**3. Results**

### *3.1. Initial Surface and Material Analysis*

The initial surface was measured by WLI, and characteristic sections were analyzed by fast Fourier transformation (FFT). The surfaces exhibit two kinds of marks, which are a result of the mechanical preparation process. The directions of the grinding marks are strongly heterogeneous but exhibit a clearly dominant orientation. Therefore, the profile analysis shows significant differences in the spatial frequency spectrum for horizontal or vertical sections through the initial surface. While Figure 4 focuses primarily on the macroroughness of the surface at spatial frequencies smaller than 50 mm<sup>−</sup>1, Figure 5 primarily highlights the micro-roughness up to spatial frequencies of 200 mm<sup>−</sup>1. Figure 4 gives a representative impression of the initial surface, while the results displayed in Figure 5 will be used for an in-depth analysis of changes in micro-roughness and their corresponding spatial frequencies and wavelengths, respectively.

**Figure 4.** (**a**) WLI image of initial surface roughness; (**b**) Profile of cross-section along horizontal white dashed line; (**c**) Onedimensional (1D) Fourier analysis of profile along cross-section; (**d**) Profile of longitudinal section along vertical white dashed line; (**e**) One-dimensional (1D) Fourier analysis of longitudinal section.

**Figure 5.** (**a**) WLI image of initial micro-roughness; (**b**) Profile of cross-section along horizontal white dashed line; (**c**) Onedimensional (1D) Fourier analysis of profile along cross-section; (**d**) Profile of longitudinal section along vertical white dashed line; (**e**) One-dimensional (1D) Fourier analysis of longitudinal section.

In addition to surface roughness, the initial microstructure and distribution of chemical elements at the surface was investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). In contrast to the experimental investigation, the surface was manually polished before SEM and EDX to ge<sup>t</sup> a clearer picture and less errors in measurement.

Figure 6 shows SEM images in different magnifications. While Figure 6a gives an overview of the overall distribution of chromium carbides (dark gray) in the steel matrix (light gray), Figure 6b provides a close-up of chromium carbides regarding sizes and geometrical shapes.

**Figure 6.** (**<sup>a</sup>**,**b**) SEM images of manually polished surface in two different magnifications to visualize size and distribution of chromium carbides (dark gray spots).

Firstly, the dark and light gray phases were characterized by EDX, and it was confirmed that the dark gray spots were chromium-enriched carbides (C39.55; Si 0; V3.69; Cr 31.83; Mn 0.39; Mo 1.04; Fe Bal.; in weight %), while the lighter gray areas were the steel matrix of AISI D2 containing less chromium (C 14.37; Si; 0.91; V 0.35; Cr 7.06; Mn 0.18; Mo 0.44; Fe Bal. in weight %). The size of the chromium carbides was primarily in the range from one to three micrometers, while the shape was predominantly ellipsoid and often spherical (Figure 6b). However, also, agglomerations of multiple chromium carbides as well as areas with a smaller density of chromium carbides were observed (Figure 6a). The size, shape, and distribution of chromium carbides are a characteristic feature of the powder metallurgical production process of 1.2379 (AISI D2). Therefore, this enhanced version of the material is typically referred to as 1.2379+ in comparison to 1.2379 (standard).

### *3.2. Surface Topography after Pulsed Laser Remelting*

The surface topography and thus surface roughness after laser polishing were investigated in a full factorial design for three different laser beam dimensions—Q100, Q200, and Q400—and ten different laser fluences ranging from 3 to 12 J/cm<sup>2</sup> in equidistant steps of 1 J/cm2. Figure 7 shows micrographs in a matrix form for selected laser fluences and for the laser beams Q100, Q200, and Q400. Figure 7a represents the initial surface topography since a fluence of 4 J/cm<sup>2</sup> at Q100 was not sufficient to introduce any topographical change in the initial surface.

**Figure 7.** Micrographs of surfaces after laser micro polishing using tools Q100 (**<sup>a</sup>**–**f**), Q200 (**g**–**l**), and Q400 (**<sup>m</sup>**–**<sup>r</sup>**) for laser fluences ranging from 4 to 12 J/cm2.

The smallest laser fluence investigated was *F* = 3 J/cm2, but this fluence was not sufficient for any of the laser beam dimensions to remelt the surface at least partially. Remelting starts at a fluence of approximately *F* = 4 J/cm<sup>2</sup> for Q200 and Q400, while a fluence of approximately *F* = 6 J/cm<sup>2</sup> is required for Q100. The micrographs for these fluences show that various small, circular craters were formed during the remelting process. In general, it is observed that surface topography is significantly different for the same fluence at different laser beam dimensions (e.g., Figure 7b,h,n). A purely topographical description shows that small craters are preferably observed for smaller laser beams and fluences (Figure 7b,g,m). Larger laser beams and higher fluences, on the other hand, tend to

produce dents, i.e., comparatively long-wavelength dimples or depressions (Figure 7j,o,p), which clearly differ in their topology from the craters at small beam sizes. At small fluences, basically only comparatively small, circular craters are visible in the laser-polished surface. A very large crater density was observed particularly at Q100 and 10 J/cm<sup>2</sup> (Figure 7e). Using larger beam sizes, surfaces with wavy depressions were formed. In particular, at large laser beam sizes and high fluences, surface ripples are formed in addition to the characteristic stripe texture (Figure 7q,r).

Additionally, visual process observation enables identifying certain characteristic process regimes and points toward different mechanisms of surface structure formation in the remelting process. Particularly, material ablation is a significant effect that is visible for all laser beam sizes at high fluences. However, the fluence required for material ablation strongly depends on the laser beam dimensions. Significant evaporation is visible for Q100 at approximately 12 J/cm2, in comparison to approximately 10 J/cm<sup>2</sup> at Q200, and approximately 9 J/cm<sup>2</sup> at Q400. Laser ablation often coincides with clearly visible, parallel stripes on the surface (Figure 7k,l,q,r). The distance between the stripes corresponds to the track offset and is typically an indication for a continuous remelting process. These characteristic stripes ge<sup>t</sup> only visible for Q100 at the highest fluence of 12 J/cm<sup>2</sup> (Figure 7f). For Q200, these stripes are already hardly observable at 8 J/cm<sup>2</sup> (Figure 7j) but ge<sup>t</sup> clearly visible at 10 J/cm<sup>2</sup> (Figure 7k). Qualitatively, the same is true for Q400, but for lower fluences. Remelting stripes ge<sup>t</sup> hardly visible at approximately 7 J/cm<sup>2</sup> (Figure 7o) but ge<sup>t</sup> more pronounced for fluences of approximately 8–10 J/cm<sup>2</sup> (Figure 7p,q).

In particular, the surfaces remelted at high laser fluence appear to radiate a greater gloss (Figure 7q,r), i.e., to be significantly smoother at the micro-roughness level. The observed evaporation might lead to a comparatively small micro-roughness at the same time.
