Objective With the rapid development of China's aerospace industry, particularly considering the implementation of a series of national programs such as “Project Moonshot” and the “Large Aircraft Program”, the standards for the strength, modulus, wear resistance, and temperature resistance of materials are increasingly high. Ti and its alloys are widely used in the aerospace, biomedical, and chemical industries because of their high specific strength, excellent corrosion resistance, good heat resistance, and high biocompatibility. However, the poor friction performance and low hardness of Ti alloys limit their application in some fields. Particle-reinforced titanium matrix composites (TMCs) can maintain the excellent properties of Ti alloys. These composites have a higher specific strength and specific modulus than Ti alloys; thus, they are expected to become essential structural metal materials in the aerospace industry. In recent years, TMCs have attracted significant interest from researchers in the field of materials. TiB is a ceramic reinforcement with high hardness, and its melting point is as high as 2000 ℃; moreover, there is a very small density difference between TiB and Ti. TiB particles can be produced by an in-situ reaction between Ti and TiB2, which leads to the formation of good interfacial bonding between the matrix and the TiB reinforcement. Recently, selective laser melting (SLM), as a newly developed additive manufacturing technology, has attracted extensive attention because it can directly process powder into materials with complex shapes and high precision. At present, the researches on the formation of TiB-reinforced TMCs via SLM mainly focus on the microstructure, distribution of the reinforcement phase, and microhardness; however, there are a few studies focusing on the mechanical properties of these materials, such as the tensile strength and plasticity. Therefore, analyzing the microstructure, phase composition, and mechanical properties of TiB-reinforced TMCs formed by SLM is necessary.
Methods TiB2/Ti-6Al-4V mixed powders after ball milling were used as raw materials. TiB-reinforced TMCs with different B contents were prepared by SLM. The phase composition, microstructure, electron backscatter diffraction (EBSD) results, and α lamella size of the TMCs with different B contents were analyzed by X-Ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM), and the results were compared. The microhardness and tensile properties of the TMCs were analyzed by hardness and tensile tests at room temperature. The reasons for the decrease in the α lamella size and the increase in the strength of the TMCs are provided.
Results and Discussions The results show that TiB diffraction peaks are observed in the XRD patterns of the TMCs. The microstructure of the TMCs is compact, and the semi-elliptic molten pool is tightly packed to form a good metallurgical bond. The needle-like TiB reinforcement phase is observed under a scanning electron microscope. The EBSD results show that compared with that of Ti-6Al-4V, the α phase of the TMC is noticeably refined. In addition, the microhardness, tensile strength, and yield strength of the TiB/Ti-6Al-4V composites are significantly improved.
Conclusions Based on the above results, the main conclusions of this paper are as follows:
After mixed powder ball grinding, Ti-6Al-4V powder particles still showed a spherical morphology, and the TiB2 particles were uniformly distributed on the surface of the spherical powder. XRD and SEM studies confirmed that Ti and TiB2 could react to produce needle-like TiB particles during the SLM process. TiB has a B27 structure, where the B atoms have a zigzag and continuous arrangement in a serrated continuous pattern with strong B—B bonds in the [010] direction. Furthermore, TiB has a highly asymmetric atomic structure and a high binding strength; hence, its growth rate is higher in the [010] direction than in the [100], [101], and [001] directions. Therefore, the TiB reinforcement phase tends to have a needle-shaped/rod-shaped morphology. In the TMCs sample with a high B content, the needle-like TiB reinforcement phases are clustered together, and the phenomena of cluster growth and coupling growth are observed. Because of the presence of B and the rapid solidification step of the SLM process, the α lamella size of the TMCs significantly decreases. Compared with those of Ti-6Al-4V prepared by SLM, the microhardness, tensile strength, and yield strength of the TMCs are significantly improved. The excellent mechanical properties of the TMCs are attributed to the hardening and strengthening effects of the TiB particles and the grain refinement of the matrix. When the mass fraction of elemental B is 0.5%,the average size of the α lamellar is 0.49 μm. Compared with those of Ti-6Al-4V, the tensile strength and yield strength of the TMC increase by 25.7% (1396.4 MPa) and 30.8% (1322.2 MPa), respectively.
