## 4 Results and Discussion  
The simulation results of the laser matter irradiation are demonstrated in Figures 3 and 4, for a pulse energy of 0.7 mJ and a fluence of 2 J/cm\({}^{2}\), above the melting threshold of the Al alloy, which was computed to be 1.5 J/cm\({}^{2}\). For the simulated laser fluence, no ablation phenomena occurred and the structural changes of the target due to laser irradiation were restricted in the plastic and melting regimes. Thus, structural changes related to plastic and melting effects occurred in the submicrometer scale. Figure 3 presents results for five models where sequential downscaling of the surface roughness level was performed. Roughness was neglected in the flat surface model. To perform the downscaling, the average roughness profile, presented in Figure 1e, was processed in MATLAB using a Gaussian smoothing filter that resulted in the cyan, dashed curve, presented in Figure 3a. Based on the geometrical characteristics of this curve, a linear approximation was performed and the flat surface model was firstly downscaled to describe a step of 3 \(\upmu\)m. Then, this model was downscaled to describe two steps of 1.5 \(\upmu\)m each, following the geometry of the cyan, dashed curve. Following this procedure, two more downscaled models were built and resulted in a total of four downscaled models having a geometrical step accuracy from 3 to 0.5 \(\upmu\)m. Each one of the sequentially developed multiscale models could simulate the thermal-mechanical laser-solid interactions, according to the irradiated area of interest and the surface roughness profile therein, with a predefined geometrical accuracy. The identification of these positions, where changes in surface roughness took place after their interaction with a single laser pulse in this energy regime, is of high importance, especially for research works that concern changes of matter of micro- or nanoscale order, which affect the material structure, since this is where different types of cracks initialize [26], ripples may be formed [27; 28], and mechanical and optical materials' properties change [29; 30]. Figure 3 presents the simulation results for the five models 100 ns after the laser beam irradiation. According to the developed permanent plastic strain distributions, the maximum value was about 0.04 for all. At 100 ns the maximum temperature decreased to \(\sim\)50 \({}^{\circ}\)C, reaching the environmental temperature. Furthermore, in Figure 3b the permanent nodal positions on the top surface along the central horizontal \(x\)-axis are plotted. The maximum depth due to material fracture was 0.1 \(\upmu\)m for all the models. The maximum computed temperature values were lower than 900 \({}^{\circ}\)C at 16 ns, securing that ablation phenomena did not occur. The total fractured volume of the downscaled model to 0.5-\(\upmu\)m step accuracy was computed to be \(\sim\)340 \(\upmu\)m\({}^{3}\). while for the rest of the models this value was gradually reduced to \(\sim\)310 \(\upmu\)m\({}^{3}\) (model neglecting roughness). It was obvious that as the geometrical accuracy of the roughness approximation was downscaled to the maximum of 0.5 \(\upmu\)m, the submicron regions, where fracture occurs, permanent deformations are developed, and the material properties change, were more precisely identified. Representative FEM results of temperature and Von Mises stresses for the model downscaled to the 0.5-\(\upmu\)m step accuracy, are presented in Figure 4 at 10 ns, 20 ns, and 50 ns and for laser fluence of 2 J/cm\({}^{2}\). The heated sample at 10 ns was in the thermoelastic regime. Plastic effects started to develop 2 ns later and, at 16 ns, the maximum surface temperature exceeded the melting point. At 20 ns the heated material was in the melting regime and from this temporal moment and afterwards the maximum surface temperature started to decrease.