A system for on line monitoring of pulsed laser welding processes

Abstract

In the field of laser welding, the lack of suited inspection means generally leads users to develop their own systems. From this observation, an adaptable system to all welding conditions, dedicated to on line inspection of pulsed laser welding operations, has been devised. Its principle is very close to Quality Control by Artificial Vision systems. It is based on the recording and the treatment of signals resulting from laser–material interaction. The use of classification techniques and signal processing tools such as the wavelet transform, introduce an innovating approach of on line inspection. Thus, defects detection by discriminant parameters determination becomes automatic and nonsubjective. The presented method allows to determine the weld spot position (pulse number) and the type of defects and to estimate the reliability of the detection. This article presents the principle of the system and its validation on some classical defects in laser welding. The final objective is, in particular, to detect penetration variations during welding hollow parts, at the Valduc center of the French Atomic Energy Agency. In most cases penetration can be easily estimated by observing (with a photodiode or charge-coupled camera for example) the heating of the back of the welded part. Unfortunately in case of hollow parts, the sensors can only be placed at the outside!