**2. Materials and Methods**

The selection of process parameters in our study was a result of a comprehensive approach. Initially, we extensively reviewed the existing literature in the field to identify commonly used and well-established process parameters employed in similar studies. These parameters served as a foundation for our research. Furthermore, we conducted preliminary experiments to evaluate the performance and feasibility of different process parameters. Via these experiments, we assessed their impact on the desired outcomes and determined the most appropriate range for our specific experimental setup. Practical constraints, such as equipment limitations, were also taken into account during the finalization of parameter selection. While we recognize that there are numerous process parameters that could potentially influence the outcomes, we carefully chose a subset that we believed would yield meaningful insights within the scope of our research objective.

The base material used for this research work was water-atomized 17-4PH stainless steel powder supplied by Oerlikon. The average particle size was 40 μm, which is widely used in AM and approved by both the German Federal Office of Civil Aeronautics and the German Federal Office of Defence Technology and Procurement. The morphology of the powder is mainly spherical, which is favorable for most additive manufacturing processes due to its excellent flowability and its high apparent density. However, as shown in Figure 1, the powder contained some particles with an irregular shape, which reduced the flowability and the apparent density of the powder bed. Furthermore, this can negatively affect the compactness of the 3D-printed parts. Yet, these particles are similar in shapes and sizes to those observed in the work of P. Ponnusamy et al. [16], in which the same alloy was considered. The official certificate (specified in the datasheet of the powder alloy) and the

measured (ICP or inductively coupled plasma method) chemical composition of the initial powder are shown in Table 2.

**Figure 1.** Morphology of 17-4 PH SS powder at 500×magnification.

**Table 2.** Chemical composition of 17-4 PH stainless steel powder (wt. %).


The LPBF machine used in this experiment was an Orlas–Creator SLM machine, using an Yb fiber, a 1070 nm wavelength laser as an energy source, nitrogen shield gas and a ~40 μm average accuracy. The printing strategy is a scan of parallel lines with a 45◦ clockwise rotation for each deposited layer, thus creating a crosshatch pattern, as depicted in Figure 2. This strategy is typically used in 3D printing [16,17] to improve the strength and quality of printed parts.

**Figure 2.** Crosshatch pattern achieved by 45-degree-clockwise rotation: (**a**) first deposited layer; (**b**) second deposited layer; (**c**) third deposited layer.

The selected pattern improves the adhesion between the layers and prevents delamination. Delamination is a serious production flaw that occurs when there is improper bonding between successive layers, leading to permanent deformation due to residual stresses. In order to execute this approach, it is necessary to adjust the settings of the 3D printer software to position the heat source at a 45-degree angle relative to the previous scanning direction. This configuration involves a complete 360-degree rotation every eight consecutive layers. This method enables the machine to maintain control over the desired structural integrity, resulting in improved mechanical properties and a more seamless layer construction. Additionally, the powder feed was supplied using a rubber coater, which moved in counterclockwise circular motions. Tensile specimens in the shape of rectangular dog bones and Charpy specimens were printed, adhering to the dimensions depicted in Figure 3. In the case of the Charpy specimens, the V notch was machined after the printing.

**Figure 3.** Dimensions and shapes of the printed (**a**) and tensile (**b**) Charpy specimen.

All printed samples were made on a horizontal (0◦) orientation as it has been proven to be the favorable orientation for this type of testing [18]. Six as-printed tensile specimens and Charpy specimens were solution-heat-treated (annealed) at 1038 ◦C for 30 min in argon atmosphere and then quenched with water. Three of the annealed specimens were subsequently aged at 480 ◦C for 60 min in argon atmosphere. The heating rate was 10 ◦C/min in both heat treatment processes. The density of the samples was determined by the Archimedes method. The hardness of the samples was measured using the Vickers method (HV10) [19] on a Wolpert UH930. Instron 5982 equipment was utilized for the tensile tests at room temperature. Tensile tests were conducted according to ASTM E8/E8M standard [20]. Charpy tests were conducted according to ASTM E23-16b [21], using a Schenck-Trebel of 300 J capacity. Three tests were carried out on each condition. The strain rate was 3 mm/min. The microstructure and the broken surface of the tested samples were investigated using optical microscopy (OM) and scanning electron microscopy (SEM) with C. Zeiss Axio Image and a C. Zeiss EVO MA 10 equipment, respectively. The experimental flowchart of the additively manufactured 17–4PH stainless steel is shown in Figure 4.

**Figure 4.** Experimental procedure conducted on the (AM)17-4PH stainless steel tensile test specimens and Charpy specimens. The arrows represent the life cycle of each test piece.
