Mechanism, modeling and application of plasma current in deep penetration laser welding

The theoretical and experimental study on the mechanism of plasma current during deep penetration laser welding is made in this paper. The plasma above the workpiece surface expands to nozzle, driven by the particle's consistency gradient, so an electric potential is formed between workpiece and nozzle due to the great difference in the diffusion velocities for ions and electrons. A kind of plasma-induced current can be obtained by circuiting the nozzle and workpiece, which current value is increased by adding a negative external voltage. As the external voltage increases, the plasma current increases until to a certain saturation value. As the workpiece-to-nozzle distance increases, the plasma current decreases. The decreasing rate becomes lower gradually until the plasma current becomes zero. Based on above analyzing, closed-loop control of workpiece-to-distance was realized based on the monotonous relationship between the plasma current and the distance. By applying a transversal magnetic field, the plasma above the keyhole can be somewhat eliminated, therefore the attenuation of plasma to the incident laser power can be lessen and the penetration depth can be increased.