Laser welding using high-power laser forms a keyhole inside a molten pool to obtain a deep penetration. However, a spatter which is the scattering of a portion of the molten pool occurs causing welding defects. Therefore, it is necessary to elucidate the mechanism of spatter generation. In our previous study, it has been found the spatter generation is due to the variability of the keyhole. However, it is difficult to understand the spatter generation from keyhole observation only. In addition to the keyhole variability, real-time observation of molten pool dynamics would be understood the spatter generation mechanism.

In this study, the longitudinal cross-section of the molten pool was directly observed using the glass transmission method to measure the temporal variation of the molten pool area in keyhole welding. A bead-on-plate welding test was conducted for the specimen with a 16 kW disk laser when the SS304 specimen butted against a glass plate was set in a vacuum chamber. The vacuum chamber was evacuated at the pressure of 10 Pa and then filled with Ar gas to the desired pressure. As the results, the average molten pool area was 17 mm² at 10⁵ Pa, which generate a large amount of spatter, while, it was 29 mm² at 10 Pa, where the spatter generation was small. Furthermore, the standard deviation of the molten pool area was 0.9 mm² at 10⁵ Pa, and 0.36 mm² at 10 Pa, it was indicated a large molten pool time variation was influenced to generate the spatter.