Aluminium specifications rely on a four-digit index system to designate both cast and wrought aluminium alloys. This standardized aluminium alloy designation system makes it possible to identify, classify, and select the right material for a given application with confidence.
These two broad classes of aluminium alloys can be subdivided further into families based on their chemical composition and temper designation. Because pure aluminium is alloyed with many other metals to achieve a wide range of physical and mechanical properties, the alloying elements serve as the foundation for sorting aluminium alloys into two main categories: non-heat-treatable and heat-treatable.
Most aluminium specifications designate alloys using a consistent four-digit format. The first digit identifies the principal alloying constituent (or constituents), while the second digit indicates variations of the initial alloy. The third and fourth digits identify individual alloy variations; these numbers carry no specific meaning on their own but are unique to each alloy.
Figure 1: Explanation of the Four-Digit Aluminium Alloy Designation System
The wrought aluminium alloys are organized into the following series, each defined by its primary alloying element:
The designation system for cast aluminium alloys resembles the wrought system in several respects. It also uses four digits, and the first digit again specifies the major alloying constituent. The key difference is that a decimal point separates the third and fourth digits. This decimal clarifies whether the designation refers to an alloy in the form of a casting (.0) or a foundry ingot (.1 or .2).
A letter placed before the numerical designation signals a modification of the original alloy or a change in impurity limits. These serial letters are assigned in alphabetical order beginning with A, but they omit I, O, Q, and X, with X reserved for experimental alloys.
The cast aluminium alloys are grouped as follows:
Tempers play a critical role in aluminium specifications because they directly influence material properties. The temper designation system is based on the sequence of mechanical treatments, thermal treatments, or a combination of both used to produce a given temper.
The temper designation always appears immediately after the alloy designation, separated by a hyphen—for example, 2014-T6, 3003-H14, or 1350-H19 (extra hard). The first character in the temper designation is a capital letter that indicates the general class of treatment:
It is important to note that temper designations differ between non-heat-treatable and heat-treatable alloys. Their meanings are explained below.
For non-heat-treatable alloys, the letter "H" is always followed by two or three digits. The first digit identifies the method used to obtain the temper:
The second digit indicates the degree of hardness: 2 is quarter hard, 4 is half hard, 6 is three-quarter hard, 8 is full hard, and 9 is extra hard. Any additional digit indicates a modification of standard practice.
Non-heat-treatable alloys are found mainly in the 1xxx, 3xxx, 4xxx, and 5xxx series, depending on their major alloying elements.
For heat-treatable alloys, the letter "T" is always followed by one or more digits that indicate the method used to produce the stable temper.
Common examples include:
Heat-treatable alloys are found primarily in the 2xxx, 6xxx, and 7xxx series.
This article explains the four-digit designation and temper system used to define aluminium alloys. The related guides in the Aluminium series cover the topics you will need alongside it. Aluminium Grades explains how the alloys are organised into series by their principal alloying element, and Aluminium Properties details the mechanical, physical, and performance characteristics those designations ultimately describe. Aluminium Standards sets out the frameworks that govern these designations across different regions and organisations. Together, they connect the designation system with the grades, properties, and standards behind it.
It is a standardized four-digit index system used to identify both wrought and cast aluminium alloys. The first digit indicates the principal alloying element, while the remaining digits specify alloy variations.
Both use four digits and identify the major alloying element with the first digit. However, cast aluminium alloys include a decimal point between the third and fourth digits to indicate whether the material is a casting (.0) or a foundry ingot (.1 or .2).
The temper designation follows the alloy number after a hyphen and describes the mechanical or thermal treatments applied. Common prefixes include F (as fabricated), O (annealed), H (strain hardened), W (solution heat treated), and T (thermally treated for stable tempers).
Non-heat-treatable alloys gain strength mainly through strain hardening and are found in the 1xxx, 3xxx, 4xxx, and 5xxx series. Heat-treatable alloys gain strength through heat treatment and appear primarily in the 2xxx, 6xxx, and 7xxx series.
It identifies a wrought aluminium-copper alloy (2014) that has been solution heat treated and then artificially aged (T6 temper).