Formation Mechanism and Suppression of Welding Defects in 20 kW CO_2 Laser Welding

摘要

Formation mechanism of the porosity in the deep penetration welding has been investigated using a microfocused X-ray transmission imaging system. Suppression of the porosity has also been attempted by pulse modulation of the laser beam. In addition, HAZ properties of the laser welded ultrafine grained high strength steel has been investigated. The results are summarised as follows. (1) The porosity formation is caused by instability of narrow cylindrical keyhole. The tip is broken up by instability while the depth abruptly decreases by spiking phenomenon. (2) Pulse modulation of the laser power is effective in decreasing the porosity if the frequency coincides with the eigenfrequency of the molten pool oscillation. (3) The porosity suppression effect is enhanced by controlling the waveform. (4) A narrow softening region caused by grain coarsening is clearly observed in the HAZ welded at the low welding speed. (5) The coarsening of the ultrafine grained ferrite is always observed near the fusion boundary even at the high welding speed. (6) HAZ softening is suppressed at the welding speed of more than 3.3 mm/s due to the formation of tiny dispersed MA constituents.