## 3 Experimental Results and Discussion
### Effect of Process Parameters on Surface Roughness
#### 3.2.2 Quality Assessment  
In order to highlight the interdependence of laser parameters, the computed values of roughness parameters of each material and thickness (averaged over the three profiles acquired on both sides of kerf) are illustrated in relation to cutting speed (Cs) and gas pressure (P) in Figures 17-19. It is possible to note that laser cutting of AISI 304 exhibited good surface roughness for all the tested conditions, and thickness had a major impact on Ra (Figure 20a,b). Conversely, laser cutting of AlMg3 showed the highest surface roughness, in particular at the maximum thickness. The worst conditions in absolute terms occurred when cutting 6 mm thick AlMg3 at high cutting speed and gas pressure (Figure 21a), showing that despite the low melting point of aluminum alloys, the high thermal conductivity and light reflectivity required more energy to produce a fault-free cut edge. Laser cutting of St37-2 achieved the lowest values of Ra, except for laser cutting of 5 mm thick sheet with low cutting speed and high gas pressure (Figure 22b) and 6 mm thick sheet at the same conditions. The latter indeed was found to be unsuitable, and it was excluded by the preliminary inspection. In contrast, the best cutting conditions were identified at high cutting speed and gas pressure. The lowest surface roughness in absolute terms was achieved in laser cutting of 2.5 mm thick St37-2 with low cutting speed and high pressure (Figure 22c). From the comparison of roughness parameters, it is possible to conclude that the increase of the cutting speed in the range of tested values resulted in a smooth but more irregular surface in the lower edge of the cut when cutting AISI 304, as shown in Figure 20b,c. Moreover, it led to the worsening of the cutting surface in laser cutting of AlMg3 (Figure 21a,b) and improvement of cutting surface when cutting St37-2 (Figure 22a,b). The quality assessment of cut sections was carried out in accordance with the standard ISO 17658:2015 by visual inspection. This standard defines the imperfections in laser beam cutting and the dimensional tolerances. From the microscope observations it is possible to conclude that kerf sections exhibited a good overall quality, with parallel drag lines and negligible presence of defects shown in the images below. In Figure 20c, it is possible to observe small removal of material in the lower cut face area. Since AISI 304 is rich in alloy elements, the hole formation originated from the change in composition related to the laser heat [30]. In addition, the risk of appearing of pittings in laser cutting of AISI 304 increased at high cutting speed, in line with Ref. [19]. Figure 17: Arithmetic mean surface roughness. Figure 18: Arithmetic mean surface roughness. Figure 19: Arithmetic mean surface roughness. Figure 20: AISI 304 kerf lateral surface: (a) 6 mm, Cs = 50% P = 100%; (b) 3 mm, Cs = 50% P = 100%; (c) 3 mm, Cs = 100% P = 100%. As for AlMg3, in some cases thick samples showed slag adhering to the bottom cut edge due to the cooling of molten metal at the outlet of the groove (Figure 21b). Indeed, in laser cutting of aluminum alloys the melt exhibits higher viscosity than that of molten iron since the thermal conductivity is significantly higher and heat is rapidly dissipated through the material. Moreover, the increasing in cutting speed enhances the drag line curvatures as shown in Figure 21a,b. Deviation of drag lines is found in laser cutting of St37-2. In Figure 23 it is possible to observe a pronounced advancement of drag lines in the cutting direction followed by an excessive curvature, which could be the curved-drag lines effect of the increase in cutting speed [31]. This effect was accentuated by the use of oxygen. Indeed, the oxidation reaction of oxygen and iron generates a change in the chemical composition with the formation of a lower-viscosity mixture of Fe/FeO than pure iron, Ref. [25].