In-situ characterization of melt pool evolution in laser powder bed fusion using ultrahigh-speed imaging with multidirectional illumination

Abstract Laser powder bed fusion (LPBF) is a key technology for high-precision manufacturing of complex-shaped metallic components. A detailed understanding of the spatter and melt pool behavior during the process is essential for optimizing the process parameters and improving the part quality. This study presents an observation system that combines ultrahigh-speed imaging at 1,000,000 frames per second (fps) with quad-directional (four-direction) laser illumination. This setup facilitates real-time, high-fidelity visualization of dynamic phenomena such as melt pool evolution and spatter generation, which are difficult to observe using conventional single-direction illumination, by reducing self-shadowing and clarifying feature contours. Using this system, transient melt pool behavior with and without spatter was contrasted across four laser powers of 50, 100, 150, and 200 W and four scanning speeds of 50, 100, 150, and 200 mm/s; under spatter-generating conditions, surface protrusions grew more rapidly and detached as spatter once their velocity and geometry were sufficient to overcome surface tension, whereas under nonspattering conditions, they were reabsorbed before detachment. Image analysis quantified frame-to-frame brightness fluctuations on the melt pool surface as a relative indicator of dynamic surface instability and revealed its dependence on process parameters, particularly the volumetric energy density. The results support a mechanism in which dynamic surface instability promotes spatter formation. This approach provides a powerful tool for in-situ analysis of LPBF and contributes to the development of advanced process control and quality assurance in metal additive manufacturing, including real-time spatter control and process-window optimization. Conceptual Abstract Quad-directional illumination paired with an ultrahigh-speed camera capturing one million frames per second makes rapid changes on the melt pool surface during laser powder bed fusion clearly observable. A simple measure of surface-brightness fluctuation summarizes the degree of surface instability and links it to the energy input per unit volume. When surface bumps grow quickly and maintain a favorable shape, they detach as spatter; under calmer settings they are reabsorbed. These insights help select more stable process windows and refine scan strategies to reduce spatter in metal additive manufacturing.