*3.1. Density and Microstructure*

The bulk density of the porosity-free, CP-wrought 17-4PH stainless steel was measured to be 7.75 g/cm<sup>3</sup> [22]. The density of the AM products shows generally a lower value compared to the CP metals. In conventional processes (CP), a substantial amount of plastic deformation is applied, which helps eliminate the porosity present in the initial cast ingot. In contrast, in the field of additive manufacturing (AM), the use of long pressureless sintering or hot isostatic pressing (HIP) can reduce porosity, but it is inevitable to have some remaining porosities in the structure. This can significantly reduce the deformability as well as the strength and toughness of the material. The overall density results of the produced samples can be seen in Table 3, where the measured density and the calculated relative density values were also determined. The results show that the porosity lies between 1.5 and 2.7%. This is close to the literature's data for sintering processes [23–25]. The applied heat treatments did not have any further effects on the density.


**Table 3.** Density of LPBF 17-4PH SS Charpy samples.

As depicted in Figure 5, the micrograph reveals the presence of two types of porosity within the microstructure. The majority of these voids exist on a microscale, with diameters less than 1–2 μm. However, there are a few voids that possess larger sizes but are less abundant. This type of porosity arises from inadequate melting during the 3D printing process, which will be further discussed in the analysis of fractured surfaces. Typically, the larger voids are observed along the outer rim of the specimen (Figure 5b). This phenomenon is well known in the realm of LPBF techniques as the process involves precise edge definition. With each layer, a closing contour is added, typically one (originally three). Although this contour addition is a machine-specific parameter and unavoidable, it does not significantly impact the mechanical properties.

**Figure 5.** Typical porosity morphology of the LPBF 3D-printed samples: (**a**) inside the sample; (**b**) close to the sample surface.
