The effect of defects on the mechanical response of Ti-6Al-4V cubic lattice structures fabricated by electron beam melting

• Published in: Acta materialia 2016-04, Vol.108, p.279-292
• Main Authors: Hernández-Nava, E., Smith, C.J., Derguti, F., Tammas-Williams, S., Leonard, F., Withers, P.J., Todd, I., Goodall, R.
• Format: Article
• Language: English
• Subjects: Additive manufacturing; Cellular solids; Construction; Crystal defects; Electron beam melting; Lamellar structure; Mechanical properties; Struts; Surgical implants; Titanium alloy; Titanium base alloys; X-ray computed tomography
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2016.02.029

Electron Beam Melting (EBM) as a means of Additive Manufacturing (AM), is of interest for the fabrication of intricate geometries for cellular materials in areas where complex architectures are needed, e.g. biomedical implants. Most studies have focused on specific geometries and so the effect of the structure on mechanical performance is not well understood. Many kinds of micro- and macro-scale defects can arise in additively manufactured components, so assessment of their influence on properties is needed. In this work, lattices of Ti-6Al-4V having a cubic structure have been manufactured by EBM, and the effect of heat treatments above and below the β-transus temperature on microstructure and compression response have been investigated. The former modifies only slightly the α+β structure and mechanical performance whereas the latter leads to coarse alternating α and β lamellae packets and α at the prior grain boundaries with a 10% loss in yield strength. The variation in the compressive yield stress with strut diameter is in good accord with simple models based on compressive deformation rather than shearing or buckling. Internal pores for struts aligned with the build direction are found around the edges of the solid form, in regions which seem to be associated with the EB scan pattern. Struts normal to the build direction show more significant defects but their redundancy means that they do not compromise the compressive performance in the build direction. Using a particle size in the range 45–100 μm minimum weld-track sizes were experimentally and numerically identified to be 176 and 148 μm in depth respectively with a depth-to-width ratio of 0.55. This produced a beam pass of the order of 300 μm oversizing small features (struts of 0.4 and 0.6 mm nominal diameter) when a contour around the strut periphery was applied. [Display omitted] •Heat treatments below the β-transus temperature had little effect on as-built microstructure and mechanical performance.•A 10% drop in yield strength and a coarser microstructure resulted from heat treatments above the β-transus temperature.•X-ray Computed Tomography revealed internal porosity that was distributed in proximity to the strut cross section perimeter.•Analytical models based on compressive yielding deformation, rather than shearing or buckling, agree experimental observations.•The roughness surface was found to be close to the order of the smaller particle size from the range used in the process.