*2.3. Material and Sample Preparation*

Particularly in the fields of solid forming, as well as cutting and punching tools, tool steel 1.2379 (DIN X153CrMoV12, AISI D2) is widely used, since it combines dimensional stability, high wear resistance, and toughness. Furthermore, 1.2379 (D2) is a recommended material for e.g., punches, ejectors, and tool dies [13]. Dörrenberg Edelstahl GmbH (Engelskirchen-Ründeroth, Germany) provided a special variant of 1.2379, which was powder metallurgically (PM) produced and used for all experiments. A special characteristic of the PM variant is a low content of impurities such as sulfur and phosphorus, a high homogeneity in the distribution of chemical elements, and a segregation-free microstructure. In terms of suitability for laser polishing, Ross et al. [8] found that powder metallurgically remelted 1.2379+ is preferable to the standard variant since chromium carbides are more homogenously distributed within the bulk material, on average smaller, and mostly spherically formed.

An overview of the chemical composition (wt %) of D2 is shown in Table 2.


**Table 2.** Tabular overview of chemical composition for AISI D2 (in wt %).

1 Based on supplier information.

The dimensions of the samples were approximately 50 × 75 × 15 mm3, while the initial heat treatment state was soft annealed. The surface of the samples was ground and showed an initial roughness of *Ra* = 0.33 ± 0.02 μm. Both sides of the flat samples were prepared in the same way and used for the experimental investigations.

### *2.4. Process Principle, Scan Strategy and Process Parameters*

Laser remelting of a surface shows a high degree of similarity to a micro-welding process with the exception that no additional material is required in the remelting process. Laser remelting not only requires no additional materials, but it is also non-subtractive, which means that no material is lost during the process. In contrast to additive and subtractive processes, laser remelting redistributes the material at the material's surface while it is molten. Typically, capillary, thermo-capillary, and gravitational forces lead to a smoothing of the surface. Particularly, the surface tension leads to a smooth melt pool surface and effective damping of capillary surface waves. Laser micro polishing (LμP) is a specific variant of laser remelting in which pulsed laser radiation is used to remelt a surface. (Figure 3a). The combination of pulse frequency and scan speed determines the pulse distance on the work piece. LμP is typically characterized as a discrete remelting process, which means that the melt duration is shorter than the temporal distance between laser pulses (reciprocal pulse frequency). This usually results in melt pool dimensions that show a high ratio of width to depth [21,22]. Areal processing is typically achieved by a meandering scanning strategy with a defined track offset between antiparallel remelting tracks (Figure 3b).

**Figure 3.** (**a**) Schematic of the process principle of laser micro polishing (LμP), and (**b**) schematic of a standard scanning pattern for areal laser processing.

Laser micro polishing utilizes pulsed laser radiation and was conducted using a laser beam source from Trumpf (TruMicro 7051, Ditzingen, Germany). Three similar investigations were conducted using different square laser beam sizes Q100, Q200, and Q400 and compared among each other. The experimental approach was based on an investigation on the influence of laser fluence on surface topography and surface roughness. An overview, a short description of the investigated process parameters, and the range of investigation is given in Table 3.


**Table 3.** Overview of relevant process parameters and range of investigation.

The process parameters for the different laser beam dimensions were chosen based on the following considerations. The pulse frequency of 20 kHz was the maximum available at the laser beam source. Scanning velocity was adapted to the laser beam sizes and repetitions rate, so that the spot overlap (*dL*/*dx*) in the scanning direction was 90% for all laser beams. Track offset was determined in the same way, so that track overlap was also 90%. This results in approximately 100 remelting cycles per irradiated area and strongly different area rates (polished area per unit time). Fluence is typically a decisive process parameter in LμP and was investigated in the same range for all laser beam dimensions. Therefore, the investigated range of pulse energy (and average laser power) was adapted to the laser beam dimensions. For all experiments, the inclination angle was *β* = 0◦, and the shielding gas atmosphere was a mixture of Argon and 1000 ppm residual oxygen from the ambient air in the process chamber.
