Columnar to equiaxed grain transition of laser deposited Ti6Al4V using nano-sized B₄C particles

A new laser melting deposition (LMD) method was employed for Ti6Al4V to stimulate the columnar to equiaxed transition (CET) via adding 1 wt% ~ 5 wt% nano-sized B₄C particles. Results indicate that the grains were signifcantly refined from 600 μm to 11 μm with the increasing content of B₄C; When the B₄C content was 3 wt%, the B₄C/Ti6Al4V composite achieved CET; TiB and TiC reinforcements were produced by in-situ reaction between the Ti6Al4V and B₄C, which not only precipitated at the grain boundaries to restrict the grain growth, but also acted as nucleation sites to accelerate the non-spontaneous nucleation of β grains to achieve fine grain strengthening. Tensile tests show that adding 3 wt% B₄C particles results in a signifcantly increased yield strength (1235 MPa) and tensile strength (1310 MPa) and maintains a certain amount of ductility (2.4% elongation). And the epitaxial growth of β grains in the underlying deposited layer and the CET mechanism with successive layer-upon-layer deposition were analyzed.