By utilizing various combinations of its advantageous properties such as strength, lightness, corrosion resistance, recyclability and formability, aluminum is being employed in an ever-increasing number of applications. This array of products ranges from structural materials through to thin packaging foils.

Aluminum alloys are alloys in which aluminum (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon 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.

Wrought alloys, which are initially cast as ingots or billets and subsequently hot and/or cold worked mechanically into the desired form.

Cast alloys are directly cast into their final form by one of various methods such as sand-casting, die or pressure die casting. Casting is used for complex product shapes. These alloys contain high levels of silicon to improve their cast ability. In comparison with wrought alloys, casting alloys contain larger proportions of alloying elements such as silicon and copper, which results in a largely heterogeneous cast structure (i.e., one having a substantial volume of second phases). This second phase material warrants careful study, since any coarse, sharp, and brittle constituent can create harmful internal notches and nucleate cracks when the component is later put under load. The fatigue properties are very sensitive to large heterogeneities. As is shown later, good metallurgical and foundry practices can largely prevent such defects.

The elongation and strength, especially in fatigue, of most cast products are relatively lower than those of wrought products. This is because current casting practice is as yet unable to reliably prevent casting defects. In recent years, however, innovations in casting processes such as squeeze casting have brought about some significant improvements in the consistency and level of properties of castings, and these should be taken into account in selecting casting processes for critical applications.

About 85% of aluminum is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminum 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 aluminum alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminum alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.

Temper Designations of the Wrought Alloys

The mechanical properties of one and the same alloy, in terms of composition, can vary drastically depending on the thermo-mechanical processing of the alloy during production or post-production. For this reason, when discussing or selecting a specific alloy, the temper should be specified.The temper of an alloy can be varied / tuned to meet the application requirements. There are two categories of alloys in this respect.

Non heat treatable alloys; for these alloys the mechanical properties are obtained through hot and/or cold working mechanisms during their production and in post-production work hardening operations such as strain hardening, with intermediate and possibly final annealing. This is done for the alloys belonging to the 1xxx, 3xxx, 4xxx and 5xxx series.

Heat treatable alloys; for these alloys the mechanical properties can be tuned through thermal treatment on top of the work hardening processes inherent to their production or post-production. This is possible for alloys belonging to the 2xxx, 6xxx and 7xxx series which can be precipitation or age hardened. There are many possible tempers because changing the heat treatment temperature and/or time results in a different microstructure and consequently a wide variety of mechanical properties may be obtained.

There are 3 basic temper groupings for aluminum products:

“O” – Dead soft (i.e. fully annealed)

“T” – Heat treated (i.e. for age hardening alloys)

“H” – Strain hardened (i.e. for non age hardening alloys)

T-Temper Main Designations

Alloys that are precipitation hardened gain their strength by solution heat treating (SHT) and ageing.

H-Temper Main Designations

Non-heat treatable wrought alloys are strengthened by strain hardening (cold-work). Strain hardened tempers (H) are followed by at least two digits.

Classification of the Wrought Alloy Series

A prefix is used to designate the standard AA of the Aluminum Association or EN AW for the European standard. Wrought aluminum is identified with a four digit number (xxxx) which identifies the alloying elements. The first digit indicates the alloy group. According to this there are 8 series of wrought alloys.

Classification of wrought aluminum alloys

The second indicates modifications to alloy or impurity limit. The last two identify the aluminum alloy or indicates the aluminum purity.

Cast aluminum alloys are identified with a four digit number with a decimal point (xxx.x).

Classification of cast aluminum alloys

The first digit indicates the alloy group. Second two digits indicate the minimum percentage of Al for 1xx.x series and indicate the different aluminum alloys for 2xx.x to 9xx.x series. Last digit (after decimal point) indicates product forms (1: casting, 2: ingot).

For wrought applications, the addition of alloying elements improves mostly the mechanical properties i.e. higher strengths can be obtained, but also other properties are affected. The alloys of the 8 series can have very different mechanical, physical and corrosion properties, and consequently very different behavior during service, surface treatments, welding, forming etc.

These differences are achieved through one of two ways:

1. the addition of different types, quantities and combinations of alloying elements

2. through differences in processing (temper differences)