Is aluminium more difficult to braze than copper?
Hydro Aluminium Precision Tubing: Flame brazing aluminium to aluminium is basically done exactly like copper/copper brazing, but with other filler materials and fluxes. The main challenge is that the temperature window is narrower when brazing aluminium compared to copper, which makes it necessary to maintain more accurate brazing temperatures. However, this is a matter of training operators, something which has been done with success in numerous HVACR companies.

How do I flame braze aluminium and copper?
Hydro Aluminium Precision Tubing: Joining copper and aluminium tubes with flame brazing is conducted in the same manner as when flame brazing two aluminium tubes.