Microstructure and mechanical properties of (TiB plus TiC)/Ti composites fabricated in situ via selective laser melting of Ti and B4C powders

Titanium matrix composites reinforced with titanium boride (TiB) and titanium carbide (TiC) were fabricated in situ via selective laser melting (SLM). Varying contents of boron carbide (B4C) from 0 to 5 wt% were added to pure Ti to prepare blended powders for SLM. The influence of the B4C content on the printability, microstructure and mechanical properties of SLM-printed (TiB + TiC)/Ti composites was studied. The relative densities of all the fabricated composites were greater than 97.8 %; an increase in the B4C content resulted in a decrease in their relative density. The microstructure of the composites varied from a lath-shaped structure (0 wt%) to a dendritic structure (1 wt%) and to a cellular + dendritic structure (2-5 wt%), which was determined by the change of thermal convection within melt pools. Both the dendritic and cellular structures were arranged by coalescent clusters composed of TiB whiskers and TiC particles, while the clusters were formed through a self-joining behavior of the TiB whiskers that were mechanically locked with the TiC particles. The composites with the addition of 1 wt % B4C exhibited the highest ultimate tensile strength of 946 MPa, yield strength of 762 MPa and elastic modulus of 128 GPa, which were 62.4 %, 49.2 % and 15.3 % higher than those of the Ti matrix, respectively. The remarkable enhancement in the mechanical strengths was attributed to the synergistic effect of dispersion strengthening and grain refinement strengthening. The (TiB + TiC)/Ti composites fabricated in situ by SLM with an optimized material formulation not only ensured comparable mechanical properties to those of their counterparts manufactured by conventional processes but also demonstrated the potential in manufacturing complex and/or customized parts with design freedom for industrial applications.