## 3 Results
### AFM Results  
The topography and surface roughness of the samples were investigated using an atomic force microscope. AFM's high-resolution imaging capability enables the precise location of indentation positions and their depth measurement [31]. The applied roughness analysis of each AFM image uses R\({}_{\text{a}}\), R\({}_{\text{q}}\), and R\({}_{\text{max}}\) values calculated according to the relative heights of each pixel of the topography of the studied sample [32].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No** & **Material** & **G\({}^{\text{f}}\)** & **E\({}^{\text{f}}\)** \\  & & **[GPa]** & **[GPa]** \\ \hline
1.1 & Milled Ti & 1639 & 97 \\
1.2 & Milled Ti & 1719 & 99 \\
2.1 & 3D-printed Ti-6 Al-4V & 1997 & 78 \\ \hline \hline \end{tabular} \(\text{G}^{\text{f}}\)—the maximum bending stress; E\({}^{\text{f}}\)—flexural modulus of elasticity.  
\end{table}
Table 4: Flexural strength (classical method) of the tested materials.  
Figure 16: Side view in the area of the realized maximum tensile–compressive stress of 3D-printed Ti-6 Al-4V specimens at different indenter movement speeds: (a) 1 mm/min/\(\times\)35/; (b) 60 mm/min/\(\times\)35/.  
Figure 15: Frontal view in the region of the realized maximum tensile stress of milled Ti specimens at different indenter movement speeds: (a) 1 mm/min/\(\times\)40/; (b) 60 mm/min/\(\times\)40/.  
The roughness analysis gives the value \(\rm{R_{a}}\) which is the arithmetic mean of the absolute values \(\rm{Z_{j}}\) of the surface height deviations measured from the mean plane  
\[\rm{R_{a}}=\frac{1}{n}\rm{\sum_{j=1}^{N}}\rm{|Z_{j}|} \tag{1}\]  
while \(\rm{R_{q}}\) is the root-mean-square value of the height deviations taken from the plane of the average image data [28].  
\[\rm{R_{q}}=\sqrt{\frac{\rm{\sum_{j=1}^{N}}z_{j}^{2}}{N}} \tag{2}\]  
The AFM images of milled and 3D-printed Ti-6 Al-4V samples were compared and presented in Figure 17. The morphology of the milled Ti-6 Al-4V sample with a scanning area of 10 \(\times\) 10 \(\rm{\upmu m^{2}}\) (Figure 17a) reveals a rougher surface in comparison with the morphology of the 3D-printed Ti-6 Al-4V sample with the same scan area (Figure 17b), which is smoother with a homogeneous structure. The measured roughness of the milled Ti-6 Al-4V sample for \(\rm{R_{q}}\) is 15.2 nm, and for the value of \(\rm{R_{a}}\) it is 12.0 nm. On the other hand, the roughness of the 3D-printed Ti-6 Al-4V sample for \(\rm{R_{q}}\) is 10.9 nm, and for the value of \(\rm{R_{a}}\) it is 8.65 nm, respectively.  
For a better evaluation of the morphology of the two samples, 2D and 3D images of milled Ti-6 Al-4V and 3D-printed Ti-6 Al-4V samples were compared with a scanning area of 5 \(\times\) 5 \(\rm{\upmu m^{2}}\) (Figure 18). The images show that the 3D-printed Ti-6 Al-4V sample has a smooth surface with the presence of small nanometric spheres. The diameter of these  
Figure 17: AFM topography of the samples: milled Ti-6 Al-4V (**a**) and 3D-printed Ti-6 Al-4V (**b**): 2D images and section analysis.  
spheres is from 10 nm to 25 nm. The milled Ti-6 Al-4V sample, on the other hand, has a rough surface with larger and smaller clusters.