AMP makes a tool which they will "rent" to you for an exorbitant amount of money, then you have to purchase the crimps from them for even more $$$$. AFAIK, this is the only means I know of as a permanent solution to AL/CU splicing.... otherwise...
There's the purple Ideal wirenuts that have been debated on this forum before about their fitness for the purpose of AL/CU splicing, but I believe they are listed for that purpose...
Back in the day, they used to use 3M Scotchlock wirenuts on aluminum... The outer shell is a soft plastic, and the inner spring mechanism tends to expand with the aluminum conductors under load and maintain grip vs. the hard shell wirenut which tends to push itself loose.... I had decent luck with those when installed correctly... one problem with that nowadays though...
Scotchlocks are only listed for CU wire now! But I feel this is more a matter of 3M not wanting the liability vs. an actual failure rate (purely my opinion though)
I do my best to avoid AL branch wiring and run a new circuit.
