Wrought aluminium alloys use a 4-digit system. The first digit is used to designate the primary alloying element. As an example, a 1XXX alloy indicates a mostly pure aluminium alloy, whereas a 6XXX alloy indicates that significant amounts of magnesium and silicon have been added to the aluminum. Below is a table for the different alloying elements of wrought aluminium:

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The second digit in the wrought aluminium numbering system indicates that there has been a special modification to one of the alloying elements. The third and fourth digits in the wrought alloy aluminium designation system are used to label the specific alloy. These numbers are arbitrary except for the 1XXX series of aluminium. In the 1XXX series, the last two digits specify the minimum aluminium content between 99% and 100%. For example, 1060 aluminium would have a minimum pure aluminium content of 99.60%.

T6 VS T651

T6 - “Solution Heat-Treated and Artificially Aged”
To get to the -T6 temper, the 6061-O aluminium billet is heated to about 532˚ C, then quenched in water, then aged at about 177˚ C for around 8 hours. That changes the typical yield strength from 8 ksi to about 35 ksi (ksi is a unit of stress, equivalent to 1000 lbf/in2).


T651 - “Solution Heat-Treated, Artificially Aged and Permanent Set”
Quenching in water puts residual stresses in the aluminium since there is a surface-to-centre cooling gradient. The -T651 designation means the aluminium mill took that extrusion and gave it a 1% to 3% stretching, or permanent set, to get rid of some of those residual stresses. Now we can machine it and it shouldn't distort as much.