3D pore structure characterization and hardness in a powder bed fusion-processed fully amorphous Zr-based bulk metallic glass

Abstract

Recently, fabrication of bulk metallic glasses (BMGs) components with complex shapes has been realized using laser powder bed fusion (PBF). Initial defects such as porosity inherent to the PBF process can significantly degrade the mechanical integrity of the BMGs components. In spite of intensive studies on pore structures in 3D-printed crystalline alloys, pore structures in 3D-printed fully amorphous BMGs have not been characterized due to the difficulties in achieving large-scale fully amorphous structures. In the present work, the pore characteristics of PBF-processed fully amorphous Zr-based BMGs were systematically studied. The amorphous structure of the printed parts was first verified using X-ray diffraction (XRD). A non-destructive high-resolution X-ray micro-computed tomography (micro-CT) technique was then applied to evaluate the inner 3D pore structure, while microstructures below the resolution of X-ray micro-CT were examined in 2D using scanning electron microscopy (SEM). The printed amorphous parts can reach a total volumetric porosity as small as 0.45%. The distribution of pore size and pore sphericity were evaluated for the PBF-processed BMGs samples with different porosities, where two observed pore features were irregular large lack-of-fusion pores, and smaller gas-induced round pores. Furthermore, correlations were found between porosity, pore size and morphology. Finally, hardness tests at various loads revealed that while the intrinsic properties were constant irrespective of laser processing, the overall mechanical properties of the PBF-processed BMGs samples are dominated by pore characteristics. While the densest printed BMG sample had a comparable macroscale hardness to the as-cast material, an increase in porosity of ~8% led to a decrease in HV5 hardness of ~17%. The presented results provide a detailed analysis of pore characteristics, and their impact on mechanical properties of PBF-processed BMGs, for further experimental and computational studies of structure-property relationships in this materials class.