A Study on Keyhole-Plasma Behavior and Multi-Sensor Correlation Analysis in Laser Welding

In this study, keyhole-plasma behavior during gap-affected laser welding was analyzed using a high-speed camera and a visible-light (VIS) photodiode. Four image-based metrics-image brightness (IG), plasma brightness (PG), molten pool area (MPA), and keyhole area (KA)-were extracted across nine weld segments. In conduction mode, all extracted metrics exhibited minimal fluctuation, indicating a stable welding condition. However, upon transitioning to keyhole mode, vapor pressure within the keyhole and Marangoni effect acting along its walls induced oscillation, resulting in increased variability in both brightness and area-related metrics. The gap-induced collapse of the keyhole temporarily reduced brightness and area metrics due to the disappearance of the plasma plume. However, during the reformation phase, rearward-oblique plasma ejection intensified, sharply increasing both brightness and area values. Correlation analysis revealed strong relationships between VIS and PG/MPA, indicating that VIS-PG residuals and VIS/MPA ratio can aid in detecting sensor faults, mode transitions, and keyhole defects.