In situ porosity imaging with synchrotron X-ray tomography during laser rescanning of Zr-based metallic glass by laser powder bed fusion

When laser powder bed fusion (LPBF) is applied to bulk metallic glasses rather than traditional crystalline alloys, one has to avoid conditions that could cause crystals to form. To achieve a balance between the porosity content and devitrification in the heat affected zone, it is common practice to process such material with a thin layer thickness, and thereby reduce the laser power necessary for melting. In this manufacturing regime, lack-of-fusion defects typically subsist. This work investigates how laser rescanning can densify metallic glasses while still ensuring their amorphous nature. Synchrotron X-ray Computed Tomography during LPBF allows imaging in situ the pores upon the glass construction. This non-destructive cutting-edge technique helps understanding the consolidation mechanism associated with rescanning and in particular its effect on layer surface roughness and the homogeneity of the powder recoating. Applied to the well-established Zr-Cu-Al-Nb grade, this work paves the way towards the adoption of less thermally stable glasses for LPBF, and the control of defect distribution. In particular, it is revealed that the hatch spacing effect is of primary importance in the production of viscous materials such as glasses, and that laser rescanning allows the surface of the deposited layer to be smoothed, improving consolidation without associated crystallisation.