Power modulation in deep penetration laser welding - Optimization of frequency and waveform to prevent the porosity

Abstract

High power laser is a promising tool to weld heavy section plate members efficiently. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, laser power modulation is attempted to suppress the porosity in 20 kW CO2 laser welding with the depth of 20 mm. Dynamic keyhole behaviour is analysed using a micro-focused X-ray transmission imaging system to evaluate keyhole stability. Waveform control of power modulation is effective in stabilising the keyhole and leads to not only enhancing the porosity suppression effect but also expanding the optimum frequency range to prevent the porosity. Monitoring of plasma signal is also tried using a photo diode to determine the optimum frequency. The keyhole is stabilised at the eigenfrequency of the molten pool oscillation. The narrow optimum frequency can be successfully determined by plasma monitoring.High power laser is a promising tool to weld heavy section plate members efficiently. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, laser power modulation is attempted to suppress the porosity in 20 kW CO2 laser welding with the depth of 20 mm. Dynamic keyhole behaviour is analysed using a micro-focused X-ray transmission imaging system to evaluate keyhole stability. Waveform control of power modulation is effective in stabilising the keyhole and leads to not only enhancing the porosity suppression effect but also expanding the optimum frequency range to prevent the porosity. Monitoring of plasma signal is also tried using a photo diode to determine the optimum frequency. The keyhole is stabilised at the eigenfrequency of the molten pool oscillation. The narrow optimum frequency can be successfully determined by plasma monitoring.