*3.3. Tensile Properties*

The typical tensile curves for the three different heat-treated conditions of the investigated material are depicted in Figure 8 and Table 4. It was determined that the heat treatment had a notable impact on the plastic deformation behavior of the material, as evidenced by the shape of the curves. In the case of the as-printed sample, a slight strain hardening effect can be observed between the yield strength (YS) and ultimate tensile strength (UTS). This curve exhibits the highest level of uniform plastic deformation and the lowest stress values. Following the UTS, a short period of non-uniform plastic deformation, known as necking, can be observed. Conversely, the annealed sample displays a different pattern. While the strain hardening period is short, the necking period is longer. The curve for the H900 sample exhibits a distinct curvature. After yielding, the tensile stress shows a sustained elongation phase. No further necking is observed after the UTS, and the sample fractures rapidly. The trend observed in the hardness and toughness values is also

reflected in the tensile properties. The strength of the samples increases as a result of the heat treatment, while the plastic deformability experiences a significant decrease.

**Figure 8.** Tensile curves of 17-4ph LPBF samples [31,32].


**Table 4.** Tensile properties of LPBF specimens in various conditions.

The values for yield strength (YS), ultimate tensile strength (UTS) and strain at break obtained from the tensile curves, along with the H900 value, are presented in Figure 9. The figure also includes data from the literature on CP- and AM-produced samples in different aged states. By comparing the results of the CP and AM alloy, as depicted in the diagram, it can be observed that the highest UTS can be achieved by the H900 treatment (1309 MPa), which is nearly equivalent to that of traditional manufacturing methods (1310 MPa). Generally, AM processes are often promoted as post-production free processes, but the as-built condition lacks the necessary mechanical properties for practical use. A comparison was made with similar results reported in the literature. In their respective studies, H.R. Lashgari et al. [33] and F.R. Andreacola et al. [14] applied heat treatment to their specimens, resulting in UTS values close to 1300 MPa, while the as-printed mechanical strength was approximately 900 MPa.

Figure 10 provides optical microscopy (OM) and scanning electron microscopy (SEM) micrographs of the fracture surfaces of the tensile specimens. Upon comparing the fracture surfaces, it becomes evident that the fracture mechanism differs. It is apparent that all the fractured surfaces exhibit a notable presence of porosity accompanied by unmelted powder. This is clearly visible in Figure 10d, although a similar amount can be observed on all other investigated surfaces. As indicated by the tensile curves, the curves corresponding to the as-printed and H900 aged samples display a short period of necking in comparison to the curve of the annealed sample. A short necking period implies that the fracture occurs without significant plastic deformation. This can be observed in Figure 10a,c,d,f, where cleavage fracture surfaces are depicted. Conversely, the tensile curve of the annealed sample exhibits a prolonged necking period. This substantial amount of plastic deformation is visible on the ductile-type fractured surfaces shown in Figure 10b,e.

**Figure 9.** Tensile properties of the different values reported in the literature. CP parts are marked with blue, while AM parts are marked with red [27,29,34–38].

**Figure 10.** Fractured surfaces of (**a**,**d**) as-printed, (**b**,**e**) annealed and (**c**,**f**) H900 tensile specimens.
