[DavesTent]

Beginner here, just learning TIG. Welding AC on 0.130 AL at about 120 Amps, stubby gas lens with #5 tip and 12-13 Lpm argon, 2% Lanthanated 3/32. Everlast PowerTIG 185. Running practice beads and figuring out what the machine wants to be set at. Had some decent welds then this happened? Switched to a fresh tungsten and never happened again. What causes this?

[Oscar]

It's related to post-flow. Too short of a post-flow and the tip, possibly more, will get oxidized.

[hey_allen]

I've had similar when I ended up with a leak at the torch head, and pulled air into the shield gas flow.
If bumping post-flow up a little doesn't fix it, you might start checking for leaks around the cup and back cap.

[LtBadd]

It can also happen if you have the habit of quickly pulling the torch away, the action of the torch moving fast enough that the argon can't protect the tungsten

[DavesTent]

Thanks everyone! I'm assuming that it was probably me moving the torch away too quickly as the before and after passes did't produce the same issues. That's a good tip to check for leaks at the torch as well, I'll file that one away for next time.