Aluminium is a versatile and lightweight metal used in a range of applications, from decorative and brightly-coloured trim to load-bearing aeroplane structures. The mechanical properties, formability and quality of surface are determined by the alloy elements, and temper (heat treatment).

Primary aluminium production is energy intensive due to the large amount of electrical power consumed. In the life cycle analysis (LCA) of aluminium, the carbon footprint can be heavily influenced by the indirect greenhouse gas (GHG) emissions, which are the result of energy generation. While the global average kgCO2e/kg for the production of primary aluminium is 15.1 (International Aluminium Institute), China is around 12.6 (Integrated Knowledge for our Environment) to 17.6 (Ecoinvent), North America 8.45 (the Aluminium Association) and Europe 6.7 (European Aluminium). The range is mainly due to the different sources of energy, because around two-thirds of the emissions come from the electrolysis process, one-third from alumina refining and the remainder from cast house operations. So for example, while domestically produced aluminium in Europe has a carbon footprint of 6.7, the average carbon footprint of aluminium consumed in Europe is 8.6 kgCOe/kg.

There are grades available that result in only 4 kgCO2e/kg. For example, Hydro Reduxa manufactured in Norway produced with renewable energy (hydropower in the case of Reduxa) and efficient electrolysis technology. Of course, the grades are limited to the type of aluminium cast there. Using recycled material can bring the kgCO2e/kg to below 2.5, but this can have implications on mechanical properties as well as available grades. The recycling input rate (RIR) for aluminium is around 30%. The RIR is an indicator of the proportion of recycled scrap being consumed in the production of new material in any given region.

Designations for wrought aluminium alloys use four-digit code – assigned by the US-based Aluminum Association, as follows:

1XXX series – 99% aluminium. In this case the last two digits represent the purity, expressed to the nearest 0.00 percent. The second digit indicates impurities: 0 equals natural impurities. These types are relatively soft and ductile, with low strength, and so more easily formed. They have good resistance to corrosion and high conductivity.

For the 2XXX through 8XXX series, the last two digits identify different alloys and the second digit indicates modifications, where zero is the original alloy.

2XXX series – principal alloy copper (Cu). These are some of the strongest aluminium alloys and are utilised in aerospace and demanding automotive, construction and sports equipment. Forming can be more challenging, due to the high strength, and they require surface protection, such as painting or anodising, to avoid corrosion.

3XXX series – principal alloy manganese (Mn). These are medium-strength alloys with good formability and resistance to corrosion. They are utilised in aluminium cans, for example – one of the iconic uses of aluminium.

4XXX series – principal alloy silicon (Si). These alloys have slightly improved formability compared to 3-series, and similar mechanical properties. The Si content results in a slightly darker colour when anodised; a quality utilised by architects.

5XXX series – principal alloy magnesium (Mg). These are non-heat treatable alloys that have excellent surface finish, and produce bright colours when anodised. They are used in decorative architectural and furniture applications. They are among the highest strength of the non-heat treatable alloys, and noted for their toughness (absorbing lots of energy during fracturing).

6XXX series – principal alloys magnesium (Mg) and silicon (Si). These are very popular alloys with a good balance of strength, formability and surface quality. Very-well suited to extrusion, they are utilised in a range of applications, from consumer electronics housing to automotive parts.

7XXX series – principal alloy zinc (Zn). These high-strength alloys, similar to 2-series, are utilised in aerospace, automotive and sports equipment, for example. They have low formability, which limits the complexity of profile that can be extruded.

8XXX series – other. Certain alloys in this group have high-strength, but they are not among the popular commercial types in use.

Casting alloys are identified using a similar numbering system – three digits followed by a decimal – where the first digit designates the major alloying element. The second and third digits identify the alloy in the series and the decimal indicates whether it is a casting (.0) or ingot (.1 or .2).

1XX series – unalloyed, non-heat treatable.

2XX series – principal alloy copper (Cu), heat-treatable.

3XX series – principal alloy silicon (Si) plus copper (Cu) and/or manganese (Mn), heat-treatable.

4XX principal alloy silicon (Si), heat-treatable.

5XX seires – principal alloy magnesium (Mg), non-heat treatable.

7XX series – principal alloy zinc (Zn), heat-treatable.

8XX principal alloy tin (Tn), heat-treatable.

The series classification is followed by temper designation to indicate an alloy’s hardness (or elasticity). Where as heat-treatable alloys acquire their optimal mechanical performance through a process of thermal treatment, non-heat treatable alloys are strengthened by strain hardening (cold working). The five categories are identified as follows: H, non-heat-treated (strain hardened); T, thermally-treated; W, solution heat-treated and aged; O, annealed (soft); and F, untreated (as fabricated).