Aluminium alloy

Aluminium alloys or aluminum alloys are alloys of aluminium, 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 aluminium. Bare aluminium 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 aluminium alloy is placed in proximity to stainless steel in a wet environment. Aluminium alloy and stainless steel parts should not be mixed in water-containing systems or outdoor installations.

Aluminium alloy compositions are registered with the Aluminium 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, ^[1] and the ASTM.

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Engineering use

Overview

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.

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 that 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 buckling failure.

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 than the steel parts they replace, but they possess an airy gracefulness that 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 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 hurt aluminium's reputation for this use. However, the widespread use of aluminium components in the aerospace and high-performance automotive 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 that 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 that was recently restored by the Audi factory, noted that the engine's original aluminium alloy would today be used only for lawn furniture and the like. As recently as the 1960s, the aluminium cylinder heads and crankcase of the Corvair 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.

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.

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

Alloy designations

Wrought and cast aluminium alloys use different identification systems. Wrought aluminium 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 aluminium alloy. Cast aluminium 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)

Wrought alloys

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.

• 1000 series are essenitally pure aluminium with a minimum 99% aluminium 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 aluminium alloy.
• 8000 series are a miscellaneous category

Wrought Aluminium Alloy Composition Limits (% weight)

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.

Cast alloys

The Aluminium 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 ^[4]. The main alloying elements in the AA system are as follows:

• 1xx.x series are minimum 99% aluminium
• 2xx.x series copper
• 3xx.x series silicon, copper and/or magnesium
• 4xx.x series silicon
• 5xx.x series magnesium
• 7xx.x series zinc
• 8xx.x series tin
• 9xx.x series miscellaneous

Named alloys

• Duralumin (copper, aluminium)
• Magnox (magnesium, aluminium)
• Zamak (zinc, aluminium, magnesium, copper)
• Silumin (aluminium, silicon)

Overview of use

Common aerospace alloys

These are aluminium alloys which have a long history of being used in aircraft and other aerospace structures. ^[5]

• 7075 aluminium
• 6061 aluminium
• 6063 aluminium
• 2024 aluminium
• 5052 aluminium

Other aerospace alloys

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

• 2090 aluminium
• 2124 aluminium
• 2195 aluminium - Al-Li alloy, used in Space Shuttle Super Lightweight external tank
• 2219 aluminium
• 2324 aluminium
• 6013 aluminium
• 7050 aluminium
• 7055 aluminium
• 7150 aluminium
• 7475 aluminium

Marine alloys

These alloys are used for boatbuilding and shipbuilding, and other marine and salt-water sensitive shore applications. ^[6]

• 5052 aluminium
• 5083 aluminium
• 5086 aluminium
• 6061 aluminium
• 6063 aluminium

References