*2.1. Materials*

Commercial Ti–6Al–4V alloy powder supplied by EOS GmbH was used in the experiment, meeting ISO 5832-3 and ASTM F1472. As an optimized medical material, the trace elements of Ti–6Al–4V ELI such as O, N, H, C, and Fe are relatively low in content (Table 1). As shown in Figure 1 (SEM, ΣIGMA, Zeiss, German), the powder has high sphericity and few satellite spheres with a particle size range of 15–53 μm.

**Figure 1.** (**a**) SEM micrograph and (**b**) particle size distribution of Ti–6Al–4V powder.

**Table 1.** Chemical composition of Ti–6Al–4V powder (wt. %).


## *2.2. Design and Fabrication*

Selective laser melting of a porous structure is characterized by a controllable and precise layer-wise material addition process. This method generates complex structures by selectively melting successive layers of metal powder using a focused and computer-controlled laser beam (Figure 2a). The specimens were fabricated using an LPBF machine (EOSINT M290; EOS GmbH, Munich, Germany), which was equipped with a Yb fiber laser of 400 W with a wavelength range of 1000–1100 nm and a Gaussian spot, and a building chamber filled with argon gas with an oxygen content below 200 ppm.

**Figure 2.** (**a**) Schematic of the laser powder bed fusion process; (**b**) the LPBF machine of EOS M290; (**c**) the selected scanning strategy; (**d**) the as-designed 3D model of solid struts; (**e**) the as-built Ti–6Al–4V solid struts; and (**f**) the as-built Ti–6Al–4V porous structures for the confirmatory experiment.

The volumetric energy density (*E*v) was calculated according to Equation (1) [22]:

$$E\_V = P/(\upsilon \times h \times d),\tag{1}$$

where *P* is the laser power, *h* is the hatching space, *v* is the scan speed, and *d* is the layer thickness. This equation takes the most important laser parameters into account and is suitable for calculating the thermal input during the LPBF process. An inside to outside scanning strategy with a meander hatch style was selected. The meander hatch direction rotated by an angle of 67◦ in the following layer (Figure 2c).

A porous structure is generally made up of struts with relatively small size and various angles. In this work, the sample model consisted of struts with angles of 0◦, 45◦, and 90◦, which had a square section with a side length (*L*s) of 0.4 to 1.4 mm (Figure 2d). Three sets of parameters were used in this fabrication process, resulting in different volumetric energy densities of 95.24, 51.28, and 35.09 J/mm<sup>3</sup> (Table 2 and Figure 2e). To verify the correctness of the analysis, two kinds of porous structures with as-designed *L*s values of 0.6 and 1.4 mm were fabricated via the LPBF process with *v* values of 700 and 1900 mm/s, respectively (Figure 2f).


**Table 2.** The selected process parameters for LPBF-fabrication.

#### *2.3. Measurements and Characterizations*

A micro-computed tomography (μCT) scanner (FF35 CT; YXLON International, Hamburg, Germany) with 5 μm resolution was used to scan the samples at 200 kV and 50 μA. The samples were rotated over 360◦ in steps of 0.18◦ during the acquisition. Two-dimensional (2D) projection images (*n* = 2000) were then collected. The 3D models of the fabricated samples were reconstructed through slice image data using commercially available software (VG Studio MAX 3.0; Volume Graphics GmbH, Heidelberg, Germany). The same software was also used to detect the size deviation distribution of the as-built samples compared to the 3D model with an error of 5 μm (*n* = 3). The sample section profile and defect distribution (pores) were then extracted from the CT data. After CT scanning, the struts with the same square were cut o ff together from the base plate via wire electrical discharge machining (WEDM). The relative density of struts with the same square section was measured using the Archimedes method on each set of samples. The Archimedes test results were calculated based on a combination of dry weighing and weighing in pure ethanol and on the theoretical density of 4.43 g/cm<sup>3</sup> for Ti–6Al–4V. Taking open porosity and the highly developed surface of the samples into consideration, the samples were coated with wax after the first dry weighing. The relative density ( relative) of the samples was calculated using Equation (2):

$$
\varrho\_{\text{relative}} = m\_1 / \left[ (m\_2 - m\_3) / \varrho\_{\text{ethanol}} - (m\_2 - m\_1) / \varrho\_{\text{max}} \right] \tag{2}
$$

where *m*1 is the mass of the sample without a wax coating in air, *m*2 is the mass of the sample with a wax coating in air, and *m*3 is the mass of the sample with a wax coating in pure ethanol. Due to the weight of the struts being too small, ten samples of each set were measured together, and the arithmetic mean value of relative density was calculated (*n* = 10). Taking the small size of the struts into consideration, nanoindentation tests on the polished sections of as-built struts with an angle of 90◦ were performed using a nanoindenter (G200, Agilent, Ltd., Santa Clara, CA, USA) to evaluate the mechanical properties, including the elastic modulus and nanohardness. A loading–unloading test mode was used with a maximum indentation depth of 2000 nm, a loading speed of 10 nm/s, and a hold time of 10 s (*n* = 5). The Oliver-Pharr method [27] was then applied to calculate the nanohardness and elastic modulus. The microstructure of LPBF-produced struts with an angle of 90◦ was observed using a field-emission scanning electron microscope (S-4800; Hitachi, Ltd., Tokyo, Japan) and a light microscope (GX41; Olympus, Ltd., Tokyo, Japan). To reveal the microstructure, an etchant containing 50 mL distilled water, 25 mL HNO3, and 5 mL HF was used for the polished samples. The di fference in phase composition was analyzed by X-ray di ffraction (XRD) using an X-ray di ffractometer (D/max 2500 PC, Rigaku, Ltd., Tokyo, Japan) with Cu K α radiation at 40 kV with a beam current of 100 mA. A scan speed of 2◦/min was used for the scan range of 30–80◦ in steps of 0.02◦. The porosity ( Φ) of the fabricated porous structure was obtained from Equation (3):

$$
\Phi = 1 - V\_{\text{porous}} / V\_{\text{bulk}} \tag{3}
$$

where *<sup>V</sup>*porous is the volume of the LPBF-produced structure measured from reconstructed 3D models using commercially available software (Magics 21.0.0; Materialise, Leuven, Belgium) and *V*bulk is the total volume of the solid cube that has the same outline size as the porous sample (*n* = 3). The main information of all the measuring methods is presented in Table 3.


**Table 3.** Measuring objectives and methods in this study.
