# Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing
## 3 Results and Discussion
### Moving Experiment: High-Speed Imaging
#### 3.2.1 Experimental Results  
Some typical examples of experiments with different velocities and directions are given in Figure 13a-c. The corresponding high-speed movie can be seen at: [[https://www.empa.ch/web/s204/modellinglaserpolishing1](https://www.empa.ch/web/s204/modellinglaserpolishing1)]([https://www.empa.ch/web/s204/modellinglaserpolishing1](https://www.empa.ch/web/s204/modellinglaserpolishing1)) (accessed on 8 July 2022). They represent values in the range often encountered for LP of rough workpieces. The first line of images is representative only of the first line in real process whereas the return line is representative of the rest of LP where the surface to be treated is scanned with a given overlap between the process lines. In Figure 13a, the melt pool for a first line at 35 mm/s is clearly visible in the center of the image (in dark). It is surrounded by the EDM surface that again consists of a succession of bright and dark regions due to the high roughness of the workpiece. On the left-hand side of the melt pool is the re-solidified line. This line appears much smoother as compared to the EDM surface and again shows the positive polishing effect of the process on the surface roughness. The surface roughness was reduced by a factor between 3 and 5 depending on the process parameters. The melt pool edge is not very well-defined when in contact with the EDM surface and fluctuates due to the variations of the topography. The fluctuations are better seen on the videos of the experiment available at: [[https://www.empa.ch/web/s204/modellinglaserpolishing1](https://www.empa.ch/web/s204/modellinglaserpolishing1)]([https://www.empa.ch/web/s204/modellinglaserpolishing1](https://www.empa.ch/web/s204/modellinglaserpolishing1)) (accessed on 8 July 2022). For the return line with an overlap of 30% shown in Figure 13b, the fluctuations are reduced for the part of the melt pool that is overlapping with the first line. Finally, Figure 13c shows also a second line of an experiment but at higher velocity (100 mm/s) and higher power (561 W). The increase in laser power is necessary to compensate for the increased velocity so  
Figure 12: Time evolution of the melt pool width. The inset shows the detail of the solidification. that the maximum temperature of the melt pool stays constant. In Figure 13c, it is observed that the shape of the melt pool becomes oval due to a reduction of the width and a slight increase of the length in the direction of the workpiece movement. In order to avoid the influence of the fluctuations due to the roughness, averaged images were produced for the different conditions. The average images are simply realized by stacking all steady-state images together and then for each pixel calculating the average intensity of all images. The resulting averaged images are presented in Figure 13d,e,g,h for the first and second lines at 35 and 100 mm/s, respectively. Based on these images, we see that the melt pool is better defined. In Figure 13d,g, the EDM also appears almost as a bright uniform region due to the random positions of the bright spots over the length travelled. For the return lines (Figure 13e,h), the first line is also clearly visible as the brightest region as it is very flat and so reflects most light towards the camera. The melt pools appear mostly as a uniform dark grey and have better defined shapes. Furthermore, the borders of the melt pools positioned over the first lines are very well defined. This shows that the surface of the first line is mostly flat whereas in the overlapping region, no fluctuation of the melt pool size is provoked by the topography variations. On the other hand, over the EDM, the melt pool transitioned from dark grey (the color of the melt pool) to light grey (color of the solid EDM surface). This transition region is where some fluctuations of the melt pool occur due to the fluctuations of the surface topography. Figure 13: (**a**) First line at 35 mm/s and 415 W in the steady state regime.