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A sensing and control system of process quality in C02 laser deep penetration welding has been developed, including plasma photo sensor (PS) and plasma charge sensor (PCS), signal processing circuit, personal computer, and optimization and control software. By using the information of plasma radiation and plasma charge, the focusing lens is adjusted to the optimum focal point position (at which the maximum penetration can be achieved ) and closed-loop control is realized to assure the focal point position and penetration depth constant. In laser welding of the workpiece aslant placed with larger angle, the variation offocal point position is less than 0.2mm and the fluctuation ofpenetration depth is less than 0.05mm. The control system has been preliminarily applied in laser precision welding of 2.5m long zircaloy fuel channel used in nuclear heat-supply reactor.

Plasma charge current distribution during deep penetration CO2 laser welding was analyzed theoretically and experimentally. The laser-induced plasma above the workpiece surface expands up to the nozzle, driven by the particle concentration gradient, forming an electric potential between the workpiece and the nozzle due to the large difference between the diffusion velocities of the ions and the electrons. The plasma-induced current obtained by electrically connecting the nozzle and the workpiece can be increased by adding a negative external voltage. For a fixed set of welding conditions, the plasma charge current increases with the external voltage to a saturation value. The plasma charge current decreases as the nozzle-to-workpiece distance increases. Therefore, closed-loop control of the nozzle-to-workpiece distance for laser welding can be based on the linear relationship between the plasma charge current and the distance. In addition, the amount of plasma above the keyhole can be reduced by a transverse magnetic field, which reduces the attenuation of the incident laser power by the plasma so as to increase the laser welding thermal efficiency.