I'm a journeyman welder with a lot of aluminum experience so here it goes. Yes it can be welded. Yes The aluminum will have to be spotless inside and out. Just welding over the crack will work, but will not last. The crack will still be there. Unless you grind out the crack and do a full penetration weld it will crack a lot faster. It might take 1 hour, it might take 100 hours. It will just depend on the vibrations. Even a full penetration weld will be a lot better, but there is still a chance it will crack. Putting a pad over the crack and welding around it is a worse idea. The crack will continue to grow until it gets to the outside and keep growing after that. You could drill holes in the crack before putting on the pad but there is good chance you will not drill into the end of the crack. For example the crack could be an inch longer than where you drill the hole. You just would not be able to see yet. Aluminum does not flex like steel does, its brittle and cracks. That's why its not used for anything structural in something like a building. Same reason why every ones light weight aluminum ATV trailer is cracked or cracking. So there is all the info. Its up to you to decide what you want to do now. I would fix it once and see how long it lasts. $600 for a used b pipe is 10 repairs. If you are will to do the work to remove the pipe and reinstall it. Never had a ski with a b pipe so I don't know how difficult it is.
And no do not use JB weld.
