Fig. 1. Images of the molten pool. (a) External image of the molten pool during the laser welding process; (b) internal image of the molten pool during the laser welding process; (c) internal image of the molten pool during the laser powder bed fusion process

Fig. 2. Sparks and spatters during laser powder bed fusion. (a) Schematic; (b) actual diagram

Fig. 3. Three classifications of spatters^[34]

Fig. 4. Simulation of powder particle size and heat flow^[41-42]. (a) Powder droplet model; (b) duplicate powder layer thickness for CFD calculation; (c) simulation of single layer powder heat flow; (d) simulation of two layers powder heat flow

Fig. 5. Experiment and simulation of laser-driven powder spatter process

Fig. 6. Powder particles fall on the powder bed. (a) Balling^[30]; (b) satellite^[30]; (c) air-entrained powders reside in trac^[26]

Fig. 7. Spatter particles are embedded in the microstructure. (a) Un-melted powders in residual pores; (b) large pore; (c) spatter particles falling into large pores

Fig. 8. Spatter in single-track scanning (scanning direction from left to right) and motion image of airflow during the spatter process. (a)(b) Laser power is 50 W and scanning speed is 0.1 m/s; (c)(d) laser power is100 W and scanning speed is 0.5 m/s; (e)(f) laser power is 200 W and scanning speed is 1 m/s; (g) motion image of airflow during the spatter process

Fig. 9. Spatter distribution on both sides of the laser scanning track^[29]

Fig. 10. The spatter difference between 316L stainless steel aerosolized powder and water atomized powder (orange arrow indicates laser scanning direction, red arrow indicates approximate laser position, moving particles are circled, green and yellow arrows indicate backward and forward movement, respectively). (a) Aerosolized powder; (b) water atomized powder

Fig. 11. Comparison of spatter of different materials. (a) 316L stainless steel; (b) Al4047

Fig. 12. Camera-based surveillance system architecture^[61]. (a) Coaxial architecture; (b) paraxial architecture

Fig. 13. Motion tracking of spatter particles^[69]

Table 1. Imaging features of several types of high-speed cameras