## 4 Conclusions  
According to the results, the differences between laser engraving PBF and cold-rolled steel sheets seem to be negligible, or at most very small. The material removal rate, including drilling time and removed volume of material, was 0-10% higher for the PBF sheets. Generally, the holes drilled in PBF sheets were slightly bigger in diameter, and had a slightly higher tapering, up to 0.7 percentage points. The width of the BE on the material surface at the entry side was up to 0.24 mm smaller for the PBF sheets, which is a significant difference, since the drilled holes were only 0.6 mm in diameter. The difference in BE widths were attributed to the different surface topographies. From the magnified images of the PBF material surface, it can be easily seen that the surface of the material is very rough. The surface consists of small round particles with an average particle size of 32 \(\upmu\)m (100 measurements), which is very close to the raw powder average particle size of 40 \(\upmu\)m. The much higher effective surface area of PBF sheets likely dissipates heat more effectively, which results in a smaller BE. It should also help with the initial absorption of the laser beam, but in deep engraving this is not very significant.  
Some defects were found when analyzing the microstructure of the PBF sheet. Semicircular black traces were seen at the start and end points of the laser lines, as well as near the surfaces of the PBF material. These were found to be shallow recesses in the material. It is unclear whether they were produced by the PBF-building process, or by the grinding and polishing when the samples were prepared for the micrographs. A likely explanation would be that there are deviations in energy input at the start and end points of the laser lines that causes detrimental changes in the material structure, or causes some of the powder material to not properly melt. The nearly loose material that was not fully fused was then removed when the sample was prepared for analysis. Near the surfaces of the material, these circular defects are not semicircular anymore, but rather full circles of shallow recesses. It seems as if a few powder particles were fused together, but were not fully fused to the surrounding material. Thus, two types of visible defects were noticed in the PBF material compared with the cold-rolled material: (1) powder particles near the surfaces of the object that are not fully fused to the surrounding material, and (2) semicircular defects at the end and/or start points of the laser lines. However, it can be concluded that it is very unlikely that these had a significant effect on the laser engraving process, since the recesses are so small and sparsely located.  
This study was conducted in Finland as part of the research project \"Micro- and millistructured reactors for catalytic oxidation reactions\", abbreviated MICATOX. The financial support of The Academy of Finland and the participants of the project are gratefully acknowledged.  
[PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data and wrote the paper.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.