**1. Introduction**

Manual polishing of metallic surfaces is still a widespread and common manufacturing process, which primarily aims to significantly reduce the surface roughness. However, the polishing result often depends heavily on the condition and skill of the person who conducts the manual polishing. Additionally, the manual polishing process gets increasingly cost intensive and time consuming when the surface to be polished is not a plane but an arbitrarily formed surface with small surface features and small curvature radii. Laser polishing is a new manufacturing process with the potential to at least partially replace manual polishing for small and complex shaped surfaces [1]. This is underscored e.g., by the proven potential for full automation, by the flexible use of wear-free "polishing tools", and by the high spatial resolution of the laser polishing process [2]. Therefore, it is understandable that an increasing number of researchers and studies aim to strengthen the fundamental understanding of the laser polishing process.

The special research focus of this study lies on the surface roughness evolution after laser micro polishing (LμP) of cold working steel 1.2379 (DIN X153CrMoV12, AISI D2)

**Citation:** Temmler, A.; Cortina, M.; Ross, I.; Küpper, M.E.; Rittinghaus, S.-K. Laser Micro Polishing of Tool Steel 1.2379 (AISI D2): Influence of Intensity Distribution, Laser Beam Size, and Fluence on Surface Roughness and Area Rate. *Metals* **2021**, *11*, 1445. https://doi.org/ 10.3390/met11091445

Academic Editors: Sergey N. Grigoriev, Aleksander Lisiecki, Marina A. Volosova and Anna A. Okunkova

Received: 5 July 2021 Accepted: 7 September 2021 Published: 13 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

using square intensity distributions of different sizes. D2 is typically used for deep drawing tools since it contains high amounts of carbon and chromium. Adapted heat treatment cycles usually lead to the precipitation of M23C6 and M7C3 carbides within the martensitic steel matrix [3]. In conventional forming operations, this results in a high resistance against the adhesive and abrasive wear of deep drawing tools made from this material (Sing et al. [3]). Within a DFG priority program of the German Research Association SPP 1676 ("Dry metal forming"), the cold working steel was selected as one 'standard material'. Therefore, laser processing and its effects on surface topography, mechanical properties, and wear received broad attention and were investigated, e.g., for rotary swaging [4,5], cold massive forming [6], cold extrusion [7,8], or deep drawing [9–12].

Since laser polishing is a highly localized process including extraordinary temperature gradients of up to 10<sup>9</sup> K s<sup>−</sup>1, an intense interaction of radiation and material is typical for this energy beam-based process [13]. An introduction to the topic of energy beams for surface modifications including laser polishing was given by Deng et al. [14]. The review article of Krishnan and Fang [15] focuses more on the specifics of the laser-polishing process and presents a decent collection of relevant studies and works on laser polishing. Although already a few years old, Bordatchev et al. [16] also delivers a systematic evaluation of achieved laser polishing results for a wide range of metals. Bhaduri et al. [17] lists some key publications on laser polishing (using cw and pulsed laser radiation) for various materials with high relevance in industrial applications. Concerning specifically LμP, the studies of Temmler et al. [18,19] include a more detailed introduction to the specifics and characteristics of LμP for metals. A more general introduction on the interaction of pulsed laser radiation and material was recently compiled by Li and Guan [20] and gives a good overview on theoretical fundamentals, which are also partially relevant for pulsed laser remelting. However, LμP typically aims not to work in the ablation regime, so that a reduction of surface roughness is achieved by the redistribution of molten material [21]. Thus, the melt duration is decisive for the effective reduction of spatial frequencies or wavelengths in LμP [22]. Kuisat et al. [8] found even for the direct laser interference patterning (DLIP on Ti64 and Scalmalloy©) process using ns laser pulses that this smoothing effect is inherent to the remelting process and occurs simultaneously to the DLIP process. This is similar to the WaveShape process, which achieves structuring and polishing in one process step [23] or even utilizes a spatially adapted laser power modulation for the reduction of waviness on a surface (Oreshkin et al. [24]). Overall, the laser polishing process can be seen as a spatial low-pass filtering of a surface [25,26], resulting in an effective reduction of surface roughness. Therefore, the reduction of surface roughness in LμP is specifically pronounced for the micro-roughness, while the waviness of the surface stays typically unaffected [27]. Thus, special tools for surface roughness analysis are usually required such as spatial frequency [28] or spatial wavelength analysis [27] based on an adapted fast Fourier transformation. The longest spatial wavelength or smallest spatial frequency that is effectively reduced by LμP is referred to as critical wavelength [29] and critical frequency [30], respectively. Nonetheless, a fundamental and complete understanding of the specific roughness evolution for an arbitrary material is still not in sight.

In addition to surface roughness, specifically, the mechanical properties after LμP have been the focus of many studies. A detailed and extensive study on material properties after laser remelting (continuous wave (cw) and pulsed) of tool steel was presented by Temmler et al. [27]. Guan et al. [31] investigated the effect of pulse duration and heat transfer for laser pulses in the microsecond domain on Mg alloy AZ91D and found that discrete laser remelting occurs with a characteristic, homogeneous microstructure. In addition to surface structuring, Ma et al. [32] showed for additively manufactured Ti alloys that laser polishing is an effective method to reduce surface roughness, increase gloss, and enhance microhardness. Morrow and Pfefferkorn [13] found local hardness variations due to a heterogenic microstructure after pulsed laser remelting on tool steel S7. In the context of surface hardening in pulsed surface treatment, Maharjan et al. [33] compared the hardness

of 50CrMo4 steel after laser treatment using fs, ps, ns, ms, and cw laser radiation and found that the most pronounced hardening effect was achieved for longer pulse duration and cw laser radiation. Furthermore, alone, the change in microstructure leads to characteristic surface features resulting from the track overlap of the scanning strategy. Both Morrow and Pfefferkorn [13] as well as Ma et al. [33] found a characteristic backtempering effect from overlapping laser pulses due to carbon diffusion [27]. Li et al. [34] found through a thermal history analysis of the laser-polishing process that martensitic phase formation in the remelted layer leads to significant effects on fatigue, residual stress, and strength. Temmler et al. [27] found particularly high residual stresses after LμP of tool steel H11 in an Argon environment, while Bhaduri et al. [35] found high tensile stresses after LμP of AlSi10Mg parts in an atmospheric environment. Liang et al. [36] found for laser polishing of additively manufactured Ti6Al4V an increase in hardness due to reduced porosity, an improved cycle fatigue life, and an improved cell biocompatibility.

Although different approaches for theoretical and FEM models of the LμP process exist [37] (e.g., by Mai et al. [38], Ukar et al. [39], Chow et al. [40], Vadali et al. [25], Ma et al. [41], or Richter et al. [42]), a precise prediction of surface roughness is only possible for a few selected materials and a narrow range of process parameters [25]. A reason for this is the complex interactions between laser beam and material based on their multiple properties, which makes the integration of all aspects into a complete and coherent model a very challenging task [37]. Particularly, process-inherent surface structure formation, resulting from specific material properties or from the selected process parameters and processing strategy as described by Nüsser et al. [43], is not considered in any model. However, these process-inherent surface features often remain on the surface and lead to a considerable surface roughness after laser polishing.

Additionally, the specific shape of the intensity distribution (ID) of a laser beam might be important for a laser-based process. The effects of the laser beam intensity distribution for laser welding were discussed by e.g., Kaplan [44]. If possible, the ID should be specifically tailored to the requirements of the laser process, the material, and its application, so that an adapted time-dependent temperature profile can be created (Völl et al. [45]). With regard to squared IDs in laser-based processing, Khare et al. [46] came to the conclusion that squared IDs enable higher process speeds and reduce the risk of centerline solidification cracking. Furthermore, in comparison to circular laser beams, the squared ID leads to changes in the microstructural growth, which is dominantly axial instead of columnar (Kou [47]). More specifically regarding laser polishing, Nüsser et al. [48] found that a squared ID might be beneficial for LμP of Ti6Al4V.

In this context, this study contributes to expand and strengthen the empirical data basis for laser micro polishing of high-carbon steels. This study provides an in-depth analysis of an LμP polished surfaces of 1.2379+ tool steel and demonstrates the transition from a discrete, pulsed to continuous remelting process for high fluences. Additionally, it provides a systematical analysis by directional 1D FFT analyses and shows different smoothing behaviors in the x and y directions. Furthermore, this study investigates a correlation between carbides and the formation of craters in pulsed laser processing. Finally, this study contains results and discussion regarding achievable area rates in LμP, and regarding a correlation of micro-hardness evolution and process characteristics in laser polishing (LμP or cw laser polishing). Overall, the material 1.2379+ poses multiple challenges for laser polishing, which were systematically identified, and potential mechanisms were discussed. Therefore, this work joins the steadily increasing list of experimental studies on laser polishing over the past decade.
