Coaxial wire laser directed energy deposition of equiaxed structured titanium alloy with ultrahigh strength and minimal mechanical anisotropy

Directed energy deposition (DED) of titanium alloys is frequently accompanied by strong thermal gradients, resulting in coarse columnar prior β grains and pronounced mechanical anisotropy. Achieving a columnar-to-equiaxed transition (CET) in near-β titanium alloys therefore remains a critical solidification-related challenge. This study systematically investigates the solidification behavior and microstructural evolution of a near-β titanium alloy (Ti-5Al-2Sn-2Zr-4Mo-4Cr, TC17) fabricated via a six-laser coaxial wire-fed directed energy deposition (WLDED) process. Owing to the spatially distributed multi-laser energy input, the melt pool thermal field and fluid flow behavior are significantly modified, leading to enhanced convection and altered local solidification conditions. As a result, a pronounced CET is achieved directly in the as-deposited condition without compositional modification or external field assistance, producing nearly equiaxed prior β grains with an average size of approximately 126 μm. Furthermore, the high solidification rate inherent to the WLDED process promotes the formation of a hierarchical nanoscale α+β lamellar microstructure. The combined effects of equiaxed prior β grains and refined lamellar architecture result in a high ultimate tensile strength of approximately 1.3 GPa and low in-plane mechanical anisotropy (%IPA<0.4%). These results demonstrate that process-induced thermal-fluid-solidification coupling plays a decisive role in driving CET in near-β titanium alloys under non-equilibrium DED conditions.