The wrought alloy designation system consists of four numerical digits, sometimes preceded by a capital letter. The first digit indicates the principal alloying elements, as described next; the second digit is the variation of that alloy; and the last two digits represent the specific alloy designation.
1xxx – Pure Aluminium
The alloys in this group have relatively low strength, even when strain hardened; however, they have extremely high ductility and formability and so may be readily worked or formed. The 1xxx series aluminiums also have exceptionally high electrical conductivity and resistance to all types of corrosive environments and may be readily joined by a number of commercial processes.
2xxx – Copper
As the principal alloying element in this series, copper provides relatively high strength because it yields solution strengthening and the ability to precipitation harden. Many commercial aluminium alloys contain copper as the principal alloying constituent in concentrations from 1 to 10%. Because these alloys naturally age at room temperature, it is advantageous to do any required working or forming of the metal soon after quenching from solution heat treatment. If a delay is needed, it may be desirable to cool them until the mechanical work can be performed.
In the fully hardened (age-hardened) condition, the ductility of 2xxx alloys is generally lower than for some other alloys (except in special variations), and their resistance to atmospheric corrosion is not as good as that of pure aluminium or most non-heat treatable alloys.
Unless given special treatments, 2xxx alloys in the T3 and T4 conditions may be susceptible to stress-corrosion cracking (SCC) when stressed in the short-transverse direction (i.e., normal to the principal plane of grain flow). Precipitation hardening improves resistance to SCC but reduces ductility and toughness.
Most aluminium-copper alloys are not readily welded by commercial processes, but a few alloys such as 2219 and 2195 have been developed especially for applications requiring welding.
3xxx – Manganese
Manganese provides only modest strength increase even when strain hardened but relatively high formability and ductility, and very high resistance to corrosion in almost all environments. Alloys of the 3xxx series are readily weldable and are among the best for brazing and soldering applications.
Commercial aluminium-manganese alloys contain up to 1.2% manganese, but it is appropriate to note that manganese is commonly employed as a supplementary alloying constituent in alloys of the other series to enhance strength.
4xxx – Silicon
There are two types of silicon-bearing aluminium alloys: those with silicon alone, which are not very strong but provide excellent flow and finishing characteristics, and those that also include copper and/or magnesium as well as silicon and so gain strength by solution heat treatment and aging.
The 4xxx alloys are not highly resistant to atmospheric corrosion and tend to “gray” with time in humid environments. Interestingly, this characteristic is used to advantage with finishing techniques such as anodizing to obtain a variety of rich gray shades.
Because silicon adds to their “flow” characteristics during working, some 4xxx alloys (e.g., 4032) are used for complex or finely detailed forgings such as pistons. The 4xxx alloys are readily welded and, in fact, include some of the mostly widely used weld filler alloys, another result of their high fluidity.
5xxx – Magnesium
Magnesium additions to aluminium provide among the highest strength non-heat-treatable alloys. These alloys also are exceptionally tough, absorbing lots of energy during fracture, and so can be used in critical applications where superior toughness is vital. Alloys of the 5xxx series are readily welded by commercial procedures.
Generally, the 5xxx alloys also have excellent resistance to atmospheric and seawater corrosion to the point that they may be used in severe marine environments. However, alloys with more than 3% Mg are not recommended for service in which significant exposure to high temperature may be encountered because some sensitization to SCC may develop. For these types of applications, alloys such as 5052, 5454, and 5754 containing less magnesium are recommended.
6xxx – Magnesium & Silicon
With both magnesium and silicon present, aluminium forms a quasi-binary section with the Mg2Si phase of the magnesium-silicon system, which in turn provides excellent precipitation-hardening capability. This results in modestly higher strengths than possible with non-heat-treatable alloys, combined with generally excellent corrosion resistance.
Alloys of the 6xxx type are among the easiest of aluminium alloys to extrude, and are thus widely used for complex (e.g., multihollow or finned) shapes produced in this manner. In addition, they are readily joined by almost all commercial processes.
As with the 2xxx series, some natural aging begins immediately after solution heat treatment, so forming operations should be scheduled soon after the material is quenched.
7xxx – Zinc
Zinc-bearing aluminium alloys, especially when combined with copper and magnesium, provide the highest strengths of any commercial series.
As a group, these alloys possess relatively poorer atmospheric corrosion resistance compared with other aluminium alloys and, except for some special versions, are less tough and more susceptible to stress-corrosion cracking under short-transverse stressing. Special treatments have been developed to deal with these characteristics and are especially important when the alloys would be subjected to high short transverse stresses in service).
As with the 2xxx and 6xxx series, 7xxx alloys naturally age following heat treatment, so scheduling of any intended forming operations is essential.
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