Fig. 1. Schematic of experimental setup for high spatial-temporal resolution in situ imaging during SLM

Fig. 2. Time series snapshots of evolution of molten pool and spatter behavior as a function of laser energy density (E_v) during SLM. (a1)-(a5) No obvious droplet column ejection occurs with E_v of 27.5 J·mm^－3; (b1)-(b5) a subthreshold ejection occurs where the droplet column absorbs back into the molten pool with E_v of 59.0 J·mm^－3; (c1)-(c5) a droplet column oscillates and eventually breaks up into spatter particles with E_v of 90.4 J·mm^－3

Fig. 3. Droplet column ejection velocity and ejection angle as a function of time during SLM (the droplet column breaks up into a large number of droplet spatters when the ejection velocity is greater than the threshold of escape velocity. The discontinuity of curve represents breaking up of droplet column, E_v＝90.4 J·mm^－3)

Fig. 4. Powder spatter behavior driven by metal vapor entrainment during SLM. Powder particles P1, P2, and P3 agglomerate on the substrate; P1 and P2 eject as large spatters and enter entrained inert gas flow at 600 μs; P2 and small spatters (P4-P8) enter the metal vapor plume; P3 locates at the denudation zone adjacent to the melt track (E_v＝50.9 J·mm^－3)

Fig. 5. Evolution of ejection angle θ and ejection velocity u of the large spatters (P1 and P2) and small spatters (P4-P8) driven by entrained inert gas flow during SLM. The ejection angle of P2 is in good agreement with average ejection angle of small spatters (P4-P8) during t＝980-1100 μs, which indicates P2 enters into the metal vapor plume

Fig. 6. Schematic of spatter ejection process and interaction between metal vapor and spatter in SLM

Fig. 7. Upper surface of a typical spatter is irradiated by laser, resulting in vapor recoil and deflection of spatter trajectory during SLM. t＝100-300 μs is vapor lifting dominant stage, and t＝350-450 μs is vapor recoil dominant stage (E_v＝50.9 J·mm^－3)

Fig. 8. Vapor recoil pressure-driven spatter (P9) behavior during SLM. (a) Deflection of spatter trajectory; (b) ejection angle θ and ejection velocity u of spatter as a function of time