Objective The Ti6Al4V titanium alloy exhibits specific advantages such as low density, high specific strength, good corrosion resistance, and strong biocompatibility. Its potential applications cover a wide range of technological fields, including aerospace, automobile manufacturing, shipbuilding, biomedicine, national defense, and military industry. However, its poor wear resistance and low hardness have restricted its application in industry. In recent years, particle-reinforced titanium-based composite materials have been fabricated by introducing second-phase particles into the titanium and titanium alloy matrix. These materials are expected to improve the hardness and overall performance of the matrix. Ceramic-reinforced titanium-based composite materials are one of the particle-reinforced titanium-based composite materials. Compared with traditional titanium alloy materials, the hardness, wear resistance, and some mechanical properties of the ceramic-reinforced titanium-based composite materials can be improved by adding reinforcing phases. Among all ceramic materials, B and C in B₄C have a strong affinity for Ti. During the laser high-energy-density melting process, in situ reaction between Ti and B₄C may occur, resulting in the production of TiB, TiB₂, TiC, and other high-hardness, high-thermal stability compounds. Compared with other forming methods, the selective laser melting technology, which is an additive-type method, is applied to directly shape metal components after laying a powder bed. The advantage of this technology is the laser high-energy density, which can promote generation of an in situ reaction between powders. Using this technology, shape control and the control of complex structural parts can be achieved.

Methods Ti6Al4V-10%B₄C composites were prepared using selective laser melting technology. First, the surface roughness and relative density of the composites were investigated by applying different laser energy densities (58.33, 66.67, 75, 87.5, 100, and 114.3 J/mm ³). Then, the degree of the in situ reaction between Ti and B₄C and the composition of the reactants were investigated. The form of existence of the reinforcing phase in the composite materials and its effect on the microstructure of Ti6Al4V titanium alloy matrix were also analyzed. In addition, the microhardness of the Ti6Al4V-10%B₄C composites was tested and the effect of the B₄C addition on the hardness of the Ti6Al4V titanium alloy matrix was analyzed.

Results and Discussions During the forming process of the Ti6Al4V-10%B₄C composites, an in situ formation reaction between Ti and B₄C occurred, and the reactants were TiB₂ and TiC. These reactants can be considered reinforcing phases that refine the Ti6Al4V titanium alloy matrix grain and considerably improve the hardness of the Ti6Al4V titanium alloy matrix. The surface roughness and relative density of the Ti6Al4V-10%B₄C composites prepared under different laser energy densities were compared. When the laser energy was too high or too low, the forming surface of the composite material exhibited an apparent spheroidization, resulting in an increase in the surface roughness of the composite material. High surface roughness indicates bad forming quality of the next layer, which results in a decrease in the density of the composite material (Fig. 4). An analysis of the phase composition and microstructure morphology of the composite material revealed that the phase compositions of the composite material were mainly α-Ti, β-Ti, TiB₂and TiC, indicating that an in situ formation reaction occurred between Ti and B₄C. The TiB₂ and TiC existed in the Ti6Al4V titanium alloy matrix mainly in clustered or reticular associated organization (Fig. 7), which significantly refined the Ti6Al4V titanium alloy grain (Fig. 8). Microhardness testing of the Ti6Al4V-10%B₄C composites showed that the addition of B₄C could significantly improve the hardness of the Ti6Al4V titanium alloy. For laser energy density of 114.3 J/mm ³, the microhardness of the composite material could reach up to 910 HV (Fig. 9). This is a 153.5% enhancement compared with pure Ti6Al4V titanium alloy matrix.

Conclusions In the present study, Ti6Al4V-10%B₄C composites were successfully prepared using selective laser melting technology. TiB₂ and TiC were generated in situ from Ti and B₄C. These enhanced phases significantly refined the grains of the Ti6Al4V matrix and enhanced its microhardness. A comparison of the forming surface roughness and the relative density of the Ti6Al4V-10%B₄C composite materials prepared under different laser energy densities revealed that for a laser energy density of 66.67 J/mm ³, the surface roughness of the composites exhibited its lowest value, the value of R_a was 37.2 μm, the value of R_z was 216 μm, and the relative density reached its highest value (95.55%). During the preparation of the Ti6Al4V-10%B₄C composite via selective laser melting, an in situ formation reaction occurred between Ti and B₄C, and the reactants were TiB₂ and TiC. The grain size of TiB₂ approached the nanometer level. The TiB₂ and TiC reactants exhibited clustered or reticular associated TiB₂-TiC organization. The microhardness of the Ti6Al4V-10%B₄C composite material was significantly higher than that of a pure Ti6Al4V titanium alloy material and its highest microhardness value was 910 HV, which is 153.5% enhancement compared with the titanium alloy matrix.