6.4 Results and discussion
6.4.2 Spatial and temporal investigation of LIPAA process
Based on the confidence gained from the validation of the simulated results, further investigation on the numerical model was carried out. Spatial thermal analysis was performed at various positions along the x-direction and the y-direction. The temporal
analysis was also carried out in the course of the pulse-on time (heating) and pulse-off time (cooling).
Figure 6.11 (a) Temperature distribution and (b) Temperature contour of LIPAA process along x-direction for the chosen set of process condition
The spatial temperature contour along the x-direction at different distances from the interface of the two materials are shown in figure 6.11. The plot is shown for the process condition of pulse power density of 3.06 MW/cm2, pulse repetition rate of 40 Hz, pulse duration of 4 ms and scanning speed of 4 mm/s. It is observed that, at A1 and A′1
i.e. at a distance of 50 µm from the interface towards the PC and the aluminium sheet respectively, the temperature at A′1 is much higher than that at A1. It may be due to the higher thermal conductivity of aluminium compared to the thermal conductivity of the PC. Thus, most of the laser heat is conducted into the aluminium sheet. Similar variance of the temperature was also observed for A2 and A′2 i.e. at a distance 100 µm on both sides from the interface. Further, it is observed that, at the end of pulse-on time (4 ms), a temperature of maximum value 657 K is obtained at A0, i.e., at the interface of the two materials. At the same time, a highest temperature of 683 K is generated within the aluminium sheet. But then again, the temperature at the interface is beyond the melting temperature of the PC and hence material removal is obtained. At 50 µm distance from the interface towards the PC, the maximum temperature attained was 334 K which is below the melting temperature of the PC. Thus, there was no material removal. Similar
observations were also seen for 100 µm distance from the interface. The simulated results show good agreement with the FESEM image of the machined PC surface. The same can be observed from the experimentally fabricated microchannels, shown by the FESEM image in figure 6.12.
Figure 6.12Comparison of temperature distribution and its effect on material removal (a) experimental results and (b) computational results
Figure 6.13 (a) Temperature distribution and (b) Temperature contour of LIPAA process along y-direction for the chosen set of process condition
The spatial temperature distribution in the y-direction on the X-Y plane is shown in figure 6.13. The distributions are shown at three different distances from the axisymmetric axis. It is observed that, a maximum temperature of 657 K is obtained at Bo i.e. at the axisymmetric axis and lies at the interface of the PC and the aluminium
sheet. The temperature at the interface of the axisymmetric axis is thus found to surpass the melting temperature of the PC and consequently, the material removal on PC sheet is achieved. Moving to a distance of 125 µm and 250 µm from the axisymmetric axis i.e. at B1 and B2, decrease in the temperature distribution was observed. The maximum temperature at B1 and B2 was less than the melting temperature of the PC and thus, no material removal was obtained. Identical observation was found from the experiments too, as shown by the FESEM image of the microchannel on PC for the same set of parameters in figure 6.14.
Figure 6.14Comparison of temperature distribution and its effect on material removal (a) experimental results and (b) computational results
It can also be observed from figure 6.14 that, at equal distance from the interface towards both PC and aluminium, the temperature on the aluminium is higher than that on PC, resulting in a non-axisymmetric temperature contour (along the y-axis). It is because of the application of lower value of laser process parameters (3.06 MW/cm2 pulse power density, 40 Hz pulse repetition rate and 4 ms pulse duration). At low value of laser parameters, plasma generation is less and thus, laser beam irradiation plays a significant role in temperature generation in both the materials. The PC being transmissible to the laser beam is subjected to very less thermal effect. Thus, it can be understood that, the aluminium metal target is more thermally affected and as such, at equal distance from the interface towards both PC and aluminium, the temperature on the aluminium is higher than that on PC, resulting in a non-axisymmetric temperature contour.
For the same set of process condition, the temporal investigation of the LIPAA process on PC was also carried out. The peak temperature during pulse‒on time and pulse‒off time of the process was obtained as a function of time as shown in figure 6.15.
As observed from the figure, the temperature rises at a high rate during pulse‒on time and reaches to maximum value of 683 K at its end i.e. at 4 ms. The pulse‒on time is also referred to as the heating time. However, the temperature after 4 ms starts decreasing at a slow rate and at the end of pulse‒off time i.e. 25 ms, reaches a value very close to the initial temperature of the materials. The pulse‒off time is hence, referred to as the cooling time. In a time period, the cooling time is thus found to be much longer than the heating time, thereby resulting in a slower cooling rate. But depending on the repetition rate and the scanning speed, the cooling period of the former laser pulse is overlapped with the heating time of the upcoming laser pulse, hence resulting in the formation of microchannels.
Figure 6.15 Time versus temperature for pulse-on time of 4 ms and pulse-off time of 21 ms 6.4.3 Influence of laser parameters on microchannel fabrication
A parametric analysis was performed to understand the contribution of the laser parameters such as pulse power density, pulse duration and pulse repetition rate on the responses such as microchannel width, microchannel depth and eventually the machining rate. Figure 6.16(a) and 6.16(b) shows the variation of temperature along the x-axis and y-axis at varying pulse power density. From the figures, it can be seen that as the pulse power density increases keeping the other parameters fixed at 60 Hz pulse repetition rate and 4 ms pulse duration, values of both the width and depth of the fabricated microchannel increase. And hence, an increase in the machining rate is also obtained.
Similarly, the effect of the increase in the pulse duration on the width and depth of the microchannel is shown in figure 6.16(c) and 6.16(d). In this case, the pulse duration was increased from 2 ms to 6 ms while the other parameters were fixed at 60 Hz pulse repetition rate and 4.6 MW/cm2 pulse power density. It is noticed that as the pulse
duration is increased, a growth in the width and depth of the microchannel is achieved, thereby resulting in the increase of the machining rate too. The width and depth of the microchannel is also affected by the variation in pulse repetition rate, as shown in figure 6.16(e) and 6.16(f). On increasing the value of pulse repetition rate from 40 Hz to 80 Hz, keeping the other parameters fixed at 4 ms pulse duration and 4.6 MW/cm2 pulse power density, there was a growth in the microchannel width and depth and as a result of which, increase in the machining rate was also obtained.
Figure 6.16Influence of pulse power density, pulse repetition rate and pulse duration on microchannel width and depth
Overall, the graphs portray that, as the laser parameters are increased, an increase in the microchannel width and depth and the machining rate of the process is obtained. It is because; an enriched plasma is produced with the rise in the laser parameters.
Correspondingly, the plasma attains a temperature of 2500 K to about 50000 K, with a pressure of approximately 500 MPa [27]. The plasma thus has a large influence on the rear side of the PC sheet. A temperature beyond the melting temperature of the PC will be produced on its surface, thus, resulting in an increased microchannel width and microchannel depth.
Again in figure 6.16(b), (d) and (f), it is observed that the temperature contours on PC and aluminium are nearly axisymmetric (i.e. at equal distance from the interface towards PC and aluminium, the temperature on both the material is nearly the same). The reason for such observation is that, the input laser process parameters applied are at comparatively higher levels. As the laser parameters are increased, there is an increase in the laser intensity resulting in dynamic interaction of the plasma with the laser irradiation. This causes a rapid expansion of the plasma. As such, at higher levels of laser parameters, the plasma plays a major role in temperature generation in both the materials. Again, we know that the thermal conductivity of PC being low (~0.4 W/mK) generates higher temperature near the irradiated zone with narrower temperature distribution. While thermal conductivity of aluminium being high (~250 W/mK), due to higher conduction of heat into the material, it results in less temperature generation near the irradiated zone with a much wider temperature distribution. It can thus be witnessed that, at higher value of laser parameters, the temperature at equal distance from the interface are more or less similar, thereby resulting in a near axisymmetric temperature contour. Moreover, it can also be witnessed that as we move away from the irradiated zone on both side of the interface, the temperature profile are not axisymmetric in nature.
The scanning speed during the simulation of the LIPAA process was however kept constant at a value of 4 mm/s to form the channel. It is known that for a channel to form, the overlap of the consecutive pulses must be larger than zero. The overlap distance and its percentage between two pulses is denoted by DL and O and can be expressed as:
L
D v
f (6.17)
L 100%
O D
(6.18)
where v is the laser scanning speed, f if the repetition rate and w is the laser spot diameter. Scanning speed of 4 mm/s results in the minimum overlap of 50 % to a maximum value of 75 % for our selected levels of laser process parameters where pulse power density was varied from 3.06 MW/cm2 to 6.1 MW/cm2, pulse duration from 2 ms to 6 ms and pulse repetition rate from 40 Hz to 80 Hz. The focused beam diameter was considered to be 200 µm.
It is also found that the nature of change in both width and depth of the microchannel with the change in the laser process parameters is similar, maintaining a constant aspect ratio of the microchannel. Thus, it can be said that, a very narrow microchannel to a wide microchannel with a nearly constant aspect ratio can be obtained by the technique of LIPAA.
6.5 Estimation of channel index (CI) for microchannel fabrication during