Metallurgical Materials Science and Alloy Design. Aluminium is the third most plentiful element known to man, only oxygen and silicon exist in greater quantities. The element aluminium, chemical symbol Al, has the atomic number. The majority of aluminum base alloys can be successfully arc welded without cracking related problems, however, using the most appropriate filler alloy and conducting. Cost of Corrosion to US Industries Corrosion is the deterioration of a material often a metal due to interaction with its environment. A study in 1998. Aluminium alloys are normally supplied as semifinished products such as sheet, plate, coil, extrusions, tube or wire. All forms can then be readily fabricated into. Lightness is an essential property of aluminium. The metal has an atomic weight of 2. The electrical conductivity of 9. C is 6. 3. 8 of the. International Annealed Copper Standard IACS. Because of its low mass density, the mass electrical conductivity of pure aluminium is more than twice that of annealed copper and greater. The resistivity at 2. C is 2. 6. 9 u. Ohm cm. The electrical conductivity which is the reciprocal ofresistivity, is one of the more sensitive properties of aluminium being affected by both, changes in composition and thermal treatment. The addition of other metals in aluminium alloys. Heat treatment also. The thermal conductivity, of 9. Wm. K for the temperature range 0 10C which is 6. IACS, and again because of its low mass density its mass thermal conductivity is twice that of copper. When exposed to air, a layer of aluminum oxide forms almost instantaneously on the surface of the aluminum. This layer has. excellent resistance to corrosion. It is fairly resistant to most acids but less resistant to alkalis. Hence, aluminium has a higher resistance to corrosion than most other construction metals owing to the protection conferred by the thin but tenacious and very dense oxide film. This oxide layer is always present on the surface of aluminium in oxygen atmospheres. Mg, Cu, Ag, Zn and Si are alloying elements in aluminium, which have high solid solubility in the Al matrix. In contrast, Cr, Mn and Zr are mainly used to form compounds to control grain structure. Details on specific alloying elements Antimony occurs in trace amounts 0. Antimony has very small solid solubility in aluminum lt 0. Some bearing alloys contain up to 4 to 6 Sb. Antimony can be used instead of bismuth to counteract hot cracking in aluminum magnesium alloys. Beryllium occurs in aluminum alloys containing magnesium to reduce oxidation at elevated temperatures. Up to 0. 1 Be is used in aluminizing baths for steel to improve adhesion of the aluminum film. Bismuth. The low melting point metals such as bismuth, lead, tin, and cadmium are added to aluminum to make free machining alloys. These elements have a restricted solubility in solid aluminum and. An advantage of bismuth is that its expansion on solidification compensates for the shrinkage of. A 1 to 1 lead bismuth ratio is used in the aluminum copper alloy, 2. Mg Si alloy, 6. 26. Small additions of bismuth 2. Boron is used as alloying element in aluminum and its alloys as a grain refining agent and for improving conductivity by precipitating vanadium, titanium, chromium, and molybdenum. Boron can be. used alone at levels of 0. Commercial grain refiners commonly contain titanium. Cadmium is a relatively low melting element that finds limited use in aluminum. Up to 0. 3 Cd may be added to aluminum copper alloys to accelerate the rate of age hardening, increase strength, and. At levels of 0. 0. Calcium has very low solubility in aluminum and forms the intermetallic Ca. Al. 4. An interesting group of alloys containing about 5 Ca and 5 Zn have superplastic properties. Calcium combines with. Ca. Si. 2, which is almost insoluble in aluminum and therefore will increase the conductivity of commercial grade metal slightly. In aluminum magnesium silicon alloys, calcium will. Its effect on aluminum silicon alloys is to increase strength and decrease elongation, but it does not make these alloys heat treatable. Carbon occurs in aluminium as an impurity element as oxycarbides and carbides, of which the most common is A1. C3, but carbide formation with other impurities such as titanium is possible. A1. 4C3. decomposes in the presence of water and water vapor, and this may lead to surface pitting. Cerium, mostly in the form of mischmetal rare earths with 5. Ce, has been added experimentally to casting alloys to increase fluidity and reduce die sticking. Chromium occurs as a minor impurity in commercial purity aluminum 5 to 5. It has a large effect on electrical resistivity. Chromium is a common addition to many alloys of the. In excess of these limits, it tends to form very. Chromium has a slow diffusion rate and forms fine dispersed phases in wrought products. These dispersed. phases inhibit nucleation and grain growth. Chromium is used to control grain structure, to prevent grain growth in aluminum magnesium alloys, and to prevent recrystallization in. Chromium is thus. Chromium will also reduce stress corrosion susceptibility and improves toughness. Cobalt is an uncommon addition to aluminum alloys. It has been added to some aluminum silicon alloys containing iron, where it transforms the acicular aluminum iron silicon into a more rounded. Aluminum zinc magnesium copper alloys containing 0. Co are produced by powder metallurgy. Copper. The aluminum copper alloys typically contain between 2 to 1. The copper provides substantial increases in strength and facilitates precipitation hardening. The introduction of copper to aluminum can also reduce ductility and corrosion. The susceptibility to solidification cracking of aluminum copper alloys is increased consequently, some of these alloys can be the most challenging aluminum alloys to weld. These. alloys include some of the highest strength heat treatable aluminum alloys. The most common applications for the 2xxx series alloys are aerospace, military vehicles and rocket fins. Both cast and wrought aluminum copper alloys respond to solution heat treatment and subsequent aging with an increase in. The strengthening is maximum between 4 and 6 Cu, depending upon the influence of other constituents present. Copper magnesium. The main benefit of adding magnesium to aluminum copper alloys is the increased strength possible following solution heat treatment and quenching. In wrought material of certain. On artificial aging, a further increase in strength, especially in yield strength can. Copper magnesium plus other elements. The cast aluminum copper magnesium alloys containing iron are characterized by dimensional stability and improved bearing characteristics, as well as by high. However, in a wrought Al 4Cu 0. Mg alloy, iron in concentrations as low as 0. Fe. Si constituent. Gallium is an impurity in aluminum and is usually present at levels of 0. At these levels its effect on mechanical properties is quite small. At the 0. 2 level, gallium has been found. Hydrogen has a higher solubility in the liquid state at the melting point than in the solid at the same temperature. Because of this, gas porosity can form during solidification. Hydrogen is. produced by the reduction of water vapor in the atmosphere by aluminum and by the decomposition of hydrocarbons. In addition to causing primary porosity in casting, hydrogen causes secondary. It probably plays a role in grain boundary decohesion during stress corrosion. Its level in melts is controlled by fluxing with hydrogen free gases or by vacuum degassing. Indium. Small amounts 0. Cu. Iron is the most common impurity found in aluminum. It has a high solubility in molten aluminum and is therefore easily dissolved at all molten stages of production. The solubility of iron in the. Lead. Normally present only as a trace element in commercial purity aluminum, lead is added at about the 0. Lithium. The addition of lithium to aluminum can substantially increase strength and, Youngs modulus, provide. The impurity level of lithium is of the order of a few ppm, but at a level of less than 5 ppm it can promote the. Traces of lithium greatly increase the oxidation rate of molten aluminum and alter the surface characteristics of wrought. Magnesium is the major alloying element in the 5xxx series of alloys. Its maximum solid solubility in aluminum is 1. The addition of magnesium markedly increases the strength of aluminum without unduly decreasing the ductility. Corrosion resistance and weldability are good. The addition of magnesium to aluminum increases strength through solid solution strengthening and improves their strain. These alloys are the highest strength nonheat treatable aluminum alloys and are, therefore, used extensively for structural applications. The 5xxx series alloys are produced. The reason for this is that these alloys strain harden quickly and, are, therefore difficult and expensive to extrude.
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