<i>In-Situ</i> Observation of Periodic Melting Behavior in Pure Titanium Powder during Laser Powder Bed Fusion

This study investigates the development of an image processing method for evaluating periodic melting behavior, referred to as wrapping behavior during laser powder bed fusion of metals under support-free conditions. Support-free conditions are essential for fabricating hollow, lattice and porous structures. Pure titanium powder was irradiated with a 100-W laser beam at a laser scanning speed of 10 mm/s. The melting behavior including wrapping was monitored using a high-speed camera at 4000 frames per second. The obtained images were assigned to a region of interest (ROI), and the temporal evolution of average brightness was measured as a feature of the wrapping behavior. Average brightness was analyzed using the short-time Fourier transformation method. The resulting time–frequency components included noise below 50 Hz. The noise was attributed to the metallic luster of powder particles, surface irregularities of the powder bed, and the appearance of a large melt pool in the ROI. To ensure reliable evaluation of wrapping frequency, the analysis was limited to times after t = 0.35 s, when wrapping behavior had stabilized. Under these conditions, the median wrapping frequency was approximately 150 Hz.