A real-time observation system for a longitudinal section of molten pool and temperature distribution was developed directly in real time by a combined system with the glass transmission method and two-color temperature analysis to clarify the mechanism of spatter generation during laser welding. A stainless-steel-type 304 (SS304) was employed in this study because of its application in many industries. The SS304 was set in a vacuum chamber and then a disk laser with an output power of 10 kW was irradiated and scanned on it to form a weld bead. At the same time, the behavior of the molten pool and keyhole was captured with the real-time temperature observation system and an x-ray transmission system, respectively. As a result, the temperature of the longitudinal section of the molten pool formed by laser irradiation under atmospheric pressure was about 1500 °C; however, at 10 Pa, it reached about 1700 °C, indicating that the temperature increases with a decrease in the ambient pressure. Furthermore, the fluctuation in the longitudinal sectional area of the keyhole and molten pool was evaluated. It was found that the average fluctuation in the longitudinal sectional area of the molten pool was 1.7% at 10 Pa and that of the molten pool was 18.8% under atmospheric pressure.

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