PREVENTION OF WELD DEFECT BY POWERMODULATION IN HIGH POWER LASER WELDING

摘要

Deep penetration welding in excess of 20 mm has been carried out using a 20 kW CO CO2 laserrnfacility. Dynamic keyhole behaviour is observed using a micro-focused x–ray transmissionrnimaging system to elucidate the formation mechanism of the porosity. The bubble is generatedrnat the keyhole tip during welding by capillary instability of the cylindrical keyhole. Stabilisingrnthe keyhole is essential to prevent the porosity. Laser power modulation is effectivernin stabilising the keyhole and then preventing the porosity, if the modulation frequency coincidesrnwith that of the molten pool oscillation. The porosity prevention effect is enhanced andrnthe optimum frequency range is expanded by controlling the waveform. The optimum frequencyrnrange to prevent the porosity can successfully determined by monitoring the plasmarnemission.