1. Aluminum and copper gum up so they'd be slower, and won't spark. It's definitely steel, just not hard steel.
2. Second, that belt grinder is running really fast. You can hear it and see a lot of sparks getting carried back to the cut along the entire belt
3. It's an extremely low grit belt meant for this type of fast material removal.
Even with all of that, it's a pretty impressive sanding belt.