Effect of Trace B on Microstructure and Mechanical Properties of Additive Manufactured Near β Titanium Alloy Ti55531

The refinement of β grains is an effective approach to optimize the grain boundary α phase and enhance the mechanical properties for laser-directed energy deposited (L-DED) titanium alloys. In this study, the primary β grain size is refined by adding 0.05 and 0.10 wt% boron in Ti55531, respectively. It was found that the addition of trace boron can not only reduce the primary β grain size during molten pool solidification process, but also can suppress the primary β grain coarsening during in-situ thermal cycling process. The Ti55531 + 0.05B alloy exhibited higher strength and elongation. This is attributed to the enhanced coordinated deformation ability resulting from the refined β grains and little harmful effect resulting from the presence of needle TiB. In contrast, when the boron content increased to 0.10 wt%, the harmful effect of excessive TiB whiskers at the grain boundaries outweighed the favorable effect of the further β grain refinement, which leads to the decrease in plasticity. This study suggests that an appropriate boron content added can refine primary β grain significantly and meanwhile avoid too much TiB precipitation, achieving superior comprehensive mechanical properties for additive manufacturing near β titanium alloy.