2005 Volume 23 Issue 2 Pages 259-264
High power laser is a promising tool to weld heavy section plates. One of the major problems, in this case, is formation of some weld defects such as porosity and hot cracking. A final goal of a series of our investigations is to develop defect free laser welding process with the depth of more than 20mm. In the present study, characteristics of deep penetration welding have been investigated using a 20kW CO2 laser facility. Deep penetration of more than 20nm can be attained even at a high welding speed 16.7mm/s. However, porosities are always observed in the longitudinal sections, more significantly for upper focusing conditions. The porosity is not observed, on the other hand, with nitrogen shielding, although large plasma periodically formed during welding and leads to significant reduction in the penetration depth. Pulse modulation of the laser power at the same frequency and duty of the periodical plasma formation can effectively decrease the porosity formation even with helium shielding. This suggests the possibility of porosity prevention by laser power modulation. Dissolution of nitrogen in the molten pool has also good effect to prevent the porosity with nitrogen shielding.