## 4 Conclusions  
In this work, the effects on the microstructural and mechanical properties of L-DED 17-4 PH single-track depositions induced by specifically designed heat treatment routes, comprising a solution step followed by quenching and subsequent aging, were investigated and compared to those caused by long-term aging treatments or by direct aging.  
Based on the results, the following conclusions can be drawn:  
* In the as-built condition, the single tracks exhibited a microstructure composed of a lathy martensitic matrix with a high amount of lathy \(\delta\)-ferrite, mainly located at the interface between the substrate and the depositions, with an average microhardness value of 392 \(\pm\) 17 HV0.05.
* Five different PFHTs were settled and successfully applied to the 17-4 PH single tracks. The direct aging strategy demonstrated its efficacy in improving the mechanical properties of the depositions, with increments up to 35% as compared to the as-built condition. The best results were obtained after direct aging conducted for 3 h at 480 \({}^{\circ}\)C.
* XRD analyses confirmed that distinguishing \(\delta\)-ferrite peaks from \(\alpha^{\prime}\)-martensite ones in the diffraction pattern of 17-4 PH steel is challenging due to the low C concentration and their similar crystalline structures. Irrespective of the heat treatment route, the \(\alpha^{\prime}\)-martensite peaks may encompass both phases, namely either \(\delta\)-ferrite or \(\alpha^{\prime}\)-martensite.
* TEM analyses performed on specimens subjected to long-time aging of 100 h were able to reveal the presence of Cu-rich nanometric particles and Nb carbides. In the directly aged specimens, the precipitation mechanism was different from that observed in the specimens subjected to solution heat treatment before aging. In the directly aged specimens, the Nb carbides were finer and closer to each other than those in the specimens solubilized prior to aging. These findings further justified the measured microhardness values.  
The understanding of the mechanism and sequence of precipitation of the strengthening particles in L-DED 17-4 PH depositions remains a challenge. A deeper investigation is ongoing, and the results will be compared to the ones discussed in this paper.  
Figure 10: Mean Vickers microhardness values (HV0.05) \(\pm\) standard deviation of all deposited regions.  
**Author Contributions:** Conceptualization, C.M. and M.M.; methodology, C.M., C.S., M.F. and M.M.; validation, C.M., C.S., M.F. and M.M.; investigation, C.M., M.F. and M.M.; resources, M.F. and M.M.; data curation, C.M. and M.M.; writing--original draft preparation, M.M. and C.M.; writing--review and editing, M.M., C.M., A.F., M.F. and C.S.; supervision, M.M.; project administration, M.M.; funding acquisition, M.M. and M.F. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the BiRex Corporation (Project AN-MEC--L'Additive Manufacturing nella filiera produttiva dell'industria meccanica: dallo sviluppo del processo alla definizione del business model per la produzione di nuovi componenti\", CUP C4I)20000030008).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The authors confirm that the processed data supporting the findings of this study are available within the article. The raw data not explicitly reported in the article are available on request.  
**Acknowledgments:** The authors wish to acknowledge [PERSON] for providing the samples analyzed in the present research. The authors also acknowledge [PERSON] for his support in the XRD analyses.  
**Conflicts of Interest:** The authors declare that this study received funding from BiRex Corporation. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.