## 3 Results and Discussion
### Hardness  
The 3D-printed DMLS samples and bar after plastic working--the PW samples were prepared and analyzed to assess their mechanical properties. The results of the hardness measurements are presented in Figure 13.  
From the results obtained, it was observed that the printed samples (DMLS) in their initial state showed a higher hardness than the bar after plastic working the PW samples. Heat treatment and steam sterilization contributed to an increase in hardness in both cases (\(p<0.05\)) compared to samples in the initial state. The higher hardness of the DMLS samples (DMLS 1,2) in the initial state, compared to the bar after plastic working the samples (PW 1,2) may be due to a minimal difference in the chemical composition of the Ti6 Al4V alloy. The titanium powder used for 3D printing additionally contains the element Yttrium (Y), which is not present in PW samples [28; 33]. Figure 12: Examples of bar and 3D printing specimen surfaces: (**a**,**c**) before pitting corrosion testing, (**b**,**d**) after pitting corrosion testing. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample Name**} & \multicolumn{2}{c}{\(i_{cone}\)} & \multicolumn{2}{c}{\(E_{corr}\)} & \multicolumn{2}{c}{\(R_{p}\)} \\  & **[mA/cm\({}^{2}\)]** & & **[V]** & & **[k\(\Omega\)\(\cdot\)cm\({}^{2}\)]** \\ \cline{2-6}  & **Av.** & **SD** & **Av.** & **SD** & **Av.** & **SD** \\ \hline PW 1,2 & 275 & 20 & \(-\)0.17 & 0.07 & 120 & 20 \\ \hline PW 1,2,4 & 181 & 2 & \(-\)0.15 & 0.03 & 149 & 86 \\ \hline PW 1,2,3 & 365 & 148 & \(-\)0.05 & 0.01 & 235 & 2 \\ \hline PW 1,2,3 & 286 & 89 & \(-\)0.03 & 0.04 & 129 & 66 \\ \hline DMLS 1,2 & 238 & 22 & \(-\)0.133 & 0.05 & 81 & 7 \\ \hline DMLS 1,2,4 & 473 & 65 & \(-\)0.135 & 0.04 & 65 & 16 \\ \hline DMLS 1,2,3 & 162 & 45 & \(-\)0.120 & 0.02 & 66 & 43 \\ \hline DMLS 1,2,3,4 & 249 & 82 & \(-\)0.116 & 0.01 & 78 & 21 \\ \hline \end{tabular} Av.—average; SD—standard deviation.  
\end{table} 
Table 3: Pitting corrosion resistance results for samples from two groups. The Ti6 Al4V titanium alloy is currently one of the most widely used biomaterials for orthopedic implants. In recent years, the rapid development of 3D printing technology, particularly the SLM and DMSL methods, has significantly accelerated and optimized the manufacturing process of implants and medical devices [23; 24; 25; 29; 30; 31; 32; 23]. All samples were sandblasted before testing. Mechanical grinding and polishing, heat treatment (hardening), and steam sterilization were carried out to modify their surfaces. The sandblasted and mechanically polished samples were marked as baseline materials for analysis. Microstructure images of the Ti6 Al4V alloy after etching showed an \(\alpha\)-phase with equiaxial spherical grains and a \(\beta\)-phase at 1000\(\times\) magnification. This microstructure is consistent with the results presented by the authors [41; 42]. A two-phase lamellar structure was observed for the \"DMLS\" samples [33]. As regards the heat-treated samples, no significant change in microstructure was observed for the PW 1,2,3 and DMS 1,2,3 samples (sandblasted, mechanically polished and heat-treated samples). This may be due to the slow cooling of the furnace from 820 \({}^{\circ}\)C to room temperature (\(\pm\)20 \({}^{\circ}\)C), which most likely resulted in the restoration of the pre-hardening structure. Based on SEM microscopic observations, no significant differences were observed in the surface of the 3D-printed samples and the samples obtained from the bar after plastic working. This may be since the samples in their initial state had already undergone sandblasting and mechanical polishing. EDS analysis also showed no significant differences in the chemical composition between the materials being tested. In [26], the authors observed the presence of small spheres on the surface of the examined samples, which is most likely the result of incomplete melting of the Ti6 Al4V powder used for implant production.