In-process monitoring of penetration depth in 20kW-class CO2 laser welding of thick sections

Light emission was detected by a multi-photosensor system consist of ten photo-diodes with different aiming angles between 0 and 86 degree to distinguish the light intensities emitted from plume, keyhole opening and the finger part of keyhole. Bead-on-plate welding of thick sheets with a CO2 laser of 20 kW power level was carried out at different laser powers and welding velocities. It was found that the light emission from the finger Dart was much smaller than that from the keyhole opening. The power spectrum of the light emission from the finger part was correlated to the penetration depth based on an empirical formula. The thus estimated penetration depth is found to agree well with the experimental penetration depth at various test conditions.