High strength and ductility of titanium matrix composites by nanoscale design in selective laser melting

Abstract

• SLM parameters play a major role in densification and microstructure. • Nanosized secondary particle additions promote densification. • Nanosized BN additions produce fine, high aspect ratio TiB reinforcement. • TiB morphological evolution during SLM and heat treatment impacts properties. • High tensile strength and ductility through high aspect ratio TiB nanowhiskers. The use of selective laser melting (SLM) to produce titanium matrix composites (TMCs) with high strength while retaining sufficient tensile ductility suitable for structural applications is emerging as an attractive opportunity in the field of advanced manufacturing. However, the presence of coarse ceramic reinforcements as well as difficulties in optimizing the SLM process is a barrier to the application of TMCs. In this study, we demonstrated the production of TMCs reinforced with in situ high aspect ratio TiB nanowhiskers by selective laser melting using nanosized BN powder additions. Pure Ti with 2.5 vol.% nanosized BN powder showed promise for producing high performance TMCs with retained ductility. BN acted to produce TiB nanowhiskers with diameter < 50 nm. Further, by controlling post process furnace annealing TiB retained a low diameter but exhibited a high aspect ratio, up to 400. In addition to TiB refinement, nanosized BN addition promoted grain refinement during SLM, both acting as a solute to induce nucleation events and, as TiB is formed, providing nucleation sites leading to an ultrafine grain structure in as printed samples and after annealing. The produced TMCs exhibit high tensile yield strength, up to 1392 MPa, while retaining tensile ductility up to 10%. This study has shown how nanoscale design in powder bed fusion additive manufacturing techniques can be used to produce high performance TMCs through a combination of refined grain structure and high aspect ratio TiB leading to TMCs with significant improvement in strength, isotropic properties and retained tensile ductility.