1.2 GMAW — Gas Metal Arc Welding (MIG/MAG)
Key Takeaways
- GMAW feeds continuous solid wire with external gas on a constant-voltage (CV) source; wire-feed speed sets amperage
- Four transfer modes: short-circuit (all-position, low heat), globular (flat, spatter), spray (≥80% Ar, high deposition), pulsed-spray (all-position, controlled heat)
- MIG = inert gas (Ar/He); MAG = active gas (CO₂ or Ar/CO₂); C25 (75/25) is the common steel mix
- Spray transfer requires ≥80% argon and current above the transition current (~220–250 A for 0.045" wire)
- GMAW deposition efficiency is 90–95%, far above SMAW's 60–65%, but short-circuit mode risks cold lap
Gas Metal Arc Welding (GMAW)
Gas Metal Arc Welding (GMAW) — MIG (Metal Inert Gas) or MAG (Metal Active Gas) — is a semi-automatic process feeding a continuous solid wire electrode through a gun while an externally supplied shielding gas protects the pool. It dominates shop fabrication because it deposits fast (90–95% efficiency, no slag to chip) and is easily mechanized or robotized.
The gun's contact tip energizes the wire; the arc forms between wire and work; gas flows through the nozzle. The MIG/MAG distinction is purely about the gas:
- MIG uses inert gas (argon, helium) — non-ferrous metals such as aluminum and (with additions) stainless.
- MAG uses active gas (CO₂, or Ar + CO₂/O₂) — carbon and low-alloy steels. The small CO₂ or O₂ fraction is deliberately "active": it reacts in the arc to stabilize it and improve wetting, but it also oxidizes alloy, which is why steel wires carry extra silicon and manganese deoxidizers to compensate.
Power Source — Constant Voltage
GMAW uses a constant-voltage (CV) power source, the opposite of SMAW. The welder sets voltage and wire-feed speed; the CV supply self-regulates burn-off so the arc length stays constant. Wire-feed speed sets amperage — feed faster and current rises. This self-correcting "burnback" behavior is what makes GMAW forgiving and automatable.
| Component | Function |
|---|---|
| Power source (CV) | Holds voltage; regulates arc length automatically |
| Wire feeder | Drives solid wire at a set speed (sets amperage) |
| Gun / contact tip | Delivers current, wire, and gas; tip transfers current |
| Gas cylinder & regulator | Supplies and meters shielding gas (typ. 20–45 CFH) |
The contact tip is a wear item the inspector should not overlook: a worn tip causes erratic current transfer and arc instability, and spatter buildup in the nozzle restricts gas flow and invites porosity, so nozzle and tip condition are part of routine equipment checks before production welding.
Metal Transfer Modes
The four transfer modes — set by voltage, current, and gas — are core exam material. The transition current is the threshold above which globular flips to spray; for ~0.045" steel wire in 90/10 Ar/CO₂ it is roughly 220–250 A.
| Mode | V / A | Gas | Behavior | Best for |
|---|---|---|---|---|
| Short-circuit | Low (≈16–22 V) | CO₂ or 75/25 | Wire dips into pool, shorts and re-arcs 50–200×/sec; coolest | Thin steel, root, all positions |
| Globular | Medium | CO₂ | Drops larger than wire fall by gravity; heavy spatter | Carbon steel, flat/horizontal; rarely chosen on purpose |
| Spray | High (≥24 V, above transition) | ≥80% Ar | Fine droplets stream axially; smooth, high deposition | Thick steel, flat/horizontal, production |
| Pulsed-spray | Pulsed peak/background | ≥80% Ar | Pulses above/below transition; spray quality at lower average heat | All positions, medium-thick |
Wire Classification and Shielding Gas
The carbon-steel wire spec is AWS A5.18. Example ER70S-6: E = electrode, R = rod, 70 = 70 ksi tensile, S = solid, 6 = chemistry (high Si/Mn deoxidizers, tolerates light mill scale). ER70S-3 is the cleaner-steel general-purpose wire.
| Gas | Behavior | Use |
|---|---|---|
| 100% CO₂ | Deep penetration, most spatter, cheap | Carbon steel, short-circuit, outdoors |
| 75% Ar / 25% CO₂ (C25) | Balanced penetration/spatter | Most common all-around steel mix |
| 90% Ar / 10% CO₂ | Low spatter, supports spray | Carbon steel spray transfer |
| 100% Argon | Smooth, shallow | Aluminum, (with He) stainless |
| Ar/He | Hotter, deeper | Thick aluminum, copper |
Exam trap: Pure spray transfer needs ≥80% argon — it will NOT occur in 100% CO₂. And remember CV power, not CC. A worked heat input: 250 A, 28 V, 14 ipm → (28 × 250 × 60)/14 = 30,000 J/in (30 kJ/in).
Variants, Variables, and Inspection
GMAW has several named variants the exam may reference. 1 does not prequalify GMAW-S — it must be qualified by testing — precisely because of its incomplete-fusion tendency. Other related modes include the surface-tension-transfer and other controlled-short-circuit waveforms that modern inverters provide for root passes. Beyond gas and mode, the welder/WPS controls wire-feed speed (amperage), voltage, contact-tip-to-work distance (CTWD/stickout), gun travel and work angles, and gas flow rate (too low invites porosity; too high causes turbulence that pulls in air).
Common GMAW discontinuities the CWI watches for: cold lap / incomplete fusion in short-circuit mode (low heat), wormhole/whisker porosity and wire stubbing, excessive convexity in vertical-up welds, and porosity when wind, low flow, a clogged nozzle, or spatter buildup disrupts gas coverage. Because GMAW leaves no slag, surface inspection is easier than SMAW or FCAW, but incomplete fusion hidden beneath a sound-looking cap is the chronic concern — one reason production codes often require volumetric NDE (RT or UT) on critical short-circuit GMAW joints.
| Advantages | Limitations |
|---|---|
| High deposition; 90–95% efficiency | Wind disrupts gas shielding |
| Continuous wire, little downtime | More equipment than SMAW |
| Lower skill than SMAW; easily robotized | Cold-lap risk in short-circuit mode |
What type of power source does GMAW require?
Which transfer mode allows all-position welding using low voltage with the wire repeatedly shorting to the pool?
Spray transfer in GMAW requires a shielding gas that is at least:
In ER70S-6, what does the "S" designate?