## 3 Experimental Results and Discussion  
### Effect of Process Parameters on Kerf Geometry  
A preliminary inspection was conducted to identify nonvalid cuts (i.e., presence of holes, material projections or uncut areas). This leads to the exclusion from the subsequent analysis of those cutting conditions which do not guarantee suitable kerfs. More specifically, cuts performed on 6 mm thick St37-2 steel using 50 and 100% of gas pressure and cutting speed, respectively, were found to be incongruuous, and consequently their kerf width was omitted. Top and bottom kerf widths are illustrated in relation to cutting speed (Cs) and gas pressure (P) in Figures 5-7. It can be observed that the kerf width of each material decreased at the decrease of thickness, in accordance with the behavior found in literature [23]. Since the amount of material to be removed increased with thickness, in laser cutting of thick sections, a greater energy contribution was required to obtain through-thickness cuts, which leads to a longer interaction time and increases the thermal erosion. Furthermore, the efficiency of laser beam decreased with increasing workpiece thickness, owing to conductive losses. Therefore, as it was expected, the minimum kerf width of each material was obtained at the thinnest sections. The comparison between top and bottom kerf widths highlights different shapes of kerf profiles. In most cases (and more precisely in the totality of cases when relating to AISI 304), kerf profiles exhibited the typical slight v-profile or parallel-sided sections. More rarely, the kerf profiles widened slightly at the bottom edge of the workpiece. Figure 4: Schematic of profile acquisition. Figure 5: Kerf width for AlMg3 in relation to cutting conditions and thickness. ANOVA Results for Kerf Geometry  
The ANOVA results for geometrical variables are summarized in Table 6, where significant items are indicated in bold. The ANOVA assumes that observations are independent and normally distributed with equal variances between treatments (homoscedasticity). These assumptions were successfully checked using the graphical examination of residuals [24]. However, for the sake of brevity, its results were not included in this analysis. The statistical analysis proved that all main effects were significant at a confidence level of 95%, with the exception of gas pressure and cutting speed for Tk and S, respectively. It is noticed that the greatest effect or significance (which corresponds to the highest F-value) in relation to the top kerf width was certainly obtained by varying the piece thickness. Conversely, Bk was strongly influenced by the material type, followed by thickness and gas pressure. Figure 6: Kerf width for St37-2 in relation to cutting conditions and thickness. Figure 7: Kerf width for AISI 304 in relation to cutting conditions and thickness. Figures 8 and 9 illustrate the main effects plot for Tk and Bk. The increase in the cutting speed reduced the top kerf width, whereas in contrast the bottom kerf width increased. Indeed, at high cutting speed, the irradiation time between the laser and the material was reduced, and consequently, Tk decreased as the result of the lower energy provided. Conversely, the increase in Bk as the cutting speed increased, although more contained, was unexpected. In the range of tested values for cutting speed, this result could be explained by the change in cut front inclination. At the maximum cutting speed (industrial relevant high cutting speed), the cut front was entirely irradiated by the laser beam, and the temperature of the molten metal at the bottom edge of the workpiece increased [25]. Moreover, Bk decreased when the assisting gas pressure increased, owning to the enhancement of the drag force, which facilitates the effective removal of melt from the groove. As it was expected, position was a significant factor in both Tk and Bk models. From the main effects plot in Figure 8, it is possible to observe that Tk first decreased and then increased gradually with position.