Aluminum

In general, there are three grades of aluminum commonly used for combat robotics.

6061-T6 - A common grade of general purpose aluminum
2024-T3 - Aircraft grade
7075-T6 - Higher performance aircraft grade aluminum

7075-T6 aluminum has nearly twice the strength of 6061-T6. 2024-T3 is someplace in between.

6063-T5 aluminum is referred to as, "structural aluminum." Most commonly available aluminum angle and tubing are made of this alloy. It is only about half as strong as 6061-T6. Still, its reduced strength may be outweighed by its availability in useful pre-made shapes.

Specific properties for a few alloys are available on the materials page.

This comprehensive list of aluminum alloys and properties has even more data.

The 4 digit number specifies the, "recipe" used for the aluminum. 6XXX refers to silicon alloys. 2XXX refers to copper based allows. 7XXX grades are alloys high in zinc.

The, "T" refers to the tempering (hardening) process used.

Often aluminum is provided in an unhardened or annealed state to make working with it easier. This may be indicated by an "O" after the alloy number, such as, "7075-O".

Annealed aluminum often has a small fraction of the strength of hardened aluminum. It should only be used for parts which will be hardened later, or where strength isn't critical.

Often when purchasing scrap aluminum its alloy and temper are not clearly labeled. If you bang two pieces of metal sheet together on their sides, the softer one will become more dented. By this method, you can approximate what grade you are dealing with if you have a known sample.

That is, if an unknown piece of aluminum dents a known piece of 6061-T6, it's likely 2024-T3 or something better.