**2. Experimental Procedure**

SLM Ti-6Al-4V (the source of the Ti-6Al-4V powder particles is EOS GmbH (Electro-Optical Systems) was used in this study, for which the chemical composition and process parameters are shown in Tables 1 and 2, respectively. The test specimens were removed by the electrical discharge machining (EDM) by cutting the wire from the support, and no post-treatment was implemented before various tests. As-SLM Ti-6Al-4V is called AS in this study. The original test specimens were held for 4 h in a tubular furnace (Deng Yng, new Taipei City, Taiwan) in argon atmosphere at 400 ◦C, 600 ◦C, and 800 ◦C and subjected to air cooling and induced phase transformation. The test specimens were labeled 400-AC, 600-AC, and 800-AC, respectively.

The normal direction (ND) of the specimen was set as parallel to the laser direction, where the direction vertical to the laser direction was called the side direction (SD). After being polished with SiC paper (from #80 to #5000), Al2O3 aqueous solution (1 and 0.3 μm), and a 0.04 μm SiO2 polishing solution, the specimens were etched with Keller's reagen<sup>t</sup> (1 mL HF + 1.5 mL HCL + 2.5 mL HNO3 + 95 mL H2O) to examine the microstructure. The microstructure of the specimens was observed using optical microscopy (OM, OLYMPUS BX41M-LED, Tokyo, Japan), and X-ray diffractometry (XRD, Bruker AXS GmbH, Karlsruhe, Germany) was used for identification of the microstructural phases. Hardness measurements were valuated using the Rockwell hardness test (Mitutoyo, Kawasaki-shi, Japan). The measurement conditions for the HR test followed the C-scale (the indenting load 150 kg), and the mean value for five impressions was taken as the hardness of the corresponding condition.





The dimensions of the SLM Ti-6Al-4V tensile specimen are shown in Figure 1. The tensile test was performed with an universal testing machine (HT-8336, Hung Ta, Taichung, Taiwan), with the crosshead speed at 1 mm/min, which corresponded to the initial strain rate of 18.33 × 10−<sup>4</sup> s<sup>−</sup>1. The AS and the heat-treated different temperatures specimens were subjected to a room temperature tensile test to analyze the mechanical properties of SLM Ti-6Al-4V. There were at least three specimens for each test and the mean value of the test specimens was taken as the tensile results of the corresponding condition.

**Figure 1.** Dimensions of the selective laser melting (SLM) Ti-6Al-4V tensile specimen.

Regarding the high temperature tensile properties, titanium alloys are often applied in a high temperature environment of 250–400 ◦C [17]; therefore, tensile tests were carried out at 250 ◦C, 300 ◦C, 350 ◦C, and 400 ◦C to investigate the influence of temperature on the mechanical properties of SLM Ti-6Al-4V. There are at least three specimens for each test and the mean value of the test specimens was taken as the tensile results of the corresponding condition.

The equipment used in the erosion test is shown in Figure 2. Al2O3 particles were used, for which the average particle size was approximately 450 μm and scanning electron microscope (SEM) morphology is shown in Figure 3. The specimens were polished with #80 to #1000 SiC papers to remove the oxidized layer and soaked in acetone for ultrasonic cleaning before the erosion test. Then, 200 g of the erosion particles under a compressed air flow of 3 kg/cm<sup>2</sup> (0.29 MPa) were subjected to the erosion test [14,15], for which the impact angles were 15◦, 30◦, 45◦, 60◦, 75◦, and 90◦ to compare the erosion behavior of needle-like α' phase in the AS specimen and the plate-like α + β phase in 800-AC specimen. According to previous reports, the maximum erosion rate of the general ductile material takes place at about 20◦–30◦, but brittle materials, such as ceramics and glass, have maximum erosion rates at about 90◦ [14,15]. The erosion rate (ER% = δW/Wtotal particles) of a specimen is defined as its weight loss (δW) divided by the weight of the total erosion particles (Wtotal particles). Finally, optical microscopy and a scanning electron microscope (SEM, HITACHI SU-5000, HITACHI, Tokyo, Japan) were used to examine the surface and subsurface of the erosion specimens and to determine the erosion mechanism. All heat treatment conditions and measurements are shown in Table 3.


**Table 3.** Heat treatment condition and measurements.

**Figure 2.** The particle erosion test apparatus. ( **A**) Compressed air flow, (**B**) erosion particle supplier, ( **C**) erodent nozzle, ( **D**) specimen, and (**E**) specimen holder; θ = impact angle.

**Figure 3.** Scanning electron microscope (SEM) morphology of Al2O3 erosion particles.

## **3. Results and Discussion**

## *3.1. Microstructures and Phases*

Figure 4 shows the microstructures of the as-SLM Ti-6Al-4V subjected to heat treatment at different holding temperatures. Figure 4a shows the normal direction microstructure of AS, where it can be observed that a large number of needle-like phases are surrounded by a network profile with a diameter of approximately 80 μm. Figure 4b shows the same needle-like structure, therefore the AS is a needle-like structure as a whole. The normal direction and side direction microstructures were subjected to heat treatment at 400 ◦C for 4 h, as shown in Figure 4c,d, respectively. The microstructure of 400-AC is similar to that of AS, which is comprised of needle-like α' phases as a whole. Similar microstructures can also be seen at 600-AC, as shown in Figure 4e,f. In addition, white plate-like α phases were produced at 600 ◦C. Figure 4g,h shows that the microstructure of 800-AC is different from that of AS, 400-AC, and 600-AC. At 800 ◦C, a significant phase transformation occurred, and the α' phases disappeared and were substituted with continuous lamellar α + β phases comprising white α phases surrounded by black β phases.

**Figure 4.** *Cont*.

**Figure 4.** Microstructures in the (**a**) normal direction of as-SLM Ti-6Al-4V (AS), (**b**) side direction of AS, (**c**) normal direction of 400-AC, (**d**) side direction of 400-AC, (**e**) normal direction of 600-AC, (**f**) side direction of 600-AC, (**g**) normal direction of 800-AC, and (**h**) side direction of 800-AC.

The β peak at 600 ◦C and the presence of the α/α'and β peaks at 800 ◦C were confirmed with a XRD analysis, as shown in Figure 5. The XRD patterns are referenced from a previous report [18]. This indicates that when the heat treatment temperature increases up to 600 ◦C, the β-phase will be generated. According to a previous report [19,20], as shown in Figures 4 and 5, there is only an α' phase in AS. The equiaxed β phases were preferentially formed above the β-transform temperature during the cooling process. Because of the extremely high temperature gradient in quenching [21,22], the β phases were completely transformed into the needle-like martensitic α' phase. According to

the Xu et al. report [19], when the AS specimen was heated to 400 ◦C, the α phases were precipitated, where the phase transformation mechanism was α' → α' + α. When AS specimen is heated to 600 ◦C, the temperature is higher than the martensitic transformation temperature (575 ◦C), plate-like α phases precipitate around the α' phases, and a small amount of β phases precipitate on the α phase boundaries as hold time increases, where the mechanism of phase transformation is α' → α' + α + β. When AS specimen is heated to 800 ◦C, the α' phases completely disappear, and transformation to the continuous lamellar α + β phase occurs, for which the phase transformation mechanism is α' → α + β.

**Figure 5.** X-ray diffraction pattern of AS, 400-AC, 600-AC, and 800-AC specimens.
