## 5 Conclusions  
The structure, morphology, and mechanical properties of two titanium alloys, produced by two different methods (milled and 3D printed by means of selective laser melting), were investigated and compared. A tensile test, a three-point bending test, nanoindentation, and atomic force microscopy were used for determination of the mechanical properties and surface roughness of the investigated titanium alloys. The results showed that the 3D-printed Ti-6 Al-4V sample has a higher indentation hardness, indentation modulus, tensile strength, flexural strength, and smaller roughness than the milled one.  
The structure and morphology of the samples were investigated by means of SEM and XRD. A significant difference in the size of the crystallites of the Al\({}_{0.15}\)Ti\({}_{0.85}\) main phase was observed; in the milled titanium sample, the crystallites were 37% larger than the crystallites of the printed titanium sample. For the secondary phases, approximately 20% larger crystallites were observed in the milled titanium sample. According to the Hall-Petch relationship [40], the smaller crystallite size of the 3D-printed sample could be one of the reasons for its higher indentation hardness.  
From a clinical perspective, the higher modulus of elasticity as well as tensile and flexural strengths of printed titanium make it suitable for the fabrication of larger prosthetic structures, especially implant-supported ones. Its smoother surface is an advantage when polishing, especially on areas in contact with soft tissues.  
When the Ti-6 Al-4V alloy is used to make orthodontic retainers, milled titanium is more suitable. It has a lower indentation hardness and indentation modulus, which would allow, with the same parameters as printed Ti, greater individual tooth mobility in the retention period of teeth fixed with a retainer. Its rougher surface is a disadvantage that can be removed with subsequent processing of the metal.  
Although extensive research was conducted using six types of experiments and statistical analyses, bearing in mind that some authors observed large residual stresses in the SLM-induced samples, we plan to conduct numerical simulations using the finite element method of the nanoindentation process in the future. Furthermore, in the future it would be useful to investigate the relaxation curve and strain relaxation of these materials, which were not the subject of the present study.  
Conceptualization, V.P. ([PERSON]); methodology, S.C. and V.P. ([PERSON]); formal analysis, S.C. and V.P. ([PERSON]); investigation, S.C., V.P. ([PERSON]), V.B. and S.S.; resources, V.P. ([PERSON]); data curation, V.B.; writing--original draft preparation, S.C.; writing--review and editing, V.P. ([PERSON]); visualization, S.S., V.B. and V.P. ([PERSON]); supervision, S.C. and V.P. ([PERSON]); project administration, S.C.; funding acquisition, S.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the European Regional Development Fund within the OP \"Science and Education for Smart Growth 2014-2020\", Project CoE \"National Center of Mechatronics and Clean Technologies\", \(N\) BG05M2 OP001-1.001-0008-C01.  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.