AWS Gas Metal Arc Welding (GMAW/MIG) 2 — Questions and Answers

Question 1: What metal transfer mode is used in GMAW short-circuit transfer?

The droplet transfers across the arc without touching the base metal
The wire physically touches and short-circuits into the weld pool at low voltage (Correct answer)
Droplets are propelled by electromagnetic force in a spray
Metal is transferred in a rotating manner at very high currents

Correct answer: The wire physically touches and short-circuits into the weld pool at low voltage

In short-circuit transfer (SCT), the wire tip contacts the molten pool, causing a short circuit; the current surges, melts the wire tip, and breaks the short — repeating 20–200 times per second at low voltage.

Short-circuit transfer (AWS term) occurs at low voltage (14–22V) and low current. The wire periodically contacts the weld pool, creating a short circuit that momentarily drops voltage. Current surges, the wire tip pinches off into the pool (pinch effect), the arc reignites, and the cycle repeats. This produces a small, fast-freezing weld pool ideal for thin materials and out-of-position welding. However, it has higher risk of cold lap (incomplete fusion) if parameters aren't carefully controlled.

Question 2: In GMAW spray transfer, what gas mixture is most commonly used for carbon steel?

100% CO2
100% Argon
Ar/CO2 blend (typically 75-95% Ar) (Correct answer)
He/CO2 blend

Correct answer: Ar/CO2 blend (typically 75-95% Ar)

Spray transfer requires a high-argon shielding gas (typically 75–95% Ar with 5–25% CO2) to maintain the stable axial spray arc; 100% CO2 doesn't support true spray transfer.

True spray transfer requires argon content above the transition threshold (approximately 85% Ar for carbon steel). 100% CO2 supports only globular or short-circuit transfer at normal currents. With high-argon mixtures, current exceeds the transition current, and droplets become smaller than the wire diameter and project axially into the pool at high velocity — spray transfer. This mode produces high deposition rates, excellent penetration, minimal spatter, and is ideal for flat and horizontal welding of thick steel.

Question 3: The 'burnback' setting on a GMAW welding machine controls:

The rate at which the wire retracts after the arc extinguishes (Correct answer)
The preflow time for shielding gas before arc initiation
The slope of the volt-ampere curve
The wire feed speed during run-in

Correct answer: The rate at which the wire retracts after the arc extinguishes

Burnback (also called burn-back time or retract) is the brief delay between wire feed stopping and the power source shutting off, which burns back the wire to prevent it from freezing in the puddle.

When a GMAW weld terminates, wire feed stops before the power source completely shuts off. This brief period of arc-on-with-no-feed melts back (burns back) the wire slightly, preventing the electrode from sticking in the solidifying pool. Too little burnback results in the wire tip sticking in the crater; too much burnback causes the arc to wander back into the contact tip. Proper burnback setting is important for clean weld terminations and protecting contact tip life.

Question 4: Globular transfer in GMAW is characterized by:

Droplets smaller than the wire diameter transferring axially
Large, irregular droplets that fall off the wire due to gravity — high spatter (Correct answer)
Wire touching the pool in short circuits at low voltage
Fine droplets rotating around the wire axis at very high current

Correct answer: Large, irregular droplets that fall off the wire due to gravity — high spatter

Globular transfer produces droplets larger than the electrode diameter that detach irregularly due to gravity, resulting in high spatter levels, and is generally avoided in production welding.

Globular transfer occurs in the intermediate voltage range — above short-circuit but below spray transition current. Droplets grow larger than the wire diameter before detaching, driven primarily by gravity. This causes erratic arc behavior and high spatter. CO2 shielding promotes globular transfer due to its lower ionization potential. While not preferred for production, globular transfer can be used in flat position on thicker steels. Welders typically avoid it in favor of short-circuit (for thin/out-of-position) or spray (for flat/horizontal on thick material).

Question 5: What is the purpose of the inductance setting on a GMAW CV power source?

Controls the wire feed acceleration rate
Smooths the arc by controlling the rate of current rise during short circuits (Correct answer)
Adjusts the no-load voltage
Sets the maximum current output

Correct answer: Smooths the arc by controlling the rate of current rise during short circuits

Inductance controls how quickly current rises during short-circuit events in GMAW. Higher inductance slows the current rise, producing a softer arc with less spatter and a flatter bead.

In GMAW short-circuit transfer, each short circuit causes a rapid current surge. The inductance setting acts as an electrical 'shock absorber' — a higher inductance value slows the rate of current rise (dI/dt), reducing the violence of the short-circuit rupture and decreasing spatter. Lower inductance produces a crisper, more penetrating arc but more spatter. Most modern GMAW machines allow inductance adjustment as part of arc quality tuning, particularly important for short-circuit transfer on thin materials.

Question 6: For GMAW on austenitic stainless steel, the recommended shielding gas is typically:

100% CO2
100% Argon
Ar/He/CO2 trimix or Ar/CO2 with ≤2% CO2 (Correct answer)
75% Ar / 25% CO2

Correct answer: Ar/He/CO2 trimix or Ar/CO2 with ≤2% CO2

Stainless steel welding requires very low CO2 content (≤2%) or tri-mix (Ar/He/CO2) to minimize carbon pickup (sensitization) and carbide precipitation in the HAZ.

Carbon pickup from CO2 decomposition can cause sensitization (chromium carbide precipitation at grain boundaries) in austenitic stainless steel, reducing corrosion resistance. Therefore, stainless steel GMAW typically uses Ar/1-2% CO2 or Ar/He/CO2 trimix (e.g., 90% Ar / 7.5% He / 2.5% CO2) to maintain arc stability while minimizing carbon exposure. 100% Ar alone can cause arc wander on stainless. The specific gas is called out in the WPS and qualified per AWS D1.6.

AWS Gas Metal Arc Welding (GMAW/MIG) 2

What metal transfer mode is used in GMAW short-circuit transfer?