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 β columnar grains and continuous grain boundary α-laths (α_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 β grains and α 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 β 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 α-laths were reduced by 43% and 33%, respectively, owing to the refinement of β grains and the heterogeneous nucleation of α phases on TiB particles. With an increase in boron content, the strength anisotropy gradually decreased for the disappearance of α_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.