# Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing
## 3 Results and Discussion
### Moving Experiment: Microstructure Observation and Adjustment of the Absorption
#### 3.3.4 Comparisons of Experimental and Simulation Results  
The first simulation was performed with a constant absorption coefficient equal to 0.35. Figure 17 shows the melting pool and the HAZ (top), and their corresponding bounding boxes (bottom). The reason for using bounding boxes instead of the isotherm line corresponding to the liquidus temperature (1'505 \({}^{\circ}\)C) and the austenitization temperature (950 \({}^{\circ}\)C) is due to the fact that the experimental results are measured after the laser treatment, whereas the areas of interest in the numerical simulation are computed during the treatment. In other words, during the simulation, the deeper part of these two regions does not necessarily correspond to their widest part. As it can be seen in Figure 17, the sizes of the melting pool and HAZ are largely over-estimated.  
The absorption coefficient of the liquid was then decreased in order to obtain a solution closer to the experimental results. A good solution was found for a liquid absorption coefficient of 0.25 and is presented in Figure 18. In this case, the size of the melting pool better fits the experimental result.  
With this liquid absorption, the size of the melt pool fits better to the experimental result. The size of the HAZ is still slightly exaggerated. This, however, can be explained, as the austenitization temperature is from the phase diagram where the phases are given without taking into account the kinetics of the reaction. In this present case, the heating by the laser and subsequent cooling only last for a fraction of a second. It is probable that in these conditions, a quite high overheating is needed to provide the driving force for the phase transformation to occur. From the simulations it appears that in these conditions, the austenitization occurs only above 1'100-1'200 \({}^{\circ}\)C.  
Figure 16: Melt pool viewed form the top and from the side (perpendicular to the laser direction). (**a**) For a constant absorption of \(a_{s}=a_{l}=35\%\), (**b**) for \(a_{s}=35\%\) and \(a_{l}=20\%\), (**c**) for \(a_{s}=35\%\) and \(a_{s}=25\%\), and (**d**) for \(a_{s}=35\%\) and \(a_{l}=30\%\)).  
A drop in the absorption as the material melts was also reported by [PERSON] et al. [25] during conduction welding. The explanation for a decrease of the absorption is the decrease of the roughness of the metal in the liquid state.  
Figure 17: Melting pool (blue) and HAZ (red) with \(a_{l}\) = 35%. Top: top and side view of the iso-volumes. Bottom: bounding boxes overlaid on experimental results.  
Figure 18: Melting pool (blue) and HAZ (red) with \(a_{l}\) = 25%. Top: top and side view of the iso-volumes. Bottom: bounding boxes overlaid on experimental results.  
Finally, based on the results presented in this paper, we can conclude that the thermal model is sufficiently accurate to model the depth and size of the melt pool. It provides a useful tool to estimate the changes on the melt pool when the laser parameters (velocity or power) are varied. In a second part of our research, a flow model will be developed and solved inside the melt regions modelled with the current approach, as illustrated in Figure 19. A weak coupling is used between the thermal solution and a computational fluid dynamics (CFD) calculation.