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What is tunnel/wormhole defect in FSW?

Wormhole is a defect that can be encountered in friction stir welding (FSW). It can occur on FSW butt or lap welds.
Visible to the naked eye, the wormhole is an internal cavity in the welded joint. This porosity is present along all or part of the length of the weld. Mechanically, this weld defect behaves like a crack.
The wormhole is mainly due to a bad definition of the welding parameters.