Pretty much the title. I know it's the bottom of the product stack with a relatively wimpy heatsink and a single fan so I assume it's entirely aluminum. 
 
I just bought one cheaply on eBay for a build I'm doing for my brother, and the seller said it works fine but it runs hot and a bit loud. Just curious if it's an aluminum heatsink or copper/nickel plated copper so I can put liquid metal on it. 
 
I still plan on repasting it with 'normal' thermal paste if it's aluminum, but if it's not I need to track down some clear nail polish to cover the SMDs around the die.
 
Thanks!