Abstract: The iterative equation to extract the keyhole depth is founded according to the energy balance met by evaporation of the keyhole front wall and the extracted radial keyhole dimension in coaxial visual monitoring, and the weld penetration is monitored in the Nd：YAG laser deep penetration welding process based on the extracted keyhole depth. The studied re-sults show that the monitored weld penetration accords well with that of experiment and the monitored error is less than 12% for partial penetration. While for full penetration, the monitored weld penetration is much greater than the thickness of the workpiece, which is used to judge whether the workpiece is fully penetrated or not. The experimental results indicate that it is effective to monitor weld penetration in laser deep penetra-tion welding based on the energy balance model of keyhole front wall by using coaxial visual sensing system.

Key words: Weld penetration, Coaxial visual sensing, Energy balance, Nd：YAG laser deep penetration welding Keyhole