Deformation mechanisms and fracture of electron beam melted Ti-6Al-4V

This study presents an exploratory characterization on the effect of build orientation on deformation behavior of Ti-6Al-4V rods produced through electron beam melting (EBM). Microstructural differences in horizontally and vertically printed rods were characterized using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). A typical microstructure of the alloy consists of primary-alpha and vanadium-enriched beta-phases. Compared to the horizontal samples, vertically printed ones exhibited finer lamellar microstructure with thin prior beta-boundary, reduced thickness of a-laths, grain-boundary alpha (alpha(GB)), and smaller alpha-colony size. Most of the basal poles were aligned either parallel or perpendicular to the build direction in the horizontal sample, whereas they were tilted away from the build direction in the vertical sample. Uniaxial tensile tests were conducted on both horizontal and vertical samples, where the loading axis was perpendicular and parallel to the build direction, respectively. Vertical specimens exhibited higher yield strength (similar to 30 MPa), ultimate tensile strength (similar to 50 MPa), and tensile elongation (similar to 1.8%) than those of the horizontal ones. While the strength attributed to the less interlamellar spacing, the difference in elongation was due to the near-parallel orientation of prior beta-boundaries to the tensile direction in the vertical samples. Videos of the strain fields generated by digital image correlation (DIC) revealed the regions of local strain concentrations in the horizontal specimen, whereas more homogeneous strain distribution was observed in the vertical specimen. EBSD Schmid factor distribution maps of the deformed specimens indicated that all possible slip deformation modes are favorable to operate to some extent in the horizontal sample. However, In the vertical sample, deformation occurred mainly from the contribution of the first-order pyramidal slip.