**4. Summary**

In this study, different mixtures of coarse (180 μm APS) and fine (3 μm APS) Cp-Ti 2 powders were used for the production of a pure Ti material. The fine powder was milled from the initial coarse powder by use of high-energy planetary ball milling.

During the mixing process, the finely milled flakes were trapped and filled the pores of the coarse spongy particles and covered their surface. Cylindrical samples were produced from the different mixtures via uniaxial cold pressing. In order to minimize the porosity of the green samples, the various feedstock mixtures (0–10 wt.% fine powder) were coldpressed at an extremely high uniaxial pressure (1.6 GPa).

The relative density of the cold-pressed samples varied between 97% and 94% depending on the proportion of fine particles. These density values are particularly high compared to literature data (52–69%), which is due to the good deformability of the rough and soft titanium sponge, which accounts for 90–98 wt.% of the produced samples.

This high green density allows the use of low-temperature and rapid sintering (800–950 ◦C, 60 min in 6.0 argon atmosphere), as long-distance atomic movement is not required during the densification process. The relative density of the sintered samples varied between 95.6% and 97.8%, which is comparable with corresponding results from the literature (93.5 to 97.5%).

Due to the applied sintering conditions, the coarsening of the initial dual-scale structure has not occurred, and the comparable hardness (max. 350 HB) and compressive properties (max. 1492 MPa YS) of the Ti6Al4V alloy were measured. The production parameters of the optimal strength–strain version of the alloy were 6 wt.% amount of milled Ti and 850 ◦C sintering temperature. It is possible that the inhomogeneous microstructure with large and fine features could contribute to differences between the compression and tensile test results. The presence of these features could cause variations in stress distribution and lead to localized deformation or failure during testing. Additionally, the presence of large features could affect the sample geometry and cause stress concentrations, which could also influence the test results. This constitutes a matter for further research.

In summary, a quasi-pure Ti material was produced with comparable to the Ti6Al4V alloy properties by use of a production method which is cheaper, more energy efficient, and consequently more environmentally friendly than conventional techniques. The latter factor is due to the low temperature and the rapid sintering applied. Moreover, the produced Ti parts are more easily recyclable due to the absence of alloying elements. The latter advantages make our material a potential candidate for consideration in aerospace applications requiring high strength, as the above characteristics contribute to the goals and aims of sustainable and circular aviation [48,49].

A complete LCA and LCC analysis of the production route could be the subject of further research in order to quantify the exact environmental and cost gains of the whole process compared to conventional production processes. Moreover, further research will be needed, including tensile and fatigue tests, to address the complete mechanical characterization of novel materials. The measuring of the effect of contamination by, for example, iron or oxygen should be part of a future research. Finally, further optimization of the production process will probably boost and lead to more uniform properties.

**Author Contributions:** Conceptualization, T.M., Z.G. and D.M.; methodology, T.M.; software, T.M.; validation, T.M. and D.P.; formal analysis, D.P. and G.G.; investigation, T.M.; resources, Z.G. and G.G.; data curation, T.M.; writing—original draft preparation, T.M., D.M. and D.P.; writing—review and editing, T.M., D.M., D.P. and Z.G.; visualization, T.M. and Z.G.; supervision, Z.G.; project administration, G.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the UMA<sup>3</sup> project, funded by the European Union's Horizon 2020 research and innovation program under grant agreement No. 952463.

**Conflicts of Interest:** The authors declare no conflict of interest.
