Welding Tips and Tricks

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?

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

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.

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

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.