Wire Feed and Tungsten Arc Processes (GMAW, FCAW, GTAW)
This domain tests the setup logic of the wire-feed and tungsten-arc processes: transfer mode, shielding gas, polarity, wire-feed variables, tungsten preparation, and the defects that follow from a wrong setting.
GMAW and FCAW
GMAW uses a continuous bare wire, a constant-voltage power supply, and DCEP. Wire-feed speed sets current; voltage sets arc length. Short-circuit transfer suits thin sheet, globular transfer is rough and generally avoided, and spray transfer needs an argon-rich gas and higher current. FCAW uses a flux-cored wire: gas-shielded FCAW usually runs DCEP with CO2 or Ar/CO2, while self-shielded FCAW often runs DCEN and needs no external gas but is vulnerable to wind. Keep CTWD/stickout short enough for stable current and gas coverage.
GTAW
GTAW uses a nonconsumable tungsten electrode and a separate filler. DCEN puts heat into the work and is used for carbon steel, stainless, and most alloys; AC is used for aluminum because it provides oxide cleaning; DCEP is rarely used because it overheats the tungsten. Grind a taper for DC, ball the tip for AC aluminum, and never touch the tungsten to the puddle-contamination causes arc wander and inclusions. Use argon or argon/helium and allow preflow and postflow.
Worked example: An operator TIG welding 1/8 in aluminum gets a sooty, dull bead and the tungsten is balled and discolored. Reasoning: aluminum is welded on AC so the half-cycle provides oxide cleaning; a balled tip is normal for AC, but discoloration means the tungsten touched the puddle or shielding was lost. Regrind or replace the tungsten, clean the oxide with a stainless brush, restore argon flow (about 15–25 cfh), and recheck postflow. Switching to DCEN would stop the cleaning action and make the problem worse.
Traps: GMAW is not normally run DCEN; short-circuit transfer is not the choice for thick plate; self-shielded FCAW is not protected by a gas curtain; a contaminated tungsten should be replaced or reground, not ignored.