# Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing
## 3 Results and Discussion
### Moving Experiment: High-Speed Imaging
#### 3.2.3 Comparisons of Experimental and Simulation Results  
In Table 1, the measured widths and lengths of the melt pool sizes are given in normal font whereas the simulated results are in _blue italic font_ (a line below) for the same laser conditions. The width of the melt pool is measured perpendicularly to the moving direction; the length is measured parallel to the moving direction. The edge shift was also measured in the moving direction and is the difference between the leading edge of the laser spot with the leading edge of the melt pool. It is measured as positive if the melt pool is in front of the laser spot (see first experiment in Table 1), and negative if the melt pool is behind the laser spot.  
The first experiment in Table 1 corresponds to exaggerated conditions with low velocity and high power to create extreme conditions to test the model. This condition is not suitable for LP as it will create a too deep melt pool far from the shallow melting desired for LP. The other three conditions are conditions suitable for LP and the laser velocities of 35 and 100 mm/s are the same conditions as in Figure 13. As can be seen, the first condition creates a markedly larger melt pool than the spot size. It is also the only condition where the melt pool is in front of the laser spot. Due to its length, the region trailing the spot size (length \(-\) laser diameter \(-\) shift edge = 1460 \(-\) 900 \(-\) 142 = 418 \(\mu\)m) is still bigger than the region in front (142 \(\upmu\)m). The other conditions produce a rounder melt pool at low velocity and one which is more elongated at higher velocity. They also all trail the laser spot and the edge shift increases with velocity.  
As evident from Table 1, most of the values obtained from the simulations are below the experimental values. The reason could be similar to the static experiment and could come from the absence of the strong coupling with convective flow inside the melt pool. Another explanation could come from the absorption. Indeed, the absorptivity for the EDM surface was measured at room temperature and taken as a constant for the solid (\(a_{s}\) = 35%). It is certainly possible that the absorptivity changes with temperature and/or also with the laser intensity. The absorptivity of the liquid was unknown and must be taken between \(a_{l}\) = 25-30% after fitting with the experimental results.  
The static simulations are able to predict both the elongation of the melt pools and the edge shift with velocity. The order of magnitude of the sizes is acceptable and some  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Velocity** & **Power** & **Width Melt Pool** & **Length Melt Pool** & **Shift Edge** \\ \hline
**[mm/s]** & **[W]** & **[\(\upmu\)m]** & **[\(\upmu\)m]** & **[\(\upmu\)m]** \\ \hline
20 & 615 & 1259 & 1460 & 142 \\
20 & 615 & _985_ & _1159_ & –32 \\
35 & 415 & 891 & 922 & –252 \\ _35_ & _415_ & _760_ & _782_ & –199 \\
50 & 454 & 726 & 790 & –309 \\ _50_ & 454 & _731_ & _158_ & –239 \\
100 & 561 & 588 & 810 & –385 \\ _100_ & _561_ & _712_ & _712_ & –291 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Laser parameters, with the melt pool characteristics measured and simulated.  
Figure 14: Simulation of the melt pool temperature and shape and relative position to the laser beam (red line) for **(a)** 20 mm/s and 401 W, **(b)** 35 mm/s and 415 W, **(c)** 50 mm/s and 454 W and **(d)** 100 mm/s and 561 W.  
imprecision on the size measurements are also possible as they are coming from the high-speed imaging where one pixel is approximately 6 \(\upmu\)m and observed with an angle.