Quantitative prediction of a novel grain refinement law in TC11-xB alloys prepared by laser-directed energy deposition

Laser-directed energy deposition is extensively used for the production of titanium alloy components. These additively manufactured titanium alloys tend to show anisotropy owing to the coarse columnar prior-β grains formed under the ultra-high thermal gradient. Boron addition has proved to be a powerful method for controlling solidification grains. However, it is still limited due to the lack of quantitative regulation on the grain morphology. To explore the relationship between boron content and the grain morphology, boron-modified α + β titanium alloys Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) were laser-directed energy deposited as multilayer walls. Columnar-to-equiaxed transition (CET) and obvious grain refinement occur when boron content reaches 0.35 wt%. At high growth restriction factor Q, a nonlinear relationship between grain sizes and 1/Q is found. Therefore, a model extending the interdependence theory is developed to interpret the phenomena. This work proposes a model to predict grain sizes and aspect ratios of additively manufactured titanium alloys under extreme solidification conditions.