## 2 Materials and Methods
### Measured Properties  
Material removal rates, entry and exit hole diameters, hole tapers, and the widths of the burning effect (BE) on the surface, near the entry and exit holes, were measured and compared. Additionally,  
Figure 3: (**a**) Hatch pattern illustration for the engraving, not in scale. Red lines denote laser beam movement; (**b**) Photographs of the drilled sheets, drilled holes were 2 mm apart. Cold rolled sheet on top, PBF sheet on bottom.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**\#** & **Pulse Length, ns** & **Pulse Energy, mJ** & **PRR, kHz** & **Pulse Overlap, \%** & **Beam Speed, mm/s** & **Waveform** \\ \hline
1 & 8 & 0.100 & 200 & 75 & 2000 \\ \hline
2 & 14 & 0.160 & 125 & 60 & 2000 \\ \hline
3 & 20 & 0.190 & 105 & 52 & 2000 \\ \hline
4 & 30 & 0.235 & 85 & 50 & 1700 \\ \hline
5 & 50 & 0.333 & 60 & 50 & 1200 \\ \hline
6 & 100 & 0.500 & 40 & 50 & 800 \\ \hline
7 & 200 & 1.000 & 20 & 50 & 400 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Used laser parameters and waveforms for the experiments. PRR: pulse repetition rates.  
some microstructural analysis was carried out. Figure 4 shows where the hole diameter and BE measurements were taken, with example images taken from the drilled cold rolled sheet. For the purpose of microstructural analysis, the two sample sheets were ground and polished as close to the middle line as possible to show the cross-sections of the holes. Polished samples were then etched to show the microstructure of the material. A light microscope and a scanning electron microscope were used to take magnified images of the hole profiles.  
Eight measurements were taken of each drilled hole (see Figure 4a), and the average value was used for the hole diameter calculations. Three measurements were taken of the BE (see Figure 4b) where the width of the BE was determined visually from the magnified images, and the average value was used. It was reasonably clear from the magnified images where the base material began to change color to yellow, but since the measuring process was done manually, there is likely some error present. The amount of possible error was estimated by measuring the distance from 'where the color of base material has clearly changed' to 'where the color of base material has clearly not changed'. The error estimation was done for each BE measurement. Entry and exit holes were measured separately for both the hole diameter and BE. Hole tapers in degrees were calculated by a simple trigonometric equation:  
\[\text{Hole taper}=(180/\pi)\tan^{-1}\,(r_{1}\,-r_{2})/\text{h} \tag{1}\]  
where \(r_{1}\) and \(r_{2}\) are the radii of the entry and exit holes, and h is the thickness of the material.  
Material removal rate was calculated by the removed volume and measured drilling time. Since the drilled holes were shaped almost exactly like circular truncated cones, the volume was easy to calculate from knowing the entry and exit hole radii, and the thickness of the material. Exact material thicknesses were measured from the micrographs and used in the calculations. Since the cold rolled steel sheet was found to be slightly thicker, the exit hole diameters for the cold rolled samples were measured at a depth corresponding to the thickness of the PBF sample. Volume was then divided by the drilling time to get the material removal rate (MRR) in cubic millimeters per minute.  
Scanning electron microscope (SEM) images of both the PBF and cold-rolled steel samples were taken to examine any significant differences in the material microstructure. A Hitachi SU3500 scanning electron microscope was used for the imaging.  
Figure 4: Images showing where the measurements were taken. (**a**) Eight measurements for the hole diameter; (**b**) Three measurements for the burning effect (BE) width.