Operando tomographic microscopy during laser-based powder bed fusion of alumina

Abstract

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.

Advances and challenges in direct additive manufacturing of dense ceramic oxides

“…These adjustments prolonged the molten pool lifetime and decreased its viscosity due to elevated liquid temperature, facilitating gas bubble escape and porosity reduction. However, exceeding critical LED can be counterproductive, leading to an unstable molten pool or excessive material evaporation [190,194]. In addition to LED, other parameters such as scanning strategy, feedstock powder size, and substrate material also significantly impact porosity as demonstrated in [16,80,157].…”

“…Compared to other pore defects, LOF pores are less frequently reported in dAMed ceramics, likely attributed to their ease of avoidance through laser parameter tuning. For example, the studies on Al 2 O 3 and SiO 2 ceramics [107,190] reported cases of LOF defects between the deposited layer and substrate (figure 14(a)), which can be eliminated via either increasing laser power or reducing layer thickness. In contrast to the LOF defect, gas pores are originated from entrapped gas in the ceramic melt, and display a spherical or ellipsoidal shape with a smooth inner wall as shown in figure 14(b).…”

“…This technology provides enhanced penetration power and spatiotemporal resolution, allowing for detailed examination of the internal structures and dynamics of molten pool. For instance, Makowska et al [190] achieved a stunning 3D visualization of the Al 2 O 3 melt through operando x-ray computed tomography, as depicted in figure 16(g). They observed, in real-time, the influence of laser power on balling, powder denudation, and pore formation.…”

“…Figure 16. In-situ monitoring of dAMed ceramic oxides: (a) thermal image depicting the preheating area in L-PBF of Al 2 O 3 -ZrO 2 ceramic [196]; (b) average temperature evolution over time in L-PBF of ZrO 2 ceramic [97]; (c) evolution of signal with different laser powers in L-PBF of Al 2 O 3 ceramic [199]; (d) high-speed video image illustrating the phenomenon of melt flowing out in L-PBF of Al 2 O 3 -ZrO 2 ceramic[16]; (e) high-speed video image demonstrating the evolution of crack defects in L-PBFed Al 2 O 3 ceramic[198]; (f) high-speed video image in two adjacent Al 2 O 3 scan tracks, revealing powder denudation zones[26]; (g) 3D visualization of Al 2 O 3 molten pool using operando x-ray computed tomography[190]; (h) time-series radiographs acquired in L-PBF for absorber-doped SiO 2 ceramic using different scan speeds[169]. (a) Reprinted from[196], Copyright © 2010 Published by Elsevier B.V. (b) Reprinted from[97], Copyright © 2015 Elsevier B.V. All rights reserved.…”