Ti6Al4V ELI microlattices produced by electron beam powder bed fusion: an investigation into the process-structure-property relationship across a wide range of strain rates

Abstract

The Ti6Al4V ELI (extra-low interstitial) alloy is a prominent choice for use in additively manufactured lattice structures due to its mechanical properties, low density and enhanced osseointegration. However, there is a lack of data on the impact behavior of these Ti6Al4V lattices produced by electron beam powder bed fusion (PBF-EB). Despite the significant progress in the field, printed titanium alloys still suffer from defects such as porosity and unfavorable brittle martensite located within coarse columnar β grains. Using combined experimental and computational approaches, this study investigated the mechanical properties of PBF-EB Ti6Al4V ELI lattice structures under a wide range of strain rates with a focus on the effects of processing on the structure at the meso- (design) and microscopic scales (defects, microstructure). Body-Centered Cubic (BCC) structures and BCC structures with vertical supports (BCCZ) were considered. Hot isostatic pressing was also applied to minimize PBF-EB-induced defects and to improve the microstructure. The effect of such is reported. Inclined struts exhibited poorer surface quality with distinct microstructures when compared to the vertical struts and this was attributed to their thermal histories during printing. Hot isostatic pressing significantly reduced porosity, refined surface quality, and coarsened the microstructure, leading to improved ductility and energy absorption but having little effect on strength. It was found that BCCZ microlattices showed superior mechanical properties over a range of compression velocities, with approximately double the compressive strength, Young's modulus and energy absorption. Both BCCZ and BCC microlattices demonstrated weak strain rate sensitivity.