Metastable β titanium alloy has a high melting point, high activity, low thermal conductivity, and high deformation resistance. However, traditional manufacturing methods face several problems when dealing with the complex components of metastable β titanium alloys, such as numerous processes, long cycles, high cost, and low yield. Laser selective melting (SLM) is a new manufacturing technology that uses a laser as the heat source to melt metal powders layer-by-layer to manufacture solid parts. Owing to its super-complex structure forming ability, high material utilization rate, and rapid prototyping manufacturing ability, SLM provides an excellent solution for the manufacturing of titanium alloy parts with complex structures in aerospace. For the initial manufacturing process of metastable β titanium alloy, the cooling rate range of SLM is 10³?10⁸ K/s^-1, while that of traditional vacuum arc melting (VAM) is 10¹?10² K/s^-1. Non-equilibrium solidification resulting from rapid cooling is advantageous for grain refinement. The grain sizes of the SLM samples are significantly smaller than those of the VAM samples, and the grain size has a significant impact on their mechanical properties. The VAM Ti-1023 alloy is used to simulate the as-cast microstructure of a Ti-1023 alloy. The differences in the microstructures and mechanical properties of the SLM and VAM Ti-1023 alloys are compared. The effects of rapid cooling conditions on the microstructure and properties of Ti-1023 alloy are systematically studied and provide a theoretical basis for the additive manufacturing of complex components of metastable β titanium alloy.

The SLM samples are fabricated using the Ti-1023 alloy powder prepared by gas atomization through laser selective melting. Figure 1 illustrates the morphology and particle size of the Ti-1023 powder. A laser metal powder 3D printer is used to produce the SLM samples. The laser spot diameter is 70 μm, and 20 mm×20 mm×10 mm blocks are directly deposited on a commercial Ti-6Al-4V substrate as shown in Fig. 2(a). Table 1 lists the elemental contents of both the Ti-6Al-4V substrate and Ti-1023 powder. In the preparation process, a laser power of 250 W, scanning speed of 1100 mm/s, scanning spacing of 60 μm, powder thickness of 30 μm, and rotation scanning strategy of 67° are employed as illustrated in Fig. 2(b). The sample with a relative density of 99.9% can be obtained by using Archimedes method.

The VAM samples are prepared on a commercial Ti-1023 alloy plate. The elemental composition of the commercial Ti-1023 alloy plate is listed in Table 1. The treated raw materials are placed in a water-cooled copper crucible in a VAM furnace. The furnace is first vacuumed to a gas pressure of 5×10^-4 Pa, and then filled with argon . After three times, the ambient oxygen content is reduce to avoid sample oxidation. The surfaces of the VAM and SLM samples are then treated using a vibration-polishing equipment. The vibration-polishing frequency and polishing time are set to 56 Hz and 16 h, respectively. Next, the microstructures of the samples are observed and analyzed using an X-ray diffractometer, optical microscope, scanning electron microscope, and electron backscatter diffraction equipment. The tensile properties of the Ti-1023 samples under different processes are tested using a universal mechanical testing machine. The tensile tests are performed at room temperature at a tensile speed of 0.1 mm/min.

As shown in Fig. 3, the phase composition of the VAM sample is α+β phases, and the SLM sample is mainly composed of β phase. It is shown that the SLM rapid cooling condition inhibits the phase transition process of β→α and a full β-phase structure forms, while the VAM sample is composed of an α+β dual phase structure. Under rapid SLM cooling, the grain size is approximately 1/10 that of VAM as shown in Fig. 4. The acicular α phase with volume fraction of 3.11% forms in the VAM sample during cooling. Simultaneously, the acicular α phase accumulates at the grain boundary of the β phase and disperses inside the β grains as shown in Fig. 8. Although the SLM sample lacks a high-hardness α phase, the high-density dislocation caused by the rapid cooling conditions is 2.1 times that of the VAM sample as shown in Fig. 7. The dislocation grid is an immovable dislocation, and its main role is to coordinate the lattice interface and maintain material continuity. Dislocation movement is hindered during the deformation process, thereby improving the yield strength of Ti-1023. The full β-phase structure of the SLM sample avoids the formation of the α/β interface and produces stress-induced martensitic transformation during deformation as shown in Fig. 9. The stress-induced martensitic transformation can increase the fracture elongation by more than five times that of the VAM sample as shown in Fig. 5.

During the cooling process of VAM Ti-1023 alloy, the phase transition from β to α occurs, and the acicular α phase with volume fraction of 3.11% is produced. The microstructure at room temperature is composed of α+β phases. Because of the rapid cooling of the SLM Ti-1023 alloy, the formation of the β phase is inhibited, and the room-temperature structure is composed of single β phase.

SLM samples lack a high hardness α phase and their yield strength is comparable to that of VAM samples owing to the presence of high-density dislocations, which inhibit dislocation movement under rapid cooling conditions.

The full β-phase structure of the SLM sample avoids the formation of the α/β interface. As a result, stress-induced martensitic transformation occurs during deformation, leading to an increase in fracture elongation by more than five times that of the VAM samples.