Operando tomographic microscopy during laser-based powder bed fusion

Abstract

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes, which are impossible to achieve using conventional manufacturing routes. However, mechanical properties of dense LPBF-manufactured ceramics are very poor due to large amount of structural defects. To acquaint deeper understanding of the underlying mechanisms, operando tomographic microscopy during LPBF of a magnetite-modified alumina using a pulsed, green laser has been carried out. Operando 3-dimensional information provides an insight into phenomena not accessible with other techniques. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms is investigated. Using increasing power results in 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 leads to avoid the lack of 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.