## 3 Results and Discussions
### Optimal Laser Fluence Range  
SEM analysis of the microchannels revealed that in general the pulse overlap of 66.66% resulted in better surface at the bottom of the microchannel as compared to other pulse overlaps. The microchannels with 58.33% overlap resulted in craters at the bottom surface of the channel especially at lower levels of peak fluence. The higher pulse overlap of 75% resulted in V-shaped microchannels  
Figure 8: Effect of peak fluence and pulse overlap on MRR. Figure 7: Effect of peak fluence and pulse overlap on depth per scan. as shown in Figure 3. In order to find the optimal laser fluence range for the accurate fabrication of microchannels, the pulse overlap was fixed at 66.66% and laser fluence was varied in minimum possible increments with the help of the Flashlamp current as shown in Figure 2. The microchannels of dimension 200 \(\upmu\)m width and 5 mm length were machined in the lower fluence range. The combination of lower fluence and increased number of laser scans was selected so as have a small depth of cut. Table 3 shows the design array of the microchannels fabricated by varying peak fluence at a fixed pulse overlap of 66.66% for 48 laser scans. In this, experimental strategy microchannel geometry varied with respect to peak fluence similar to the study in the previous section but peak fluence greater than 140 J/cm\({}^{2}\) resulted in V-shaped channels. This is because of the increase in the number of scans which results in increased depth and hence a V-shape. The microchannel width varied within a 210-229 \(\upmu\)m range in the selected fluence range and the variation of TWE with varying peak fluence is shown in Figure 9a. The fluence range of 125-130 J/cm\({}^{2}\) resulted in channels with width closer to the target width of 200 \(\upmu\)m. The bottom width of the channel however varied drastically with peak fluence up to a certain fluence range after which any increase in peak fluence resulted in V-shaped microchannels. The variation of BWE with respect to peak fluence is shown in Figure 9b. The maximum width obtained at the bottom of the channel was 142 \(\upmu\)m at a peak fluence of 125.76 J/cm\({}^{2}\). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**S.no**} & **Fluence** & **Channel** & **Channel** & **TWE \(\upmu\)m** & **BWE \(\upmu\)m** & **D/S \(\upmu\)m** & **MRR** \\  & **J/cm\({}^{2}\)** & **Width \(\upmu\)m** & **Depth \(\upmu\)m** & & & **mm\({}^{3}\)/min** \\ \hline
1 & 125.768 & 211.7 & 144.0 & 11.7 & 58.6 & 3.0 & 0.0607 \\
2 & 126.455 & 211.7 & 178.3 & 11.7 & 82.1 & 3.7 & 0.0701 \\
3 & 127.324 & 210.4 & 195.3 & 10.4 & 110.3 & 4.1 & 0.0725 \\
4 & 132.590 & 217.7 & 262.6 & 17.7 & 141.4 & 5.5 & 0.0811 \\
5 & 135.287 & 213.6 & 267.8 & 13.6 & 141.9 & 5.6 & 0.0843 \\
6 & 138.227 & 218.4 & 301.0 & 18.4 & 173.7 & 6.3 & 0.0895 \\
7 & 141.471 & 219.5 & 336.4 & 19.5 & & 7.0 & 0.0951 \\
8 & 145.244 & 215.2 & 340.1 & 15.2 & & 7.1 & 0.0952 \\
9 & 148.733 & 221.2 & 366.8 & 21.2 & & 7.6 & 0.1034 \\
10 & 152.841 & 220.6 & 371.2 & 20.6 & & 7.7 & 0.1065 \\
11 & 157.302 & 227.9 & 400.0 & 27.9 & & 8.3 & 0.1161 \\
12 & 162.124 & 216.5 & 409.2 & 16.5 & & 8.5 & 0.1156 \\
13 & 169.765 & 229.8 & 409.1 & 29.8 & & 8.5 & 0.1199 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Design array of the microchannels fabricated by varying the laser fluence for 48 laser scans at a fixed pulse overlap of 66.66%. Figure 9: \(Cont.\)  
Depth per each laser scan increases with the increase in peak fluence as shown in Figure 9c. The increase is higher in the lower fluence range because the energy absorption into the material is higher at lower fluence, whereas in the high fluence range it reaches saturation limit hence there is minimum variation in depth removed. Depth per scan and total number of laser scans define the final depth of the channel. It is important in order to have the accurate depth of the channel. Depth per scan of less than 5 \(\upmu\)m was found to give favorable results considering the TWE and BWE.