The term "alloy" is used to describe a mixture of a base metal with one or more alloying elements, which can be either metallic or non-metallic (like silicon). The aim is to improve the properties of the base metal, especially its strength and corrosion resistance. Thus, like practically all industrially used metals, aluminium is also used mainly in the form of alloys, the exceptions being unalloyed aluminium and refined aluminium.
In European standards, different designations are possible. Aluminium alloys have the prefix EN AW (EN = European standard, AW = Aluminium Wrought Alloy) or EN AC (Aluminium Cast Alloy). There is an alphanumerical system that starts with the letters for the alloy base (Al for aluminium) followed by the chemical symbols of the alloying elements with their contents in per cent in descending order of alloy content. For example: AlMg4,5Mn0,7 is an aluminium alloy containing 4.5 per cent magnesium and 0.7 per cent manganese.
There is also a numerical system with alloy numbers (wrought alloys have four digits, cast alloys have five). This avoids possible misunderstandings between the old DIN system and the new EN system. Thus, the alloy mentioned above has the designation EN AW-5083. The cast alloy EN AC-43300 can also be referred to as EN AC-AlSi9Mg.
Sometimes product names are also used, for example Duralumin for certain aluminium"manganese alloys.
Aluminium alloys are made by melting, sintering or mechanical mixing. Depending on the most suitable method of processing one differentiates between casting and wrought alloys, and in both cases between age-hardenable and non-age-hardenable alloys:
- The degree of cold working (cold forming) has a marked influence on strength.
- In non-age-hardenable alloys, the alloying elements (at low concentration) are completely dissolved. These materials have good formability.
- In age-hardenable alloys, the alloying elements added are present at room temperature in the form of precipitates. Their distribution determines the strength of the alloy. By solution heat treating at 450-550 °C, the elements are taken completely into solution. This state is frozen in by quenching the alloy. Controlled precipitation at room temperature or a slightly elevated temperature (approx. 140-190 °C) increases the strength to a level that is far in excess of that achievable by forming.
These are alloys with silicon, magnesium and copper that can only be shaped by casting and which contain up to 20 per cent of alloying elements. Castings are usually not processed further. The base metal is mostly recycled aluminium ("secondary aluminium"). Primary aluminium is only used for special applications. Good castability (thanks to silicon) takes precedence over strength.
These alloys, which are intended to have good formability (for example for extrusion, forging or rolling), contain up to ten per cent of alloying elements. Magnesium additions produce alloys that are non-age-hardenable but seawater-resistant. An example of an age-hardenable alloy is the classical alloy Duralumin (brand name) that was developed in 1906 by the German Alfred Wilm and which contains copper, manganese and magnesium.
- Mechanical alloying: if melting or sintering is not possible, for example with ceramics, powders of aluminium and the alloying element are ground together in high-energy ball mills, whereby a part of the powder welds together and alloys. It is also possible to alloy under explosive pressure, for example with titanium powder to produce strong lightweight alloys.
- Aluminium"lithium alloys have a lower density than unalloyed aluminium and are thus of interest, for example, for space travel. They are normally only used in such applications because of their high manufacturing costs.