Ti-6Al-4V(TC4) titanium alloy exhibits impressive characteristics, including excellent corrosion resistance, high specific strength, and yield ratio, and thus, widely used in several fields, such as aerospace, navigation, and biomedical industries. Selective laser melting (SLM), a common method for manufacturing metal alloys, has multiple advantages, such as short production time, low cost, high integration, and high precision. Therefore, SLM is preferred for the fabrication of TC4 alloys with high density and good mechanical properties. However, the molten pool of SLM undergoes rapid melting and solidification under high temperature gradient. The microstructure of SLM TC4 is mainly composed of β columnar crystals, imparting poor plasticity and obvious anisotropy to the samples. Therefore, the deposited samples fail to meet the standard of commercial engineering and the simulation requirements of the titanium alloy. In this study, the TC4 samples prepared by SLM were heat-treated under three different conditions of solution aging, cyclic annealing, and cyclic spheroidizing annealing with solution aging. The effects of the three heat treatments on the microstructures and mechanical properties of the deposited TC4 were investigated with an optical microscope, a scanning electron microscope, and a tensile strength testing machine. The mechanisms of change in the microstructure and anisotropy during the three heat treatments are analyzed, providing a research basis for the application of TC4 in the aerospace field.

In our study, TC4 alloy powder with the particle size of 15-65 μm was used along with the rolled TC4 substrate. Before the experiment, the powder was placed in an oven and dried at 120 °C for 24 h to remove the moisture from the powder. The formation process and heat treatment were performed in the argon gas atmosphere. The size of the horizontal sample was 45 mm×8 mm×8 mm, while that of the vertical specimen was Φ8 mm×45 mm. According to the national standard GB/T228.1-2010, the samples were processed into standard tensile specimens and subjected to the tensile strength test at room temperature. Later, the samples were ground and polished, and the polished surface was corroded with Kroll reagent [V(HF)∶V(HNO₃)∶V(H₂O)=1∶3∶50 ]for 5 s. KEYENCE VH-600 optical microscope and TESCAN MIRA 3 LMH field emission scanning electron microscope were used to observe the microstructure and fracture.

The microstructure study showed that the substructure of SLM TC4 mainly comprised of martensite α′ and α″. The aspect ratio of martensite was greater than 20, and the dislocations and stresses produced by rapid heating and cooling provided sites for the nucleation of martensite. Post HM1[950 ℃/1 h/ air cooling (AC)+ 550 ℃/4 h/ AC] heat treatment, a bimodal microstructure was observed for SLM TC4 with the aspect ratio of lath α phase being about seven. During the heat preservation at 950°C, martensite α′ transformed into the lath α phase by downhill diffusion, the grain boundary α truncated when lath α phase grew, β phase nucleated and grew preferentially at the grain boundary, and the grain boundary broke under the combined influence of the above factors. During the aging process, small strips of secondary α phase precipitated from the metastable β phase. After HM2[(920 ℃×5 min+ 700 ℃×10 min)×5 / AC] heat treatment, the β phase in the microstructure of TC4 almost completely transformed into lath and equiaxed α phase, with the aspect ratio of the lath α phase being about 4, and the transformation mainly occurred as α′+ α″→α+ β. The spheroidization is mainly caused by thermal grooving and boundary splitting mechanisms. After five cycles, the volume fraction of α phase and the degree of spheroidization increased. After HM3[(920 ℃×10 min+ 800 ℃×30 min-550 ℃)×4/ furnace cooling (FC)+ 920 ℃×1 h/ AC+ 550 ℃×4 h/ AC) ]heat treatment, the microstructure of SLM TC4 mainly composed of lath α phase, equiaxed α phase, and net basket-like secondary precipitated α phase. The Burgers orientation relationship was satisfied during the phase change process. Due to the long heating time of cyclic spheroidizing annealing, the grain strength decreased more severely, and the subsequent solution aging made up for the strength loss during the cycle annealing process (Figs.3-4). As per the tensile strength test results, the tensile strength and the yield strength of SLM deposited TC4 were obtained as 1223.0 and 1054.2 MPa, respectively. The grain boundary hindered the movement of dislocations, therefore, the deposited sample exhibited high strength. After HM1 heat treatment, the tensile and the yield strengths of the TC4 depositions were 957.5 and 865.4 MPa, respectively, and the elongation was approximately 17%. Among the three heat treatments, the highest elongation of the deposited TC4 was observed after HM3 heat treatment, reaching up to 18.4%. After the heat treatment, the grain boundary α phase transformed into a continuous equiaxed α phase, which weakened the crack resistance, so that the strength decreased and elongation increased (Table 3). Analyzing the anisotropy data of the different mechanical properties of the deposited TC4 in the four states(As-builted、HM1、HM2、HM3), the anisotropy of the elongation and the reduction of the TC4 deposition area was relatively high (12.21% and 5.63%, respectively). (Figs.5-6). The anisotropy of tensile strength and yield strength of TC4 after heat treatment of HM1, HM2 and HM3 is less than 2%, and the anisotropy of elongation and area shrinkage decreases compared with that in the deposited state.

The deposited microstructure of SLM TC4 constitutes coarse β columnar crystals with martensite α′ and α″ as the intragranular substructures. The microstructure of SLM TC4 after solution aging heat treatment is bimodal, and a secondary α phase is generated during the aging process. During the cyclic annealing process, parts of the grain boundary α phase and lath α phase are broken to form an equiaxed α phase. After the cyclic spheroidizing annealing and solution aging treatment, the microstructure of the sample is mainly composed of the lath α phase, equiaxed α phase, and secondary α phase; the volume fraction of the equiaxed α phase is relatively high. The standard deviation of the mechanical property data of SLM TC4 is generally large, which is significantly reduced after the heat treatment. The strengths of the samples treated either solely by solution aging or a combination of cyclic spheroidizing annealing and solution aging are roughly equivalent. The sample plasticity obtained by the combined cyclic spheroidizing annealing and solution aging is better than that obtained by solution aging alone. After the heat treatment methods, the mechanical properties of the samples exceed the national forging standard. The plastic anisotropy of SLM TC4 depositions is high. However, the three heat treatment processes significantly reduce the plastic anisotropy. Among these processes, the solution aging treatment provides the deposited TC4 with the lowest anisotropy of the mechanical properties. SLM TC4 depositions have mixed fracture morphology, while the deposited TC4 after heat treatment shows a typical ductile fracture.