Laser powder bed fusion with metallic materials as a heat source (PBF-LB/M) is an additive manufacturing (AM) technique that has been applied in various industrial fields to reduce component weight, improve functionality, lower manufacturing costs, and reduce lead times. However, detailed characterization of the PBF-LB/M phenomenon is challenging because of the mutual influence of laser parameters and chamber environment. In PBF-LB/M, the powder is repeatedly melted and solidified by laser irradiation. However, the hot spatter generated in the process causes defects and insufficient melting. In this study, we use a high-speed camera to observe hot spatter ejected from the laser-irradiated area of a commercial PBF-LB/M system and investigate the effects of inert gas flow and laser scanning strategy on hot spatter behavior. We found that the ejection velocity of hot spatter immediately after ejection from the melt pool decreases as the particle size increases and is not affected by gas flow velocity. Furthermore, we observed that hot spatter is always ejected behind the laser scanning direction, but the ejection direction of the hot spatter changes over time. Particularly, when the laser scanning direction follows the gas flow direction, the spatter ejected in the backward direction of the scanning direction may follow a large curve over time to the front of the scanning direction and deposit on the build part. Based on the results of these investigations, we drew conclusions on the effect of the laser scanning direction with respect to the gas flow direction on the build quality and found that scanning the laser in the opposite direction to the gas flow is more effective in improving the surface quality.

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