ONLINE ESTIMATION OF LASER WELD PENETRATION

High-power CO2 laser welding has been widely used in the industry beca use of its high productivity and excellent weld quality. In order to t ap the potential of this process completely, it is important to have o n-line weld quality inspection methods to improve the process producti vity and reliability by achieving 100 percent weld inspection. Weld pe netration is one of the most important factors critical to the quality of a laser weld. However, it is very difficult to directly measure th e extent of penetration without sectioning the workpiece. In th is pap er a model-based penetration depth estimation technique suitable for t he production environment is developed The proposed model relates the temperature measured on the bottom surface of the workpiece, weld bead width, laser beam power and welding speed to penetration depth. The c losed-loop depth estimator combines the model and a model-error compen sator to compensate for the uncertainty in the measurement of the lase r power and absorptivity. Other effects considered are the averaging d ue to the finite size of the sensor delay based on the sensor location and the process and sensor dynamics. Several bend-on-plate and butt w elds were made on low carbon steel plates to validate the static proce ss models and the depth estimation scheme. Temperatures on the bottom surface of the workpiece during welding were measured using infrared t hermocouples. The welds were sectioned longitudinally to obtain the pe netration profile. The penetration profiles estimated by the depth est imator matched satisfactorily with the measured penetration profiles. The results validate the capability of the proposed depth estimator to estimate penetration depth and its ability to trace the dynamic chang es in penetration depth.