GMAW Process Execution: Parameters, Gas, and Technique
Domain 4 - GMAW welding process execution. This domain covers transfer modes, parameter relationships, shielding gas, torch manipulation, and the four positions - the core skill the rest of the exam supports.
Transfer modes
Short-circuit: low voltage and current, small wire, all positions, but colder and prone to lack of fusion. Globular: coarse and spattery, best avoided. Spray: high current with argon-rich gas, deep penetration, flat and horizontal only. Pulsed spray gives spray-like transfer at lower average current for out-of-position work.
Parameter relationships
In CV GMAW, wire feed speed sets amperage and voltage sets arc length.
- Voltage too high: long arc, wide flat bead, spatter, undercut, poor gas coverage.
- Voltage too low: stubbing, cold lap, incomplete fusion, ropey bead.
- Travel too fast: narrow high crown, undercut, incomplete penetration.
- Travel too slow: wide bead, excess penetration, burn-through, excess heat. Stickout is normally 6–13 mm; a longer stickout lowers current and invites porosity.
Shielding gas
Flow about 12–20 L/min. Too little gives porosity and a dirty arc; too much causes turbulence that also drags in air. Draughts, fans, and open doors destroy coverage - screen the work instead of turning up the flow.
Torch technique and positions
Push (forehand) flattens and widens the bead; drag (backhand) increases penetration. On fillets hold roughly a 45° work angle and watch both legs. Keep weaves modest, about 2.5–3 times wire diameter, or fusion suffers at the edges. Vertical-up gives penetration; vertical-down suits thin sheet. Overhead needs a short arc, small puddle, and correct PPE.
Worked example: A bead shows undercut at both toes, heavy spatter, and scattered porosity. Reason: arc too long (voltage high) plus lost gas coverage. Fix: reduce voltage, clean the nozzle, reset flow, screen the draught, and run a test bead.
Traps: raising gas flow to cure porosity caused by draught; using spray transfer in the vertical position; ignoring stickout as a variable.