Influence of trace boron addition on microstructure, tensile properties and their anisotropy of Ti6Al4V fabricated by laser directed energy deposition

The mechanical anisotropy of laser directed energy deposition (L-DED) titanium alloys using laser solid forming (LSF) is significantly deteriorated owing to the coarse prior beta columnar grains and continuous grain boundary alpha-laths (alpha(GB)). The influence mechanisms of trace boron (up to 0.25 wt%) on the microstructure, tensile properties and anisotropy of Ti6Al4V manufactured by LSF were investigated. The sizes of beta grains and a phases both decreased with increasing boron content. Compared with the Ti6Al4V deposit, when the addition of boron was 0.08 wt%, the average width of beta grains was reduced by about one order of magnitude owing to the growth restricting effect caused by boron and Zener pinning by TiB. Besides, the average length and aspect ratio of alpha-laths were reduced by 43% and 33%, respectively, owing to the refinement of beta grains and the heterogeneous nucleation of alpha phases on TiB particles. With an increase in boron content, the strength anisotropy gradually decreased for the disappearance of alpha(GB), the elongation anisotropy initially increased and then significantly decreased for the poor deformation compatibility between the TiB and the matrix. A better combination of strength and elongation could be obtained with the addition of 0.08 wt% boron to Ti6Al4V. (C) 2019 The Authors. Published by Elsevier Ltd.