AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is evaluating a friction stir weld (FSW) on an aluminum alloy aerospace component. The weld exhibits a 'tunnel' defect, characterized by an elongated void running parallel to the weld direction beneath the surface. Which of the following is the most likely cause of this defect?
- AToo high rotation speed.
- BInsufficient shoulder plunge depth.
- CInsufficient clamping force.
- DExcessive travel speed.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive travel speed.
A 'tunnel' defect in FSW is typically caused by insufficient heat input and/or insufficient material flow, often a result of excessively high travel speed. When the travel speed is too high, the material does not have enough time to soften and flow around the pin, leading to void formation beneath the surface.
Why the other options are wrong
- A. Too high rotation speed generally increases heat input and material flow, which typically would reduce the likelihood of a tunnel defect, though it could lead to other issues like excessive flash or nugget coarsening.
- B. Insufficient shoulder plunge depth usually leads to surface defects, such as a groove along the weld line, rather than a subsurface tunnel defect.
- C. Insufficient clamping force can lead to sheet separation, poor heat transfer, and potentially surface defects or inconsistent material flow, but a specific subsurface tunnel defect is more directly linked to material flow issues caused by travel speed.
FSW Tunnel Defect
A tunnel defect in Friction Stir Welding (FSW) is a subsurface void running parallel to the weld, often caused by inadequate material flow and consolidation.
- Subsurface void, elongated.
- Caused by insufficient heat input or material flow.
- Commonly due to high travel speed.
- Weakens the weld structurally.
Memory trick: Fast travel, tunnel forms, the material didn't flow through the storms.