Might be thinking wrong here but.... as alloy expands more than steel, surely heating it up will make it even tighter in the frame than it was originally...???
You're right about the expansion properties, but I wasn't heating it up - it's an exothermic reaction so the alloy was heating itself up! I wasn't trying to remove it at the same time so it made no difference; the plan is to melt the alloy away enough, then let it cool and clean it up before trying to remove it.
Adding the solid to the water is exothermic. How did you do it, make the solution or just fill it with pellets/powder and add some water ?
I would recommend having Citric Acid (solid) to hand in case you spill it on the floor, a bit of that and it'll neutralise the solution.
Though dilute it slightly and pour down you drain to clear it out. It is drain unblocker (a bit weaker still and it's oven cleaner)
*EDIT, don't stick the citric acid in the frame, while the sodium hydroxide will react with the aluminium and not the steel, acids will.