AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is overseeing the fabrication of high-precision, thin-walled stainless steel components. The WPS specifies Plasma Arc Welding (PAW) in the keyhole mode. The welder reports frequent instances of porosity and occasional melt-through. Upon inspection, the CWI notes an inconsistent keyhole opening and erratic penetration. Which of the following adjustments is most likely to resolve these issues, assuming gas flow rates are adequate?
- ADecreasing the travel speed.
- BIncreasing the arc voltage.
- CIncreasing the shielding gas flow rate.
- DDecreasing the plasma gas flow rate.
Show answer & explanationAnswer & explanation
Correct answer: D. Decreasing the plasma gas flow rate.
In PAW keyhole mode, the plasma gas flow rate is critical. Too high a plasma gas flow rate can create an overly aggressive keyhole, making it difficult to control, leading to melt-through and porosity due to excessive pressure and turbulent molten metal. Decreasing it allows for a more stable keyhole and better control.
Why the other options are wrong
- A. Decreasing travel speed would increase heat input per unit length, likely exacerbating melt-through issues in thin-walled components and potentially making keyhole control more difficult due to a larger, more persistent molten pool.
- B. Increasing arc voltage (without adjusting current) would widen the arc cone and potentially increase heat input, which could worsen melt-through and keyhole control.
- C. Increasing shielding gas flow rate might help with atmospheric contamination but won't directly address keyhole instability or melt-through caused by plasma gas pressure.
PAW Keyhole Mode Control
Keyhole mode Plasma Arc Welding (PAW) relies on a precise balance of parameters, especially plasma gas flow, to maintain a stable, full-penetration keyhole without melt-through or porosity.
- Plasma gas forms the keyhole.
- Too much plasma gas: unstable keyhole, melt-through, porosity.
- Too little plasma gas: no keyhole, insufficient penetration.
- Travel speed, current also critical.
Memory trick: Plasma gas is the key, too much makes the hole flee.