I know Dad used to silver solder almost anything to anything but I'm not sure I ever saw him do Al. Looking around there appears to be fluxes out there that will do the job.
http://www.cupalloys.co.uk/joining-a...m-c100014.html
http://www.youtube.com/watch?v=UGsQP-wX1lI
http://www.hobartwelders.com/weldtal...p/t-31260.html