In-process monitoring of laser welding by the analysis of ripples in the plasma emission

A novel in-process monitoring system employing two detectors set above the workpiece at different aiming angles of 5° and 75° has been developed to detect whether or not CO2 laser welding fully penetrates through to the back surface of steel sheets. The acquired signal contained a.c. components of the emission of the laser-induced plasma in the plume and in the keyhole with frequencies up to approximately 10 kHz. The mean square value of the a.c. signal obtained by using the 75° sensor during full penetration welding was much larger than that of the partial penetration welding, showing that full penetration welding can be monitored with a high accuracy by using the 75° sensor.