Originally Posted by DrNautica
Also, that is a poor area for a weld which is to be stressed. That weld is made at the apex of a bend. The material on the upper side of the bend will be stretched, and therefore, thinner at that point. The material thickness at the crack looks suspiciously thin to me. I've drilled a few holes in my ND tower and I can tell you that it is easily twice that thick (at least if your photo shows it right.)
I agree it is a poor design and was most likely not analyzed by an engineer. The combination of a major stress riser (bend apex) with a saddle welded structural support is just not a good idea. Now factor in the metal properties of the T6061 (I assume), the load, and the constant flexing of the "zero flex" you get the failure shown.
I too would replace the entire lower structure if possible. Proper welding will only buy you time as the tube will most likely crack again somwhere near the repair weld.