**3. Results**

#### *3.1. Morphology Features, Relative Density and Defects*

The results of the geometric profile error comparison between the as-built samples and the as-designed 3D model are presented in Figure 3 and Table 4. As illustrated in Figure 3, the solid struts with small *L*s (0.4, 0.6, 0.8 mm) more easily gained a positive error in size. The higher *v* made this phenomenon more obvious. The solid struts with larger *L*s (>1 mm) had a more stable size. This was in agreemen<sup>t</sup> with the results shown in Table 4. Under di fferent *v* values of 1900, 1300, and 700 mm/s, the struts with an *L*s value of 0.4 mm had sizes of 412 ± 20, 465 ± 27, and 489 ± 34 μm, respectively, and the strut with an *L*s value of 1.4 mm had sizes of 1432 ± 15, 1421 ± 23, and 1430 ± 51 μm, respectively. For the overhanging struts with an angle of 0◦, the actual size was larger than that of the vertical struts. This was because the molten pool would usually generate many tumor forms on the bottom surface, owing to the permeation e ffect, causing an uneven geometric profile and over-dimension as a condition of low scan speed (Figure 5).

**Figure 3.** Size deviation distribution of the as-built samples compared to the as-designed 3D model.


**Table 4.** The average values of measured sizes of the solid struts with *L*s values of 0.4 and 1.4 mm (unit:μm, *n* = 10).

The relative density of the as-built samples is depicted in Figure 4. Under the process conditions of di fferent scan speeds, the struts presented inconsistent variation trends with increasing geometric characteristic size. On one hand, for the struts with small *L*s (0.4, 0.6, and 0.8 mm), the relative densities of the samples with a *v* of 1900 mm/s were highest, separately reaching 96.8%, 96.5%, and 96%. The relative density of samples with a *v* of 700 mm/s was lowest, only reaching 86%, 90%, and 93%. On the other hand, for the struts with an *L*s value of 1.4 mm, the relative density of the samples with a *v* of 1300 mm/s was highest, reaching 98.1%, and the relative density of samples with a *v* of 1900 mm/s was lowest at 95%. Thus, it could be seen that the relative density was sensitive to not only scan speed but also to the geometric characteristic size.

**Figure 4.** Arithmetic mean value of relative density of struts with different *L*s and *v* values.

In the thresholding CT images shown in Figure 5, the pore distribution of the as-built struts with different dimensions, angles, and scan speeds is roughly demonstrated. This result echoes the relative density, as described in Figure 4. It is apparent that the number of pores decreased with the increase in scan speed for the struts with an *L*s value of 0.4 mm, and the struts with an angle of 45◦ or 0◦ tended to generate pores. However, the struts with an *L*s value of 1.4 mm had the least pores in the condition of a *v* value of 1300 mm/s.

**Figure 5.** Thresholding single projection CT images of struts with *L*s values of 0.4 and 1.4 mm.

#### *3.2. Phase Identification and Microstructure*

Figure 6 shows the X-ray diffraction analysis performed to identify phases of the struts with *L*s values of 0.4 and 1.4 mm under different process conditions. As shown in Figure 6a, a brief observation of four struts within a wide 2θ range of 30◦ to 80◦ revealed that the LPBF-built Ti–6Al–4V samples' phase was mainly composed of α/α' phase. It is known that all of these phases are α' phases, due to the large cooling rate caused by laser melting and solidification [28]. For the struts with an *L*s value of 1.4 mm, when the scan speed increased, the diffraction peaks broadened considerably and the intensity decreased, which implied the formation of considerable refined crystal [29]. The XRD characterization with a small 2θ range of 39.5◦ to 41.5◦ is depicted in Figure 6b. It shows that the refined 2θ locations of peaks of struts with an *L*s value of 1.4 mm generally shifted to the higher 2θ with the increase in scan speed. In addition, under the process condition of a *v* of 700 mm/s, the peak of the strut with an *L*s value of 0.4 mm became thinner than that of the strut with an *L*s value of 1.4 mm. This means that the strut with an *L*s value of 0.4 mm had a coarser microstructure than did the strut with an *L*s value of 1.4 mm. The relatively small geometric characteristic size might have affected the heat dissipation of newly produced solid struts.

**Figure 6.** X-ray diffraction analysis results of as-built samples: (**a**) diffraction angle 2θ of 30◦–80◦; (**b**) diffraction angle 2θ of 39.5◦ to 41.5◦.

To further understand the microstructural differences, observation using an optical metallographic microscope and SEM was conducted on the initial microstructure of the X–Y planes perpendicular to the building direction, and the results are provided in Figures 7 and 8. Figure 7 reveals that the section microstructure was composed of very fine acicular martensites α' and primary columnar β grains with different shapes, which grew along the building direction. The interior of the primary columnar β grains mainly consisted of relatively coarse acicular martensites α' throughout the entire grain. This result is consistent with the findings of the XRD analysis in Figure 6a. Due to the optical metallographic microscope having relatively low magnification and the generation of multilevel martensites α' caused by the repetitive thermal loading of layer-wise laser melting, there was no obvious difference observed in the acicular martensites of struts with different process conditions. However, the primary columnar β grains became thicker and coarser with increasing scan speed. Moreover, as SEM images with 2000× magnification, shown in Figure 8, under the same condition of a *v* of 700 mm/s, the martensites α' in the strut with an *L*s value of 0.4 mm were obviously coarser than that of the struts with an *L*s value of 1.4 mm, on the whole.

**Figure 7.** Optical images of microstructures of solid struts on the X–Y plane: (**a**) *v* = 1900 mm/s, *L*s = 1.4 mm; (**b**) *v* = 1300 mm/s, *L*s = 1.4 mm; and (**c**) *v* = 700 mm/s, *L*s = 1.4 mm.

**Figure 8.** SEM images of the solid struts microstructure of martensites α' on the X–Y plane: (**a**) *v* = 700 mm/s, *L*s = 1.4 mm; (**b**) *v* = 700 mm/s, *L*s = 0.4 mm.
