456A MIG/FCAW (GMAW & Flux-Cored Arc Welding) 3 — Questions and Answers
Question 1: What is the correct procedure when porosity is found in a GMAW weld due to suspected shielding gas loss?
- Check gas flow rate, hose connections, and nozzle for blockage; test on scrap before resuming production welding (Correct answer)
- Increase wire feed speed and voltage to burn out the porosity
- Add backing gas to the back side of the joint
- The weld is acceptable if porosity is below 3mm diameter
Correct answer: Check gas flow rate, hose connections, and nozzle for blockage; test on scrap before resuming production welding
When porosity indicates shielding gas issues, the welder must systematically check: flow rate (typically 15–20 L/min for C25), hose connections for leaks, nozzle cleanliness, and draft/wind conditions. Testing on scrap confirms the fix before resuming production welding to avoid rejectable welds.
Question 2: What is the minimum recommended shielding gas flow rate for GMAW carbon steel welding (C25) to ensure adequate coverage?
- 15–20 L/min (30–40 CFH) (Correct answer)
- 5–8 L/min (10–16 CFH)
- 35–45 L/min (70–90 CFH)
- Gas flow rate does not affect weld quality
Correct answer: 15–20 L/min (30–40 CFH)
A flow rate of 15–20 L/min (approximately 30–40 CFH) is the standard range for GMAW with C25 shielding gas. Too low a flow rate provides inadequate protection; too high a flow rate causes turbulence that can draw in atmospheric air, both resulting in porosity.
Question 3: What is 'lack of fusion' (LOF) in GMAW and what typically causes it?
- Weld metal that does not bond to the base metal or previous pass; caused by insufficient heat input or improper technique (Correct answer)
- Incomplete filling of the groove joint
- Weld metal that solidifies before reaching the root
- An undersize weld that does not meet the drawing requirements
Correct answer: Weld metal that does not bond to the base metal or previous pass; caused by insufficient heat input or improper technique
Lack of fusion occurs when the weld metal and base metal (or adjacent weld passes) are in contact but not metallurgically bonded. Causes include too low an amperage/voltage, too fast a travel speed, improper electrode angle directing the arc away from the fusion face, or a dirty/oxidized base metal surface.
Question 4: When welding with an E71T-1C flux-cored electrode, what does the 'C' suffix indicate?
- CO2 shielding gas is required (Correct answer)
- Calcium-based flux formulation
- Cold weather application only
- Canadian Standards Association classification
Correct answer: CO2 shielding gas is required
In the AWS A5.20 (and CSA equivalent) flux-cored electrode classification, the 'C' suffix on FCAW electrodes (e.g., E71T-1C) indicates that pure CO2 shielding gas is required. An 'M' suffix indicates mixed gas (Ar/CO2). Some electrodes are rated for both (C/M).
Question 5: What is the main advantage of FCAW-G over SMAW for structural welding?
- Higher deposition rate and continuous wire feed allowing longer welds without stopping to change electrodes (Correct answer)
- Lower heat input reducing distortion
- Better performance in windy outdoor conditions
- No slag to remove after welding
Correct answer: Higher deposition rate and continuous wire feed allowing longer welds without stopping to change electrodes
FCAW-G offers significantly higher deposition rates than SMAW (up to 4–5× faster) and uses continuous wire so the welder does not need to stop to change electrodes. This increases productivity, reduces labour costs, and is why FCAW-G has largely replaced SMAW in structural fabrication shops.
Question 6: What does the term 'duty cycle' mean for a GMAW power source?
- The percentage of a 10-minute period that the machine can weld at a rated amperage without overheating (Correct answer)
- The number of arc starts the machine can perform per hour
- The maximum current the machine can output continuously
- The ratio of wire consumed to wire deposited
Correct answer: The percentage of a 10-minute period that the machine can weld at a rated amperage without overheating
Duty cycle is expressed as a percentage of a 10-minute interval. A machine rated at 300A at 60% duty cycle can weld at 300A for 6 minutes out of every 10 minutes. Exceeding the duty cycle activates thermal protection (overtemp cutout). Higher duty cycles are needed for production FCAW and GMAW applications.
456A MIG/FCAW (GMAW & Flux-Cored Arc Welding) 3
What is the correct procedure when porosity is found in a GMAW weld due to suspected shielding gas loss?