The TC17 alloy has excellent mechanical strength, fracture toughness, and corrosion resistance and is primarily used in the manufacturing of aero engine disk components. Directed energy deposition (DED) can achieve specific location repairs of damaged parts and 3D printing of complex and large parts. DED of TC17 alloy has been successfully applied to such parts as integral blade disks. However, the low hardness and poor wear resistance of TC17 alloy make it susceptible to fatigue fracture under the harsh use environment of aero engines, which limits its application in the aerospace field. At present, the problems of poor bonding strength, introduction of new defects, and inability to meet various performance requirements in the spraying of hard coatings on titanium alloy surfaces present potential risks and limitations in aerospace applications. Optimized heat treatment has unique advantages in solving these problems. Optimized heat treatment can adjust material properties by adjusting the microstructure. Several studies have been conducted on improving the tensile properties of TC17 by optimizing the heat treatment process, but further research is needed on whether optimized heat treatments can improve the wear resistance of deposited TC17 alloy. Therefore, in this study, the effects of annealing, post-annealing solid solution, and post-annealing solid solution aging treatments on the microstructure, hardness, and tribological properties of deposited TC17 alloy are investigated. The evolution of the structure during heat treatment and the comprehensive wear behavior under 20 N dry sliding wear are analyzed. The results provide a reference for optimizing the tribological properties and heat treatment process of TC17 alloy.

The experimental material is TC17 spherical powder, with an average particle size of 66.6 μm (Fig. 1). The experiment uses the semiconductor laser to generate lasers, and the robot, equipped with coaxial powder feeding, deposits the TC17 powder on the polished TC4 substrate in the argon environment (Fig. 2). The process parameters are optimized: laser power of 1600 W, scanning speed of 10 mm/s, powder feeding rate of 11 g/min, overlap rate of 45%, and center protection gas flow rate of 11 L/min. A TC17 alloy sample (size of 75 mm×35 mm×12 mm) is obtained using an N-type scanning path. Samples are taken along the direction of laser deposition, with a sample size of 6 mm×6 mm×6 mm. The TC17 deposition samples are subjected to pre-annealing, annealing solid solution treatment, and annealing solid solution aging treatment (Fig. 3), and the microstructure and wear properties of the TC17 deposition and heat-treated samples are characterized by the X-ray diffraction (XRD), energy-dispersive X-ray spectroscope (EDS), scanning electron microscope (SEM), hardness tester, and pin-disk-type friction wear testing machine.

The experimental results indicate that the deposited TC17 alloy consists of α and β phases, displaying a basketweave structure (Fig. 6). After annealing at 840 ℃, some of the fine α phase dissolves because of the high temperature. After solid solution treatment at 800 ℃, the primary α phase (α_P) in the grain interior gradually flattens. During the annealing and post-annealing solid solution treatment stages (Fig. 7), affected by the diffusion rates of different elements, the grain boundary α phase (α_GB) is divided into continuous α_GB and discontinuous α_GB. A phase-free zone (PFZ) appears around the continuous α_GB because of the insufficient concentration of α stabilizing elements at the low-angle grain boundary. After aging at 580 ℃ based on the post-annealing solution treatment, a large amount of fine needle-like secondary α phase (α_S) precipitates, and the PFZ disappears. When the temperature rises to 630 ℃, some of the ultrafine α_S redissolves in the β matrix. After aging at 680 ℃, PFZ reappears with only partial coarsening α_S interspersed between α_P (Fig. 9). The α_S precipitates inside the β grains, and the randomness of orientation makes the size and quantity of α_S very sensitive to changes in aging temperature. After solution treatment, the average microhardness reaches 425.45 HV, which is higher than the hardness in the as-deposited state (Fig. 12). This is attributed to the precipitation and growth of α_P, which increase the volume fraction of the phase, thereby improving the microhardness. After aging treatment, the hardness is further increased, reaching its highest value (~486.93 HV) after aging at 580 ℃. This is caused by the large amount of α_S precipitation, which achieves the strongest dispersion-strengthening effect. The wear test results show that the wear properties of the heat treatment state are superior to those of the deposition state (Fig. 13). Table 3 shows the maximum wear widths and depths in different states. The tribological properties are the best after aging at 580 ℃, which is attributed to the significant increase in hardness and a large number of secondary phases inhibiting dislocation movement and crack expansion. After heat treatment, a variety of wear mechanisms coexist, and the wear rate and wear morphology depend on the changes in the microstructure and oxide layer.

A TC17 sample is prepared by DED and then heat-treated. The changes in phase composition, microstructure, microhardness, and tribological properties during heat treatment are studied. The results show that the main microstructure evolution of TC17 alloy during heat treatment includes the growth and coarsening of α_P; the continuous and discontinuous growth of α_GB, where the width of PFZ is positively related to the continuous growth of α_GB; and the precipitation and growth of α_S, where, as the aging temperature increases, some α_S dissolves and some grows to have a clear phase boundary with the β phase. The hardness after heat treatment is higher than that of the deposited state. In the solid solution stage after annealing, the hardness increases with the increase in α phase volume fraction. After further aging, the precipitation of α_S achieves dispersion strengthening, and the strengthening effect weakens with the dissolution of α_S. The hardening effect is higher at 580 ℃, and the hardness is increased by 20.1% compared with that of deposition state. The tribological performance after heat treatment is better than that in the deposition state. The increase in hardness, secondary phase precipitation, and hard oxide formation are the main reasons for the improvement of the tribological properties. Optimal wear resistance is achieved in the heat treatment systems of 840 ℃/1 h, air cooling+800 ℃/4 h, water quenching +580 ℃/8 h, and air cooling, with a friction coefficient (wear rate) of 0.422 (0.0451 mg/m).