456A GMAW MIG — Questions and Answers
Question 1: What are the two main modes of metal transfer in GMAW (MIG) welding?
- Stick transfer and flux transfer
- Short-circuit (short arc) transfer and spray transfer — with globular and pulsed spray as additional modes (Correct answer)
- Gravity transfer and magnetic transfer
- Continuous transfer and intermittent transfer
Correct answer: Short-circuit (short arc) transfer and spray transfer — with globular and pulsed spray as additional modes
The primary GMAW transfer modes are: short-circuit (wire touches the puddle, metal transfers during short circuits — used for thin material and all positions) and spray transfer (fine droplets are propelled across the arc — used for thicker material in flat and horizontal positions). Globular and pulsed spray are intermediate and advanced modes.
Question 2: What shielding gas or gas mixture is most commonly used for GMAW welding of carbon steel?
- Pure argon (100%)
- 75% argon / 25% CO₂ (C25) or 90% argon / 10% CO₂ (Correct answer)
- 100% CO₂
- Pure helium
Correct answer: 75% argon / 25% CO₂ (C25) or 90% argon / 10% CO₂
The most common shielding gas mixture for GMAW on carbon steel is 75% argon / 25% CO₂ (often called C25). This mixture provides good arc stability, minimal spatter, good weld appearance, and adequate penetration. Pure CO₂ is cheaper and provides deeper penetration but produces more spatter. Higher argon ratios (90/10) give smoother transfer.
Question 3: What is wire feed speed (WFS) in GMAW and what does it control?
- The speed the welder moves along the joint
- The rate at which the filler wire is fed to the arc; it directly controls the welding amperage in constant-voltage systems (Correct answer)
- The speed of the shielding gas flow
- The speed of the cooling water flow
Correct answer: The rate at which the filler wire is fed to the arc; it directly controls the welding amperage in constant-voltage systems
Wire feed speed (WFS) is the rate (in m/min or in/min) at which the consumable electrode wire is fed from the spool through the gun to the arc. In constant-voltage (CV) GMAW systems, WFS is the primary control for welding current — increasing WFS increases amperage, and decreasing WFS decreases amperage.
Question 4: What is the function of the contact tip in a GMAW gun?
- To start the arc by creating a spark
- To transfer welding current to the wire and guide it precisely to the weld joint (Correct answer)
- To regulate the shielding gas flow
- To cool the welding wire
Correct answer: To transfer welding current to the wire and guide it precisely to the weld joint
The contact tip is a copper or copper-alloy tube inside the GMAW gun nozzle that serves two functions: it transfers welding current from the power cable to the moving wire electrode, and it guides the wire to maintain a consistent stick-out and arc location. Contact tips are consumable and must be replaced when worn.
Question 5: What is the correct contact-tip-to-work distance (CTWD) or stick-out for GMAW on carbon steel?
- 3 to 6 mm (1/8 to 1/4 in.)
- 10 to 19 mm (3/8 to 3/4 in.) (Correct answer)
- 25 to 38 mm (1 to 1-1/2 in.)
- 50 mm (2 in.) minimum
Correct answer: 10 to 19 mm (3/8 to 3/4 in.)
The recommended CTWD for GMAW on carbon steel is typically 10 to 19 mm (3/8 to 3/4 inch). Too short a stick-out causes excessive spatter and can damage the contact tip. Too long a stick-out reduces shielding effectiveness, increases resistance (preheating the wire), and can cause irregular arc behaviour.
Question 6: What is a bird's nest or wire tangle in GMAW, and what causes it?
- A decorative weld pattern
- A tangle of wire at the drive rolls caused by a blockage in the liner, worn contact tip, excessive drive roll pressure, or wire feeding issues (Correct answer)
- A type of weld defect in the finished joint
- A pattern used for vertical-up welding
Correct answer: A tangle of wire at the drive rolls caused by a blockage in the liner, worn contact tip, excessive drive roll pressure, or wire feeding issues
A bird's nest is a tangled mass of wire that bunches up at the drive rolls. It occurs when the wire cannot feed through the gun — causes include a kinked or clogged liner, worn or wrong-sized contact tip, incorrect drive roll tension, contaminated wire, or a sharp bend in the gun cable. It requires stopping, cutting the tangle, and fixing the root cause.
456A GMAW MIG
What are the two main modes of metal transfer in GMAW (MIG) welding?