Vol. 33, Issue 2-3, pp. 433-443

Abstract

The dynamics of the plasma plume produced during laser welding is quite complex. The keyhole wall oscillates and this results in oscillations of the plasma plume over the keyhole mouth. The metal vapour, which appears in irregular bursts, interacts with the shielding gas flowing from the opposite direction. In the present work, temporary electron densities and temperatures are determined in the peaks of plasma bursts during welding with a continuous wave CO₂ laser. It has been found that during strong bursts the plasma plume over the keyhole consists of metal vapour only, without being diluted by the shielding gas. The results, together with the analysis of the colour pictures from streak camera, allow interpretation of the dynamics of the plasma plume. No apparent mixing of metal vapour and the shielding gas has been observed. In typical bursts the electron density determined from the Stark broadening of Ar I lines varies from 0.9×10²³ m^–3 near the metal surface to 0.5×10²³ m^–3 at a distance of 1.5 mm from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermal equilibrium these electron densities correspond to temperatures 12.7 kK and 11.5 kK, respectively. In strong bursts the electron density varies, along the same distance, from 1.6×10²³ m^–3 to 0.6×10²³ m^–3 , which corresponds to the temperatures of 14.2 kK and 11.8 kK, respectively.