## 4 Results and Discussion
Furthermore, the maximum developed Von Mises stresses of 380 MPa are presented in Figure 4.  
Figure 3: Downscaling for surface roughness from 0 to 0.5 \(\upmu\)m. The computed nodal positions and the plastic strains 100 ns after the laser beam irradiation are presented. (**a**) A step-like linear approximation of the roughness profile is shown and the original flat surface was gradually downscaled to reach the resulting roughness curve (dashed, cyan line) after the Gaussian filtering. (**b**) The permanent nodal positions on the top surface along the central horizontal \(x\)-axis 100 ns after the laser beam irradiation. were observed at 18 ns. The advantage of such a high geometrical accuracy for surface roughness level was clearly visible at 50 ns, where the residual stresses that existed due to the plastic deformation of the heated samples were apparent and were above the yield stress of the Al alloy, which is 324 MPa [25]. The effect of the laser pulse on the surface roughness of the workpiece is experimentally demonstrated in Figure 5 for the same pulse energy of 0.7 mJ and fluence of 2 J/cm\({}^{2}\). Experiments performed with pulse energies below 0.6 mJ showed that no detectable effect of the laser pulse shot was measurable on the surface of the target. Figure 5 shows two-dimensional images of the area of interest and cropped images, at a position of the target with the maximum roughness variation (\(\sim\)3 \(\upmu\)m depth), on which the laser elliptical spot was focused. The cropped images cover a length of \(x=220\)\(\upmu\)m. The two-dimensional graphs of Figure 5 show the surface roughness (height) plotted against the x dimension, passing through the center of the irradiated region as determined by the experiment. For every \(x\) position, the roughness values at every point along the \(y\) direction were averaged, for a length \(y=4\)\(\upmu\)m within the yellow rectangular area. The graphs produced exhibit the average surface roughness (\(z\) dimension, height) plotted along the \(x\) direction. The images refer to the same area of interest before and after the laser pulse interacts with the surface. Figure 4: Representative FEM results of temperature and Von Mises stress distribution for the model with 0.5-\(\upmu\)m step accuracy at 10 ns, 20 ns, and 50 ns and for laser fluence of 2 J/cm\({}^{2}\). Figure 5 demonstrates that the laser pulse affected the interaction area of the surface, while the roughness profile line-outs revealed the relative changes on the position of the affected peaks, as well as on their height and/or depth before and after the interaction with the laser pulse, reaching the micrometer and submicrometer scale. By processing the images before and after laser irradiation, the average difference in height and/or depth experienced by the peaks due to the influence of the laser pulse was found to be less than 0.3 \(\upmu\)m. Some resulting laser-affected roughness peaks showed differences reaching up to 1 \(\upmu\)m. Such spikes that existed also outside the laser-affected region of the sample were considered as artefacts (measurement noise) and were attempted to be kept low through the averaging performed on the roughness values. These artefacts limited the direct correlation of the detailed features of the roughness profiles to the corresponding simulation outcomes. The experimental findings regarding the fractured volume agreed with the simulation results of the downscaled model to 0.5-\(\upmu\)m step accuracy, since from the roughness profiles of Figure 5 the extracted value of the fractured volume was ~360 \(\upmu\)m\({}^{3}\). It is worth noting that, through the averaging performed on the roughness values, large fluctuations arising from local surface discontinuities were also smoothened, while information on the location and the type of the changes experienced by the roughness peaks was less influenced, compared to the case where a single line was chosen for plotting the roughness profile, by its exact position relative to the pulse spot center.