Processing of hydroxyapatite (HA)–Ti–6Al–4V composite powders via laser powder bed fusion (LPBF): effect of HA particle size and content on the microstructure and mechanical properties

This research study examines the influence of hydroxyapatite (HA) powder particle size and content on the processability, microstructure, and mechanical properties of the laser powder bed fusion (LPBF) fabricated Ti–6Al–4V (Ti64)-HA composites. For this purpose, various Ti64-HA composite powders were produced, and the effects of the HA particle size and content on the optical reflectance were investigated. Composite powders and the monolithic Ti64 powder (as the reference) were subjected to the LPBF process within a wide range of volumetric energy densities by varying the LPBF process parameters. Parts with the highest relative density for each material were assessed by studying their microstructure, nanohardness, and nanoindentation-derived yield strength. Results revealed that the incorporation of the highly reflective HA powder into the Ti64 slightly enhanced the reflectance of Ti64-HA composite powders over that of the monolithic Ti64 powder. The LPBF processability of the composite powders was found to highly depend on the content of the HA constituent. For any given volumetric energy density employed in this study, composites containing 1 wt%HA were crack-free. However, composite systems containing higher HA content (2.5 wt%) featured randomly distributed transgranular cracks. Addition of 1 and 2.5 wt%HA to the Ti64 resulted in a significant improvement in the nanohardness (∼30 and 75%) and yield strength (∼20 and 37%). The dominant strengthening mechanisms in composite samples were the Hall-Petch and geometrically necessary dislocations strengthening, accounting for 57–66% and 13–20% of the total yield strength improvement.