Originally Posted by
brokencase
You make it sound like the entire integrity of the tube would be compromised by the application of 19mm long welding bead and that is simply not the case. I know that this bead would be laid down in less that a second and the heat generated would have only a minimal impact on the surrounding material. The important thing is to stop the propagation of the crack. Will it be strong as original? No, but it may never be an issue again after the repair. I think the OP can access that risk.
You are laying down a bead, and melting the base metal, thus destroying the heat treatment, in a spot that has
already failed. Sure, it may only weaken it slightly, but
it will weaken something that wasn't strong enough to begin with. Your comment that you would try to have an aluminum bike frame repaired with a MIG welder says a lot more about your lack of experience than anything I have said does about mine.
FWIW, I have learned to TIG weld, stick weld, MIG weld, and OA braze and weld. I have little experience actually applying these techniques outside of the welding class, but I have
some idea how and when to apply them... you seem not to.