Rather than using WD-40 on the threads, there is a product call anti-seize lubricant that is available at your local auto parts store that is specifically made for dissimilar metals. Available in small packets, up to a large (4oz) bottle with brush. I use this on just about anything where different metals are in contact, and have never had a problem with removal of bolts/nuts that I've used it on.

If indeed you have galled the threads in the aluminum, there are kits available for thread renewal, look for heli-coil. Available in SAE (U.S. common) and metric, in most sizes from 8-24 up to 3/4-10. Be warned, the initial kit can be pricey, but coil refills are also available. The kit normally includes a special sized drill bit, a tap for the resized hole, an installation tool, and several heli-coils. The heli-coil insert puts new stainless steel threads into your assembly, so the use of anti-seize is recommended -- that darned dissimilar metals thing again -- galvanic action.