Objective The Ti6AI4V 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 Ti6AI4V-10 yoB,C 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 Ti6AI4V titanium alloy matrix were also analyzed. In addition, the microhardness of the Ti6AI4V-10 B4C composites was tested and the effect of the B4C addition on the hardness of the Ti6AI4V titanium alloy matrix was analyzed. Results and Discussions During the forming process of the Ti6AI4V-10 yo 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 Ti6AI4V titanium alloy matrix grain and considerably improve the hardness of the Ti6AI4V titanium alloy matrix. The surface roughness and relative density of the Ti6A14V-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 alpha-Ti, P-Ti, TiB2 and TiC, indicating that an in situ formation reaction occurred between Ti and B4C. The TiB2 and TiC existed in the Ti6AI4V titanium alloy matrix mainly in clustered or reticular associated organization (Fig. 7), which significantly refined the Ti6AI4V titanium alloy grain (Fig. 8). Microhardness testing of the Ti6AI4V-10 YoB,C composites showed that the addition of B4C could significantly improve the hardness of the Ti6AI4V 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 Ti6A14V titanium alloy matrix. Conclusions In the present study, Ti6A14V-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 Ti6A14V matrix and enhanced its microhardness. A comparison of the forming surface roughness and the relative density of the Ti6A14V-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 mu m, the value of R, was 216 mu m, and the relative density reached its highest value (95.55%). During the preparation of the Ti6A14V-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 Ti6A14V-10 oRiC composite material was significantly higher than that of a pure Ti6A14V titanium alloy material and its highest microhardness value was 910 HV, which is 153.5 % enhancement compared with the titanium alloy matrix.
