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Aluminium alloys (or aluminum alloys; see spelling differences) are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al–Si, where the high levels of silicon (4.0–13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.
Alloys composed mostly of aluminium have been very important in aerospace manufacturing since the introduction of metal skinned aircraft. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium.
Aluminium alloy surfaces will formulate a white, protective layer of corrosion aluminium oxide if left unprotected by anodizing and/or correct painting procedures. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminium, and an electrolyte is present that allows ion exchange. Referred to as dissimilar metal corrosion this process can occur as exfoliation or intergranular corrosion. Aluminium alloys can be improperly heat treated. This causes internal element separation and the metal corrodes from the inside out. Aircraft mechanics deal daily with aluminium alloy corrosion.
Aluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International.
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). Selecting the right alloy for a given application entails considerations of its tensile strength, density, ductility, formability, workability, weldability, and corrosion resistance, to name a few. A brief historical overview of alloys and manufacturing technologies is given in Ref. Aluminium alloys are used extensively in aircraft due to their high strength-to-weight ratio. On the other hand, pure aluminium metal is much too soft for such uses, and it does not have the high tensile strength that is needed for airplanes and helicopters.
Aluminium alloys typically have an elastic modulus of about 70 GPa, which is about one-third of the elastic modulus of most kinds of steel and steel alloys. Therefore, for a given load, a component or unit made of an aluminium alloy will experience a greater elastic deformation than a steel part of the identical size and shape. Though there are aluminium alloys with somewhat-higher tensile strengths than the commonly used kinds of steel, simply replacing a steel part with an aluminium alloy might lead to problems.
With completely new metal products, the design choices are often governed by the choice of manufacturing technology. Extrusions are particularly important in this regard, owing to the ease with which aluminium alloys, particularly the Al–Mg–Si series, can be extruded to form complex profiles.
In general, stiffer and lighter designs can be achieved with aluminium alloys than is feasible with steels. For instance, consider the bending of a thin-walled tube: the second moment of area is inversely related to the stress in the tube wall, i.e. stresses are lower for larger values. The second moment of area is proportional to the cube of the radius times the wall thickness, thus increasing the radius (and weight) by 26% will lead to a halving of the wall stress. For this reason, bicycle frames made of aluminium alloys make use of larger tube diameters than steel or titanium in order to yield the desired stiffness and strength. In automotive engineering, cars made of aluminium alloys employ space frames made of extruded profiles to ensure rigidity. This represents a radical change from the common approach for current steel car design, which depend on the body shells for stiffness, known as unibody design.
Aluminium alloys are widely used in automotive engines, particularly in cylinder blocks and crankcases due to the weight savings that are possible. Since aluminium alloys are susceptible to warping at elevated temperatures, the cooling system of such engines is critical. Manufacturing techniques and metallurgical advancements have also been instrumental for the successful application in automotive engines. In the 1960s, the aluminium cylinder heads of the Corvair earned a reputation for failure and stripping of threads, which is not seen in current aluminium cylinder heads.
An important structural limitation of aluminium alloys is their lower fatigue strength compared to steel. In controlled laboratory conditions, steels display a fatigue limit, which is the stress amplitude below which no failures occur – the metal does not continue to weaken with extended stress cycles. Aluminium alloys do not have this lower fatigue limit and will continue to weaken with continued stress cycles. Aluminium alloys are therefore sparsely used in parts that require high fatigue strength in the high cycle regime (more than 107 stress cycles).
Often, the metal's sensitivity to heat must also be considered. Even a relatively routine workshop procedure involving heating is complicated by the fact that aluminium, unlike steel, will melt without first glowing red. Forming operations where a blow torch is used can reverse or remove heat treating, therefore is not advised whatsoever. No visual signs reveal how the material is internally damaged. Much like welding heat treated, high strength link chain, all strength is now lost by heat of the torch. The chain is dangerous and must be discarded.
Aluminium also is subject to internal stresses and strains when it is overheated; the tendency of the metal to creep under these stresses tends to result in delayed distortions. For example, the warping or cracking of overheated aluminium automobile cylinder heads is commonly observed, sometimes years later, as is the tendency of improperly welded aluminium bicycle frames to gradually twist out of alignment from the stresses of the welding process. Thus, the aerospace industry avoids heat altogether by joining parts with rivets of like metal composition, other fasteners, or adhesives.
Stresses in overheated aluminium can be relieved by heat-treating the parts in an oven and gradually cooling it—in effect annealing the stresses. Yet these parts may still become distorted, so that heat-treating of welded bicycle frames, for instance, can result in a significant fraction becoming misaligned. If the misalignment is not too severe, the cooled parts may be bent into alignment. Of course, if the frame is properly designed for rigidity (see above), that bending will require enormous force.
Aluminium's intolerance to high temperatures has not precluded its 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; in fact the extremely high thermal conductivity of aluminium prevented the throat from reaching the melting point even under massive heat flux, resulting in a reliable lightweight component.
Because of its high conductivity and relatively low price compared with copper in the 1960s, aluminium was introduced at that time for household electrical wiring in North America, even though many fixtures had not been designed to accept aluminium wire. But the new use brought some problems:
All of this resulted in overheated and loose connections, and this in turn resulted in some fires. Builders then became wary of using the wire, and many jurisdictions outlawed its use in very small sizes, in new construction. Yet newer fixtures eventually were introduced with connections designed to avoid loosening and overheating. At first they were marked "Al/Cu", but they now bear a "CO/ALR" coding.
Another way to forestall the heating problem is to crimp the aluminium wire to a short "pigtail" of copper wire. A properly done high-pressure crimp by the proper tool is tight enough to reduce any thermal expansion of the aluminium. Today, new alloys, designs, and methods are used for aluminium wiring in combination with aluminium terminations.
Wrought and cast aluminium alloys use different identification systems. Wrought aluminium is identified with a four digit number which identifies the alloying elements.
Cast aluminium alloys use a four to five digit number with a decimal point. The digit in the hundreds place indicates the alloying elements, while the digit after the decimal point indicates the form (cast shape or ingot).
Note: -W is a relatively soft intermediary designation that applies after heat treat and before aging is completed. The -W condition can be extended at extremely low temperatures but not indefinitely and depending on the material will typically last no longer than 15 minutes at ambient temperatures.
The International Alloy Designation System is the most widely accepted naming scheme for wrought alloys. Each alloy is given a four-digit number, where the first digit indicates the major alloying elements.
|1100||0.95 Si+Fe||0.05–0.20||0.05||0.10||0.05||0.15||99.0 min|
|†Manganese plus chromium must be between 0.12–0.50%.|
††This column lists the limits that apply to all elements, whether a table column exists for them or not, for which no other limits are specified.
The Aluminum Association (AA) has adopted a nomenclature similar to that of wrought alloys. British Standard and DIN have different designations. In the AA system, the second two digits reveal the minimum percentage of aluminium, e.g. 150.x correspond to a minimum of 99.50% aluminium. The digit after the decimal point takes a value of 0 or 1, denoting casting and ingot respectively. The main alloying elements in the AA system are as follows:
|Alloy type||Temper||Tensile strength (min) [ksi] ([MPa])||Yield strength (min) [ksi] ([MPa])||Elongation in 2 in [%]|
|201.0||A02010||T7||60.0 (414)||50.0 (345)||3.0|
|204.0||A02040||T4||45.0 (310)||28.0 (193)||6.0|
|T61||32.0 (221)||20.0 (138)||N/A|
|295.0||A02950||T4||29.0 (200)||13.0 (90)||6.0|
|T6||32.0 (221)||20.0 (138)||3.0|
|T62||36.0 (248)||28.0 (193)||N/A|
|T7||29.0 (200)||16.0 (110)||3.0|
|319.0||A03190||F||23.0 (159)||13.0 (90)||1.5|
|T6||31.0 (214)||20.0 (138)||1.5|
|328.0||A03280||F||25.0 (172)||14.0 (97)||1.0|
|T6||34.0 (234)||21.0 (145)||1.0|
|355.0||A03550||T6||32.0 (221)||20.0 (138)||2.0|
|T51||25.0 (172)||18.0 (124)||N/A|
|T71||30.0 (207)||22.0 (152)||N/A|
|C355.0||A33550||T6||36.0 (248)||25.0 (172)||2.5|
|356.0||A03560||F||19.0 (131)||9.5 (66)||2.0|
|T6||30.0 (207)||20.0 (138)||3.0|
|T51||23.0 (159)||16.0 (110)||N/A|
|T71||25.0 (172)||18.0 (124)||3.0|
|A356.0||A13560||T6||34.0 (234)||24.0 (165)||3.5|
|T61||35.0 (241)||26.0 (179)||1.0|
|443.0||A04430||F||17.0 (117)||7.0 (48)||3.0|
|B443.0||A24430||F||17.0 (117)||6.0 (41)||3.0|
|512.0||A05120||F||17.0 (117)||10.0 (69)||N/A|
|514.0||A05140||F||22.0 (152)||9.0 (62)||6.0|
|520.0||A05200||T4||42.0 (290)||22.0 (152)||12.0|
|535.0||A05350||F||35.0 (241)||18.0 (124)||9.0|
|705.0||A07050||T5||30.0 (207)||17.0 (117)†||5.0|
|707.0||A07070||T7||37.0 (255)||30.0 (207)†||1.0|
|710.0||A07100||T5||32.0 (221)||20.0 (138)||2.0|
|712.0||A07120||T5||34.0 (234)||25.0 (172)†||4.0|
|713.0||A07130||T5||32.0 (221)||22.0 (152)||3.0|
|771.0||A07710||T5||42.0 (290)||38.0 (262)||1.5|
|T51||32.0 (221)||27.0 (186)||3.0|
|T52||36.0 (248)||30.0 (207)||1.5|
|T6||42.0 (290)||35.0 (241)||5.0|
|T71||48.0 (331)||45.0 (310)||5.0|
|852.0||A08520||T5||24.0 (165)||18.0 (124)||N/A|
|†Only when requested by the customer|
The addition of scandium to aluminium creates nanoscale Al3Sc precipitates which limit the excessive grain growth that occurs in the heat-affected zone of welded aluminium components. This has two beneficial effects: the precipitated Al3Sc forms smaller crystals than are formed in other aluminium alloys and the width of precipitate-free zones that normally exist at the grain boundaries of age-hardenable aluminium alloys is reduced. Scandium is also a potent grain refiner in cast aluminium alloys, and atom for atom, the most potent strengthener in aluminium, both as a result of grain refinement and precipitation strengthening. However, titanium alloys, which are stronger but heavier, are cheaper and much more widely used.
The main application of metallic scandium by weight is in aluminium-scandium alloys for minor aerospace industry components. These alloys contain between 0.1% and 0.5% (by weight) of scandium. They were used in the Russian military aircraft Mig 21 and Mig 29.
Some items of sports equipment, which rely on high performance materials, have been made with scandium-aluminium alloys, including baseball bats, lacrosse sticks, as well as bicycle frames and components, and tent poles. U.S. gunmaker Smith & Wesson produces revolvers with frames composed of scandium alloy and cylinders of titanium. 
These alloys are used for boat building and shipbuilding, and other marine and salt-water sensitive shore applications.
4043, 5183, 6005A, 6082 also used in marine constructions and off shore applications.
These alloys are used for cycling frames and components
6111 aluminium and 2008 aluminium alloy are extensively used for external automotive body panels, with 5083 and 5754 used for inner body panels. Hoods have been manufactured from 2036, 6016, and 6111 alloys. Truck and trailer body panels have used 5456 aluminum.
Wheels have been cast from A356.0 aluminium or formed 5xxx sheet.