Selective laser melting (SLM) technology applied to TC4 alloys is widely used in the aerospace and aircraft industries. Heat treatment is used to regulate the microstructure of selective laser-melted (SLMed) TC4 alloys, which has important implications for current industrial production. The majority of related research has focused on strengthening the mechanical properties of SLMed TC4 alloys through heat treatment. However, no systematic studies have been conducted to determine the effects of various heat treatment systems on the microstructure and tensile properties of SLMed TC4 alloys. This study investigated the effects of annealing, solid solution treatment, solid solution + aging, and other processes on the microstructure and mechanical properties of SLMed TC4 alloys, clarified the microstructural evolution mechanism of SLMed TC4 alloys under different thermal environments, and provided a reference for selecting heat-treatment systems for SLMed TC4 alloy components.

The morphology and particle size distribution of the powders are examined using scanning electron microscopy (SEM), as shown in Fig. 1. The chemical compositions of the TC4 powders is listed in Table 1. The densely SLMed TC4 alloy samples are heat-treated using an optimized process in an argon shielding chamber. Annealing, solid solution, and solid solution + aging are performed to study the effects of heat treatment. The macroscopic microstructure and fracture morphology are characterized using optical microscopy and SEM (Figs. 3?5 and 12). The grain morphology, orientation, and local misorientation of the bonding interface are characterized by electron backscatter diffraction (EBSD). The tensile tests are conducted using an electronic universal testing machine.

A dense SLMed TC4 alloy is obtained under the following conditions: 300 W laser power, 1200 mm/s scanning speed, 0.1 mm scanning pitch, and 0.06 mm layer thickness; however, a few defects are observed. The heat treatment does not significantly improve the samples' metallurgical quality (Fig. 2). The SLMed TC4 alloy samples are composed of coarse β columnar crystals. The internal structure of the columnar crystals primarily includes a large amount of fine α′ phase needle-like martensite and a small amount of β phase particles between the α stripes (Figs. 3, 4, and 8). The as-deposited specimens have an average yield strength of 1080.00 MPa, tensile strength of 1238.75 MPa, and 8.85% elongation after fracture (Table 2, Figs. 9?11). The fracture mode is a mixed ductile-brittle fracture (Fig. 12). The annealed SLMed TC4 alloy, heat-treated at 800 ℃/4 h/AC (Air cooling), has a reduced internal stress, a more uniform microstructure, and decreased tensile and yield strength to 990.00 MPa and 881.80 MPa, respectively. The elongation increased to 14.34%. The fractures show significant ductile characteristics (Fig. 12). The microstructure of the solution-treated SLMed TC4 titanium alloy heat-treated at 920 ℃/2 h reveals an interlaced basket-weave structure, with coarse α phase rods arranged in a relatively regular manner. The transformation from the α phase to the β phase is incomplete, resulting in partial α phase coarsening (Figs. 6 and 10). The yield strength is 799.40 MPa, the tensile strength is 928.40 MPa, and the elongation after the fracture is 15.62%. The solution-aged specimens heat-treated with a combination of 920 ℃/2 h/WQ (Water quenching) and 540 ℃/4 h/AC exhibit a relatively uniform distribution of the (α+β) phase, with coarse α phase laths and β phase distributed around them (Figs. 7 and 11). The yield and tensile strengths are 829.60 MPa and 954.00 MPa, respectively, and the elongation after fracture reached 15.98%. The specimens' tensile fractures exhibit significant ductile characteristics when compared to those in the annealed state, with deeper and larger dimples and smaller dimples interspersed within the larger ones. The as-deposited and heat-treated SLMed TC4 samples show no texture overall. The dislocation distribution is primarily concentrated near the grain boundaries. After annealing, the grain size decreases whereas the dislocation density increases. After solution treatment, the α phase size increases, and dislocations are primarily distributed at the grain boundaries. After solution treatment followed by aging, the dislocation density increases and the dislocations become more evenly distributed (Fig. 12).

This study systematically investigates the formation process, microstructure, grain texture, dislocation density, tensile properties, and fracture morphology of SLMed TC4 alloys both in the as-deposited state and after various heat treatments. The results also clarified the microstructural evolution mechanism of SLMed TC4 samples subjected to different heat treatment processes. Dense SLMed TC4 was obtained with a laser power of 300 W, scanning speed of 1200 mm/s, scanning pitch of 0.1 mm, and layer thickness of 0.06 mm, but there were a few defects. Heat treatment did not significantly improve the samples' metallurgical quality. The SLMed TC4 titanium alloy specimens consist of coarse β columnar crystals. The interiors of the columnar crystals primarily consist of a large amount of fine α′ phase needle-shaped martensite with a small amount of β phase particles between the α strips. The average yield strength of the as-deposited specimens was 1080.00 MPa, tensile strength was 1238.75 MPa, and elongation after breakage was only 8.85%, indicating a mixed ductile-brittle fracture mode. The tensile and yield strengths of the annealed samples decreased, whereas their plasticity increased. The fracture mode changed to ductile fracture. After solid solution heat treatment, the specimens exhibit α phase coarsening and form short rods with a consistent overall arrangement. The transformation from the α phase to the β phase is incomplete, with some of the α phase coarsening. Tensile and yield strength decreased further, while plasticity increased. After solid solution aging treatment, the specimens exhibited a relatively uniform distribution of the (α+β) phase, with the α phase coarsening into large laths. The β phase was distributed around the α phase and has the highest toughness. There was no texture in either the as-deposited or heat-treated specimens. The dislocation distribution was primarily focused on the interfaces between the grains. After annealing, solid-solution treatment, and solid-solution aging treatment, the dislocation density gradually increased, and the distribution became more uniform.