase solubilities. These treatments include solution heat treatment, quenching, and precipitation, or age, hardening. For either casting or wrought alloys, such alloys are described as heat treatable. A large number of other wrought compositions rely instead on work hardening through mechanical reduction, usually in combination with various annealing procedures for property development. These alloys are referred to as work hardening.Some casting alloys are essentially not heat treatable and are used only in as-cast or in thermally modified conditions unrelated to solution or precipitation effects.Cast and wrought alloy nomenclatures have been developed. The Aluminum Association system is most widely recognized in the United States. Their alloy identification system employs different nomenclatures for wrought and cast alloys, but divides alloys into families for simplification (see the article "Alloy and Temper Designation Systems for Aluminum and Aluminum Alloys" in this Volume for details). For wrought alloys a four-digit system is used to produce a list of wrought composition families as follows:· 1xxx Controlled unalloyed (pure) compositions· 2xxx Alloys in which copper is the principal alloying element, though other elements, notably magnesium, may be specified· 3xxx Alloys in which manganese is the principal alloying element· 4xxx Alloys in which silicon is the principal alloying element· 5xxx Alloys in which magnesium is the principal alloying element· 6xxx Alloys in which magnesium and silicon are principal alloying elements· 7xxx Alloys in which zinc is the principal alloying element, but other elements such as copper, magnesium, chromium, and zirconium may be specified· 8xxx Alloys including tin and some lithium compositions characterizing miscellaneous compositions· 9xxx Reserved for future useCasting compositions are described by a three-digit system followed by a decimal value. The decimal .0 in all cases pertains to casting alloy limits. Decimals .1, and .2 concern ingot compositions, which after melting and processing should result in chemistries conforming to casting specification requirements. Alloy families for casting compositions are:· 1xx.x Controlled unalloyed (pure) compositions, especially for rotor manufacture· 2xx.x Alloys in which copper is the principal alloying element, but other alloying elements may be specified· 3xx.x Alloys in which silicon is the principal alloying element, but other alloying elements such as copper and magnesium are specified· 4xx.x Alloys in which silicon is the principal alloying element· 5xx.x Alloys in which magnesium is the principal alloying element· 6xx.x Unused· 7xx.x Alloys in which zinc is the principal alloying element, but other alloying elements such as copper and magnesium may be specified· 8xx.x Alloys in which tin is the principal alloying element· 9xx.x UnusedManufactured FormsAluminum and its alloys may be cast or formed by virtually all known processes. Manufactured forms of aluminum and aluminum alloys can be broken down into two groups. Standardized products include sheet, plate, foil, rod, bar, wire, tube, pipe, and structural forms. Engineered products are those designed for specific applications and include extruded shapes, forgings, impacts, castings, stampings, powder metallurgy (P/M) parts, machined parts, and metal-matrix composites. A percentage distribution of major aluminum products is presented in Fig. 4. Properties and applications of the various aluminum product forms can be found in the articles "Aluminum Mill and Engineered Wrought Products" and "Aluminum Foundry Products" that follow.Fig. 4 Percentage distribution of major aluminum products in 1988. Source: Aluminum Association, Inc.Standardized ProductsFlat-rolled products include plate (thickness equal to or greater than 6.25 mm, or 0.25 in.), sheet (thickness 0.15 mm through 6.25 mm, or 0.006 through 0.25 in.), and foil (thickness less than 0.15 mm, or 0.006 in.). These products are semifabricated to rectangular cross section by sequential reductions in the thickness of cast ingot by hot and cold rolling.Properties in work-hardened tempers are controlled by degree of cold reduction, partial or full annealing, and the use of stabilizing treatments. Plate, sheet, and foil produced in heat-treatable compositions may be solution heat treated, quenched, precipitation hardened, and thermally or mechanically stress relieved.Sheet and foil may be rolled with textured surfaces. Sheet and plate rolled with specially prepared work rolls may be embossed to produce products such as tread plate. By roll forming, sheet in corrugated or other contoured configurations can be produced for such applications as roofing, siding, ducts, and gutters.While the vast majority of flat-rolled products are produced by conventional rolling mill, continuous processes are now in use to convert molten alloy directly to reroll gages (Fig. 5). Strip casters employ counterrotating water-cooled cylinders or rolls to solidify and partially work coilable gage reroll stock in line. Slab casters of either twin-belt or moving block design cast stock typically 19 mm (0.75 in.) in thickness which is reduced in thickness by in-line hot reduction mill(s) toproduce coilable reroll. Future developments based on technological and operational advances in continuous processes may be expected to globally affect industry expansions in flat-rolled product manufacture.Fig. 5 Facility for producing aluminum sheet reroll directly from molten aluminumWire, rod, and bar are produced from cast stock by extrusion, rolling, or combinations of these processes. Wire may be of any cross section in which distance between parallel faces or opposing surfaces is less than 9.4 mm (0.375 in.). Rod exceeds 9.4 mm (0.375 in.) in diameter and bar in square, rectangular, or regular hexagonal or octagonal cross section is greater than 9.4 mm (0.375 in.) between any parallel or opposing faces.An increasingly large proportion of rod and wire production is derived from continuous processes in which molten alloy is cast in water-cooled wheel/mold-belt units to produce a continuous length of solidified bar which is rolled in line to approximately 9.4 to 12 mm (0.375 to 0.50 in.) diameter.Engineered ProductsAluminum alloy castings are routinely produced by pressure-die, permanent-mold, green- and dry-sand, investment, and plaster casting. Shipment statistics are provided in Fig. 6. Process variations include vacuum, low-pressure, centrifugal, and pattern-related processes such as lost foam. Castings are produced by filling molds with molten aluminum and are used for products with intricate contours and hollow or cored areas. The choice of castings over other product forms is often based on net shape considerations. Reinforcing ribs, internal passageways, and complex design features, which would be costly to machine in a part made from a wrought product, can often be cast by appropriate pattern and mold or die design. Premium engineered castings display extreme integrity, close dimensional tolerances, and consistently controlled mechanical properties in the upper range of existing high-strength capabilities for selected alloys and tempers.Fig. 6 U.S. casting shipments from 1978 through 1988. Source: Aluminum Association, Inc.Extrusions are produced by forcing solid metal through aperture dies. Designs that are symmetrical around one axis are especially adaptable to production in extruded form. With current technology, it is also possible to extrude complex, mandrel-cored, and asymmetrical configurations. Precision extrusions display exceptional dimensional control and surface finish. Major dimensions usually require no machining; tolerance of the as-extruded product often permits completion of part manufacture with simple cutoff, drilling, broaching, or other minor machining operations. Extrudedand extruded/drawn seamless tube competes with mechanically seamed and welded tube.Forgings are produced by inducing plastic flow through the application of kinetic, mechanical, or hydraulic forces in either closed or open dies. Hand forgings are simple geometric shapes, formable between flat or modestly contoured open dies such as rectangles, cylinders (multiface rounds), disks (biscuits), or limited variations of these shapes. These forgings fill a frequent need in industry when only a limited number of pieces is required, or when prototype designs are to be proven.Most aluminum forgings are produced in closed dies to produce parts with good surface finish, dimensional control, and exceptional soundness and properties. Precision forgings emphasize near net shape objectives, which incorporate reduced draft and more precise dimensional accuracy. Forgings are also available as rolled or mandrel-forged rings.Impacts are formed in a confining die from a lubricated slug, usually cold, by a single-stroke application of force through a metal punch causing the metal to flow around the punch and/or through an opening in the punch or die. The process lends itself to high production rates with a precision part being produced to exacting quality and dimensional standards. Impacts are a combination of both cold extrusion and cold forging and, as such, combine advantages of each process.There are three basic types of impact forming--reverse impacting, forward impacting, and a combination of the two—each of which may be used in aluminum fabrication. Reverse impacting is used to make shells with a forged base and extruded sidewalls. The slug is placed in a die cavity and struck by a punch, which forces the metal to flow back (upward) around the punch, through the opening between the punch and die, to form a simple shell. Forward impacting somewhat resembles conventional extrusion. Metal is forced through an orifice in the die by the action of a punch, causing the metal to flow in the direction of pressure application. Punch/die clearance limits flash formation. Forward impacting with a flatface punch is used to form round, contoured, straight, and ribbed rods. With a stop-race punch, thin-walled parallel or tapered sidewall tubes with one or both ends open may be formed. In the combination method, the punch is smaller than an orificed die resulting in both reverse and forward metal flow.Powder metallurgy (P/M) parts are formed by a variety of processes. For less demanding applications, metal powder is compressed in a shaped die to produce green compacts, and then the compacts are sintered (diffusion bonded) at elevated temperature under protective atmosphere. During sintering, the compacts consolidate and strengthen. The density of sintered compacts may be increased by re-pressing. When re-pressing is performed primarily to improve dimensional accuracy, it is termed "sizing;" when performed to alter configuration, it is termed "coining." Re-pressing may be followed by resintering, which relieves stresses induced by cold work and may further consolidate the structure.By pressing and sintering only, parts having densities of greater than 80% theoretical density can be produced. By repressing, with or without resintering, parts of 90% theoretical density or more can be produced. Additional information on conventionally pressed and sintered aluminum P/M products can be found in the Appendix to the article "High-Strength Aluminum P/M Alloys" in this Volume.For more demanding applications, such as aerospace parts or components requiring enhanced resistance to stresscorrosion cracking, rapidly solidified or mechanically attrited aluminum powders are consolidated by more advanced techniques that result in close to 100% of theoretical density. These consolidation methods include hot isostatic pressing, rapid omnidirectional compaction, ultra-high strain rate (dynamic) compaction, and spray deposition techniques. Using advanced P/M processing methods, alloys that cannot be produced through conventional ingot metallurgy methods are routinely manufactured. The aforementioned article "High-Strength Aluminum Powder Metallurgy Alloys" provides detailed information on advanced P/M processing.Powder metallurgy parts may be competitive with forgings, castings, stampings, machined components, and fabricated assemblies. Certain metal products can be produced only by powder metallurgy; among these are oxide-dispersioned strengthened alloys and materials whose porosity (number distribution and size of pores) is controlled (filter elements and self-lubricating bearings).Metal-matrix composites (MMCs) basically consist of a nonmetallic reinforcement incorporated into a metallic matrix. The combination of light weight, corrosion resistance, and useful mechanical properties, which has made aluminum alloys so popular, lends itself well to aluminum MMCs. The melting point of aluminum is high enough to satisfy many application requirements, yet is low enough to render composite processing reasonably convenient.Aluminum can also accommodate a variety of reinforcing agents. Reinforcements, characterized as either continuous or discontinuous fibers, typically constitute 20 vol% or more of the composite. The family of aluminum MMC reinforcements includes continuous boron; aluminum oxide; silicon carbide and graphite fibers; and various particles, short fibers, and whiskers. Figure 7 shows a variety of parts produced from aluminum MMCs. Information on the processing and properties of these materials can be found in the article "Metal".