Basic rules for alloy classification

The classification of aluminium alloys must take into account three different levels, mutually interacting:

1. Chemical composition – Refers to alloying elements’ type and amount;

2. Process – Some alloys are suitable for the various kinds of hot and/or cold deformation processes (extrusion, rolling, forging, stamping)—they are the so-called wrought alloys—and some others are adequate only for foundry processes—they are the so-called casting alloys;

3. Metallurgical state – Some alloys can be microstructurally (and mechanically) optimized by proper heat treatments (the so-called heat treatable alloys), while others can improve their microstructure and mechanical behaviour by cold working.

Thus, very clear rules are necessary for the proper designation of alloys, allowing, at the same time, the identification of chemical composition, processing attitude, and metallurgical state.

(1) Chemical composition

Aluminium can be alloyed by adding five main alloying elements—copper, manganese, silicon, magnesium, and zinc—along with other specific elements added on a case-by-case basis. This process is summarized in Figure 1.

_{Figure 1. The main effects of each minor alloying element.}

In more detail, the addition of copper and/or zinc is targeted at maximizing mechanical behaviour, in both wrought and cast alloys, by means of heat treatment. Manganese and/or silicon and/or magnesium are used in various combinations to achieve a well-balanced compromise of key properties in alloys, such as weldability, corrosion resistance, and formability.

A lot of other elements (minor elements) may be added, in specific cases, to achieve special results; e.g. sodium or strontium modify (and improve) microstructure in casting alloys, while titanium and boron are excellent grain refiners.

Iron’s role is worth mentioning. It can be considered an impurity in almost all cases, negatively affecting the mechanical behaviour and ductility of alloys; however, it is the main contaminant deriving from recycling processes, so it must be carefully monitored.

You can explore all these features through our interactive infographic in Step 1.14 Explore aluminium alloys.

(2) Processing attitude as a result of composition

For these reasons, wrought and casting alloys must be approached separately when establishing a designation system.

(3) Heat treatment and metallurgical state

When developing the Zeppelin dirigibles (around 1906), German researchers discovered that proper heat treatment, primarily involving heating to about 500°C, water quenching and then ageing for a few hours at 150-200°C (temperatures depending on the specific alloy)significantly enhances the mechanical properties of aluminium-copper alloys. In the following years, the same behaviour was observed in aluminium-zinc and aluminium-silicon-magnesium alloys. This improvement is attributed to the evolution of the alloy’s microstructure:

• Heating at about 500°C leads to a solid solution, in which all alloying elements are dissolved within the aluminium matrix;
• Water quenching allows to “freeze” such microstructure, producing a super-saturated solid solution;
• Ageing at relatively low temperatures (150-200°C, but in few cases even at room temperature!) activates, by means of solid-state diffusion, the formation of very small chemical compounds (e.g. CuAl2 or Mg2Si), also called precipitates, which are coherent with the aluminium matrix and strengthen the alloy.

If their composition is favourable (Al-Cu, Al-Zn, Al-Si-Mg), alloys are defined as heat treatable, and various heat treatment options are available. Heat treatment is a fundamental step in their manufacturing route and must be mentioned when the alloy is selected and used.

In all other cases, i.e. pure aluminium and the systems Al-Mn, Al-Si, Al-Mg, alloys are not heat treatable. Their mechanical behaviour is usually assessed by cold deformation (work hardening), which can be performed to various extents.

Thus, when an alloy is designated, its metallurgical state (heat-treated or work-hardened) must be specific. For instance, the most common heat treatments are associated with the codes T6 (artificial ageing) and T4 (natural ageing), and the degree of work hardening is defined, in the simplest cases, as H1, H2, H3 or H4.

The most adopted aluminium alloy designation systems

As a direct consequence of the issues presented above, aluminium alloy designation systems take into account:

• Processing attitude – We have different standards for wrought and casting alloys
• Composition – Designation codes allow immediate identification of main constituents
• Metallurgical state – Designation codes must be followed by the code associated with heat treatment or work hardening conditions.

The simplest rules are those related to wrought alloys, whose designation system is common worldwide, based on four digits:

Figure 2 describes the designation system for wrought aluminium alloys.

_{Figure 2. Designation system for wrought aluminium alloys}

The designation system for casting alloys is different between Europe (EN standards) and North America (ASTM-AA standards), as displayed in Figures 3 and 4. EN standards identify alloys suitable for foundry processes by means of 5 digits; similarly to wrought alloys, we have:

Other systems are not used, and thus not mentioned. Due to their widespread use and adaptability to various foundry processes, there are 8 sub-groups of Al-Si casting alloys, from 41XXX to 48XXX.

_{Figure 3. European designation system for casting alloys}

North American standards follow a different approach with 3+1 digits, differentiating Al-Si alloys based on their response to heat treatment: 3XX.X are heat-treatable alloys, while 2XX.X are not.

_{Figure 4. North American designation system for casting alloys}

More detailed information about rules for aluminium alloy classification can be found in the related links and will appear at the bottom of the step under the heading See also.

At this link you can find the complete catalogue of European standards for aluminium.