PLASMA CHARGE SENSOR FOR IN-PROCESS, NONCONTACT 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 plasm a behaviour has been observed during welding through measurement of th e space charge voltage induced on an electrically insulated welding no zzle, the plasma charge sensor (PCS). It is shown both theoretically a nd experimentally that the induced voltage is a measure of plasma temp erature and thus of the welding performance. In particular, it is show n under laboratory conditions that the PCS signal can measure weld pen etration and detect a wide range of weld defects. Results have also be en 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% suc cess rate has been achieved in detecting and classifying defects in hi gh-speed industrial laser can welding. The PCS is shown to be a reliab le and robust non-contact method of on-line weld monitoring well suite d to installation on existing laser welding stations.