## 3 Experimental Results and Discussion
### Effect of Process Parameters on Surface Roughness
#### 3.2.1 ANOVA Results for Surface Roughness  
The ANOVA results for surface roughness are summarized in Tables 7 and 8, where significant items are indicated in bold. Ra_L1 was found to be affected by material, thickness and mostly M*Cs (%) interaction. Ra_L2 in addition was influenced by the cutting speed, whereas Ra_L3 was dependent only on material. Rz_L1, Rz_L2 and Rz_3 exhibited the same behaviors of Ra; therefore, their statistical analysis was not reported in this work for the sake of brevity. RSm_L1 was only influenced by material and thickness, whereas RSm_L2 was also affected by cutting speed, pressure and t*P (%) interaction. Finally, RSm_L3 was dependent on all the main factors and M*Cs (%) interaction. Figure 11: Main effects plot for kerf section. Figure 12: Interaction plot for top kerf width. The main effect plots for Ra_L1, Ra_L2 and Ra_L3 are reported in Figure 13. It is noticed that laser cutting of AlMg3 resulted in the highest value of Ra, whereas laser cutting of AISI 304 exhibited the lowest. Furthermore, the arithmetic surface roughness of AlMg3 significantly increased along the laser beam direction, whereas that of AISI 304 remained practically unchanged. St37-2 shows only a moderate increase in Ra. As the melt flows along the laser beam direction, the melt film thickness increased, and the hydrodynamic phenomena in the lower workpiece surface became increasingly important. When the melt viscosity and surface tension were high, the melt accumulated at the bottom of the cut groove where it solidified, creating surface ripples. The main power losses were related to thermal conduction followed by convective cooling performed by the assisting gas [27]. Therefore, this effect was less evident in laser cutting of stainless steel due to its relatively low thermal conductivity. As expected, the lowest arithmetic surface roughness was achieved in laser cutting of thin sections. However, Ra_L3 was found to be unaffected by thickness, since complex phenomena such as overheating, turbulence and slag formation in lower edge of cut can hide the surface roughness. Since Ra is generally measured at one-third of the thickness, this result agrees with previous research [28]. A general decrease in Ra_L2 (that is indicative of the mean surface roughness) was observed when the cutting speed increased. This experimental result, which is apparently in contrast with those of other research studies, can be examined in greater detail in the interaction plot reported in Figure 14. As it was expected, in laser cutting of AlMg3 the lowest value of Ra_L2 (2.68 \(\upmu\)m) was achieved at low cutting speed (compared to Ra_L2 = 4.25 \(\upmu\)m obtained at high cutting speed), in accordance with the results given in Ref. [19]. In contrast, in laser cutting of St37-2, a higher speed provided the lowest value of Ra_L2 (of \(\sim\)1.01 compared to 4.41 \(\upmu\)m). This behavior could be explained in view of the outcomes described in Ref. [10]. The trend of surface roughness may depend on the range of cutting speed being considered. Indeed, there was a specific range of cutting speed at which Ra was minimum. The arithmetic surface roughness decreased with cutting speed until it reached that optimum, to increase once such threshold was exceeded. Another possible explanation lies in the occurrence of side burning at lower cutting speed, as proposed in Ref. [14]. Finally, AISI 304 was found