GMAW (Gas Metal Arc Welding) Fundamentals
This domain tests GMAW fundamentals: wire feed, polarity, shielding gas, transfer mode, contact-tip-to-work distance (CTWD), inductance and troubleshooting. Expect questions that ask why a mode or gas is chosen for a thickness and position.
Transfer modes
- Short-circuit transfer uses low current and small droplets that short to the pool; it suits thin sheet and out-of-position work but has low heat input and more spatter.
- Globular transfer has large droplets and is generally less stable.
- Spray transfer needs a high current and an argon-rich gas; it gives high deposition and deep penetration but is usually limited to flat or horizontal positions.
- Pulsed transfer alternates peak and background current to give spray-like transfer at a lower average current.
Worked example
A 6 mm carbon steel plate is to be welded flat with solid wire. Reasoning: spray transfer with an Ar/CO2 mix and DCEP can give good penetration and deposition. Set wire-feed speed to obtain current above the spray transition for that wire diameter, set voltage to match arc length, and keep CTWD short and steady. If the job were thin sheet in the vertical-up position, short-circuit transfer would be more controllable.
Traps
Solid wire GMAW normally uses DCEP; wrong polarity gives a poor arc. CO2 does not produce a true spray arc. Too-long CTWD reduces current and gas coverage. Do not blame gas flow for porosity before checking dirty metal, wind and a worn contact tip.