3.1 Comparison of geometric morphology and structure for DLM and WJALM
Fig.5, Fig.6 and Fig.7 exhibit the geometric morphologies and structural parameters of microchannels fabricated by DLM and WJALM with same laser power and scanning speed. It can be clearly observed from Fig.5 and Fig.6 that the total-depth and top-width fabricated by WJALM are smaller than those by DLM, which indicates that partial laser energy is lost due to the presence of low-pressure waterjet. Researches have shown that the energy loss magnitude is mainly determined by the absorptivity and reflectivity of liquid water [28]. According to Beer Lambert’s law [29], for the laser wavelength of 1064nm used in this paper, the energy attenuation coefficient and absorption length of the laser in pure water are approximately 34.24 and 29.20mm, respectively. Then, if the water layer thickness is given of 1mm, the reduced laser energy can be calculated to be about 3.4%. It should be emphasized that the thicker the water layer, the greater the loss of laser energy, which is attributed to corresponding optical path of the laser transmission underwater turns to increase.
Comparing Fig.5(a) and Fig.7(a) with Fig.5(b) and Fig.7(b), the heat-affected zone (HAZ) formed of HAR microchannel fabricated with DLM is larger than that of WJALM, and thermal damage and recast layer phenomenon are also relatively severe. Irregular curve edges of the microchannel with blocky molten slags and large-sized cracks around them can be observed for DLM. The reason for this phenomenon is that molten materials induced by the laser thermal effect are not promptly removed but accumulates inside the channel or solidifies again to form recast layers [30], and as the total-depth increases, the discharge of molten materials becomes more difficult. Furthermore, these recast layers, on the one hand, would deteriorate the laser processing accuracy to result in irregular processing edges. Meanwhile, they would scatter and block the subsequent laser beam to reduce the laser ablation efficiency. This is also the main reason for the narrower instantaneous channel width along the depth direction in Fig. 5(a). The micropores and microcracks generated are principally because thermal stress effect during the resolidification process of molten materials.
In contrast to DLM, it can be found from Fig.5(b) and Fig.7(b) that the phenomenon of recasting layer and thermal effect is not obvious under the assistance of waterjet. The top surfaces of HAR microchannel are mainly composed of some molten structures, spatters and smaller micropores. This phenomenon can be explained that when the waterjet is applied to the inner surfaces of HAR microchannel at a certain angle and pressure, most of the molten materials are quickly flushed away by flowing liquid water before they have completely solidified into recast layers on channel surfaces than that by DLM. Moreover, the kinetic energy carried by the waterjet can play an auxiliary role in fabricating of HAR microchannel. And the cooling effect of liquid water itself is beneficial for suppressing the heat transfer to the interior of silicon carbide material, to further reduce the heat affected zone. Therefore, to some extent, the machining efficiency and surface quality of HAR SiC ceramic microchannels by using WJALM can be improved significantly.
3.2 Influence of processing parameters on removal ability of HAR microchannels
3.1.1 Geometric profile evolution for varying laser powers
Fig.8 and Fig.9 show the profile characteristic variations and the aspect-ratios of HAR SiC ceramic microchannels under varying laser powers. It can be seen that as the average laser power increases from 18W to 30W, the total-depth, top-width, and effective ablation area of HAR microchannels increase from 0.693mm, 0.167mm, and 0.058mm2 to 1.125mm, 0.211mm, and 0.117mm2, respectively, but their growth rates are not entirely identical.
For HAR microchannels fabricated by utilizing WJALM, the laser beam successively passes through an air layer and a thin-flowing water layer before reaching workpiece surface. During this process, some laser energies are absorbed by the air and water medium thin film, and then there is also a portion of laser energy reflected by the water medium and workpiece surface. The laser energy absorbed by water medium is mainly related to the thickness of the thin film and the wavelength of the nanosecond laser, which is independent of the laser intensity [23]. Therefore, under given conditions, the change in average laser power basically cannot cause additional laser energy attenuation. As average laser power increases, more energy is irradiated on per unit area due to an increase in single-pulse energy density, making it easier to reach the damage threshold of SiC ceramic material. Consequently, more material can be ablated within the same time to enlarge the effective ablation area, as shown in Fig.8. On the other hand, the surface to be processed moves downwards along with continuous laser ablation, and effective energy irradiated on machined surface is on the decline due to Gaussian distribution characteristics of the laser. One effective solution for improving the total-depth is to place the laser focal plane below the workpiece surface (i.e. negative defocus). Particularly, research has reported that the laser focus turned to shift downward due to the refractive effect of the water medium film [23], which is beneficial for achieving HAR microchannels from this perspective. Whereas, the channel top-width gradually becomes smaller due to the photo-chemical effect and laser reflection, and the deeper the microchannel depth is, the larger the taper angle of channel sidewall turns to be, which is also a crucial factor for the difficulty in guaranteeing the processing accuracy of HAR microchannels. It can be also discovered from Fig.9 that a slight growth happens to the aspect-ratios for different laser powers, which mainly because of the inconsistent changes in total-depth and top-width described above.
3.1.2 Geometric profile evolution for varying scanning speeds
Fig.10 and Fig.11 illustrate structural feature changes and aspect-ratios of HAR microchannels with varying laser scanning speeds. It can be clearly observed that all the microchannel depth, top-width, and ablation area show a slow decreasing trend with an increasing scanning speed. The larger the scanning speed is, the faster the decrease rate becomes. This phenomenon can be interpreted that the overlap rate of laser spots is responsible for geometric morphology and contour characteristics of ablated microchannels [31], owing to it is inversely proportional to the scanning speed at constant spot diameter and repetition rate. The overlapping area of pulse laser deposition on the processed surface is on the decline with a decrease in the laser overlap rate induced by a larger laser scanning speed. Then, within the same pulse duration, the laser interaction time is shortened, resulting in a sharp reduction in laser energy reaching deeper into HAR microchannel. Meanwhile, some laser energy is absorbed and reflected by the water layer, which further suppresses the effective ablation of HAR microchannels. It can be obtained from Fig.10 that the total-depth, top-width, and effective ablation area are 1.139mm, 0.266mm, 0.151mm2 for 600mm/s, and 0.816mm, 0.173mm, and 0.071mm2 for 1400mm/s, respectively. The corresponding decrease rate is 28.4%, 35%, and 53.4%, respectively, along with scanning speed changes from 600mm/s to 1400mm/s. Notwithstanding, considering that insufficient material ablation has a negative impact on machining accuracy, it is still difficult to meet practical application requirements though the depth-to-width ratios of HAR microchannels for higher scanning speeds are greater than 4, as presented in Fig.11. Therefore, appropriate scanning speed is crucial for ensuring effective ablation and large aspect-ratio of HAR SiC ceramic microchannels.
3.1.3 Geometric profile evolution for varying waterjet velocities
Fig.12 and Fig.13 is the effect of waterjet velocities on structural feature changes and aspect-ratios of HAR microchannels. It can be seen that the total-depth, top-width, and ablation area show an increasing trend when waterjet velocity changes from 4m/s to 16m/s, and then decrease as it continues to increase to 20m/s. The primary reason for this phenomenon is the combined effect of the cooling and impact of the waterjet. At low waterjet velocity of 4m/s, due to the dominant cooling effect, the energy loss gradually becomes large with the increase of water flow rate per unit time. Moreover, the waterjet’s impact force is relatively weak at this moment, and materials debris cannot be promptly flushed out of HAR microchannel but accumulate on sidewall surfaces. The phenomenon for an increase in channel-depth, top-width, and ablation area from 4m/s to 16m/s in Fig.12, can be explained that on the one hand, the waterjet’s impact effect keeps synchronous growth with an increased waterjet velocity, further promoting more effective scouring effect to act on molten materials and recast layers. Thereby, it significantly alleviates the blockage and absorption of subsequent laser energy, which is conducive to achieving deeper microchannels. On the other hand, although the cooling effect of low-pressure waterjet continues to enhance at faster jet velocity, the increase magnitude in average heat transfer coefficient is on the decline. In other words, the influence of laser energy loss caused by waterjet’s cooling effect on laser machining process turns to slow down. Furthermore, as mentioned in section 2.1 above, when high-energy nanosecond laser penetrates liquid water, high-temperature and high-pressure plasma is generated in the irradiation area. The laser-induced plasma shock waves play an auxiliary machining role in the fabrication of microchannel surfaces [32]. Meanwhile, when bubbles induced by the outward expansion of plasma collapse around the sidewalls, micro-jets and shock-waves are also generated under the pressure difference between the inside and outside of the bubbles, further enhancing the interaction between the laser and SiC material with the assistance of waterjet [33].
At the waterjet velocity of around 16m/s in Fig.12, the geometric contour parameters reach their peak state, and there is an unexpected decrease in geometric structural parameters after 16m/s. The reason for this phenomenon is that the higher waterjet velocity no longer effectively assists laser machining, and the cooling effect once again dominates. In addition, as waterjet velocity increases from 4m/s to 16m/s, the growth rates of microchannel depth and top-width are 28.4% and 31.9%, respectively, which indicates that the top-width is more sensitive to the increase in waterjet velocity than that of total-depth. This is also the critical reason for the decrease in the depth-to-width ratio of microchannels from 16m/s to 20m/s in Fig.13.
3.3 Influence of processing parameters on sidewall surface quality of HAR microchannels
3.3.1 Sidewall surface quality for varying laser powers
Fig. 14 exhibits sidewall surface morphology of HAR SiC ceramic microchannels for varying laser powers. Almost no recast layer and cracks are found on the middle and upper sidewall surfaces of microchannels, and these surfaces turns to be uniform and smooth. It can be observed from Fig. 14 that some resolidified columnar structures are formed along the depth direction to the channel bottom. This phenomenon can be explained that the laser pulse in overlapping area can be strengthened due to its inherent characteristics of nanosecond pulse laser, resulting in columnar structures with different spacing sizes. Meanwhile, the attenuation of the laser energy induced by Gaussian distribution characteristics is responsible for the insufficient ablation along the depth direction, causing some silicon carbide materials failing to reach their ablation threshold and re-solidifying on the inner surface. On the other hand, the faster laser scanning speed combined with waterjet’s cooling effect leads to a rapid decrease in temperature at the bottom of HAR microchannels. The molten material immediately solidifies within a short period of time, even with the assistance of low-pressure waterjet, also making it difficult to effectively remove material debrics formed at the bottom.
Noteworthy, many irregularly distributed micropores are discovered on and around columnar structures. The formation reason of micropores is attributed to the random damage by shock-waves and micro-jets generated after collapsing of cavitation bubbles [33]. For WJALM, many bubbles simultaneously produce with the laser-induced plasma when nanosecond pulse laser penetrates liquid water, and then they undergo multiple pulsations by continuously absorbing laser energy to cause an increasing pressure difference between the inside and outside of themselves. When these bubbles happen to collapse inside the microchannel, a recoil pressure is acted on sidewall surfaces to form cavitation phenomenon.
Fig. 15 presents the variation curves of areal surface roughness on the sidewalls for different laser powers. Combining Fig. 15 with Fig. 14(a), it can be observed the although some residues that is not fully ablated, the bottom sidewall surface of HAR microchannels is relatively flat with small areal surface roughness of about 20.4μm at low laser power of 18W. As the laser power increases to 30W in Fig. 14, resolidification phenomenon becomes a little more pronounced, and the spacing between two columnar structures gradually decreases. However, although the total depth is increasing, the machining quality of bottom sidewall surface becomes worse, as shown in Fig. 15. The reason for this phenomenon is that an increase in microchannel depth is more sensitive to larger laser power, and due to the bottom space is relatively narrower, making it more difficult to effectively discharge material debrics from inside the HAR microchannel. Even worse, the waterjet cannot play an effective impact effect during laser machining process at this moment.
3.3.2 Sidewall surface quality for varying scanning speeds
Sidewall surface morphology and areal surface roughness variation curves at varying laser scanning speeds are presented in Fig.16 and Fig.17. It can be seen from Fig.16(a) that at scanning speed of 600mm/s, there is only a small number of irregularly distributed micropores on the bottom surface, and almost no obvious recast layers are produced. The sidewall surface is relatively flat and smooth with the areal surface roughness of about 10.4μm, as noticed from Fig.17. As scanning speed increases, not only does the total-depth decrease, but also the machining quality of sidewall surfaces turns to deteriorate. It can be evidently seen that some regular columnar structures gradually form and continue to extend upward along the sidewall surface. Significantly, effective number of pulses per unit area is on the decline at higher scanning speed, resulting in the inability of laser energy to be transmitted downwards along the interior of the microchannel and insufficient removal of molten material. On the contrary, under waterjet’s cooling effect, the molten material rapidly solidifies to form bubble-like resolidified structure, which is the primary reason for a gradual increase in areal surface roughness in Fig.17.
3.3.3 Sidewall surface quality for varying waterjet velocities
The influence of laser powers on sidewall surface morphology and areal surface roughness are displayed in Fig. 18 and Fig. 19, respectively. It can be seen from Fig. 18(a)-(b) some microcracks and pits are appeared at lower waterjet velocity. The sidewall surface quality of HAR SiC ceramic microchannels is relatively poorer, and the areal surface roughness of bottom surface at 4m/s is about 20.5μm in Fig. 19. The insufficient material ablation of HAR microchannel is responsible for the above phenomenon. This is because the waterjet’s scouring effect applied on the HAR microchannel is not significant due to small impact force at this moment, while the dominant effect of waterjet also results in partial laser energy to dissipate. Subsequently, the impact effect of waterjet gradually becomes dominant, and the sidewall surface becomes smooth and flat with almost no slags and recast layer, as shown in Fig. 18(c). And the areal surface roughness obtained from Fig. 19 is only about 8.7 μm. Comparing Fig. 18(d) and (e), it can be noticed that there are some significant blocky slags and irregular pits produced on sidewall surface, and the machining quality turns to be worse. This phenomenon can be explained that slight shaking of the processing device caused by excessive waterjet velocity, which leads to the deviation of the waterjet impact center and the unstable focusing of the laser spot. Furthermore, as mentioned in section 3.1.3, the waterjet impact effect gradually loses its assisting role at waterjet velocity of 20m/s. In conclusion, although an increase in sidewall surface roughness is observed when waterjet velocity changes from 12m/s to 20m/s, the machined surface quality is still better than the lower velocity of 4m/s-8m/s.