3.3 MIG/MAG (GMAW) & Flux-Cored Arc Welding

3.3 MIG/MAG (GMAW) & Flux-Cored Arc Welding

Quick Answer: GMAW uses continuous solid wire + external gas on a CV source (MIG = inert gas, process 131; MAG = active gas, process 135). FCAW uses flux-cored tubular wire, either gas-shielded (136 family) or self-shielded (114). Transfer mode, gas, voltage, and wire speed must match the WPS—inspectors verify them as process controls.

GMAW Process Fundamentals

Gas Metal Arc Welding feeds a continuous solid electrode wire through a gun. Current transfers at the contact tip; shielding gas flows through the nozzle around the arc. Because the electrode is continuous, deposition rates and duty cycles are high, slag is absent with solid wire, and the process is readily mechanised or robotised.

Power source: Constant voltage (CV). The operator sets voltage and wire-feed speed (WFS). WFS is the primary control on current; the CV characteristic self-adjusts burn-off to hold arc length for a given contact-tip-to-work distance (stick-out).

MIG vs MAG

The “active” fraction in MAG deliberately improves arc stability and wetting on steels but also oxidises the pool; solid steel wires therefore carry deoxidizers (Mn, Si). Using pure CO₂ often increases spatter and changes bead profile versus argon-rich mixes. Wrong gas for the wire/base metal is a frequent root cause of porosity, oxidation, and mechanical property issues.

ISO 4063 references you should recognise:

• 131 — MIG with solid wire, inert gas
• 135 — MAG with solid wire, active gas
• 136 — MAG with flux-cored wire (gas-shielded FCAW)
• 114 — self-shielded tubular cored wire (no external gas)

Metal Transfer Modes

Transfer mode describes how molten metal moves from wire to pool. Mode depends on current, voltage, wire diameter, gas, and power-source waveform (including pulsed options).

Short-circuit (dip) transfer

• Wire tip periodically short-circuits into the pool; surface tension and magnetic forces clear the neck.
• Low heat input, good for thin material and some out-of-position work.
• Can produce spatter and, if parameters or technique are poor, lack of fusion on thicker sections or cold sidewalls.
• Common with CO₂ or mixed gases at lower currents.

Globular transfer

• Large droplets form and detach under gravity/electromagnetic force at intermediate parameters.
• Often high spatter, irregular arc; usually avoided as a production target mode on quality work.
• May appear when voltage/current are poorly matched for spray or short-circuit.

Spray transfer

• Stream of fine droplets projected axially across the arc above a transition current (gas and diameter dependent).
• Smooth arc, higher deposition, deeper penetration—typically flat and horizontal on steels with argon-rich gases.
• Heat input is higher; not automatically suitable for thin sheet or all positions without pulsed variants.

Pulsed transfer

• Current pulses between background and peak so average heat is controlled while still detaching droplets in a spray-like manner.
• Improves positional welding, thin-to-thick transitions, and spatter control on modern inverter sets.
• WPS must list pulsed parameters or approved programs when pulsing is essential to qualification.

Inspector insight: Transfer mode is not a decorative label—it is a process outcome of the parameter set. If the WPS implies short-circuit for thin wall but the welder runs hot spray parameters, heat input, distortion, and fusion behaviour leave the qualified envelope.

Equipment and Parameter Windows

Critical set-up items:

• Contact tip size and condition — worn tips cause unstable current pickup.
• Nozzle cleanliness — spatter buildup restricts gas and causes porosity.
• Stick-out / CTWD — longer stick-out increases resistance heating of the wire, lowers current for a given WFS, and can disturb shielding.
• Gas flow rate — too low invites air aspiration; too high can cause turbulence and entrain air. Follow WPS (often on the order of typical shop flow rates for the nozzle size—do not invent values).
• Wire quality — rust, oil, or wrong cast/helix feed problems create bird-nesting and arc instability.

Flux-Cored Arc Welding (FCAW)

FCAW uses a tubular wire filled with flux (and sometimes metal powders). It keeps continuous-feed productivity while adding slag and gas generation from the core.

Gas-shielded FCAW (FCAW-G)

• External gas (often CO₂ or Ar/CO₂) plus core reactions.
• Smooth beads, good toughness potential with the right wire, common in shop structural and heavy fabrication.
• Still vulnerable to wind and nozzle blockage like GMAW.
• Slag must be removed between passes.

Self-shielded FCAW (FCAW-S)

• Shielding generated only by core decomposition—no gas cylinder.
• Favoured for outdoor erection and windy sites where gas shielding would be stripped.
• Fume levels can be high; health controls matter.
• Weld metal chemistry and hydrogen behaviour depend strongly on wire design—use only WPS-specified classifications.

FCAW combines GMAW’s CV parameter logic with MMA-like slag traps. Incomplete deslagging, wrong voltage (long arc), or excessive stick-out are common defect generators.

Common Defects from Wrong Mode, Gas, or Parameters

Inspector Parameter Verification

Before and during welding, verify against the WPS and ITP:

1. Process number and mode — 131/135 vs 136 vs 114; pulsed program if specified.
2. Power source — CV (or correct synergistic program); polarity typically DCEP for most GMAW/FCAW steel wires—confirm on WPS.
3. Wire — classification, diameter, heat/lot traceability where required; dry undamaged packaging for cored wires.
4. Shielding gas — type, mixture percentage, flow rate, and hose integrity; for self-shielded FCAW, confirm no external gas is required (and that none is incorrectly fitted in a way that confuses operators).
5. Electrical parameters — voltage, WFS/current, stick-out within qualified ranges; meters readable and credible.
6. Travel speed and technique — as needed for heat input limits when specified.
7. In-process evidence — stable arc sound/appearance appropriate to intended transfer mode; no systematic porosity or undercut developing.

Productivity vs Control

GMAW and FCAW dominate modern fabrication because they weld fast. That speed is exactly why parameter drift is dangerous: a robot or semi-automatic operator can deposit metres of non-compliant weld before visual inspection catches a pattern. IWI-S competence includes stopping work, raising NCRs, and requiring return to qualified settings—not merely noting “arc looked a bit long” after the fact.

Bridge to Heat Input and Consumable Standards

Chapter 4 deepens heat input, arc energy, and ISO consumable designations. For now, remember: transfer mode + gas + CV parameters define GMAW quality, and cored wire type + slag discipline define FCAW quality. Both live or die by WPS fidelity.