Aluminium alloy

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Aluminium alloys or aluminum alloys are alloys of aluminum, often with copper, zinc, manganese, silicon, or magnesium. They are much lighter and more corrosion resistant than plain carbon steel, but not quite as corrosion resistant as pure aluminum. Bare aluminum alloy surfaces will keep their apparent shine in a dry environment, but light amounts of corrosion products rub off easily onto skin when touched. Galvanic corrosion can be rapid when aluminum alloy is placed in proximity to stainless steel in a wet environment. Aluminum alloy and stainless steel parts should not be mixed in water-containing systems or outdoor installations.

Aluminum alloy compositions are registered with the Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminum alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, [1] and the ASTM.

Contents

[edit] Overview: engineering use and the problem of replacing other metals with Al alloy

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 strength, ductility, formability, weldability and corrosion resistance to name a few. A brief historical overview of alloys and manufacturing technologies is given in Ref.[2] Aluminium is used extensively in modern aircraft due to its high strength to weight ratio.

[edit] Flexibility considerations

Improper use of aluminium may result in problems, particularly in contrast to iron or steel, which appear "better behaved" to the intuitive designer, mechanic, or technician. The reduction by two thirds of the weight of an aluminium part compared with a similarly sized iron or steel part seems enormously attractive, but it must be noted that this replacement is accompanied by a reduction by two thirds in the stiffness of the part. Therefore, although direct replacement of an iron or steel part with a duplicate made from aluminium may still give acceptable strength to withstand peak loads, the increased flexibility will cause three times more deflection in the part.

Where failure is not an issue but excessive flex is undesirable due to requirements for precision of location, or efficiency of transmission of power, simple replacement of steel tubing with similarly sized aluminium tubing will result in a degree of flex which is undesirable; for instance, the increased flex under operating loads caused by replacing steel bicycle frame tubing with aluminium tubing of identical dimensions will cause misalignment of the power-train as well as absorbing the operating force. To increase the rigidity by increasing the thickness of the walls of the tubing increases the weight proportionately, so that the advantages of lighter weight are lost as the rigidity is restored.

In such cases, aluminium may best be used by redesigning the dimension of the part to suit its characteristics; for instance making a bicycle frame of aluminium tubing which has an oversize diameter rather than thicker walls. In this way, rigidity can be restored or even enhanced without increasing weight.[3] The limit to this process is the increase in susceptibility to what is termed "buckling" failure, where the deviation of the force from any direction other than directly along the axis of the tubing, causes folding of the walls of the tubing.

The latest models of the Corvette automobile, among others, are a good example of redesigning parts to make best use of aluminium's advantages. The aluminium chassis members and suspension parts of these cars have large overall dimensions for stiffness but are lightened by reducing cross-sectional area and removing unneeded metal; as a result, they are not only equally or more durable and stiff as the usual steel parts, but they possess an airy gracefulness which most people find attractive. Similarly, aluminium bicycle frames can be optimally designed so as to provide rigidity where required, yet exhibit some extra flexibility which functions as a natural shock-absorber for the rider.

The strength and durability of aluminium varies widely, not only as a result of the components of the specific alloy, but also as a result of the particular manufacturing process. This variability, plus a learning-curve in employing it, has from time to time gained aluminium a bad reputation. For instance, a high frequency of failure in many poorly-designed early aluminium bicycle frames in the 1970s, temporarily hurt aluminium's reputation for this use. However, the widespread use of aluminium components in the aerospace and automotive high performance industries, where huge stresses are withstood with vanishingly small failure rates, illustrates that properly-built aluminium bicycle components need not be intrinsically unreliable. Time and experience has subsequently proven this to be the case.

Similarly, use of aluminium in automotive applications, particularly in engine parts which must survive in difficult conditions, has benefited from development over time. An Audi engineer, in commenting about the V12 engine, producing over 500 horsepower (370 kW), of an Auto Union race car of the 1930s which was recently restored by the Audi factory, that the aluminium alloy of which the engine was constructed would today be used only for lawn furniture and the like. Even the aluminium cylinder heads and crankcase of the Corvair, built as recently as the 1960s, earned a reputation for failure and stripping of threads in holes, even as large as spark plug holes, which is not seen in current aluminium cylinder heads.

One important structural limitation of an aluminium alloy is its fatigue properties. While steel has a high fatigue limit (the structure can theoretically withstand an infinite number of cyclical loadings at this stress), aluminium's fatigue limit is near zero, meaning that it will eventually fail under even very small cyclic loadings, but for small stresses this can take an exceedingly long time.

[edit] Heat sensitivity considerations

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 therefore requires some expertise, since no visual signs reveal how close the material is to melting.

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 instance, the warping or cracking of overheated aluminium automobile cylinder heads is commonly observed, sometimes years later, as is the tendency of 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 adhesives or mechanical fasteners. Adhesive bonding was used in some bicycle frames in the 1970s, with unfortunate results when the aluminium tubing corroded slightly, loosening the adhesive and collapsing the frame.

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.

[edit] The special problem of household wiring

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 the United States, even though many fixtures had not been designed to accept aluminium wire. But the new use brought some problems:

  • The greater coefficient of thermal expansion of aluminium causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection.
  • Pure aluminium has a tendency to "creep" under steady sustained pressure (to a greater degree as the temperature rises), again loosening the connection.
  • Galvanic corrosion from the dissimilar metals increases the electrical resistance of the connection.

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.

See also: Aluminium wire

[edit] Alloy designations

Wrought and cast aluminum alloys use different identification systems. Wrought aluminum is identified with a four digit number which identifies the alloying elements, followed by a dash, a letter identifying the type of heat treatment and a 1 to 4 digit number identifying the specific temper, e.g. 6061-T6, the most common free-machining aluminum alloy. Cast aluminum alloys use a four to five digit number with a decimal point. The digit in the hundred's place indicates the alloying elements, while the digit after the decimal point indicates the form (cast shape or ingot)

[edit] Wrought alloys

  • 1000 series are essenitally pure aluminum with a minimum 99% aluminum content by weight and can be work hardened
  • 2000 series are alloyed with copper, can be precipitation hardened to strengths comparable to steel. Formerly referred to as duralumin, they were once the most common aerospace alloys, but were susceptible to stress corrosion cracking and are increasingly replaced by 7000 series in new designs.
  • 3000 series are alloyed with manganese, and can be work hardened
  • 4000 series are alloyed with silicon. They are also known as silumin
  • 5000 series are alloyed with magnesium, derive most of their strength from solution hardening, and can also be work hardened to strengths comparable to steel
  • 6000 series are alloyed with magnesium and silicon, are easy to machine, and can be precipitation hardened, but not to the high strengths that 2000, 5000 and 7000 can reach.
  • 7000 series are alloyed with zinc, and can be precipitation hardened to the highest strengths of any aluminum alloy.
  • 8000 series are a miscellaneous category
Wrought Aluminum Alloy Composition Limits (% weight)
Alloy Si Fe Cu Mn Mg Cr Zn V Ti Bi Ga Pb Zr Other Al
each total
1060 0.25 0.35 0.05 0.03 0.03 0.03 0.05 0.05 0.03 0.03 0.03 0.03 0.03 0.03 99.6 min
1100 0.95 Si+Fe 0.05-0.20 0.05 0.10 0.05 0.15 99.0 min
2014 0.50-1.2 0.7 3.9-5.0 0.40-1.2 0.20-0.8 0.10 0.25 0.15 0.05 0.15 remainder
2024 0.50 0.50 3.8-4.9 0.30-0.9 1.2-1.8 0.10 0.25 0.15 0.05 0.15 remainder
2219 0.20 0.30 5.8-6.8 0.20-0.40 0.02 0.10 0.05-0.15 0.02-0.10 0.10-0.25 0.05 0.15 remainder
3003 0.6 0.7 0.05-0.20 1.0-1.5 0.10 0.05 0.15 remainder
3004 0.30 0.7 0.25 1.0-1.5 0.8-1.3 0.25 0.05 0.15 remainder
3102 0.40 0.7 0.10 0.05-0.40 0.30 0.10 0.05 0.15 remainder
5052 0.25 0.40 0.10 0.10 2.2-2.8 0.15-0.35 0.10 0.05 0.15 remainder
5083 0.40 0.40 0.10 0.40-1.0 4.0-4.9 0.05-0.25 0.25 0.15 0.05 0.15 remainder
5086 0.40 0.50 0.10 0.20-0.7 3.5-4.5 0.05-0.25 0.25 0.15 0.05 0.15 remainder
5154 0.25 0.40 0.10 0.10 3.1-3.9 0.15-0.35 0.20 0.20 0.05 0.15 remainder
5454 0.25 0.40 0.10 0.50-1.0 2.4-3.0 0.05-0.20 0.25 0.20 0.05 0.15 remainder
5456 0.25 0.40 0.10 0.50-1.0 4.7-5.5 0.05-0.20 0.25 0.20 0.05 0.15 remainder
6005 0.6-0.9 0.35 0.10 0.10 0.40-0.6 0.10 0.10 0.10 0.05 0.15 remainder
6005A 0.50-0.9 0.35 0.30 0.50 0.40-0.7 0.30 0.20 0.10 0.05 0.15 remainder
6060 0.30-0.6 0.10-0.30 0.10 0.10 0.35-0.6 0.5 0.15 0.10 0.05 0.15 remainder
6061 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 0.05 0.15 remainder
6063 0.20-0.6 0.35 0.10 0.10 0.45-0.9 0.10 0.10 0.10 0.05 0.15 remainder
6066 0.9-1.8 0.50 0.7-1.2 0.6-1.1 0.8-1.4 0.40 0.25 0.20 0.05 0.15 remainder
6070 1.0-1.7 0.50 0.15-0.40 0.40-1.0 0.50-1.2 0.10 0.25 0.15 0.05 0.15 remainder
6105 0.6-1.0 0.35 0.10 0.10 0.45-0.8 0.10 0.10 0.10 0.05 0.15 remainder
6162 0.40-0.8 0.50 0.20 0.10 0.7-1.1 0.10 0.25 0.10 0.05 0.15 remainder
6262 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.14 0.25 0.15 0.40-0.7 0.40-0.7 0.05 0.15 remainder
6351 0.7-1.3 0.50 0.10 0.40-0.8 0.40-0.8 0.20 0.20 0.05 0.15 remainder
6463 0.20-0.6 0.15 0.20 0.05 0.45-0.9 0.05 0.05 0.15 remainder
7005 0.35 0.40 0.10 0.20-0.7 1.0-1.8 0.06-0.20 4.0-5.0 0.01-0.06 0.08-0.20 0.05 0.15 remainder
7072 0.7 Si+Fe 0.10 0.10 0.10 0.8-1.3 0.05 0.15 remainder
7075 0.40 0.50 1.2-2.0 0.30 2.1-2.9 0.18-0.28 5.1-6.1 0.20 0.05 0.15 remainder
7116 0.15 0.30 0.50-1.1 0.05 0.8-1.4 4.2-5.2 0.05 0.05 0.03 0.05 0.15 remainder
7129 0.15 0.30 0.50-0.9 0.10 1.3-2.0 0.10 4.2-5.2 0.05 0.05 0.03 0.05 0.15 remainder
7178 0.40 0.50 1.6-2.4 0.30 2.4-3.1 0.18-0.28 6.3-7.3 0.20 0.05 0.15 remainder

The "other" limits apply to all elements, whether a table column exists for them or not, for which no other limit is specified. alloy 6005A has another limit not shown above: the manganese plus chromium must be in the range of 0.12-0.50.

[edit] Cast alloys

  • x1xx.x series are minimum 99% aluminum
  • x2xx.x series copper
  • x3xx.x series silicon, copper and/or magnesium
  • x4xx.x series silicon
  • x5xx.x series magnesium
  • x7xx.x series zinc
  • x8xx.x series tin
  • x9xx.x series miscellaneous

[edit] Named alloys

[edit] Common aluminum alloys

These alloys are among the most commonly found alloys.

  • 2011: Wire, rod, and bar for screw machine products. Applications where good machinability and good strength are required.
  • 2014: Heavy-duty forgings, plate, and extrusions for aircraft fittings, wheels, and major structural components, space booster tankage and structure, truck frame and suspension components. Applications requiring high strength and hardness including service at elevated temperatures.
  • 2024: Aircraft structures, rivets, hardware, truck wheels, screw machine products, and other miscellaneous structural applications. The first age-hardened alloy ever discovered.
  • 2036: Sheet for auto body panels.
  • 2048: Sheet and plate in structural components for aerospace application and military equipment.
  • 2141: Plate in thicknesses of 40 to 150 mm (1.5 to 6.0 in.) for aircraft structures.
  • 2218: Forgings; aircraft and diesel engine pistons; aircraft engine cylinder heads; jet engine impellers and compressor rings.
  • 2219: Welded space booster oxidizer and fuel tanks, supersonic aircraft skin and structure components. Readily weldable and useful for applications over temperature range of -270 to 300 °C (-450 to 600 °F). Has high fracture toughness, and the T8 temper is highly resistant to stress-corrosion cracking.
  • 2618: Die and hand forgings. Pistons and rotating aircraft engine parts for operation at elevated temperatures. Tire molds.

[edit] Common aerospace alloys

These are aluminum alloys which have a long history of being used in aircraft and other aerospace structures. [4]

[edit] Other aerospace alloys

These are currently produced, but less widely used, aluminum alloys for aerospace applications.

[edit] Marine alloys

These alloys are used for boatbuilding and shipbuilding, and other marine and salt-water sensitive shore applications. [5]

[edit] References

  1. ^ SAE Aluminum specifications list, accessed Oct 8, 2006. Also SAE Aerospace Council, accessed Oct 8, 2006.
  2. ^ R.E. Sanders, Technology Innovation in Aluminium Products, The Journal of The Minerals, 53(2):21–25, 2001. Online ed.
  3. ^ For the technically minded, for a tube of constant wall thickness, stiffness scales as the cube of the diameter, whereas mass scales proportionally. So an aluminium tube with twice the diameter of steel tube, but the same wall thickness, will be roughly 8/3 stiffer and 2/3 the weight. If 1.5 times the diameter, it will be roughly the same stiffness and half the weight, and so on.
  4. ^ Fundamentals of Flight, Shevell, Richard S., 1989, Englewood Cliffs, Prentice Hall, ISBN 0-13-339060-8, Ch 18, pp 373-386.
  5. ^ Boatbuilding with Aluminum, Stephen F. Pollard, 1993, International Marine, ISBN 0-07-050426-1

6064 aluminium alloy is the hardest by 0.012%

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