2021 Volume 62 Issue 2 Pages 261-270
We used laser irradiation to create surface fine crevice structures, which induced region-selective super-spread wetting. These crevice structures enabled joining of Cu–Cu. We recently reported that laser irradiation can form surface fine crevice structures, not only on Cu but also on Fe surfaces. However, the formation mechanism of such structures is not yet understood. To clarify this mechanism, we performed in situ observations of Cu and Fe surfaces during laser irradiation, with the use of a high-speed camera and laser illumination. In this way we directly observed swelling and spattering caused by ejection of molten metal from laser-irradiated regions. We conclude that the formation of the surface fine crevice structure was caused by the accumulation of ejected molten metal, that is, entanglement of protuberances and spatter.
Fig. 12 Illustration of formation mechanism for the surface fine crevice structure; (a) metal surface during a single line scanning of laser irradiation process and (b) expanded view of region surrounding keyhole which shows the phenomena of swelling and spattering, (c) metal surface during parallel lines scanning of laser irradiation process (b) expanded view of region surrounding keyhole which shows the phenomena of accumulation of re-melted protuberance/newly ejected melts, previously formed protuberances-spatters. (e) 3-dimentional view of surface fine crevice structure.
Our group has been studying the use of laser irradiation to produce surface fine crevice structures on metal surfaces.1–5) Such structures comprise small asperities and micro-crevice gaps above and below the outermost surface, respectively, which facilitate super-spread wetting over a selected region. We have previously reported on the morphologies of surface fine crevice structures created by laser irradiation of Cu and Fe surfaces.1,3) Such surface structures enable super-spread wetting of liquid Bi, Sn on solid Cu and also wetting of liquid Sn, In on solid Fe. Furthermore, we were able to join Cu to Cu through microjoining without a solder fillet formation.1,2) We also successfully joined dissimilar Cu and Fe using this phenomenon.5) We posit that super-spread wetting of liquid metal on the surface fine crevice structure derived from capillary action through the three-dimensional connected microcrevice gaps.4) We consider that the fine surface structure mediates capillary action, that is, wetting behavior; nevertheless, the formation mechanism of the surface fine crevice structures produced by laser irradiation remains poorly understood. To further develop applications of this method, it is necessary to understand the formation mechanism of the fine structure during laser irradiation. This is of great importance for finding effective, controllable ways of generating surface fine crevice structures with good super-spread wetting performance.
In this study, we focus on the surface fine crevice structure of Cu and Fe, for which we have previously confirmed super-spread wetting behaviors.1–5) We investigate the formation mechanism of the surface fine crevice structures induced by laser irradiation of these metals. Visualization of the phenomena occurring at the metal surface during laser irradiation is the first step to understanding the formation mechanism. A high-speed camera enabled us to observe fast-moving objects in slow-motion, providing detailed insights into phenomena, such as fragmentation, penetration, and defect formation in the materials. This technique is commonly used to understand the dynamic of similar processes.6–8) We performed in situ observations of dynamic behavior, such as melting, spattering, solidification, at metal surfaces under Nd:YAG laser irradiation, through the use of a high speed camera in combination with laser illumination. We then investigated correlations between the observed dynamic behaviors on metal surfaces and the as-formed fine structures. Finally, a mechanism for the by laser-induced formation of the surface fine crevice structures is proposed.
Figure 1 shows a schematic illustration of the experimental setup used for in situ observations of the metal surface under a laser irradiation. Cu (99.96% purity) and Fe (99.5% purity) substrates were used as the materials. They were cut into 10 mm × 15 mm substrates and were cleaned by an ultrasonic machine with ethanol. A Q-switched Nd:YAG laser (Miyachi Corporation, ML-7062A) operating at 1064 nm was used to fabricate the surface fine crevice structure on the metal surface. The distance from the metal substrate surface to the scan lens was 110 mm. The laser spot diameter on the surface was 0.1 mm. The laser pulse was 160 µs with a frequency of 6.0 kHz, and the maximum output power of the laser was 50 W. The scanning rate of the laser irradiation was 10 mm/s. Single or multiple parallel lines were scanned along the X-direction. The scanning interval for each line along Y-direction was 0.01 mm.
(a) Schematic illustration of experimental set-up for in-situ observation of metal surface under the laser irradiation and (b), (c) direction of the observation and laser beam.: Sample surface is observed (b) from diagonally upward with respect to laser beam scanning in x direction, and (c) form side-view point with respect to laser beam scanning in y direction.
To observe the metal surfaces during the laser irradiation, a high-speed camera (Phantom, VED410L) was combined with laser illumination (Nobby Tech, Cavilux HF). The Nd:YAG laser irradiated area was illuminated at 810 nm and 500 W, that is, a higher energy than those of the process light. In addition, a narrow band pass filter was used with a central wavelength of 810 nm and half bandwidth of 12 nm. The filter was placed in front of the camera lens to block out all other wavelengths, including the intense laser light at 1064 nm. The formation process of surface fine crevice structures was recorded at a frame rate 10,000 f/s with a 10 µs illumination time. After laser irradiation of a single line and 100 parallel lines, the surface microstructures formed on Cu and Fe substrates were analyzed with a scanning electron microscope (SEM) and a three-dimensional (3D) laser scanning microscope (Keyence, VK-9700).
Figures 2 and 3 show high-speed camera photographs of the single-line irradiation process on a Cu surface, acquired without illumination and with illumination, respectively. In both cases, the observations were made diagonally upward from the direction of laser scanning, as illustrated in Fig. 1(b). In the non-illuminated photographs (Fig. 2) a large irradiation spot appeared over a large region; however, the surrounding area also had high-brightness, indicating that the laser irradiation process itself emits intense light. Figure 3 shows illuminated photographs. Here, the laser provided intense illumination at 810 nm and light reflected light by the Cu surface was captured. Light from the above-mentioned process laser was completely eliminated by blocking all other unwanted wavelengths, including 1064-nm laser light, with a narrow band pass filter. Thus, it was possible to observe changes to the Cu surface appearances during laser irradiation.
High-speed observation images of Cu surface under laser-irradiation process at single line scanning taken without illumination.
High-speed observation images of Cu surface under laser-irradiation process at single line scanning taken with illumination. (a) laser beam hits on the tip of the laser irradiated track indicated by red marked zone. Time after impact of (a): (b) 3 ms, (c) 15 ms and (d) 28 ms. (Laser beam moved from left side to right side.)
A keyhole, that is, a sunken surface filled with molten metal, is shown in Fig. 3, as a narrow region where the high energy laser beam was focused, with some movement from left to right. Figure 4 shows photographs observed from the side-view illustrated in Fig. 1(c) to illustrate the phenomenon occurred during the single-line irradiation process, i.e. side-view of the phenomenon which confirmed in Fig. 3. Figure 4(a) shows a side view of the Cu substrate immediately before the laser beam hit the surface. Figure 4(b) shows the moment when the laser beam hit the edge of the Cu substrate, and the keyhole formed downward through the plane of the image. Together, Figs. 3 and 4, show that interactions between the laser pulses and the Cu surface take place when the laser was irradiated. First, the solid-state Cu surface was rapidly heated and melted to form a keyhole. The laser irradiation conditions in this experiment included a distance of 1.6 µm between the two laser pulses and approximately 98% overlap of the individual irradiation spot diameters. Figure 4 shows that the keyhole extended downward as the melting progressed owing to overlap of the pulsed laser spots. This change of the keyhole was accompanied by dynamic motion, as follows. Figure 3 displays that a great number of microdroplets of molten Cu shown as multiple bright dots splashed in the vicinity of the keyhole edges, thereby resulting in a large amount of spatter. From Fig. 4, we confirmed that microliquid particles were torn away from the keyhole wall, as indicated by point A. These particles flew upwards and spattered, as indicated by point B. Molten Cu flowed upwards from the keyhole and formed piles at the either side of the keyhole walls, as indicated in Fig. 4(b)–(e). The consecutive upward outflow of molten Cu developed into a swelling with waves at the top of the keyhole walls. This swelling finally formed slender and sharped protuberances over the laser irradiated track boundaries in Fig. 3. The intense movement stopped when the laser passed. The features of single line irradiation in Fig. 3 and Fig. 4 suggest that two kinds of melt ejection, namely spattering and swelling, were generated when the laser beam was irradiated onto the Cu surface.
Side-view observation Cu surface under laser-irradiation process at single line scanning.: Side of the Cu surface observed (a) immediately before the laser beam hits the surface, (b), (d)–(f) while laser beam hits the surface and (c) extended image of (b).
The surface microstructure of the laser-irradiated area was observed with the use of a scanning electron microscope and a 3D laser scanning microscope, as shown in Fig. 5. In Fig. 5(a) and (b), some microparticles were found on the surface within and beside the laser-irradiated track. The spatter that flew upwards, as described in Fig. 3 and Fig. 4, would fall down due to the gravity and settle down onto the surface. Wave-shaped protuberances were also formed at the edge of the laser-irradiated track. Figure 5(c) and (d) show a 3D view and the profile along the blue dotted line in the 3D view, which indicate that the protuberances were higher than the substrate. Meanwhile, the central line of the track was below the substrate. As described above, molten Cu flowed out from the keyhole and piled up at the top of the keyhole walls. This melt-ejection phenomenon led to the formation of a depression over the track central-line with a depth of approximately 25 µm and width of approximately 35 µm, and upward protuberances with a height of approximately 15 µm.
The surface morphology of laser-irradiated area on Cu substrate.: (a) top-view, (b) expanded view observed by SEM, (c) 3-dimentional view observed by a 3D laser scanning microscope and (d) surface profile corresponding the blue-dotted line in (c).
SEM images of the laser-irradiated Cu substrate after 100 parallel lines scanning starting from the downside and ending at the upside are shown in Fig. 6(a)–(c). A complex “splashing rainwater-like” microstructure appears in the laser irradiated area. An expanded view of the last scanning track region of Fig. 6(b) shows that protuberances on the last scanning track attached to those of adjacent scanning tracks. Moreover, the microstructure comprised protuberances, such as those shown in Fig. 6(b), which deformed and grew to form the microstructure, shown in Fig. 6(c). From Fig. 6(b), (c), we confirmed that much of the micrometer-sized spatter was randomly scattered on the outermost surface of the protuberance structure. In addition, spatter that landed on the surface had a spherical or near spherical shape. We considered that both the spatter and the laser irradiated surface had sufficiently cooled down or completely solidified during the fall of spatter. When the spatter landed, the surface of the protuberances and the keyholes were not completely agglomerated with spatter because these instantly solidified. Figure 6(d) shows a 3D view of the laser-irradiated surface observed by a 3D laser scanning microscope, and Fig. 6(e) shows a cross-sectional surface profile corresponding to the blue-dotted line in Fig. 6(d). Protuberances with a height of approximately 13–30 µm and thickness of 12–25 µm were randomly distributed. Furthermore, microgaps which are the narrow space shaped like crevice with a depth of approximately 19–31 µm were found among the unevenly formed protuberances. Moreover, the slightly notched line in Fig. 6(e) indicates the presence of spatter, which covered the surface of the protuberances and crevice-like microgaps. This random spatter over the main structure consisted of protuberances, which led to a further non-uniform and complex microstructure in the gaps among the protuberances.
The surface morphology of Cu substrate after 100 parallel lines scanning of laser irradiation process.: (a) SEM image of the laser-scanned area, (b), (c) expanded view of local features, (d) 3-dimentional view observed by a 3D laser scanning microscope and (e) surface profile corresponding the blue-dotted line in (d).
Figure 7 shows photographs of the laser irradiation process for the last ten-line passes over the Cu surface. The lower part in Fig. 7(a), indicates an area subjected to a previous laser pass, where nine parallel left-to-right lines were scanned from the bottom to top in the image. In Fig. 7(a), the keyhole side of the front track can be clearly seen. The area of the previous laser passed area was out of focus and faded. However, light was reflected and scattered from the laser passed area with high intensity (brightness). The brightness suggests that a rough surface formed, which agrees well with the SEM observations in Fig. 6. Considering the 0.01-mm scanning interval between each scanning line, the images in Fig. 7(b)–(i) allow us to directly observe parallel lines of irradiation at the front track with a 90% side-overlap between two adjacent laser scanning tracks (i.e., the final and penultimate tracks). The region with high-level brightness, marked in Fig. 7(b), corresponds to the keyhole, that is, the region where the laser spot reached. The surface of the front track, which solidified after the ninth-line irradiation was re-melted by the subsequent tenth-line of irradiation. The re-melting induced by the multiple parallel lines of irradiation also accompanied ejection of molten Cu from the keyhole, in the form of spatter and swelling. These features perturbed the deformation of the previously formed surface microstructure after nine parallel scan lines. The deformation of the microstructure and protuberances were created by the repeated ejection of molten Cu and swelling. First, we consider the protuberance marked A in Fig. 7. The as-formed protuberance A started to remelt and collapse as the laser beam approached, as shown in Fig. 7(b)–(c). However, it then started to swell up again because of the upward out flow of the newly melted surface under continuous laser irradiation, to form a swelling, as shown in Fig. 7(d)–(e). Swelling A was redeposited with a vibration. Thereafter, swelling A solidified at the end of the laser pass. Another protuberance B, marked in Fig. 7 also remelted and swelled upward, as shown in Fig. 7(e)–(f). However, microliquid particles torn from swelling B, formed a liquid spatter B′ [Fig. 7(g)–(h)]. Additionally, several tiny liquid particles directly ejected from the melts appeared to be spattered in the vicinity of the keyhole, as shown in the overall laser pulses of Fig. 7. These particles fell under gravity and landed on adjacent surfaces, including the laser track area and previously irradiated areas, as indicated by C in Fig. 7(h).
Images of Cu surface (a) immediately after the ninth-lines scanning of laser irradiation and (b)–(i) under laser-irradiation at the tenth parallel line scanning. ((b) Laser beam hits on red marked zone and (c)–(i) moves to the right side.)
Taken together, the results shown in Figs. 6 and 7, indicate that the previously formed protuberances remelted and joined with newly melted Cu or other protuberances, repeatedly, owing to overlap of the laser scan tracks. This phenomenon forms connections among neighboring protuberances, as described in Fig. 6(b). In addition, a microstructure comprising protuberances, such as those shown in Fig. 6(b), deformed and grew into the microstructure, as shown in Fig. 6(c). On the basis of the above in situ observations, we suppose that this microstructural deformation is caused by repetitive protuberance re-melting, re-swelling, and formation of spattered particles, which redeposited and finally resolidified through the parallel multiple-line irradiation.
3.3 Fe surface appearance under single-line scan of laser irradiationFigure 8 shows an Fe surface observed by the high-speed camera under the same observation conditions used for imaging in Fig. 3, with the laser beam irradiated along a single line. The white part at the tip of the irradiated track indicates the keyhole being irradiated by the laser. Melt ejection of the spatter and swelling were also found for the Fe surface. As indicated by point A in Fig. 8(a), a small and round swelling was generated at the top of the wall close to the laser-irradiated keyhole. Figure 8(c) and (d) confirm that the next swelling, indicated as B developed under the advancing laser, swelling A continued to grow and wobbles owing to agglomeration of existing swelling and ejection of newly melted Fe from the keyhole. In addition, the molten Fe piled up in separate swellings A and B because of the tendency for the surface tension to reduce the surface area and unstable phenomenon, such as the flow oscillation associated with surface tension, viscosity, and thermal expansion. Figure 9 shows the surface microstructure at the Fe substrate after a single-line of laser irradiation, as measured by SEM and 3D laser scanning microscope observations at the same magnification used in Fig. 5. The surface of the laser-irradiated track was covered with microparticles, derived from spattered particles. A waved structure with undulating protuberances formed at the boundaries between the laser irradiated track and the non-irradiated smooth surface. The clear formation of these undulating protuberances reflects the separate swelling of the melts in Fig. 8(b)–(d) caused by surface tension. The surface profile of Fig. 9(c) corresponds to the blue dotted line in the 3D view of Fig. 9(b). Similar to the single line irradiation of Cu, molten Fe piled up on both sides of the keyhole walls. Therefore, as shown in Fig. 9(c), the single line scan of the Fe surface had a microstructure with protuberances 67.9 µm in height and depressions 52.1 µm in depth over a width of 84.3 µm. When compared with the results in Fig. 5, we note that the volume of the protuberances and depressions formed on the Fe surface was clearly greater than those formed on the Cu surface. Hence, a greater amount of the melt was ejected from the keyhole of the Fe surface, compared with the Cu surface. This behavior affects the morphological changes during laser irradiation and the final morphology of the microstructure. Furthermore, it eventually leads to various morphologies of the surface fine crevice structure depending on the material, as described later.
Images of Fe surface under laser-irradiation at single line scanning. Fe surface was observed (a) at moment that laser beam hits on the tip of the laser irradiated track indicated by red marked zone and (b)–(f) while laser beam moves to the right side.
The surface morphology of laser-irradiated area on Fe substrate.: (a) surface observed by SEM, (b) 3-dimentional view observed by a 3D laser scanning microscope and (c) surface profile corresponding the blue-dotted line in (b).
The surface microstructure of the Fe substrate after laser irradiation with 100 parallel lines scanning is shown in Fig. 10. In Fig. 10(a), the lower part of the image is a laser-irradiated region and the upper part shows the original unirradiated Fe surface. A “broccoli-like” coarse microstructure was formed over the laser irradiated area. From Fig. 10(b) of the expanded view, some ripples and microparticles were observed at the surface of protuberances near the last scanning track. Moreover, as shown in Fig. 10(c), broccoli-shaped protuberances formed in the middle region of the laser irradiated surface. However, no microparticles, such as those observed in Fig. 10(b), appeared although larger particles were agglomerated on surface. Figure 10(d) and (e) show a 3D view of the laser-irradiated surface observed by the 3D laser scanning microscope and the surface profile corresponding to the blue-dotted line in Fig. 10(d), respectively. Numerous blunt protuberances accumulated with the appearance of a surging wave. We note that the protuberances in the region where the laser scanning start [right side in Fig. 10(e)] had a high height of approximately 130 µm and a thickness of 90–110 µm. Moreover, deep and sharp crevice-like microgaps with a depth of approximately 62 µm were found between the protuberances. The protuberances in the region where the laser scanning finished [right side in Fig. 10(e)] had a low height of approximately 63 µm and thickness of 20–50 µm. Gaps formed between them had a shallow depth of approximately 45 µm. Thus, the protuberance structure grew and became rougher as the laser was repeatedly irradiated. Moreover, this microstructure consisted of larger protuberances than those formed on the Cu surface.
The surface morphology of Fe substrate after 100 parallel lines scanning of laser irradiation process.: (a) SEM image of the laser-scanned area, (b), (c) expanded view of local features, (d) 3-dimentional view observed by a 3D laser scanning microscope and (e) surface profile corresponding the blue-dotted line in (d).
Figure 11 shows the Fe surface observed following several parallel multiple-lines of laser-irradiation. The Fe surface observed immediately after the first-line of irradiation [Fig. 11(a)] had several protuberances along the laser irradiation track boundaries. These protuberances re-melted and the melted metal slid down following laser irradiation; however, subsequently the melts piled up at the keyhole wall, as new swellings formed with a notable waviness, marked by A and B in Fig. 11(b) and (c). The two adjacent swellings marked A and B collided through vibrational motion in their growth. Hence, these features agglomerated into a single large swelling [Fig. 11(d)]. Thereafter, the large agglomerate continued to increase in volume and change its shape, becoming round after the laser passed [Fig. 11(e)]. We compared the results in Fig. 11(f) with those in Fig. 11(a) to confirm that the wavy protuberances at the keyhole wall coarsened throughout the second irradiation line, owing to overlap of the lines. Figure 11(g)–(j) shows images of spatter, particularly that generated by being torn off from the surface of the swelling. The spatter C′ tore off from swelling C and then landed on the substrate surface. Spatter D′ was torn off from swelling D. In real-time, a neck formed between the remaining swelling and the detached liquid droplet, as shown in Fig. 11(j). This type of spatter can also cause the volume of swelling protuberances to shrink. Spatter that was also formed because of direct ejection of liquid droplets from the keyhole, was observed as many bright tiny-dots around the keyhole after completion of the laser pulses (Fig. 11). Subsequently, this spatter landed on adjacent surfaces, as indicated in Fig. 11(c), (h), (i), contributing to the formation of the microstructure.
Images of Fe surface: observed (a) immediately after the first-line irradiation, (b)–(e) under the second-line irradiation, (f) immediately after the second-line irradiation, (g)–(j) under the third-line irradiation.
In combining Figs. 10 and 11, we confirmed that, scanning of multiple parallel lines with overlapping laser scanning tracks caused re-melted liquid Fe to pile upward and subsequently stack on the surface of protuberances, forming ripples, as shown in Fig. 10(b). Because this phenomenon is accompanied by the release of spatter, as described in the in situ observations, some microparticles were also found on the protuberances surface in Fig. 10(b). The processes of repeated re-melting and re-solidification of protuberances generated a considerable accumulation of melt-features, such as spatters and ripples. As a result, the stack of the microstructure described in Fig. 10(b) was improved under multiple cycles of laser scanning. The repeated stacking of melt ripples and spatter yielded micro-aggregates with a broccoli-like shape. Hence, a microstructure with broccoli-shaped protuberances formed on the Fe surface.
3.5 Formation of surface crevice structure fineOn the basis of the above results, we propose a mechanism for the formation of the surface fine crevice structure created by laser irradiation, as follows. Figure 12 illustrates the proposed formation mechanism. First, the intense laser beam interacted with the metal surface, whereupon the metal surface melted and simultaneously evaporated. This evaporation, which is associated with the escape of the metal atoms from the surface, generates a recoil pressure on the irradiated surface.9,10) The recoil pressure, P, is expressed as follows:11)
| \begin{equation} P(T) = 0.54P_{a}\textit{exp}\left[\frac{\lambda}{k_{B}}\left(\frac{1}{T}-\frac{1}{T_{b}}\right)\right], \end{equation} | (1) |
Illustration of formation mechanism for the surface fine crevice structure; (a) metal surface during a single line scanning of laser irradiation process and (b) expanded view of region surrounding keyhole which shows the phenomena of swelling and spattering, (c) metal surface during parallel lines scanning of laser irradiation process (b) expanded view of region surrounding keyhole which shows the phenomena of accumulation of re-melted protuberance/newly ejected melts, previously formed protuberances-spatters. (e) 3-dimentional view of surface fine crevice structure.
A comparison of the single-line irradiation tracks in Fig. 5 and Fig. 9 and the surface fine crevice structures in Fig. 6 and Fig. 10 indicates differences in the microstructures formed on the surfaces of the Cu and Fe substrates. Specifically, the surface fine crevice structure of Cu had a complex “splashing rainwater-like” microstructure with thin protuberances, whereas that of Fe had a “broccoli-like” coarse microstructure with large protuberances, despite the use of the same laser irradiation conditions. These differences are attributed to phenomenon occurring at the metal surface that produce the surface fine crevice structure, which depends on the material properties. For example, liquid surface tension, melting temperature, and boiling temperature are important factors in the mechanism in the formation of the surface fine crevice structure. The surface tension of liquid Cu and Fe are expressed as a function of temperature as follows:12)
| \begin{equation} \text{Cu:}\ \sigma_{\textit{Cu}}(T) = 1374 - 0.26(T-T_{m})\,\text{mN/m}, \end{equation} | (2) |
| \begin{equation} \text{Fe:}\ \sigma_{\textit{Fe}}(T) = 1909 - 0.52(T-T_{m})\,\text{mN/m}, \end{equation} | (3) |
We performed in situ observations of the surface of Cu and Fe irradiated by a 1064-nm laser and used a high-speed camera to investigate the formation mechanisms of the surface fine crevice structure. We combined the high-speed camera with 810-nm laser illumination. By visualizing only light reflected at this wavelength, we observed dynamic motion occurring at the metal surface during a laser irradiation, without interference from the intense laser light used in the treatment process. On the basis of these observations, the formation mechanism of the surface fine crevice structure is summarized as follows:
Molten metal at the laser irradiation keyhole swells from the keyhole wall. This upward swelling of molten metal ejects from the keyhole and forms a depression along the laser irradiation line with protuberance structures. Much of the spatters ejected from the laser irradiated zone thereafter lands in the vicinity of the laser track. The spatter forms a complex microstructure, which is covered by microparticles or micro-aggregates, above the surface of the protuberance structure. The accumulation of ejected molten metal during the multiple line scanning leads to the formation of microstructures. The protuberances and spatter become entangled with crevice-like microgaps between them, that is, the surface fine crevice structure.
However, because the properties of each metal are different, the tendencies to generate swelling and spatter differ between different metals. Therefore, the morphologies of the surface fine crevice structure depend on the material. Further investigations will be performed in the future to identify which metal properties affect the formation of the surface fine crevice structure.
