I have seen hail damage taken out of car roofs by leaving the car outside in the sun till it
is really hot then placing dry ice pellets in the dent. Amazingly the dents just popped out.
Now, the sheet metal on cars is different of course than the aluminum on our aircraft, but this might work
as well. My understanding of this process is the metal which is deformed by the dent is in a state of stress
and wants to go back to its original form. The hot metal doused by the super cold of the dry ice causes the rapid contraction of the metal
and it returns to the state of relaxation.
Not sure it will work on aluminum and certainly will not work if the metal has any creases.
FWIW.
Gary