Investigations on cw CO2-laser induced welding plasmas using differential interferometry

The stability of the deep penetration laser welding process is strongly influenced by the laser-induced plasma above the workpiece surface. The plasma plume can interact with the incident laser radiation due to absorption and refraction. The effect of these interactions leads to a spatially and temporally varying intensity distribution on the workpiece and, therefore, can influence the keyhole dynamics. In order to obtain first basic informations on the importance of the different mechanisms, investigations were carried out using a differential interferometer at two different wavelengths simultaneously to obtain the refractive index distribution. The interferometer allows investigations with variable sensitivity and, hence, enables to detect the large phase shifts of the probe laser light (shifts of some wavelengths are typical). In this first step the weld was performed in a chamber filled with shielding gas to avoid any influence of the flow-field induced by a nozzle. As a result of the interferometrical experiments the refractive index distribution within the plasma plume can be calculated at the two probe-laser wavelengths as long as partial symmetry can be assumed for the plasma. The laser-induced plumes at cw-laser welding show strong temporal fluctuations concerning the geometry of the plume and, therefore, complicate the evaluation of the refractive index distributions.