Originally Posted by dsaul

I build bike frames and that is definitely an aluminum frame. As for welding it, if it were mine, I would weld it and make sure I had a long enough seat post in the future. I have personally welded an aluminum MTB frame with a similar failure due to a short seatpost. I advised the owner(a friend) that he needed a longer seat post and that the weld may crack again in the future. He is a large rider, over 6' tall and over 250lbs and that frame is still going 2 years later.

With a proper length seatpost that extends a couple of inches below the top tube, the worst that can happen from a future failure of the repair would be an annoying creaking sound and maybe some side to side rotation of the saddle.