Keyhole welding of steel plates was performed using a 50kW fiber laser, and the factors underlying process instabilities were analyzed through in situ real-time observation of welding behavior. Laser welding has attracted significant attention across various industrial sectors owing to its advantages, including deep penetration and high compatibility with automated manufacturing. When a laser is irradiated onto a steel plate, the surface is rapidly heated, and a molten pool forms once the temperature exceeds the melting point. Subsequently, a keyhole is generated due to recoil pressure induced by metal evaporation. During this process, both the molten metal flow and the geometries of the solid–liquid and liquid–vapor interfaces evolve dynamically with time.　 In particular, non-uniform molten metal flow around the keyhole and periodic oscillations of the keyhole geometry can lead to fluctuations in molten pool morphology. Such fluctuation represents a critical issue that directly related affect　the reproducibility and stability of the welding process.

In general, it is considered that as the laser power enhances the dynamic instability of the keyhole and molten pool because of stronger evaporations recoil pressure and increased generation of metal vapor. However, the fundamental mechanism governing this phenomenon remains insufficiently understood. Therefore, in this study, in situ high speed observations of molten pool and keyhole behavior were conducted under a　wide range of laser irradiation conditions using a 50kW-class fiber laser. The objective was to identify the dominant factors responsible for the increased instability observed at high laser powers and to elucidate their underlying physical mechanism.