Direct observation of keyhole behaviour during pulse modulated high-power Nd:YAG laser irradiation

When a high-power laser beam is irradiated on the surface of material, it is well known that a cavity, called a keyhole, is generated in the molten material. To examine the effect of laser beam pulse shape on the keyhole behaviour, we observed it directly with an in situ microfocused x-ray transmission imaging system. As a result, it was found that the keyhole began to be generated after about 0.6-0.7 ms and became deepest after about 1.5 ms from the laser irradiation initiation and collapsed about 1 ms after laser power declination when a pulse modulated laser beam of 1.1 ms rise time and 4.6 kW peak power was used. It was also understood that the generation and the collapse of the keyhole was repeated, synchronizing with laser beam irradiation of 100 Hz. The transition of the keyhole depth and the cross sectional area had good agreement with the transition of the irradiated beam shape. Moreover, it was observed that the porosity was frequently generated at the beam shape in which the laser power was decreased rapidly from a high peak power.