Effect of energy density on the microstructure and texture evolution of Ti-6Al-4V manufactured by laser powder bed fusion

The aim of this work is to investigate the influence of the volumetric energy density (E-V) on the microstructure, texture and Vickers hardness of Ti-6Al-4V (wt%) processed by laser powder bed fusion (LPBF), both in the asbuilt and heat-treated conditions. With this goal, Ti-6Al-4V powder was processed using energy densities ranging between 24.2 and 71.4 J/mm(3). Analysis by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that LPBF processing gives rise to high density of fine alpha/alpha'laths, whose size and orientation is highly dependent on E-V, as this parameter influences the size and orientation of prior beta grains. In particular, at low energy density (E-V < 37 J/mm(3)) the microstructure of the LPBF sample is formed by near-equiaxed prior beta grains that are densely populated with randomly oriented alpha/alpha'laths. It is proposed that, under these conditions, the presence of significant thermal gradients and internal stresses during solidification induces a high density of nucleation sites for beta grains during subsequent layer deposition. On the contrary, the use of high energy density (37 J/mm(3) < E-V < 71.4 J/mm(3)) results in the growth of columnar prior-beta grains oriented with a < 111 > direction parallel to the building direction (BD), from which alpha/alpha'laths with a strong < 11-20 >parallel to BD fiber texture form, which is associated to the scanning strategy selected in this study. This research demonstrates that energy density is a key parameter to process Ti-6Al-4V specimens with random texture and high density.