Plasma charge sensor for in-process, non-contact monitoring of the laser welding process

Laser keyhole welding is an important modern manufacturing technology. During such welding a plasma cloud is generated and the behaviour of this plasma is closely coupled to the behaviour of the weld. The plasma behaviour has been observed during welding through measurement of the space charge voltage induced on an electrically insulated welding nozzle, the plasma charge sensor (PCS). It is shown both theoretically and experimentally that the induced voltage is a measure of plasma temperature and thus of the welding performance. In particular, it is shown under laboratory conditions that the PCS signal can measure weld penetration and detect a wide range of weld defects. Results have also been obtained from the PCS installed on an industrial laser can welder. An expert system has been used to analyse the PCS signal automatically and classify weld defects. It is reported that a greater than 90% success rate has been achieved in detecting and classifying defects in high-speed industrial laser can welding. The PCS is shown to be a reliable and robust non-contact method of on-line weld monitoring well suited to installation on existing laser welding stations.