[pmangels17]

Almost all metal for FRC is usually aluminum of some sort. However, there are some applications where sometimes steel is preferred, notably in high torque gearboxes, shafts, keys, chain, and shifting mechanisms, although there are people who have accomplished this with aluminum as well. Also, many common fasteners are zinc-plated steel (or some other type of steel), so unless you want to specially source your fasteners, you'll probably use a lot of steel on nuts-and-bolts stuff.

There are some teams who have, often due to unavailability of other metals, built most of their robots with steel. Theoretically, both aluminum and steel can be used to make a great robot, it's just a matter of knowing how to use each material properly and in what situations. That being said, in practice, most often aluminum will be your best bet, especially with large robot parts (because steel is HEAVY), but this sometimes a flexible rule.

EDIT: What Mr. Wallace said about motors applies as well. You won't find many motors in FRC made of aluminum. Additionally, there are many types of both steel and aluminum, so selecting specific alloys and tempers can be important in addition to the steel vs aluminum debate.