Hey man, weld your bike..........looks good from where I'm standing.

I believe the phrase you're seeking is "facial reconstructive surgery".

Eat ozone, bicycle boy!

DCB0 - I don't think you have ever welded anything in your life.

The OP describes the crack as small. Applying a welding bead across it poses minimal risk.

And you know this because you can run a bead with a Miller wire feed MIG?

I can assure you I have worked with a number of different metals and
joining systems. I also understand at least the rudiments of metallurgy,
and in particular how it applies to bike frames and components. I
can probably still put shoes on a horse, but it's been some years,
and alloy shoes don't last very long.

I believe you genuinely misunderstand the welding process, the effects
it has on the areas adjacent to the weld, and the heat treating process
in the case of thin wall aluminum tubes. You also seem to lack some
appreciation of the stresses involved in the area in question. I don't
think you intentionally mean to hurt anyone, but the road to hell is paved ........................

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.

DCB0 - you err in your wisdom. You realize that this forum has no room for logic or science when it comes to assessing the properties of materials. You would do better to say: "Yea, I'd jest grab a stick and weld 'er up, and ride the hail out uv it!"

If you are the original owner you will qualify for new frame from Klein. Here is copy of warranty. https://www.google.com/url?sa=t&rct=j...Hc_xJwXIDtm9cg
Klein is owned by Trek now, so you may need to go thru them. good luck