Objective Selective laser melting (SLM), because of its capacity to fabricate complex precision parts with high forming accuracy, has been hailed as one of the most promising manufacturing technologies for rapid prototyping. However, the mechanical properties of metal materials formed by SLM have the characteristics of anisotropy, high strength, and low plasticity. Therefore, heat treatment is always needed to control the microstructure to meet application requirements. Annealing treatment is typically adopted to improve the mechanical properties of selective-laser-melted titanium alloys. Therefore, study of the effect of annealing temperature and holding time on the microstructure, mechanical properties, and fracture mechanism of TC11 titanium alloys formed by SLM is of great significance.

Methods The resulting microstructure, mechanical properties, and fracture morphology of selective-laser-melted TC11 titanium alloys under different heating temperature and holding time were studied, and the fracture mechanism under different conditions was explored. Firstly, compact TC11 titanium alloys were obtained by SLM. Secondly, different annealing heat treatments were performed on the samples. Thirdly, the phase composition of the different samples was analyzed by X-ray diffraction, and the microstructure morphology was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the change in the micro-hardness of different samples was tested using a micro-hardness tester, the tensile properties at room temperature were tested, and the fracture morphology was observed by SEM.

Results and Discussions The as-deposited TC11 titanium alloys are composed of hexagonal close-packed Ti (HCP/Ti), with lattice parameters a and c of 0.2934 nm and 0.46757 nm, respectively. The annealed samples consisted of HCP/Ti and body center cubic Ti (BCC/Ti), where a and c for HCP/Ti are 0.29172 nm and 0.46817 nm, respectively (Fig.3). Based on the observation of microstructure morphology by OM and SEM, it is deduced that the selective-laser-melted TC11 titanium alloys consisted of columnar grains, within which acicular α' martensite was present (Fig.4). After annealing at 850 ℃ for 4 h, fine α+β mixed structures were formed in the alloys, this was the result of the nucleation and growth of α' martensite. Due to the sufficient atomic diffusion leading to coarsening α lamellae, basket-weave structures were formed in the samples annealed at 950 ℃ for 4 h. Moreover, α clusters with the same orientation and continuous grain boundary α phase (GB α) were also observed in the grains and at the grain boundaries, respectively (Fig.5). When annealing temperature remained at 950 ℃ with holding time of 1 h or 2 h, basket-weave structures were also formed in the samples, but the width of α lamellae was about one-half and one-quarter of that in the samples annealed at 950 ℃ for 4 h, respectively. In addition, GB α phase began to transform into a discontinuous distribution (Fig.6). According to the results of the hardness test, the average micro-hardness of the as-deposited samples is about 402 HV_0.5, whereas the hardness of the samples annealed at 950 ℃ for 4 h is only about 85% that of the as-deposited samples. Moreover, the micro-hardness of the samples annealed at the same temperature for 2 h and 1 h is about 381 HV_0.5 and 393 HV_0.5, respectively. The increase in micro-hardness with the decrease in holding time could be due to the effect of fine-grained strengthening (Fig.7). The tensile strength and percentage elongation after fracture is 1557 MPa and 2.5%, respectively, because acicular martensite is characterized by high strength and poor plasticity. However, after the samples were annealed at 850 ℃ and 950 ℃ for 4 h, the tensile strength is 72% and 64% of that of the as-deposited samples, respectively. In this case, the percentage elongation after fracture is 4.5 times and 5.7 times that of the as-deposited samples, respectively. When the samples were annealed at 950 ℃, their tensile strength increased from 996 to 1051 MPa, and the percentage elongation after fracture increased from 14.3% to 19.8% (Fig.8, Table 3). This is because the fine basket-weave structures have the effect of fine grain strengthening, and GB α has an effect on the percentage elongation after fracture. Based on fracture analysis, no obvious plastic deformation was observed in the macroscopic fracture of the as-deposited samples, and the fracture exhibited granular grain surfaces with different orientations. The microscopic fracture showed the characteristics of intergranular dimples. The above analysis indicates that the as-deposited sample underwent intergranular fracture when it was stretched at room temperature (Fig.9). However, macroscopic fractures of the annealed state had obvious plastic deformation, and the microscopic fractures showed the characteristics of dimples with larger size, indicating that ductile fracture occurred in the annealed state (Fig.10).

Conclusions The effect of annealing temperature and holding time on the microstructures, mechanical properties, and fracture mechanism of selective-laser-melted TC11 titanium alloys is investigated, and the following conclusions can be drawn: The microstructures of the alloys are composed of acicular martensite within columnar grains parallel to the building direction. After annealing at 850 ℃ and 950 ℃ for 4 h, the microstructures of the alloys are fine α+β mixed and α+β basket-weave structures, respectively. As a result of decomposition of the acicular martensite, the alloys are softened, and the softening effect is more obvious with the increase in annealing time or temperature. Furthermore, the fracture mechanism changes from being intergranular to ductile, which is consistent with the regular variation of the percentage elongation after fracture. When the alloys are annealed at 950 ℃, with shorter holding time, finer lamellar α can be obtained. Continuous GB α transforms into a discontinuous distribution, finally resulting in the simultaneous increase in tensile strength and elongation after fracture. Selective-laser-melted TC11 titanium alloys with better strength and plasticity can be obtained through annealing at 950 ℃ for 1 h, with a tensile strength and percentage elongation of 1051 MPa and 19.8%, respectively, after fracture.