# Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing
## 3 Results and Discussion
### Static Experiments
#### 3.1.1 Experimental Results  
A typical example of the melting of the surface is illustrated in Figure 5 and the corresponding high-speed movie can be seen at: [[https://www.empa.ch/web/s204](https://www.empa.ch/web/s204)]([https://www.empa.ch/web/s204](https://www.empa.ch/web/s204)) /modellinglaserpolishing1 (accessed on 8 July 2022). In this figure, the evolution for a complete laser illumination (997 ms) at 317 W is shown from left to right and top to bottom. The time indicated in each picture corresponds to the start of the laser irradiation. A few frames were selected from the high-speed images; the complete sequence can be watched in the videos at the link above. Figure 4: (**a**) Schematic representation of the experimental setup of LP; (**b**) photography of the actual setup. The first frame shows the initial surface just before the start of the laser illumination. The contrast is coming from the reflection of the visible external lightning on the asperities of the rough EDM surface. At the start of the illumination, only minor changes are visible (see green arrows at \(t=0\) ms) as most of the laser light is filtered out by the low pass filter. After 10 ms of laser illumination, the surface has not changed much. The bright spots in the center appear a little wider and could be a sign of the first melting of the top surface. At 20 ms, the center of the image shows that the bright spots are spreading even more as the melting progresses. After 30 ms, an elliptical melt pool is visible. The ellipse is due to the perspective induced by the angle of the camera. As the time progresses, the melt pool becomes better defined and augments its size (50 to 996.7 ms). Moreover, the center of the melt pool rises in brightness as the time passes. This is due to thermal emissivity of the center of the melt pool as the temperature increases. The solidification sequence of the same experiment is given in Figure 6, similarly to Figure 5. 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). In this figure, the time stamps refer to the time the laser light was switched off, which was just after the last frame of Figure 5 (996.7 ms). The first frame in Figure 6 corresponds to a time of 0.3 after the laser light was turned off and this corresponds to 0.3 + 996.7 = 997.0 ms from the start of the laser illumination. The first line of images (0.3 to 1.5 ms) shows a clear cooling of the melt pool as the brightness of the center comes rapidly back to dark grey. The brightness settings of the images are the same as in Figure 5 and the cooling is already very high, 0.3 ms after the end of laser illumination, and this is evident when comparing to the last frame of Figure 5 with the first one in Figure 6. After 1.8 ms, the first traces of the solidification are visible (see green arrows at 1.8 ms). It is possible that the solidification of the edge started earlier but it is not visible on the images. After this time, the melt pool in the center decreases in size as the solidification front progresses. Solidification is finished 4 ms after the end of the laser illumination. The re-solidified region appears bright as it reflects lots of light towards the camera. It is also significantly more uniform than the EDM surface visible  
Figure 5: Evolution of the melt pool during a static experiment, with a power of 317 W for a duration of 997 ms.  
around the re-melted surface and this shows the positive effect of the polishing process on the surface roughness.