Characterisation of cw Nd : YAG laser keyhole dynamics

The paper concerns laser-matter interaction characterisation. In this work, we use a rapid CCD camera located coaxially to the laser beam and we compa re recorded images with those obtained by numerical modelling. Because imag es are difficult to understand, we compute thermal radiation emitted by a k eyhole of fixed geometry and we adjust it trying to approach the camera rec ord. The modelling treats radiative heat transfer within the keyhole and de termines the sensor illumination map. By adjusting the geometrical characte ristics of the hole, we seek to obtain the image that corresponds as well a s possible to the realised experiment. Results are compared with other expe rimental methods simultaneously performed plume characterisation with an el ectric probe and spectrometric analysis. They show the existence of two dis tinct behaviours of the keyhole: a pseudo-steady state associated with regu lar and pseudo-constant keyhole shapes, low frequencies of electric current in the plume, and generally good welding results, and a highly dynamic mod e associated with irregular and rapidly varying keyhole shapes, high freque ncies in the plume current and generally poor welding results. (C) 2001 Els evier Science Ltd. All rights reserved.