It's not just a matter of TIGing aluminum. Different aluminums respond differently to welding - that is, they retain different degrees of their pre-welded strength.
The most common aluminum alloys used in frame building fall into one of to categories: a) those that need artificial aging post-welding; b) those that need all out heat treatment post welding. The difference between the two processes seems to be primarily how hot, and how quickly cooled.
Anyway, these alloys require this additional step because welding takes a lot out of them.
Hence, I'm sure the crack can be TIGed, but I wouldn't ride the bike afterwords.