now that you have it freed off, the real long term fix you are looking for, I have found, is as follows:
First a couple of questions, and some information.
Do you know what a “tap” is? If you don't, it’s for cutting new threads from a blank drilled hole, a whole set of taps consists of three actual taps, a First, then a second and finally a Plug
, they will have 3 or 4 “Flukes” (Grooves) to carry the cuttings away from the thread as its being cut. (that is the important bit)
Most hand taps have straight flukes but machine taps (High speed) sometimes have spiral flukes, the type of fluke here is not the important bit, just that there are flukes to carry away the cuttings,
The issue with the dissimilar metals which I know you understand, is that there is no where for the aluminium oxide to go while it grows inside, except to jam up the thread clearances causing the all too familiar problems.
The fix, turn the screw either out or in whichever exposes the most thread, then cut a single
groove using a junior hacksaw across the thread to form a type of fluke in your screw, your fluke will likely form a spiral across the thread. Cut it down to thread depth
and no more, (it doesn't have to be perfect, as long as it does its job of carrying away the aluminium oxide) turn your screw all the way in the other direction and try to complete your “Fluke” . Do the same with the second screw, use copious amounts of your favourite release fluid (WD40 will work
fine) and work the screws out and in until it is a free as it ever was, cleaning
your fluke as required.
When you are happy, clean off the release fluid and any crud from your fluke and lubricate with a good waterproof grease, working it into the alloy housing.
Cutting your fluke is not easy, just persevere, it will be worth it.
In the long term, your fluke is already made, just do the cleaning
As mentioned by someone else, never heat the screw, heat the bracket (cool the screw if you can too)
All the best
Colin (Sailing the Baltic)