## 2 Materials and Methods  
Gas-atomized 17-4 PH steel powders, provided by Bohler (Bohler-Uddeholm, Vienna, Austria), whose nominal chemical composition is reported in Table 1, were used to deposit single tracks onto a 120 \(\times\) 40 \(\times\) 10 mm\({}^{3}\) AISI 316L stainless steel plate. As declared by the supplier, the powders had a near-spherical shape with some rough agglomerated satellites and a few elongated particles, ranging from 45 to 90 \(\upmu\)m in diameter. A six-axis ABB IRB 4600 robot (ABB, Zurich, Switzerland), available at the Birex Competence Center (Bologna, Italy) and equipped with a v2.0 CLAMIR camera (CLAMIR, Madrid, Spain), was employed to fabricate the depositions. The optimal process parameters were chosen according to a preliminary investigation by the same authors [8]. Figure 1 depicts a scheme of the fabricated depositions, which consists of three 100 mm long single-track replicas (denoted as A, B, and C in Figure 1) being deposited with the selected process parameters on the AISI 316L substrate [8]. The microstructure of the deposited tracks in the as-built condition was characterized using a Leica DMi8 A (Leica, Wetzlar, Germany) optical microscope (OM) according to the transversal sectioning in Figure 1. One transversal specimen for each deposition was carefully subjected to metallographic preparation following the ASTM E3 standard, involving grinding with SiC papers ranging from 120 to 1200 \(\upmu\)m grit and subsequent polishing with diamond pastes ranging from 6 \(\upmu\)m to 1 \(\upmu\)m. After the metallographic preparation, chemical etching with Kalling's II (2g CuCl\({}_{2}\), 40 mL HCl in 40-80 mL ethanol) for 10 s was performed. Afterward, other transversal specimens were drawn from each single track and subjected to specifically settled PFHT routes. Three specimens, one from each single track, were then heat-treated according to five selected heat treatment routes, defined according to the outcomes of a preliminary investigation [8; 39] and collected in Table 2. The first heat treatment (HT1) was similar to the conventional H900 treatment for 17-4 PH steels and consisted of: (i) a heating ramp of the specimens inside the furnace from room temperature to 1040 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min; (ii) a solution treatment (SHT) performed at 1040 \({}^{\circ}\)C for 30 min, followed by water quenching; and (iii) final aging at 480 \({}^{\circ}\)C for 3 h. The \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Element** & **Si** & **Cr** & **Ni** & **Cu** & **Nb** & **Mn** & **P** & **S** & **C** & **Fe** \\ \hline Chemical composition (wt. \%) & 0.43 & 15.28 & 4.49 & 3.39 & 0.27 & 0.50 & 0.019 & 0.0003 & 0.039 & Balance \\ \hline \end{tabular}
\end{table} 
Table 1: Nominal chemical composition (wt. \(\%\)) of the 17-4 PH powders used for the single-track depositions. Figure 1: Scheme of the fabricated depositions: the yellow diamond indicates a transversal specimen drawn from one of the single tracks, whereas the yellow dotted line displays the sampling location. second heat treatment (HT2) differed from HT1 because no pre-heating was performed before SHT, i.e., the specimens were directly introduced into the oven at the settled SHT temperature of 1040 \({}^{\circ}\)C and held for 30 min. Conversely, the third heat treatment (HT3) was conducted providing for only direct aging carried out at 480 \({}^{\circ}\)C for 3 h, without a prior SHT step. In addition to these three heat treatment routes, long-time aging treatments were also established to explore the effect of aging time on the strengthening precipitation. The fourth heat treatment (HT4) was similar to HT1--involving a heating ramp of 10 \({}^{\circ}\)C/min, followed by SHT at 1040 \({}^{\circ}\)C for 30 min and subsequent water quenching--but with prolonging the aging duration up to 100 h. Lastly, the fifth heat treatment (HT5) was similar to HT3, but with providing for a direct aging duration of up to 100 h without SHT.