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The tubing on Brompton frames is not heat-treated and stays more within its properties due to the not ductile material to begin with, and less manipulated.
But those Scapin frames, IIRC, used a type of Dedacciai tubing that with some bad luck, would crack selling the tubing. You can see that is a failure along the moment of torsion, unlikely caused by the welding alone.
Brazing over that crack may slow down its run, but there's a good chance it will carry on from nearby.
Londonbikerider74
@Dogtemple
I’ve welded up a crack similar to this, where it ran in to the braze.
Just removed everything to see the whole crack and tig welded as much as I could, starting a weld pool before the crack and running it in to the crack with a filler rod. Ran it as close to the braze as possible then filled the rest up with bronze rod.
This was on a Brompton but a similar situation.
I’d forget mig on that unless you’ve done thin sheet work with a mig, it would probably just blow a hole in the tube and you would still have the braze to contend with.