We have to develop a design that will call for the use of plate material in aluminium alloys. This is a field very new to us and we have been doing some preliminary reading. There seem to be very many alloys available in plate from, all with different designations and with different categories such as T6 or H2 after them. Can you provide a simple guide for us to work through and understand more of what we are reading?

You probably feel that you have plunged into a morass and cannot see how to proceed. However it is not really all that complicated as hopefully you will see.

Essentially aluminium alloys fall into two distinct sub groups:

• Alloys designated 1xxx, 3xxx and 5xxx derive their strength from work hardening, these are the ones that have the H and a number after the four figure number. The H number relates to the final degree of cold work in the alloy and hence defines its mechanical properties.
• Alloys designated 2xxx, 6xxx and for military and high strength applications 7xxx and 8xxx derive their strength from heat treatment (solution treatment and age hardening), these are the ones that have T and a number after the 4 figure designation. The T number defines the heat treatment condition, and again this defines the mechanical properties for the alloy concerned.

The 4 figure designations are the standard numbers, originally developed in the USA which have been almost universally adopted across the world. All designations may be followed by O which indicates that the material is fully annealed.

Therefore we have a number of families of alloys and two distinct strengthening mechanisms. One depends upon cold working the metal, the other on a heat treatment. You may therefore find that you can obtain similar levels of mechanical property from different alloys in varying heat treatment or temper conditions. Generally the heat treatable 2xxx and 6xxx alloys will provide higher strengths in thicker sections than the other alloys. Also with the H designations H8, with its higher mechanical properties than H4 will have a greater degree of residual cold work so will be less formable than H4. Usually they will also be less formable than heat treatable alloy of comparable or higher strength.

Remember that even moderate heating to facilitate forming can permanently impair the mechanical properties of all aluminium alloys. The heat treatable alloys will overage and soften if heated above 120^oC for any length of time, and the component would require a full heat treatment to restore the properties. This will be expensive and may be impossible! The work hardenable alloys will anneal in short times if heated above 300-350^oC for even very short times. Their properties cannot be restored.

Further information

Light Metals Division