A buddy left water in his camper's tank last winter. It froze.
When repairing a crack, you need to make sure it's not gonna spread. The tried and true method of doing this is drilling a hole at each end of the crack
Next I wanted to bang the metal back into shape as best I could to try to close the gaps. I needed to anneal it to get it workable so it wouldn't fracture more. Aluminum doesn't change color like steel, so there's no tell tale sign you're getting it too hot. But the soot from a gas heavy flame(easy with acetylene, trickier but still do-able with propylene) works as a nice indicator:
Now with a neutral flame heat the aluminum....it'll melt the soot away right around the proper temp to anneal the base metal. Gotta be careful, especially on thinner material. It's easy to go too far and toast the al.
somehow I missed getting a pic of the tank beaten into place. I cut most of the edges a bit with a die grinder to give myself some solid "edge meat" to work with. Even so, the blades leave some crap behind in the base metal, which can contaminate the weld. Not ideal, but the best I had to work with here. Sometimes you can't do things by the book, and knowing when and how far you can push an improv is part intuition and part experience.
Not the prettiest thing out there, but I'm satisfied based on what I had to work with.