Objective Titanium matrix composites have attracted considerable attention because of their high modulus of elasticity, high specific strength, high wear resistance, and excellent high- temperature durability. Most studies on titanium matrix composites (TMCs) focus primarily on the in- situ formed TiC reinforced composites. However, few studies have focused on the direct addition of TiC-reinforced- reinforced titanium matrices. The manners in which the size, morphology, and distribution of TiC evolve during the SLM process and how they affect the microstructure and mechanical properties remain unclear. In this study, TiC/TC4 composites with directly added nanoscale TiC particles are successfully prepared by selective laser melting (SLM), and the microstructure evolution under different volume energy densities is investigated. Further, the TiC evolution during SLM and its influence on the microstructure and microhardness are analyzed. Thus, the findings of this study can provide the support for SLM preparation of titanium composites. Methods Herein, nanoscale TiC (diameter of 50-150 nm) and TC4 are selected as the reinforced phase and matrix, respectively. The composite powder with TiC uniformly embedded on the surface of the TC4 powder is obtained by low- energy ball milling. Subsequently, the TiC/TC4 composites are prepared via SLM with different volume energy densities (29-97 J/mm3). 3 ). The forming quality and microstructures at different volume energy densities are observed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped energy disperse spectroscope (EDS). The grain size and crystal orientation are investigated using electron backscattering diffractometer (EBSD), and the phase compositions are measured using X-ray- ray diffraction (XRD). Finally, the microhardness is measured using a digital microhardness tester. Results and Discussions The optimized volume energy densities for the SLM formed TiC/TC4 composites are in the range of 50 - 70 J/mm3, 3 , with a relative density of 99.7% (Fig. 3). Owing to the enrichment of TiC in the melt pool boundary zone, the microstructure of the composites exhibits a special double- sized grain distribution in the cross section (Fig. 6). Owing to the rapid cooling characteristics of the SLM process, TiC cannot be sufficiently dissolved. Therefore, the SEM and EBSD results reveal three types of reinforcement: undissolved TiC, eutectic TiC, and precipitated TiC. Undissolved TiC is distributed primarily at the boundaries of coarse beta equiaxed grains, eutectic TiC is distributed primarily in the boundaries of irregular eutectic beta grains, and precipitated TiC is distributed primarily in the grains. With an increase in volume energy density, the chain- like eutectic TiC gradually transforms to rod- like eutectic TiC (Figs. 7 and 8), the size of precipitated TiC inside the grain gradually increases, and the sizes of longitudinal and transverse alpha'-Ti- Ti gradually increase. Conclusions The optimal volume energy density for the formation of TiC/TC4 composites by SLM is 50 - 70 /mm3, 3 , and the relative density is 99.7% within this parameter range. TiC is enriched in the melt- pool boundary region under a strong temperature gradient and Marangoni convection. The microstructure of the composite has a special double- size grain distribution in the cross section, consisting of primary beta equiaxed grains and irregular eutectic regions growing on the periphery. In the longitudinal section, the molten pool is a fish scale, and some chain structures exist in the molten pool that grow from the direction of heat flow to the horizontal direction. With an increase in volume energy density, the size of primary beta equiaxed grains decreases, outer- ring irregular eutectic region expands, and morphology of fish scales becomes sharp. The microhardness initially decreases and then increases, essentially reaching 385-392 HV in the optimal molding process window. TiC in the composites is composed primarily of undissolved TiC (distributed near the primary beta grain boundaries), eutectic TiC (distributed in the eutectic beta grain boundaries in a chain or rod- like network), and precipitated TiC (distributed in the grain in a granular manner). With an increase in volume energy density, the difference in TiC size and quantity inside and outside the molten pool increases, chain distribution of eutectic TiC changes to rod, and the size of TiC in the grains increases. Further, no obvious orientation relationship between eutectic TiC and beta- Ti is observed; however, a distinct orientation relationship between eutectic and in- grain TiC and alpha'-Ti- Ti exists: {11-20} alpha'-Ti parallel to{110}TiC.- Ti parallel to{110}TiC.
