**5. Conclusions**

This work investigated laser micro polishing (LμP) of tool steel 1.2379 (AISI D2) using a square intensity distribution in three different sizes (Q100, Q200, Q400) and laser fluences ranging from 3 to 12 J/cm2. A laser fluence of 3 J/cm<sup>2</sup> was below the melting threshold, while 12 J/cm<sup>2</sup> exceeded the evaporation threshold and led to significant material evaporation for all laser beam sizes. Additionally, the experimental results for LμP of 1.2379 led to some noteworthy insights:


Overall, it is concluded that an increase in laser beam size enables the use of higher pulse energies and larger average laser power, and it also leads to significantly higher area rates and less overall energy input. Therefore, besides an increase in pulse frequency, larger pulse energies and larger laser beam sizes are a viable way to significantly increase the area rate in LμP. For industrial applications, multi-kW laser systems with suitable pulse energies and pulse frequencies are widely available, which potentially enable area rates of several m<sup>2</sup> per minute in LμP.

**Author Contributions:** Conceptualization, A.T.; methodology, A.T., I.R.; software, A.T.; validation, A.T., M.E.K.; formal analysis, A.T., M.C.; investigation, M.C., M.E.K., S.-K.R.; resources, A.T.; data curation, M.C., A.T.; writing—original draft preparation, M.C., A.T.; writing—review and editing, A.T., M.C., S.-K.R.; visualization, A.T., I.R., M.E.K.; project administration, I.R., A.T.; funding acquisition, A.T., I.R.; supervision, A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Deutsche Forschungsgemeinschaft (DFG) in the collaborative research project "Lubricant-free tribology concepts for cold extrusion by interactionminimized surface layers and structures", under gran<sup>t</sup> number PO 591/37-3 within the priority program SPP 1676 "Dry Metal Forming".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on reasonable request from the corresponding author.

**Acknowledgments:** The authors would like to personally thank Konrad Wissenbach and Edgar Willenborg from the Fraunhofer Institute for Lasertechnology ILT (Aachen, Germany) for fruitful discussions and acknowledge their passionate drive to push laser polishing to new boundaries. Finally, we thank the anonymous referees for their effort and valuable comments.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
