Synergistic effect of boron and nitrogen on strength enhancement and anisotropy mitigation in Ti6Al4V fabricated via directed energy deposition

Controlling the mechanical properties and microstructure anisotropies of additively manufactured Ti6Al4V alloys remains a challenge for improving their performance in demanding applications. Therefore, this study examined the effects of adding B and N on the microstructural evolution and mechanical properties of Ti6Al4V alloys fabricated using laser powder directed energy deposition (LP-DED). The SMART process, which is a novel composite powder fabrication method, was used to prepare feedstocks for LP-DED. This approach enabled control over the contents of B and N to optimize the microstructure and mechanical properties of the Ti6Al4V alloys. The addition of B resulted in effectively refined prior-β grains through constitutional supercooling and TiB precipitation. The addition of N resulted in the formation of a solid solution within the matrix without significant grain refinement. When B and N were added simultaneously, their individual roles in Ti6Al4V remained unchanged compared to their separate additions. However, the combined addition of B and N exhibited a synergistic effect, achieving a balanced improvement in both strength and elongation anisotropy. By deepening the understanding of B and N interactions in Ti alloys, these findings pave the way for a strategy to enhance structural reliability and suppress anisotropy in additively manufactured Ti6Al4V alloys for aerospace, automotive, energy, and biomedical applications.