After introducing the slow-heating treatment, the mass fraction of primary α phase decreases from 58% in the as-deposited state to 34%. The plate-like primary α phase transforms into short rod-like primary α phase. The aspect ratio decreases from 8.85 to 3.75. The continuous grain boundary α phase fractures, and the linear undissolved regions disappear.After the slow-heating treatment, owing to the transformation of the plate-like primary α phase into short rod-like primary α phase and the fracture of the continuous grain boundary α phase, the tensile strength of Ti-5Al-4Mo-3V-2Zr-Nb becomes 1015 MPa, which is 5.8% lower compared to that of the as-deposited state. The elongation after fracture is 15.6%, which is 59% higher compared to that of the as-deposited state.

Ti-5Al-4Mo-3V-2Zr-Nb (hereinafter referred to as Ti-5321G) is a metastable β alloy with high specific strength and excellent corrosion resistance and has a wide range of applications in the aerospace field. Laser deposition manufacturing (LDM) is a layer-by-layer manufacturing method with high material utilization, high freedom of forming and almost unlimited material types. However, Ti-5321G samples formed through LDM generally have continuous linear grain boundaries of α phase, resulting in poor strength-ductility match. Thus, the application of Ti-5321G is extremely limited. Therefore, it is necessary to perform specific heat treatment on Ti-5321G formed through LDM to disrupt the continuous linear grain boundaries of α phase and improve its strength-ductility match. Ultra-slow heating treatment(SHT) is employed to break the continuous linear grain boundaries of α phase and enhance material strength-ductility match. The influence of slow heating treatment on the microstructure and properties of Ti-5321G is investigated.

First, Ti-5321G samples are formed through LDM. Second, the as-deposited Ti-5321G samples are annealed at 870 ℃ for 0.5 h and then water-quenched. Subsequently, the samples are heated to 810 ℃ at a heating rate of 1.24 ℃/min and held for 2 h. The samples are then furnace-cooled to 730 ℃, held for 2 h, and air-cooled, followed by 580 ℃/4 h aging treatment. Third, the microstructure of Ti-5321G samples formed by LDM after ultra-slow heating treatment is observed using optical microscope (OM) and scanning electron microscope (SEM). The content and size of α phase are measured using Image pro plus 6.0. Finally, tensile tests are conducted on the samples, and the fracture morphology is observed using SEM for performance evaluation.

The microstructure of Ti-5Al-4Mo-3V-2Zr-Nb samples formed through LDM is shown in Fig. 3. It is characterized by plate-like primary α phase (α_p) and residual β phase, with secondary α phase (α_s) distributed in a needle-like manner in the gaps of α_p. Linear continuous grain boundary α phase (α_GB) can be observed at the grain boundaries. The plate-like α_p is distributed in a mesh-like structure in some areas and randomly distributed on the β matrix in other areas. Continuous undissolved regions appear on both sides of the linear α_GB. The microstructure of the as-deposited samples after SHT is shown in Fig. 4. After the SHT, the continuous linear grain boundary α phase and the internal structure become more uniform. The plate-like primary α phase is transformed into short rod-like primary α phase under high temperature annealing treatment. Some retained deposition-like mesh-like structures consist mostly of uniformly oriented structures. After SHT, the mass fraction, aspect ratio, and length of the α_p are approximately 42%, 3.75, and 0.51μm, respectively. The α_s is mainly in the form of short rods, with a small portion appearing as equiaxed. The tensile properties of the as-deposited samples and those after SHT are listed in Table 3. The tensile and yield strengths of the as-deposited samples are 1077.5 MPa and 1600.4 MPa, respectively, with an elongation of 9.8%. After SHT, the tensile strength of the samples is 1015.8 MPa, a decrease of 5.8% compared to that of the as-deposited state. The yield strength and elongation are 990.1 MPa and 15.6%, respectively, which are 7.1% lower and 59% higher than those of the as-deposited state, respectively. After SHT, the α phase is coarsened. Compared with the needle-like α phase in the as-deposited state, the short rod-like α phase after SHT has a weaker resistance to plastic deformation. The short rod-like α phase can be easily slipped by dislocations, and the effect of dislocation strengthening is reduced. However, it has better plastic deformation ability. During plastic deformation, the short rod-like α and β phases experience similar strain; thus, they must be subjected to smaller stress compared to the needle-like α phase to avoid the generation, connection, and propagation of microcracks at the α/β phase boundary. The plasticity of the material is improved. The fracture mode of the LDM-formed specimens is a mixed fracture, whereas that of the specimens after SHT is a ductile fracture.

The microstructure of Ti-5Al-4Mo-3V-2Zr-Nb alloy formed through LDM consists of plate-like αand β phases, and needle-like secondary α phase. Continuous linear grain boundary α phases appear, accompanied by linear undissolved regions. The tensile strength is 1077 MPa and the elongation is 9.8%.