probably a half round alum cutting file

i specify alum cutting because the scoops on them have better clearances for sticky materials such as aluminium

You could heat the outer tube up then push the inner tube in to it. Unless you need them to slide of course.

Rat tail file would be the cheapest
Flaring tool would be the best

They cost a bit more than a file but an adjustable reamer is made to do exactly what your talking about.