Welding process
Gas metal arc welding (GMAW)
A continuously fed solid wire electrode burns in an arc protected by shielding gas from the gun nozzle, giving high deposition and no slag to remove between passes.
How it works
A solid wire is fed continuously through a gun at a set speed while a constant-voltage power source supplies current. The arc melts the wire and the base metal, and the wire feed and burn-off rate self-balance: if the arc lengthens, current rises and burns the wire back faster, which restores arc length automatically.
Shielding gas flows from the nozzle around the wire, displacing air so oxygen and nitrogen cannot reach the molten metal. Pure argon and argon-rich mixtures give a stable arc with little oxidation, while adding carbon dioxide or oxygen increases penetration and improves wetting on carbon steel at the cost of more spatter.
How metal crosses the arc depends on voltage, current, and gas. At low settings the wire short circuits into the pool repeatedly, giving a cool, controllable arc for thin material and out-of-position work. At high settings with argon-rich gas the metal sprays across as a fine stream of droplets, giving deep penetration and high deposition in the flat and horizontal positions.
Strengths
- High deposition rate and high operating factor because the wire feeds continuously
- No slag to chip, so interpass cleaning is quick and slag inclusions are largely eliminated
- Easy to mechanise or robotise, since the process variables are set on the machine
- Clean, low-spatter welds in spray transfer with good bead appearance on prepared material
Limitations
- Shielding gas is easily blown away, so the process struggles outdoors without screening
- Short circuit transfer runs cool and is prone to incomplete fusion if settings or technique are wrong
- Gas cylinders, regulators, and a wire feeder make the equipment less portable than a stick machine
- Sensitive to surface contamination, since there is no aggressive flux to burn through rust and paint
Key variables
- Wire feed speed
- On a constant-voltage machine this sets welding current and therefore deposition rate and penetration depth.
- Voltage
- Sets arc length and bead shape. Too low gives a ropey stubbing arc; too high flattens the bead and increases spatter and undercut.
- Shielding gas
- Composition controls arc stability, penetration profile, transfer mode, and how much oxidation the pool sees.
- Contact tip to work distance
- Longer stickout raises resistance heating in the wire, which increases deposition but reduces current and penetration.
- Travel and work angle
- Pushing gives a flatter, wider bead with less penetration; dragging gives more penetration and a more convex bead.
What an inspector watches for
- Verify gas type and flow at the nozzle rather than the regulator, since a blocked nozzle starves the pool while the gauge still reads normal
- Watch for incomplete fusion in short circuit transfer, which produces a good-looking bead over a cold, unfused sidewall
- Check for silicon islands left on the bead and confirm they are removed before the next pass
- Confirm that the procedure's transfer mode matches what the welder is actually running, since the two behave very differently