Ti-6Al-4V ELI microlattice structures manufactured by electron beam melting: Effect of unit cell dimensions and morphology on mechanical behaviour

Additive manufactured titanium alloy microlattice structures are attracting the attention of numerous industrial fields, since they allow a customised and optimised design of components and structures. In the current work, Electron Beam Melting (EBM) technique was used to produce Ti6Al4V ELI rhombic dodecahedron (RD) microlattice structures. Preliminary tests on bulk specimens highlighted the effect of surface roughness on mechanical response. A confocal microscope was used to characterise surface topography and to obtain a precise evaluation of surface roughness, which is a critical point in EBM technique. Three different unit cell sizes were selected in order to investigate the effect of cell dimensions on microlattice mechanical properties. Morphological evaluations were performed through Computed Tomography (CT) technique, which allowed the comparison of the real models to the designed one. Discrepancies between actual and CAD geometries were observed, with increasing importance for lower cell sizes. Quasi-static compression tests and low velocity impact tests were performed on RD lattices, highlighting a decrease in mechanical properties with the increase of cell size, except for the specific energy absorption, which depends also on lattice density. A heat treatment was performed on lattice structures, but its effect on mechanical properties was negligible, mainly for the higher cell size lattice, proving the lack of significant residual stresses in lattices produced via EBM. Thermographic observations during compression tests allowed a clear identification of the failure instant and the deformation mode, which occurs along a 45° inclined plane.