On the characterization of roughness and geometrical irregularities of additively manufactured single titanium-alloy struts

Abstract

The purpose of this study is to expose data analysis techniques based on X-ray computed microtomography (XCT) for measuring the complex geometric characteristics of millimeter-sized specimens fabricated by additive manufacturing, and their modification induced by surface treatment. A set of Ti-6Al-4V struts was produced by Electron Beam Melting (EBM) at two angles, at 45° and vertically. They were scanned by XCT before and after electropolishing (EP). Extensive analysis allowed for measuring the amount and size of partially melted particles attached to the surface, the cross-section geometry along the axis of the strut, and the roughness over the whole surface at the micrometer scale. The variation of these characteristics was correlated to the EP conditions. The accuracy of the roughness values measured by XCT was validated by Laser Scanning Confocal Microscopy. The as-built 45° specimens showed the usual characteristics of EBM fabricated struts, such as elongated cross-sections, and a smooth overhanging surface. Less expected was the increased roughness on the lateral sides of these specimens, rather than on the underhanging sides. Similarly, the vertically fabricated struts showed a moderate, yet measurable, roughness variation over their circumference. Despite this lack of uniformity, all the specimens fabricated at a given angle were remarkably similar. The average quantities and sizes of attached particles were quantified for all conditions. The amount of removed material after EP correlated well with the total exchanged electric charge related to the formation of tetravalent Ti 4+ ions. Local SEM observations before and after EP put into evidence the removal of partially melted particles. • Novel custom methods for extensive analysis of 3D geometries applied to mm-sized EBM struts: cross-section analysis, surface unfolding, surface granulometry. • Quantitative assessment of attached particles, surface roughness and cross-sectional shape before and after electropolishing and related effects of process parameters. • Relating process parameters to the heterogeneity of attached particles, roughness, and equivalent diameters along the circumferences of full EBM struts. • Precise measurements of surface feature modification during electropolishing on mm-sized EBM specimens and its consequence on surface roughness evolutions. • Documentation of the mechanism of surface particles teared off during electropolishing leaving nanometer sized craters with crystallographic features.