AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is observing a Laser Beam Welding (LBW) operation on a high-strength aluminum alloy component. The welder reports significant issues with porosity in the weld bead, despite adequate shielding gas coverage. The CWI notes that the laser power is set quite high, and the travel speed is relatively low. What is the most probable cause of the porosity in this scenario?
- AImproper laser beam mode.
- BExcessive keyhole instability.
- CInsufficient focal point positioning.
- DContamination from the shielding gas.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive keyhole instability.
High laser power and low travel speed in LBW can lead to an overly deep and unstable keyhole. This instability can cause fluctuations in the keyhole size, leading to the entrapment of shielding gases or metal vapors as the keyhole collapses, resulting in significant porosity. Aluminum alloys are particularly susceptible due to their high thermal conductivity and reflectivity.
Why the other options are wrong
- A. Improper laser beam mode (e.g., multimode instead of single-mode for deep penetration) affects the beam's focusability and energy distribution, but keyhole instability is a more direct cause of porosity under these specific parameter conditions.
- C. While incorrect focal point positioning can cause issues with penetration and bead shape, excessive porosity due to high power/low speed is more directly linked to keyhole dynamics.
- D. The question states 'adequate shielding gas coverage', implying the gas itself is not the source of contamination, but rather gas entrapment due to keyhole dynamics.
LBW Porosity in Aluminum
Porosity in Laser Beam Welding (LBW) of aluminum alloys is a common issue, often caused by keyhole instability, which entraps gas and metal vapors during solidification.
- Aluminum is prone to porosity due to high thermal conductivity and reflectivity.
- Keyhole instability is a major cause.
- High power/low speed can exacerbate keyhole issues.
- Entrapment of shielding gas or metal vapor.
Memory trick: Keyhole is a wild beast, too much power, it won't cease.