Material Basics Aluminium Specifications: The Aluminium Alloy Designation System 5 min read • Updated September 2026 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. How the Four-Digit Aluminium Alloy Designation System Works 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 Wrought Aluminium Specifications The wrought aluminium alloys are organized into the following series, each defined by its primary alloying element: 1xxx – Pure aluminium (99.00% or greater) 2xxx – Aluminium-copper (Al-Cu) alloys 3xxx – Aluminium-manganese (Al-Mn) alloys 4xxx – Aluminium-silicon (Al-Si) alloys 5xxx – Aluminium-magnesium (Al-Mg) alloys 6xxx – Aluminium-magnesium-silicon (Al-Mg-Si) alloys Cast Aluminium Specifications 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: 1xx.x – Pure aluminium (99.00% or greater) 2xx.x – Aluminium-copper (Al-Cu) alloys 3xx.x – Aluminium-silicon with copper and/or magnesium (Al-Si + Cu and/or Mg) 4xx.x – Aluminium-silicon (Al-Si) 5xx.x – Aluminium-magnesium (Al-Mg) 7xx.x – Aluminium-zinc (Al-Zn) 8xx.x – Aluminium-tin (Al-Sn) 9xx.x – Aluminium with other elements 6xx.x – Unused series The Aluminium Alloy Temper-Designation System 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: F – As fabricated O – Annealed H – Strain hardened W – Solution heat treated T – Thermally treated to produce stable tempers other than F, O, or H It is important to note that temper designations differ between non-heat-treatable and heat-treatable alloys. Their meanings are explained below. Temper Designations for Non-Heat-Treatable Aluminium Alloys 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: H1 – Strain hardened only H2 – Strain hardened, then partially annealed H3 – Strain hardened, then stabilized 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. Temper Designations for Heat-Treatable Aluminium Alloys 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: T3 – Solution heat treated, then cold worked T4 – Solution heat treated, then naturally aged T5 – Artificially aged only T6 – Solution heat treated, then artificially aged Heat-treatable alloys are found primarily in the 2xxx, 6xxx, and 7xxx series.