Metals Selection and Specifications

Aluminium and its Alloys.

These notes provide a brief introduction to UK and US specifications for aluminium alloys, their properties and applications.

Aluminium sheet is cold rolled material with a thickness between 0.2 and 6 mm.

Most aluminium alloys used in engineering have very similar Young's modulus: about 70 GPa and density: 2700 - 2800 kg per cubic metre. Aluminium is often considered as a substitute for steel, however the lower E value means that structure stiffness will need to be checked as well as strength and manufacturing processes.
The potential savings in weight when compared to steels mean that aluminium alloys are increasingly being used in automotive and rail carriage applications

Similar specifications are in use in the UK and US for wrought alloys, based on 4 digits. Casting alloys in the US use 1XX.X etc coding system whereas in the UK LM numbers are used.

Some additional information about aluminium and it's alloys can be found on the European Aluminium Assoc. site.

Guide to Wrought Alloys

The wide range of alloys available can broadly split into two groups, the work hardening alloys and the heat treatable alloys.
In the notes below some indication is given of 0.2% proof stress and UTS, for rolled plate, but these are dependent upon processing/temper.

Work Hardening Alloys

Alloys in this group harden and increase in strength as they are cold-rolled (or worked). Any one alloy, therefore, can have different strength and ductility properties depending upon how much cold-working has been imposed. The degree of cold-working is denoted in the alloy designation by a suffix letter (H) and a digit from 1 to 8 indicating increasing strength.

Maximum ductility, often desirable for subsequent forming operations, is obtained by annealing - a process in which the metal is heated to a temperature of around 3600C to remove the effects of any cold-working. Annealed metal is denoted by the suffix '0' in the alloy designation.

1XXX: Commercially pure Aluminium, 99% pure and greater.

1050, 1080, 1200: These are easily formed and joined, but have the lowest strength charactenstics of all the alloys. Highly resistant to weathering and to chemical attack, these alloys are ideal for chemical processing plant applications where strength is not critical; pressings requiring high ductility, foil and many panelling applications.

3XXX: Manganese alloys

3103, 3105. 3103 is a popular alloy that offers higher strength than commercial purity metal but which retains excellent ductility, good corrosion resistance and joining properties. The alloy is widely used in the building and transport industries. 3003 aluminium foil 0.04, 0.06 or 0.08 mm thick is converted into honeycomb and used with outer skins of aluminium or plastic for lightweight panelling in avaition and other transport applications. 3105 can be considered to be a "Green Alloy," being manufactured mainly from recycled material. The finished sheet and strip has consistent mechanical properties and forming characteristics. This alloy is particularly suitable for painting, making it popular in the sign making and building products industries. Anodising is not recommended.

5XXX: Magnesium containing alloys.

5005: Used for decorative and architectural uses where good anodising quality is required.

5251, 5083: These two are another step up in strength. They respond well to MIG and TIG welding and have particularly good corrosion resistance to salt water Of the two, 5083 is the stronger and is ideal for high strength welded applications such as marine components, rail and road transport, cryogenic structures and a variety of pressure vessels.

Heat-Treatable Alloys

The strongest of the aluminium alloys are those that gain strength by special heat-treatment processes. The alloys are identified by the suffix 'T' plus the addition of digits denoting variation of heat treatment.

2XXX: Copper containing alloys.

2014A: One of the most widely used heattreatable alloys, it offers a high strength with excellent machinability. It is widely used in highly stressed aircraft applications where strength to weight ratio is critical. The alloy has only fair corrosion resistance and corrosion is a problem in aircraft. For other applications it is often specified with a pure aluminium cladding. It does not respond well to MIG and TIG welding.
0.2% Proof stress: 430 MPa, UTS: 480 MPa.

6XXX: Magnesium - silicon containing alloys.

6082: This medium/high strength alloy is the most popular of the heat treatable alloys. Normally supplied in the fully heat treated condition. Can also be fully annealed to allow cold working to be carried out. This alloy is readily anodised, machined and welded thus providing a good multi-purpose, durable and heat resistant alloy.

7XXX: Al-Zn-Mg-Cu containing alloy.

7075: This is a high strength Al-Zn-Mg-Cu alloy. The tensile strength is typically 565 MPa thus offering an outstanding strength to weight ratio for critical applications.
0.2% Proof stress: 420 - 480 MPa, UTS: 500 - 550 MPa.

Aluminium casting alloys - typically contain up to about 12% silicon. Copper and or magnesium and manganese are also included in most specifications.
The two most useful UK specification alloys are LM6 and LM25 (equivalent to Aluminium Association alloy: 356.0).
Alloy Si % Cu % Mg % Mn % Others - max. limits %
LM6 11.5 less than 0.1 less than 0.1 less than 0.5 0.6 Fe; 0.1 Ni; 0.1 Zn
LM25 7.0 less than 0.1 0.4 less than 0.3 0.5 Fe; 0.1 Ni; 0.1 Zn

LM25 offers marginally better mechanical properties and is a natural choice for a part that subsequently requires machining. Alloy LM6 is suitable for leak tight fittings as well as castings that require later welding. Its ductility, moreover, may allow components to be cast straight then bent to shape later. LM25 is available in four conditions of heat treatment and retains the good weldability of LM6.
Sand casting techniques can achieve a tolerance of +/- 0.4mm and a wall thickness down to 3mm.
Strict adherence to working procedures and quality control is needed to ensure optimum properties are obtained.

Aluminium 'Superforming' or 'Superplastic' alloys - in some very fine grain alloys, Al-Li and Al-Cu-Zr, high temperature (typically 470o C to 520o C) deformation may take place by extensive grain boundary sliding and diffusion or by mass diffusion. As long as strain rates are kept within limits, deforming forces can be very low. Such 'superplastic' behaviour can result in elongations of 1000's of percent. Aluminium alloys that can undergo 'superplastic' deformation include 2004, 5083 and 7475. This process is used for producing sheet metal components for transport and architectural applications.
For further information about superforming aluminium alloys link to Superform Aluminium

Suggested reading: 'Selection and Use of Engineering Materials', by J A Charles, F A A Crane and J A G Furness, Butterworth Heinemann, 1997.

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David Grieve, 3rd August 2005.