Suppression of porosity using pulse modulation of laser power in 20 kW CO₂ laser welding

Formation mechanism of the porosity has been investigated in the deep penetration laser welding with the depth of about 20 mm using a 20 kW CO₂ laser facility. Dynamic keyhole behaviour has been observed using a micro-focused X-ray transmission imaging system developed by Matsunawa et al. The results indicate that the porosity is formed by instability of the capillary keyhole. The tip of the keyhole is broken up by instability during an abrupt decrease in the depth by so called spiking phenomenon. Laser power modulation with a square wave can effectively reduce the porosity formation by matching the frequency with that of the molten pool oscillation. On the other hand, a lot of large porosities are formed at the frequency of 100 Hz. This may be caused by the resonant effect of the keyhole oscillation. The suppression effect of the porosity can be enhanced by power modulation with the modified waveform.