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Investigation of keyhole plasma during 10 kW high power fiber laser welding

During high power fiber laser deep penetration welding, the laser induced plasma/vapor has a great effect on the weld pool flow and keyhole stability. In this paper, experiments on 10 kW fiber laser welding of stainless steel have been carried out. A long keyhole and keyhole plasma have been observed. The distinct spectral lines of the keyhole plasma induced by the high power fiber laser were measured and used to calculate the parameters of the plasma. The pressure of the plasma was investigated experimentally. The welding images showed a clear long keyhole and uneven distribution of the keyhole plasma. The data demonstrated that, compared with the results during CO2 laser welding, the electron temperature was lower during fiber laser welding, while the electron density remained at a similar level. The ionization degree was found to be low. The pressure of the plasma was found to be much higher than atmospheric pressure. The results revealed that the low fiber laser absorptivity of the plasma and the high atomic density of the vapor induced by the high power fiber laser were the factors that dominated the values of the plasma parameters. In particular, the high pressure plasma was ascribed to the high atomic density of the metallic vapor.