Objective Ti-6Al-4V (TC4) titanium alloy has excellent corrosion resistance, and high specific strength and yield ratio. It is widely used in the aerospace, navigation, and biomedical industries. For structural parts with complex shapes, traditional processing techniques are complicated, resulting in low material utilization and high manufacturing costs. Laser melting deposited (LMD) TC4 alloy components have the advantages of low cost, short cycle, and high performance. TC4 titanium alloy is a dual-phase alloy, consisting of two phases: β-phase stable at high-temperature and α-phase stable at room temperature. The microstructure types consist of Widmanstatten, basket, dual-state, and equiaxed structures. Different microstructure morphology, size, phase ratio, and distribution have a significant impact on the performance of this alloy. Due to the principle of layer by layer in the laser additive manufacturing process, the temperature gradient in the deposition direction is very high, leading to the columnar grains growing along the deposition direction and continuous α-phase at the columnar grain boundaries, thereby resulting in large anisotropy of mechanical properties, especially plastic anisotropy. The structural optimization of LMD TC4 alloy through a reasonable heat treatment process can effectively improve the mechanical properties of this alloy and reduce anisotropy. Some researchers found that the matching of this alloy's strength and plasticity is best after solution-aging treatment. Thus, this study focuses on the effects of the solution-aging heat treatment process on the microstructure, mechanical properties, and anisotropy of the LMD TC4.
Methods In this study, TC4 titanium alloy powder with a particle size of 50--150 μm is used. TC4 titanium alloy block is formed by a laser additive manufacturing system. The powder is dried in a vacuum drying oven. The formation process is conducted under the protection of argon gas, and the laser scanning path is a vertical cross-reciprocating form. Use wire cutting to cut the tensile specimens in the middle of the deposition block parallel to the deposition direction (V) and perpendicular to the deposition direction (H), take five samples in each group in the H and V directions, and examine the effect of heat treatment on tensile properties. The samples were heat-treated in a quartz tube furnace under an argon atmosphere. The effects of three types of heat treatment processes on the microstructure and mechanical properties of this alloy were investigated. Air cooling (AC) was used for solution and aging, and furnace cooling (FC) was used for annealing. Use KEYENCE VH-600 optical microscope and TESCAN MIRA 3 LMH field emission scanning electron microscope to observe the microstructure and fracture. The content of the elements in the sample was qualitatively characterized by the energy dispersive spectrometer, and the sample was tested using XRD. The corrosive agent was Kroll reagent. Use HXD-1000TMC/LCD microhardness tester to measure the microhardness of different samples. The load is 1.96 N, and the duration is 15 s. Take ten points for each sample and calculate the average value.
Results and Discussions The microstructure study shows that the microstructure of LMD TC4 consists of fine α+β lamellas, primary equiaxed α-phases, and Widmanstatten α-clusters growing along the continuous grain boundaries (Fig. 1). In the single solid solution-aging system, with the increase in the solution temperature and holding time, primary α-plates are coarsened from 2 to 5 μm, and the degree of the grain boundary fracture is intensified. The degree of uniformity of the plate size increases with an increase in the solution temperature (Fig. 2). In the double solution-aging system, with the increase of the second solution temperature, the primary α-plates are coarsened from 3.5 to 5 μm, and the degree of grain boundary fracture is further intensified. After HT5, the grain boundaries totally broke off, and the edges of the primary α-plates appear to be separated. After HT8, the width of the secondary α-phase is coarsened to about 0.9 μm. However, the coarsening of the primary α-phase is not obvious (Fig. 3), and the level of hardness is in the middle. The XRD diffraction pattern showed that due to the decrease in the content of solid solution atoms in the lattice gap after heat treatment, the degree of lattice distortion is reduced, increasing the interplanar spacing, and the diffraction peaks of all heat-treated samples offset to the left in varying degrees (Fig. 5). The hardness test result showed that in the single solid solution-aging system, the hardness value increases as the solution temperature and holding time increase. After HT4, the primary α-phase is further coarsened; however, its content does not significantly change compared to HT3. Thus, the hardness value is the highest. In the double solution-aging system, the hardness shows a downward trend with the increase in the second solution temperature. The level of hardness after solution-aging + annealing heat treatment is in the middle. Although the content of α-phase in the LMD TC4 is very high, its size is small and β-phase distribution between the α-phase is relatively uniform, the ability of cooperative deformation between the two-phase is strong; thus, the hardness value is the lowest (Figs. 6, 7). The tensile test result showed that the strength of the LMD TC4 is the highest; however, the plasticity is lowest, and the plastic anisotropy is the largest. With an increase in the heating peak temperature, the strength continues to decrease and the plasticity continues to increase. The plasticity of LMD TC4 increases after HT2, and the plastic anisotropy decreases. After HT8, the sample has the largest strength loss with the largest increase in plasticity, reducing the plastic anisotropy further. After HT5, the best matching of strength and plasticity, the smallest plastic anisotropy, and the best comprehensive performance have been obtained (Table 3).
Conclusions The results showed that due to the high laser energy density, the microstructure of LMD TC4 alloy includes tiny α+β lamellar plates and Widmanstatten α-cluster along the continuous grain boundary, and the grain boundary is relatively complete. Thus, the strength is the highest, but the hardness and plasticity are the lowest, and the plastic anisotropy is the largest. In the single solid solution-aging system, with the increase in the solid solution temperature, primary α-plate continues to coarsen, the grain boundary fracture degree and hardness increase. After solution-aging + annealing, the content of the primary α-phase is the highest, the plasticity is the highest, but the strength is the lowest, and the level of hardness is in the middle. In the double solid solution-aging system, with the increase in the second solid solution temperature, primary α-plate are further coarsened, the degree of grain boundary fracture intensifies, and the hardness shows a downward trend. After HT5, the continuous grain boundary phase totally broke off, the hardness is low, the matching of strength and plasticity is suitable, and the plastic anisotropy is minimal.
