tubes butt welding fixture/issue

[Lderoly]

I am butt-welding 430FR stainless to 302 stainless tubes together. I am having a somewhat hard time keeping them concenctric and without a gap during welding. My current fixture is holding them concentric on the inside but often gaps during welding is a problem. I have been thinking of an alternate design for the fixture. The currenct fixture costs upward of $3000.

Any suggestions.

Also where welding overlaps on the tubes, I am having issues. It looks like a crater as if the top of the weld pool vaporized leaving behind a very thin weld with minimal backing. I have been looking to experiment with different ramp down settings. Welding with a continuous wave laser that has ramp up and down settings.

thanks