As an old weldor I can tell you that welding tempered aluminum will ggreatly weaken it. It is like annealing high carbon steel. it gets all soft and bendy. If looks are not an issue then an external sleeve will work. I did this for the mast on my boat because it was broken in two at the spreaders. I found a section of larger mast that would fit over the existing mast once the sail slot was cut out. then I drilled in several places and riveted it to the outside. Use SS rivets and tef gel. HTH