Laser-based Powder Bed Fusion (LPBF) of ceramics enables the fabrication of objects with complex three-dimensional shapes otherwise challenging or even not possible to produce with conventional manufacturing routes. However, the mechanical properties of LPBF-manufactured ceramics components are poor due to the large number of structural defects. Often, such defects are investigated by X-ray computed tomography. This allows studying in 3D the density and shape of pores and cracks in printed samples. Dynamic processes can be studied by fast in situ X-ray radiography, however this method lacks 3D information.

In this work, we present the first results obtained by operando tomographic microscopy during LPBF of a magnetite-modified alumina. This new technique allows tracking the 3D microstructural evolution during printing with a time resolution of 10ms. The experiments are performed at the TOMCAT beam line of the Swiss Light Source with a LPBF setup with a green laser. The effect of laser energy density on surface roughness, powder denudation zone and porosity formation mechanisms is investigated. Higher power results in a significant increase of the melt pool width, but not of its depth and no melt pool depression is observed. For the investigated ceramic system, the forces due to the recoil pressure do not significantly influence the melt pool dynamics. Increasing power allows avoiding lack of fusion porosity, but enhances the 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. The acquired information, not only provides understanding of underlying processes, but also is crucial for the development and the verification of models used for the LPBF process simulations.