Forming Gas Purging Pressure
The precisely regulated positive pressure of the shielding or forming gas on the root side, which prevents oxidation of the molten metal during the joining process.
The purge pressure on the shielding gas must be set so that the weld root is cleanly covered, but not so high that it causes turbulence in the heat-affected zone or pushes gas into the weld pool. Too little pressure means you get air at the hot spot and the root oxidizes away, which you'll see as a blue or gray color. Too much pressure doesn't give you better shielding but can cause the weld to blister or lead to lack of fusion. Set the pressure so there's a light, even gas flow at the root exit—that's all you need. If you're unsure, check the gas manufacturer's specs or do a test weld and inspect the root.