Observation of the mechanisms causing two kinds of undercut during laser hybrid arc welding

► Identification of two types of undercuts in two cases, remaining or removed mill scale (oxides). ► With remaining mill scale, the oxide caused lack of fusion defect. ► Formation mechanisms of the two kinds of undercut are observed by high speed imaging. ► In case of mill scale, imaging showed that the melt does not attach to the top but glides down. ► A base material layer is the interface for rounded undercut formation.