Originally Posted by
dddd
The reason the frame cracked is because of the heat treating that was done to make the very thin tubing stronger (higher yield strength and higher fatigue strength), so it retains alignment and lasts longer during hard use.
However, heat-treated aluminum cannot be dented without introducing precurser cracks, and is thus no longer malleable.
Any welding done after heat-treating will return the metal to it's original, weak state.
This. ^
The only plausible way to safely repair the frame would be to cut out the whole tube, weld in a replacement, and re-heat-treat the entire frame. The cost of doing this would likely approach that of a brand-new replacement frame.