Abstract
Recent progress in high power laser resonator has been extending the welding application to the heavy section plate structures in a variety of industries. One of the major problems in the deep penetration welding 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 paper, pulse modulation of the laser power has been attempted to suppress the porosity. Dynamic keyhole behaviour was observed by a micro-focused X ray transmission imaging system at the framing rate of 1000 s-1. Keyhole stability was evaluated by variation of keyhole diameter with time. The porosity formation significantly depends on the modulation frequency. If the eigenfrequency of the keyhole radial oscillation is selected as the modulation frequency, the keyhole is perturbed more significantly than the CW welding, resulting in forming a lot of large porosity. On the other hand, the porosity is effectively suppressed at the eigenfrequency of the molten pool oscillation. This is attributed to stabilizing the keyhole by controlling the laser / molten metal interaction periodically.
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