Laser powder bed fusion vs. single track laser melting of martensitic Ti-Nb: Phase and microstructure formation

The aim of this work is to explore the fabrication of α″ Ti-Nb via laser powder bed fusion (LPBF) using pre-alloyed ball-milled feedstock powders. Ti-29Nb alloy powder was prepared by mechanical alloying of elemental Ti and Nb powders, using NaCl as milling agent. Pre-alloyed powders were consolidated into bulk cuboids via LPBF and the effect of different build settings on resulting phases, microstructure and porosity was studied. Phases and microstructures of the LPBF parts were compared with those of single tracks on α″ martensitic substrates. Depending on the Nb content, LPBF leads to either planar or cellular-dendritic solidification. α′ and α″ martensites, β and α phase form in the as-built parts. Single track experiments show that planar growth is conducive for the formation of α″ martensite post-solidification. While in-situ alloying is possible for specific LPBF settings, the use of pre-alloyed powders is recommended to enlarge the build parameter space for reproducible as-built microstructures.