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

Abstract

The aim of this work is to investigate the influence of the volumetric energy density (EV) on the microstructure, texture and Vickers hardness of Ti-6Al-4V (wt%) processed by laser powder bed fusion (LPBF), both in the as-built and heat-treated conditions. With this goal, Ti-6Al-4V powder was processed using energy densities ranging between 24.2 and 71.4 J/mm3. Analysis by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that LPBF processing gives rise to high density of fine α/α′laths, whose size and orientation is highly dependent on EV, as this parameter influences the size and orientation of prior β grains. In particular, at low energy density (EV < 37 J/mm3) the microstructure of the LPBF sample is formed by near-equiaxed prior β grains that are densely populated with randomly oriented α/α′ 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 β grains during subsequent layer deposition. On the contrary, the use of high energy density (37 J/mm3 < EV < 71.4 J/mm3) results in the growth of columnar prior-β grains oriented with a 〈111〉 direction parallel to the building direction (BD), from which α/α′laths with a strong 〈11-20〉||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.