## 3 Results and Discussion
### Microstructural Analysis of the Heat-Treated Specimens  
In Figure 4, the representative microstructures of the deposited tracks after the different PFHTs are depicted. In the figure, the representative microstructures of the same tracks in the as-built condition are also reported as a reference (Figure 4a). Figure 4: Representative OM micrographs of the microstructures of all specimens at the interface between the single tracks and the substrate: (**a**) as-built condition and after (**b**) HT1, (**c**) HT2, (**d**) HT3, (**e**) HT4, and (**f**) HT5 heat treatments. Figure 3: XRD pattern recorded on the single tracks in the as-built condition. As can be observed by comparing the micrographs of Figure 4a with those of Figure 4b,c, the SHT performed at 1040 \({}^{\circ}\)C for 30 min or for a shorter time (the HT1 and HT2 treatments, respectively) determined the dissolution of the lathy \(\delta\)-ferrite. Similarly, no lathy \(\delta\)-ferrite was detected in the specimens subjected to prolonged aging times of up to 100 h (HT4 treatment) (Figure 4e). It is worth noting that the efficacy of SHT in solubilizing the \(\delta\)-ferrite phase in the austenitic matrix at a high temperature was previously reported in [46]. In addition, during SHT, most of the alloying elements are expected to dissolve in the matrix, generating a super-saturated solid solution after quenching at room temperature. In terms of the strengthening mechanism, it is well-known that, during the subsequent aging treatment, nanometric Cu-rich particles or Nb carbides precipitate in the martensitic matrix [47; 48]. Conversely, in the HT3- and HT5-treated specimens (Figure 4d,f, respectively), which were subjected only to direct aging, the lathy \(\delta\)-ferrite was still detectable, as in the as-built condition. Considering the high cooling rate induced by the L-DED process, a quasi-saturated condition for almost all alloying elements is expected, making the alloy prone to the precipitation of strengthening particles during aging. Nevertheless, as already mentioned, the detection of CRPs or Nb carbides remains challenging. Figure 5 displays the X-ray diffraction spectra of the single tracks heat-treated according to the HT1, HT2, and HT3 routes, respectively. In all cases, peaks ascribed to the presence of \(\alpha^{\prime}\)-martensite and \(\gamma\)-austenite were identified. As already discussed in Section 3.1, previous studies [17; 37; 44] have confirmed that distinguishing \(\delta\)-ferrite peaks from \(\alpha^{\prime}\)-martensite in the XRD patterns of 17-4 PH steel is difficult due to the low C concentration and their similar crystalline structures. Thus, irrespective of the heat treatment, \(\alpha^{\prime}\)-martensite peaks may encompass both phases, namely either \(\delta\)-ferrite or \(\alpha^{\prime}\)-martensite. Moreover, it should be noted that a high-intensity [1 1 1] peak corresponding to the \(\gamma\)-austenite was evident. Some studies [25; 49] have suggested that the presence of austenite in additively manufactured samples could be the result of prolonged exposure to medium to high temperatures during heat treatment, promoting the retention or the post-formation of the reverted austenite. In addition, [PERSON] and [PERSON] proposed that long-time aging heat treatments in a non-protective atmosphere foster a high nitrogen presence, acting as an austenitic stabilizer and influencing the phase composition [38]. However, considering the narrow dimensions of the deposited tracks and the type of substrate adopted in the present study, the high-intensity [1 1 1] peak of \(\gamma\)-austenite may certainly be attributed to the austenitic matrix of the AISI 316L steel plate. No peaks related to the presence of nanometric Cu-rich particles or Nb carbides were detected in all the XRD spectra, due to their small size and quantity, together with their crystallographic parameters closely resembling those of the matrix [49; 50].