Is there a successful chance to unscrew a locked screw into an alu fuselage (possibly without using the drill in order to save the thread) ?
In case the thread got damaged, is it possible replace it? How?
p.s. luckly (till now) this issue has not happened to me. Ask for a friend
Is the head still in tact? If so you can try spraying some WD40 into the bolt then use a heat gun to heat up the area where the bolt is. You might have to do this a few times. If the corrosion isn't too bad the WD40 might work it's way in far enough to loosen some of the corroded material. You can also insert a screwdriver / Allen key and give it some gentle taps with a hammer. I have had some success with this method.
Bigger problem is if you stripped the head of the screw. In that case you'll need to drill into the bolt and use a screw extractor. If the screw it both corroded and the head stripped it's a major pain to get it out if at all.
Once you get the screw out you'll need to properly clean out the thread and ideally recut it. Make sure you take it apart and clean it properly after every foil. From what I have seen once a fuselage starts corroding it only gets worse even if you try and clean out the corroded parts.
