GMAW (MIG) Welding Theory and Equipment Setup
This domain tests transfer modes, how wire-feed speed, voltage, and inductance interact, and correct setup of machine, gun, gas, and work clamp. The exam favors cause-and-effect over knob memorization.
Transfer modes
Short-circuit transfer runs at low current: the wire contacts the puddle and pinches off, suiting thin sheet and out-of-position work; C25 (75% Ar/25% CO2) or 100% CO2 is typical, heat input is low, spatter moderate. Globular transfer produces large droplets and a harsh arc and is avoided on thin auto steel. Spray transfer needs argon-rich gas (roughly 80% Ar or more) and current above the transition point, giving fine droplets, deep penetration, and high deposition in the flat position. Pulsed spray alternates peak and background current to get spray-like transfer at lower average heat - useful on thin aluminum and vertical joints.
Machine setup
The power source is constant voltage: wire-feed speed sets amperage, voltage sets arc length, and inductance shapes the short-circuit pinch. Use DCEP for steel. Drive rolls are V-groove for steel, U-groove with a Teflon liner for aluminum. ER70S-6 suits mild and HSLA steel; 0.023–0.030 in wire for thin panels, 0.035 for heavier stock. Gas flow runs about 15–25 cfh; too much causes turbulence and porosity, too little loses coverage. Hold 3/8–1/2 in stickout and push 10–15°.
Worked example
A fresh setup produces porosity on clean mild steel. Voltage and wire-feed speed look right, so check gas first: a loose regulator fitting, flow set at 40 cfh drawing air into the stream, a clogged nozzle, or wind from an open bay door. Only after the gas path is verified do you inspect wire condition or base-metal contamination. Porosity is a gas problem until proven otherwise.
Traps
Raising voltage to "add heat" - it lengthens the arc and can reduce penetration; using a CO2-heavy blend in spray mode; forgetting the work clamp must contact bare metal.