456A MIG/FCAW (GMAW & Flux-Cored Arc Welding) 1 — Questions and Answers
Question 1: What type of power source characteristic is used for GMAW (MIG) welding?
- Constant voltage (CV) (Correct answer)
- Constant current (CC)
- Pulsed current only
- AC transformer
Correct answer: Constant voltage (CV)
GMAW uses a constant voltage (CV) power source. The arc self-regulates: if the arc shortens, current increases and melts the wire faster; if the arc lengthens, current decreases and the wire catches up. This maintains a stable arc length automatically with a constant wire feed speed.
Question 2: What is the function of the shielding gas in GMAW welding?
- Protects the weld pool from atmospheric oxygen and nitrogen, affects arc characteristics and bead profile (Correct answer)
- Cools the contact tip to prevent burnback
- Carries the electrode wire to the weld pool
- Provides the ionization path for arc initiation only
Correct answer: Protects the weld pool from atmospheric oxygen and nitrogen, affects arc characteristics and bead profile
Shielding gas in GMAW protects the molten weld pool and arc from atmospheric contamination (oxygen causes porosity and oxidation; nitrogen causes porosity and embrittlement). The gas type also affects arc stability, penetration profile, and spatter levels.
Question 3: Which shielding gas mixture is most commonly used for GMAW on carbon steel in Canada?
- 75% Argon / 25% CO2 (C25) (Correct answer)
- 100% CO2
- 100% Argon
- 50% Argon / 50% Helium
Correct answer: 75% Argon / 25% CO2 (C25)
C25 (75% Ar / 25% CO2) is the most widely used shielding gas for carbon steel GMAW in Canada. It provides a good balance of arc stability, low spatter, good penetration, and bead appearance. Pure CO2 gives deeper penetration but more spatter; pure Ar is used for aluminum and some stainless applications.
Question 4: What is 'wire burnback' in GMAW and what causes it?
- The wire fuses to the contact tip; caused by the wire feed stopping while the arc is still active (Correct answer)
- Excessive spatter that clogs the gas nozzle
- Porosity from shielding gas interruption
- Electrode wire breaking inside the conduit liner
Correct answer: The wire fuses to the contact tip; caused by the wire feed stopping while the arc is still active
Burnback occurs when the wire feed slows or stops while the arc remains active, causing the wire to melt back and fuse to the contact tip. Causes include wire feed drive roll slippage, a kinked liner, or releasing the gun trigger too slowly. It requires contact tip replacement.
Question 5: In GMAW short-circuit transfer mode, what happens during the 'short circuit' phase of metal transfer?
- The wire touches the weld pool, current surges, and surface tension pulls molten metal from the wire into the pool (Correct answer)
- A droplet of metal detaches freely in the arc without touching the pool
- The wire is vaporized by a high-energy pulse
- Metal is transferred by explosive electromagnetic forces
Correct answer: The wire touches the weld pool, current surges, and surface tension pulls molten metal from the wire into the pool
In short-circuit transfer, the wire periodically contacts (shorts to) the weld pool, extinguishing the arc momentarily. Current surges, and a combination of surface tension and electromagnetic forces (pinch effect) detaches the molten droplet into the pool. The arc then re-establishes and the cycle repeats 20–200 times per second.
Question 6: What shielding gas is recommended for GMAW welding of aluminum?
- 100% Argon (Correct answer)
- 75% Argon / 25% CO2
- 100% CO2
- 100% Helium
Correct answer: 100% Argon
100% Argon is the standard shielding gas for aluminum GMAW because it provides the cathodic cleaning action needed to break up the tenacious aluminum oxide layer, maintains arc stability, and does not cause excessive oxidation. CO2-containing mixtures are not suitable for aluminum.
456A MIG/FCAW (GMAW & Flux-Cored Arc Welding) 1
What type of power source characteristic is used for GMAW (MIG) welding?