Hi Brent
Cu and Al can not be joined together. Firstly the difference in melting temperature is a problem, and secondly brittle intermetallic phases are formed. For this reason I assume that Al can also not be welded to brass.
In the past I have joined Al to Cu by first placing a "butter layer" of silver solder on the Cu, then welding with Al filler the Al to the silver solder.
Give it a try.
Regards
Niekie Jooste
Fabristruct Solutions