Fig. 3: Nanoparticle-enabled elimination of liquid breakup.
From: Controlling process instability for defect lean metal additive manufacturing
a–d X-ray images showing the liquid breakup during laser melting of Al6061. The liquid breakup is indicated by the yellow dashed circle. e–h X-ray images showing the more stable vapor depression without liquid breakup during laser melting of Al6061+4.4vol.%TiC. i, j The vapor depression depth and width evolution during laser melting. Since vapor depression depth plays a key role in determining spatter formation, the laser processing parameters were selected to achieve similar vapor depression depth in Al6061 and Al6061+4.4vol.%TiC for comparison. The comparison under the same laser processing parameter is shown in Supplementary Fig. 6. k, l Schematic of nanoparticle-enabled stabilization of the vapor depression fluctuation. The fluctuation of vapor depression creates a velocity gradient along the tangential direction of the vapor depression front wall, which results in the generation of viscous stress: τ = μdv/dx = μvc/h, where τ is the viscous shear stress; µ is the viscosity; v is the moving velocity of vapor depression front wall induced by depression fluctuation, x is the distance along vapor depression front wall, \({v}_{{{\mbox{c}}}}\) is the front wall moving velocity at the center of the fluctuation, \(h\) is the distance from center to the edge of the fluctuation. Due to the increased viscosity (μ) by nanoparticles, smaller front wall moving velocity (\({v}_{{{\mbox{c}}}}\)) is needed for generating the same viscous stress to resist recoil pressure (Pr). Therefore, the depth of the fluctuation (d = vcΔt, where Δt is the time period and is considered constant to study the deformation within the same time period) decreased, resulting in the stabilization of vapor depression.
