What is lack of penetration defect in FSW?

Lack of Penetration (LoP) occurs when the metal does not completely fill the thickness of the joint. This internal defect can occur during butt welding operations.
This lack of root penetration is mainly due to the fact that the pin does not reach the bottom of the material. Thus, a pin that is too short will not allow the entire thickness to be welded. If the length of the pin is adapted, a lack of penetration indicates an insufficient forging effort.
To avoid a lack of penetration, it is important to choose the right FSW tools and the right settings.