welding thin tubing with internal threads

Possibly try a copper or brass bolt to insert into the tube. I couldn't weld .047 wall tube without warpage, Silver solder maybe but not weld. If it were me I'd weld the tab to solid bar stock then machine the threaded hole.

ED

What, no opportunity for sarcasm or a redesign of your part?

This isn't gonna be easy. If I was doing this (and I've TIGed a ton of thin wall tubing) I'd try .035 or .040 filler. You want the smallest bead possible. I like Inconel wire for this sort of thing. The 1/16 tungsten should be OK, but an .040 might work better. But I wouldn't be too optimistic - as you've discovered, it won't take much distortion to bugger your thread. How about silver brazing, or TIG brazing with Everdur (silicon bronze) wire?

Change your joint design possibly?

Drill a few holes in the tab on the center line that will be tangent to the tube. Then "plug weld" it. To do this, you weld the hole edges to the tube from the backside and fill in the remaining divot (if any) with rod.

Probably your holes will be small enough so that you can just puddle some of the tube and plate, and then give it one dab of rod to finish the weld. May require a little hand grinding to smooth out the welds.

Other then that, your present problem indicates a difficulty controling the puddle in the joint where the tube and the plate meet. And welding a round to a flat with a TIG like that, that is not totally unexpected. You may be able to get greater control by extending the tungsten point further outside the gas cup so that the tungten electrode reaches deeped into the weld area. In a tight weld area like that, shielding gas loss shouldn't be a problem. I would try this solution first. Dave

Or use a brass bolt as a backer when welding.

I know you already have the parts made, but why not drill the holes in the tube, but don't tap it until AFTER the weld? Would that do it?

Otherwise, would it be possible to use a brazing alloy to make the joint? You could still use the TIG machine, but you would not have to get up to the temperature where the steel softens. You could also do the brazing with a propane torch.

One last possibility is to pre-heat the parts, either with the TIG or a gas torch, to the faintest redness, and then make the weld very fast, with minimum current with the TIG.

Jon