Casting and wrought alloys
Aluminium alloys are classified as casting alloys, wrought non-heat-treatable alloys or wrought heat-treatable alloys. Casting alloys are used in their as-cast condition without any mechanical or heat treatment after being cast. The mechanical properties of casting alloys are generally inferior to wrought alloys, and are not used in aircraft structures. Casting alloys are sometimes used in small, non-load-bearing components on aircraft, such as parts for control systems. However, the use of casting alloys in aircraft is rare and, therefore, these materials are not discussed.
Nearly all the aluminium used in aircraft structures is in the form of wrought heat-treatable alloys. The strength properties of wrought alloys can be improved by plastic forming (e.g. extrusion, drawing, rolling) and heat treatment. Heat treatment, in its broadest sense, refers to any heating and cooling operation used to alter the metallurgical structure (e.g. crystal structure, grain size, dislocation density, precipitates), mechanical properties (e.g. yield strength, fatigue resistance, fracture toughness), environmental durability (e.g. corrosion resistance, oxidation resistance) or the internal residual stress state. However, when the term ‘heat treatment’ is applied to wrought aluminium alloys it usually implies that heating and cooling operations are used to increase the strength via the process called age (or precipitation) hardening.
There are two major groups of wrought aluminium alloys: non-age-hardenable and age-hardenable alloys. The distinguishing characteristic of non-age-hardenable alloys is that when heat treated they cannot be strengthened by precipitation hardening. These alloys derive their strength from solution solid strengthening, work hardening and refinement of the grain structure. The yield strength of most non-age-hardenable alloys is below about 300 MPa, which is inadequate for aircraft structures. Age-hardenable alloys are characterised by their ability to be strengthened by precipitation hardening when heat treated. These alloys achieve high strength from the combined strengthening mechanisms of solid solution hardening, strain hardening, grain size control and, most importantly, precipitation hardening. The yield strength of age-hardenable alloys is typically in the range of 450 to 600 MPa. The combination of low cost, light weight, ductility, high strength and toughness makes age-hardenable alloys suitable for use in a wide variety of structural and semistructural parts on aircraft.
International alloy designation system
There are over 500 different aluminium alloys, and for convenience these are separated into categories called alloy series. The International Alloy Designation System (IADS) is a classification scheme that is used in most countries to categorise aluminium alloys according to their chemical composition. This system is used by the aerospace industry to classify the alloys used in aircraft. All aluminium alloys are allocated into one of eight series that are given in Table 8.1. The main alloying element(s) is used to determine into which one of the eight series an alloy is allocated. The main alloying element(s) for the different series are given in Table 8.1. The 8000 series is used for those alloys that cannot be allocated to the other series, although the principal alloying element is usually lithium.
Table 8.1
Wrought aluminium alloy series
Each alloy within a series has a four-digit number: XXXX. The first digit indicates the series number. For example, 1XXX indicates it is in the 1000 series, 2XXX is a 2000 series alloy, and so forth. The second digit indicates the number of modifications to the alloy type. For example, with the alloy 5352 Al the second digit (3) indicates that the alloy has been modified three times, but has a similar composition to earlier versions 5052 Al, 5152 Al and 5252 Al. The last two numbers in the four-digit system only have meaning for the 1000 series alloys. In this series, the last two digits specify the minimum purity level of the aluminium. As examples, 1200 Al has a minimum purity of 99.00% and 1145 Al is at least 99.45% pure. The last two digits in the 2000 to 8000 series has no meaningful relationship to the alloy content and serves no purpose other than to identify the different alloys in a series.
When an alloy is being developed it is prefixed with an X to signify it has not yet been fully evaluated and classified by the IADS. For example, the alloy X6785 indicates it is a new 6000 series alloy that is being tested and evaluated. When the evaluation process is complete, the prefix is dropped and the alloy is known as 6785 Al.
Most countries use the IADS to classify aluminium alloys. However, some nations use a different classification system or use the IADS together with their own system. For example, in the UK the IADS is used by the aerospace industry, although sometimes the British Standards (BS) system is also used to classify aluminium alloys. There are three principal types of specifications used in the UK: (i) BS specifications for general engineering use, (ii) BS specifications for aeronautical use (designated as the L series), and (iii) DTD (Directorate of Technical Development) specifications for specialist aeronautical applications.
Temper designation system
A system of letters and numbers known as the temper designation system is used to indicate the type of temper performed on an aluminium alloy. Temper is defined as the forging treatment (e.g. cold working, hot working) and thermal treatment (e.g. annealing, age-hardening) performed on an aluminium product to achieve the desired level of metallurgical properties. The temper designation system has been approved by the American Standards Association, and is used in the USA and most other countries. The system is applied for all wrought and cast forms of aluminium (except ingots).
Basic temper designations consist of individual capital letters, such as ‘F’ for as-fabricated and ‘T’ for age-hardened. Major subdivisions of basic tempers, when required, are indicated by one or more numbers. The letters and numbers commonly used to describe the temper of aluminium alloys are given in Table 8.2. The temper designation follows the alloy designation, and is separated from it by a hyphen. As examples, 1100 Al-O means the aluminium has been heat treated by annealing and 2024 Al-T6, shown in Table 8.3, means that the alloy has been tempered to the T6 condition, which involves solution treatment followed by artificial ageing as given in Table 8.2.
Table 8.2
Temper designations for aluminium alloys
Table 8.3
Composition of 2000 alloys used in aircraft
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