## 3 Experimental Results and Discussion
### Effect of Process Parameters on Kerf Geometry
This curvature also occurs in Figure 9, where conversely Bk first increased and then decreased with position. The curvilinear shape is explained by the existence of different shapes of the cutting profile, as previously mentioned. Furthermore, narrower kerfs were obtained at small thicknesses, beyond which the kerf width started to widen as the thickness increases. Top and bottom kerf widths showed linear trends in relation to thickness which were very similar to each other. Moreover, since aluminum and its alloys are highly reflective metals, laser cutting of AlMg3 needs more energy input to initiate the cut, thus resulting in larger Tk, in accordance with Ref. [19]. The regression model for Tk reached acceptable adequacy index R-sq of 68.93%, whereas that calculated for Bk was at 76.70%. As it was expected, the taper angle was mainly influenced by the material type. In Figure 10, it is possible to observe that laser cutting of AlMg3 generally produced parallel sided-sections, whereas laser cutting of AISI 304 resulted in the typical v-profile, and laser cutting of St37-2 led to a slight widening of kerf. The latter probably originates by the exothermal reaction. Indeed, in oxygen-assisted cutting, the high-temperature molten metal that is not ejected from the cutting area by the assisting gas adheres to the lateral surface and continues the oxidation reaction (side burning) [26]. Clearly, Ta depends on both top and bottom kerf widths since it was calculated by Equation (1). More specifically, Ta decreased when increasing cutting speed or decreasing workpiece thickness because Bk widening exceeded that of Tk, and it decreased as the gas pressure increased owing to the significant reduction of Bk. Concerning the effect of position on the taper angle, the same considerations applied as for Tk and Bk. The main effects plot for S is illustrated in Figure 11. The section of material removed linearly increased with thickness, and being related to Tk and Bk by means of Equation (2), it was strongly affected by the kerf width. Furthermore, as it was expected, S was insensible to the change in cutting speed, and the impact of position was negligible as well. The adequacy of fit for the regression models of Ta and S was 71.88 and 94.5%, respectively. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Tk**} & \multicolumn{2}{c}{**Bk**} & \multicolumn{2}{c}{**Ta**} & \multicolumn{2}{c}{**S**} \\ \hline
**Control Factor** & **F-Value** & **\(p\)-Value** & **F-Value** & **\(p\)-Value** & **F-Value** & **\(p\)-Value** & **F-Value** & **\(p\)-Value** \\ \hline M & 5.23 & **0.006** & 65.87 & **0.000** & 47.5  
The most statistically significant two-way interactions are shown in Figure 12. The widening of Tk with the decrease in cutting speed was significantly lower, or even eliminated in laser cutting of AISI 304. This effect was also observed for Bk and Ta, which was derived from the kerf width. A possible explanation lies in the range of values selected for AISI 304: as for cutting aluminum alloys, a relatively low cutting speed is more advisable (owing to the high thermal conductivity), in laser cutting of steel, the increase in cutting speed, where possible, could lead to better interaction of the laser beam. The laser beam utilized to the fullest may increase the temperature in the cutting zone, which is likely to result in kerf widening. This explanation has also been proposed with respect to the widening of Tk observed in the main effects plot. The interaction plot of material and thickness  
Figure 8: Main effects plot for top kerf width. Figure 10: Main effects plot for taper angle. Figure 9: Main effects plot for bottom kerf width. shows that, in laser cutting of AlMg3, the widening of kerf width was aggravated with the increase in thickness more than other materials, and this also applied to Bk. AlMg3 is less suitable for CO2 laser cutting of thick sections because of its high reflectivity.