The amps 'casing' as you call it, is usually it's heatsink. As that's the way it dissapates heat away from the internal components.
I never meant this to start a sh!t fight or flame war, just pointing out some things that I've noticed/learned/read.
I wasn't taking a dig at you putting a fan on your amp, LowGli.. I was just stating that it's generally better practice to mount a fan so it blows over the amp's heatsink, not just directly on one section of it.
Though, dude... you really do need to do at least a little reading & research before trying to prove a point that you're not even sure of. Yes, some parts of an amp's 'casing' are made of different metals, but as I said just before, the bulk of a car audio amp's exposed 'casing' is it's heatsink(s).
Take one apart and have a look for yourself
(providing you're confidant and your warranty period has passed
)