Laser Cutting Characteristics Using High-speed Camera of 75-㎛ Thick Stainless Steel Foil in Lightweight Fuel Cell Applications
Article information
Abstract
The automobile industry is attempting to suppress carbon emissions by developing fuel-cell electric vehicles (FCEVs), which are pollution-free vehicles. Because the fuel cell stack is manufactured by stacking numerous thin parts, reducing the thickness of each part to reduce the weight and volume is important. Therefore, precise cutting of the thin plate material is required. In this study, an IPG YLS-5000 fiber laser model, a 5-kW single-mode fiber laser with a beam size of 30 ㎛, was used to cut a thin plate STS316L material with a thickness of 0.075 mm. The cutting process was studied using a 650-nm wavelength filter and a high-speed camera with a frame rate of 30,000 frames per second. This study involved the investigation of various research components, such as the process from the formation of melt to hardening, the amount and divergence angle of the fume, and the size and speed of the spatter. In the process of forming the molten pool, shaking of the molten liquid, merging between the molten liquid, and sticking of the molten liquid were confirmed. This study will be helpful for future research on the quality improvement of laser cutting for very thin metal sheets.
1. Introduction
Laser cutting is a noncontact process, i.e., there is no cutting resistance of the workpiece and it is possible to process fine and complex shapes in combination with numerical control equipment1). Owing to these characteristics, laser cutting is widely used in many industries including automotive, shipbuilding, and electronics industries2,3). The laser process principle is as follows: a high-power laser beam irradiated on the workpiece surface. The properties of the cut material and beam intensity significantly affect the cut surface, creating a highly concentrated heat spot. This intense heat causes the material to melt and vaporize, potentially triggering chemical transformations before being removed with the help of a high-pressure assist gas4-11). Additionally, the molten metal and spatter generated during the cutting process have a significant impact on the overall quality of the workpiece12-16). Notably, fiber lasers used in more recent studies have shorter wavelengths compared to those employed in earlier laser cutting applications such as CO2 laser. Fiber lasers are a type of laser highly advantageous for field applications due to their ability to transmit laser beams through optical fibers. Additionally, they offer the advantage of focusing high- power laser beams, allowing high-density energy to be absorbed at the focal point and enabling the cutting process to proceed at a relatively fast speed. This reduces the amount of input energy required for processing the workpiece and minimizes the heat-affected zone (HAZ), thereby reducing deformation after cutting17,18).
Therefore, more precise and fast cutting is possible via using fiber lasers for laser thin plate cutting as compared to that by other thermal energy utilization cutting methods, including oxygen cutting and plasma cutting19). As shown in Fig. 1, the CO2 laser has a long wavelength of 10.6 ㎛, resulting in a low energy absorption rate of less than 10 %. By contrast, the fiber laser has a shorter wavelength of 1064 nm. Accordingly, each material has excellent energy absorption21-25). Additionally, the beam pumped from the laser diode and absorbed by the fiber core while fully reflecting through the fiber clad emits light; moreover, because the beam is amplified by fiber Bragg grating, the oscillation efficiency is as high as 25 % compared to that of CO2 lasers amplified by a resonator.
These features of laser cutting are well suited to the production of fuel cells comprising thin sheet materials. Fuel cells require high-precision sheet metal processing methods because reducing the thickness of each material sheet can help reduce their weight and improve efficiency26,27). To find the optimal process conditions, Abdel Ghany and Newish assessed the optimal Nd: YAG laser cutting process parameters for 1.2-mm austenitic stainless steel sheets28). However, in addition to laser cutting parameter variables such as laser power, cutting speed, cutting gas, etc., other variables need to be considered in thin-plate processing. During laser cutting, re-solidified layer, spatter, and dross are generated and thermal deformation occurs depending on the amount of input energy. Moreover, sagging, vibration, and bending occur during processing due to the small thickness. To solve these problems, jig design and appropriate laser processing methods need to be investigated29,30).
This study aims to investigate the flow dynamics of the molten pool formed during high-precision laser cutting of thin stainless steel sheets. To examine the molten metal’s dynamic behavior, a single-mode fiber laser was employed on a stainless steel sheet, with all conditions tightly controlled while varying laser power and cutting speed. Additionally, the dynamic morphology of the molten metal was captured using a high-speed camera. The findings from this study will offer valuable insights and guidance for optimizing high-quality laser cutting processes in the future.
2. Experimental methods
2.1 Laser beam cutting process
The laser beam cutting process involves heating the material with a laser beam to bring it to a molten state, and then using a cutting gas to remove the molten material4). For general laser cutting, a cutting process gas is used along with an optic head. In this experiment, a scan head was used instead of a gun type optic head used in general laser cutting. Scan heads can be processed faster than conventional optical heads. However, the cutting experiment was conducted under process conditions similar to ablation without a nozzle because the moving speed of the nozzle from which the cutting gas was generated was limited. The laser equipment used in this study was YLS-5000, a single-mode fiber laser system with a peak power of 5 kW, power deviation of less than 2%, and wavelength of 1080 nm. Single-mode fiber lasers are characterized by high energy localized irradiation with considerably high laser beam quality. The quality of the laser beam is represented by a beam parameter product, and the single-mode laser used in this system can achieve high energy density with a small beam spot of 30 ㎛ or less.
Fig. 2 shows the beam profile schematic diagram of a high-power fiber laser. As can be seen in the figure, the single-mode laser has a Gaussian energy distribution of TEM00 with a peaked center. In case of a multimode laser, the energy distribution is closer to a flat top, with multiple bands of light overlapping due to diffuse reflection. In this study, a single-mode fiber laser was used for laser cutting of ultra-thin plates because it provides a high energy density by concentrating a large amount of energy on a small area.
Beam path schematic of single mode and multi-mode fiber laser
2.2 Materials
The material used in this experiment included STS316L, an ultra-thin plate measuring 80 mm long × 40 mm wide x 0.075 mm thick. This material is widely used for manufacturing fuel cell separators, and the material type and thickness were selected according to the thickness of DANA VICTOR REINZ’s fuel cell products. The chemical composition is shown in Table 1. Stainless steel has excellent corrosion resistance due to the large amount of Ni and Cr added to carbon steel. However, the viscosity of the molten metal during laser cutting is higher than that of carbon steel. This contributes to low surface roughness and dross generation, that can be disadvantageous for manufacturing high-quality products.
Chemical composition of STS316L (Wt %)
2.3 Experimental method
In this study, the experimental apparatus included the following: the scanner device used in this study was KScan-300. A QD connector is used to establish a fiber connection to the scan head. The scan head is mounted on a 6-axis industrial robot with a payload of 60 kg. KScan-300 uses fused silica material with F-theta lens of F-420mm, a focal length 420 mm, a laser beam size of 30 ㎛, and a working file size of 200 × 200 mm. The laser beam was controlled by Scout vision utilizing a CCD camera and scanner. As shown in Table 2, parameters of laser cutting are categorized into two major sections: laser beam and process parameters4,6,31,34). Miroslav Radovanovich, Milos Madik32) suggested that cutting conditions such as laser power, cutting speed and gas pressure are the most commonly used parameters to study cutting quality. In this experiment, the variables selected to determine their influence on cutting quality include laser power and cutting speed. Laser Power was 2-5 kW and laser speed was 500-3,000 mm/s. In this experiment analyzing the behavior of the molten pool on the cutting process, most of the laser cutting parameters were held constant, with the exception of the aforementioned conditions. The specimen was fixed by jig pressing with bolts at both ends. A high-speed camera was used to observe the formation, movement, and solidification of the molten metal during the cutting process. Furthermore, an additional filter with a wavelength of 650 nm was incorporated in the experimental arrangement to prevent light reflection during the cutting process. The detailed experimental arrangement is illustrated in Fig. 3.
Classification of laser cutting parameters
The schematic of the experimental arrangement
3. Results and discussion
3.1 Molten Pool Behavior with Varying Laser Cutting Speed
The melting of material by the laser beam causes a flow of molten metal immediately after cutting; this flow depends on several parameters such as laser travel speed, power, and material properties. In a cutting process without cutting gas, the cutting mechanism involves a composite process where the molten metal is sufficiently melted and removed by gravity and the recoil pressure from evaporation. The molten metal that has not been removed solidifies and remains in the form of dross. In this section, to determine the characteristics of the molten metal according to the cutting rate, the difference in the aspect of metal flow was compared by fixing the laser power at 3.5 kW and changing the laser travel velocity. The flow of the molten metal according to the speed was observed; results are depicted in Fig. 4-6.
High speed imaging showing the formation model of molten metal on the wall surface of the laser cutting process
High speed imaging showing the Shape deformation due to over-heat at laser power of 3.5 kW Travel speed of 500 mm/s.; (A) 30 ms, (B) 50 ms, (C) 140 ms)
High speed imaging showing the spatter patterns based on the laser cutting speed using a high-speed camera
In this study, the primary parameters under investigation were laser velocity and output laser power; additionally, the following foundational conditions were set as nozzle-free, continuous wave emission, and a focus distance of 0 mm. Fig. 4 shows the High speed imaging data of the flow formation of the molten metal during the cutting process with a laser power of 3.5 kW; and, a cutting speed of 500, 1,500, and 2,500 mm/s using an ultrahigh-speed camera respectively. Under the condition of 500 mm/s, it is observed that the molten metal solidifies into a lumped state; there is no large flow of molten metal to form dross. Under the condition of 1,500 mm/s, the molten metal moves in the opposite direction of the cutting path, stirring and subsequently solidifying into dross. As a result, the dross is smaller in size compared to the 500 mm/s condition and is more widely distributed along the longitudinal direction. Additionally, it is observed that the molten zone expands due to the residual thermal energy left in the molten pool after the laser passes.
At 2,500mm/s, a thin layer was formed due to strong fluidity along the cut surface; additionally, a large amount of spatter occurred. There was no considerable flow of the molten metal along the cut surface. In conclusion, in this observation, the flow of the molten metal in the cutting axis direction was weak under a relatively slow cutting speed. Thus, it is observed that when the laser transition speed is slow, the flow power of the molten metal is low. It is also observed that the flow of the molten metal resulting from the laser transition speed of 1,500 mm/s is wider than that resulting from the speed of 500 mm/s in the cutting axis direction and it acts as a large force as it moves rapidly. At 2,500 mm/s, it is thought that the molten metal is scattered in the form of a spatter as the amount of the molten metal flowing along the cut surface is small and the force in the cutting axis direction becomes stronger.
Fig. 5 shows the cutting process under conditions of laser power of 3.5 kW and speed of 500 mm/s under severe thermal deformation using a high-speed camera. As can be seen in the figure, the material is melted by the laser heat and a certain amount of molten metal cannot escape and adheres to the cut-edge surface. Moreover, the material around the molten metal adhering to the cut surface is deformed over time. The high thermal energy of the adhered molten metal is transferred to the surrounding materials, causing them to deform. Furthermore, a difference between the kerf width at +20 ms and that at +110 ms is observed during cutting process. In laser cutting of stainless steel foil, the kerf width is formed through different mechanisms depending on the cutting process conditions. When relatively low-speed laser cutting is applied to ultra-thin materials, a larger molten zone is created, and despite some loss of molten material due to evaporation pressure, the remaining molten material adheres to the cut surface. The heat transfer to surrounding areas also causes significant thermal deformation in the processed material. Additionally, the slower cutting speed leads to the formation of large, droplet-shaped dross on the cut surface. These observations highlight how low-speed laser cutting affects molten zone and dross formation, emphasizing the need for precise adjustment of process parameters to achieve optimal cutting quality33).
3.2 Spatter Behavior with Varying Laser Cutting Speed
Spatter refers to sparks that scatter due to excessive energy injection during laser cutting process. It is observed that the main factors in spatter generation are high laser power and cutting speed. In this section, the behavior of spatter is compared as a function of laser cutting speed. The experiments were conducted at a fixed laser power of 3.5 kW with varying speeds, and the results are presented in Fig. 6. The velocity of the spatter was calculated as a scalar based on the distance and time traveled in the transverse direction. It is shown that, as the cutting speed increases, both the amount of spatter and its scattering speed are increased. Spatter ‘A’ was observed to move 1.72245 mm in 0.8 seconds, confirming a flow speed of over 2870 mm/s, while spatter ‘B’ moved 1.9149 mm in 0.8 seconds, confirming a flow speed exceeding 3191 mm/s. Spatter ‘C’ did not splash onto the surface of the specimen. At a cutting speed of 3,000 mm/s, the spatter was only partially scattered, with some adhering to the workpiece and stopping after traveling a certain distance.
In this observation, it was confirmed that as the cutting speed increases, the dispersion speed of the spatter also increases. At a cutting speed of approximately 3,000 mm/s, some spatter adheres to the base material, moving and accumulating on the specimen surface. This results in increased quality inconsistency in the laser-cut section when using a scanner head without a high-speed nozzle.
3.3 Influence of Laser Cutting Speed on Kerf Width Formation
In laser cutting, the kerf width refers to the distance between the cuts after the laser cutting process. There are various parameters that determine the kerf width of laser cutting; however, this section compares the characteristics of the cutting process according to the laser cutting speed. The beam size of the laser system used in this experiment is 30 ㎛, the laser focus position is fixed, and the laser power is fixed at 3.5 kW; moreover, the cutting quality according to the cutting speed is shown in Fig. 7. As shown in this figure, large partially agglomerated dross can be observed in the upper and lower regions of the kerf width at a relatively slow cutting speed of 500 mm/s. Furthermore, the upper surface photo shows that at a cutting speed of 1,000 mm/s or less, a prominent HAZ-estimated area appears as a reddish-blue surface oxidation area centered on the cut surface. In this study, kerf geometry observation confirmed that a power of 3.5 kW and a cutting speed of 2500 mm/s yield the best cutting quality. The kerf width at this best cutting condition is 159.3 ㎛, which is wider than the beam size of 30 ㎛. This is because the thickness of the cut material is considerably thin, so heat accumulation occurs easily and heat is easily transferred to the sides centered on the energy irradiation position; thus, the formed melt zone is larger than the beam size. Furthermore, this tendency to widen the kerf width is more pronounced at slower laser processing speeds. At cutting speeds less than 1,000 mm/s, thermal deformation during processing is easily observed.
Top view of the final surface of cutting specimen by speed with a laser power of 3.5 kW. ( (A) cutting speed of 500 mm/s; (B) cutting speed of 1000 mm/s; (C) cutting speed of 1,500 mm/s; (D) cutting speed of 2000 mm/s; (E) cutting speed of 2,500 mm/s; (F) cutting speed of 3000 mm/s
In conclusion, this observation shows that the higher the cutting speed, the smaller the size and variation of the dross, the narrower the kerf width, and the more uniform the cutting quality. However, with respect to the cutting quality corresponding to the fastest cutting speed (3,000 mm/s), the cutting layer has an irregular thickness compared with that at the 2,500 mm/s cutting speed. This is partly due to the excessive unevenness in the process of removing melt and spatter at extremely high speeds.
Fig. 8 presents data from cutting 0.075 mm thick STS316L sheet specimens, using various laser power levels and speeds, with kerf width measurements plotted accordingly. The laser cutting speeds range from 500 mm/s to 3,000 mm/s in 500 mm/s increments, with power levels of 2 kW and 3.5 kW. As anticipated, higher cutting speeds result in narrower kerf widths with reduced deviation from the mean. Under all speed conditions, a laser power of 3.5 kW consistently produces a wider kerf width. Importantly, cutting was unsuccessful at 2 kW and 3,000 mm/s but succeeded at 3.5 kW, underscoring the importance of sufficient power for effective cutting at higher speeds.
Experimental results of kerf width against the laser speed of STS316L with a thickness of 0.075 mm
Fig. 9 shows optical microscopy (OM) images of the cross-section of laser-cut specimens. The droplet-shaped features at the edge of the specimens represent dross structures, where molten metal adhered to the cut surface without being removed. As observed from surface images, slower cutting speeds generally lead to larger dross formation.
Cross-sectional photographs of cutting specimens by changing cutting speed with a laser power of 3.5 kW
In cases with larger dross, structural growth within the dross varies, progressing differently at the outer edge compared to the center. Notably, at speeds of 2,000 mm/s and above, some molten metal remains attached to the cut edge, but it does not exceed the thickness of the base foil, indicating relatively high cut quality. Additionally, under the high-speed condition of 3,000 mm/s, minimal solidified structure is observed at the edge, with almost no residual molten or solidified material.
3.4 Influence of Laser power on Kerf Width Formation
In the previous chapter, the cutting characteristics as a function of the cutting speed were discussed. In this chapter, the effect of laser power on the kerf width is discussed. The beam size and focus position are the same as that in the previous chapter, and the cutting speed is fixed at 3,000 mm/s; the cutting quality according to the laser power is shown in Fig. 10. As shown in this figure, no cut was made at the smallest laser power of 2 kW, the melt was hardened immediately after solidification, and no kerf width was produced. At 2.5 kW, a partial cut was made with no cut in the middle.
Photograph of surface of cutting specimen by varying laser power and a speed of 3000 mm/s; (A) Laser Power of 2 kW, (B) Laser Power of 2.5 kW, (C) Laser Power of 3.5 kW, (D) Laser Power of 5 kW
Beyond 2.5 kW, it becomes evident that higher power outputs result in wider kerf widths and a broader range of kerf widths. There is a tendency for the dross size to increase with higher outputs. However, under the 2.5 kW condition, the molten pool is not fully expelled, leading to growth that combines with dross formed on the opposite cut surface, resulting in bridge-type dross formation. Additionally, the molten pool’s area may not be sufficiently reduced to achieve complete cutting, causing defects resembling a bridge shape.
It was confirmed from the kerf geometry observation in this study that the conditions of power of 3.5 kW and cutting speed of 3,000 mm/s had the best cutting quality. The kerf width at the best cutting condition, that is, a laser power of 3.5 kW, was completely cut without partial cutting and bridge-type dross. It is observed that the kerf width is narrower at 162 ㎛ compared to the 240 ㎛ kerf width of the 5 kW high-power cut. This can be attributed to the appropriate laser power to melt the material at a cutting speed of 3,000 mm/s. Furthermore, as the power increases, the wide and uneven kerf width can be observed from the ultrahigh-speed camera observation that the molten metal becomes more abundant as the laser power increases, and that the abundant molten metals are subjected to transverse flow generated by the high cutting speed.
In short, this observation shows that lower laser power results in no cut or partial cut. The higher the power during cutting, the wider is the kerf width, the greater is the variation, and the more uneven is the quality of the cut. Therefore, setting the laser power appropriately becomes a crucial factor in ensuring high cutting quality.
Fig. 11 presents observations of the cut cross-section when laser irradiation power is varied under a cutting speed condition of 3,000 mm/s. These images provide a relatively clear view of the melted regions within the material. Under the 2 kW laser power condition, the specimen appears connected, with boundary regions indicating rapid cooling from the bottom of the molten pool following complete melting. Specimens cut at 2.5 kW exhibit similar shapes to those cut at 2 kW, but with successful cutting, indicating insufficient energy per unit area and resulting in incomplete cutting. Under the 3.5 kW condition, superior cutting quality is observed, with clean cross-sectional views where the bottom portions are melted and removed. This suggests full penetration along the cutting line and peripheral melting due to the higher input energy. At 5 kW, the cross-sectional morphology resembles that at 3.5 kW; however, dross formation is evident. This is attributed to uneven molten flow from excessive laser speed and increased molten metal generation due to the 5 kW output.
Cross-sectional photographs of cutting area by changing laser power at 3000 mm/s
Fig. 12 displays the kerf width sizes at a cutting speed of 3,000 mm/s with power outputs of 2 kW and 3.5 kW, illustrating the trend of kerf width relative to laser power. In conclusion, at 3.5 kW power and a 500 mm/s cutting speed, significant shape distortion from thermal deformation prevented successful cutting; similarly, at 2 kW power and 3,000 mm/s cutting speed, cutting was unsuccessful due to insufficient power. Under other conditions, kerf width trends showed that as power decreased and speed increased, kerf width tended to narrow; however, no drastic change in kerf width was observed.
Experimental results of kerf and dross width against the laser power of STS316L with a thickness of 0.075 mm
3.5 Cutting kerf and Dross formation mechanism
Kerf width and dross exhibit different patterns depending on the molten metal flow induced by the laser, as mentioned in the previous section. These phenomena not only reflect the material characteristics but also display various behaviors based on several parameters. In this section, we aim to investigate the changes in molten metal flow with respect to laser speed and power and their effects on kerf width and dross formation. Previous studies conducted by Hirano and Fabbro33), Molitorp34), and Pocorni35) involved the use of cameras to visualize the molten metal and solidification processes. D. Arntz confirmed that with increasing melt wave velocity, the effect of the surface tension, which can contribute to coarse structures in the melt pool and even to the constriction of the melt pool to melt streams, is reduced. Increasing the cutting velocity increases the melt flow stability, as already observed by Hirano and Fabbro33). However, when the laser cutting speed exceeds 2,000 mm/s, unstable molten metal flow tends to occur. Fig. 13 illustrates the classification summary of molten metal flow behavior as a function of varying laser cutting speeds.
High-speed imaging showing the molten pool flow based on laser cutting speed using high- speed camera
In this experiment, at the slowest speed condition (A) of 3.5 kW and 200 mm/s, the largest accumulation of molten metal was observed on the cut surface, with weak melt flow moving in the direction opposite to the cutting axis, resulting from the slow laser speed. However, despite the limited molten metal flow, the accumulation of a substantial amount of molten metal led to weakened surface tension per unit area, facilitating the merging of the molten metal.
Severe shape changes due to thermal deformation were also evident. In case of Fig. 13(B), with a laser power of 3.5 kW and speed of 500 mm/s, there was no merging of the molten metal, similar to Fig. 13 (A); however, low melt flow and significant molten metal accumulation were observed. Shape changes due to thermal deformation were also evident. At Fig. 13 (C), where the laser power was 3.5 kW and speed was 1,500 mm/s, the molten metal formed at each cutting front merged, creating bridge-type molten metal. Consequently, the cutting surfaces were connected, resulting in complete cutting failure. For Fig. 13 (D), with a laser power of 3. 5 kW and speed of 2,500 mm/s, relatively good cutting quality was observed. Although strong melt flow and high input energy induced spatter, the remaining molten pool at the cutting front formed melt streams through melt flow, creating a melt film. In Fig. 13 (E), with a laser power of 3.5 kW and a speed of 3,000 mm/s, excessive melt flow led to the complex action of excessive flow of melt streams and vaporization of spatter, resulting in unstable spatter behavior and increased attachment to the specimen surface as compared to Fig. 13 (D). The number of dross formations also increased. Fig. 14 shows the processing appearance at the laser power of 3.5 kW and speed of 200 mm/s. Unlike other conditions, the merger of molten pools between adjacent molten pools is observed on the same cut surface. This is believed to be because the newly formed molten metal has more thermal energy, thus having relatively weak surface tension. Additionally, the periphery of the previously formed molten metal is preheated, making it more vulnerable to shape changes. Therefore, the molten metal is tilted and merged by the deformation of the periphery of the previously formed molten metal mass.
High-speed imaging showing the molten pool merging in the same cutting front
Fig. 15 illustrates the processing appearance under the condition of the laser power of 3.5 kW and speed of 1,500 mm/s. Unlike other conditions, a bridge-type dross formation process, where molten metal merges face-to-face, was observed. Consequently, partial cutting occurred due to the connection of molten metal at the cutting surface. The reason for the formation of bridge-type dross, unlike other conditions, is believed to be due to the small kerf width and weak melt flow during processing, resulting in some dross remaining on the cutting surface in the form of water droplets. Fig. 16 shows schematic representations of typical molten metal flow types in laser foil cutting at various laser speeds, conducted without the use of cutting active gas. Generally, increasing the melt flow proportionally with the laser speed influences the size of dross and occurrence of spatter. In low-speed laser processing, high input energy and small melt flow lead to the formation of large molten pools. Consequently, a significant amount of material is melted, resulting in wide kerf widths and severe thermal deformation. The melted metal attaches to the cutting surface and cools down due to the small melt flow. The accumulation of excessive molten metal not only reduces surface tension but also induces positional changes in molten metal droplets due to excessive material deformation, leading to their merging. The merged molten metal increases the total energy of heat, resulting in a wider HAZ due to heat transfer to the surrounding material. It has been observed that as the size of dross formed by cooling the molten metal below the melting temperature increases, the area of the HAZ also tends to increase. In high-speed laser processing, relatively low input energy and strong melting flow occur. Consequently, a small amount of material is melted, resulting in a narrow kerf width and minimal thermal deformation. Melt metals generate spatter and produce vigorous movement due to their strong melting flow. This movement can create a thin molten stream instead of a round dropper shape; subsequently, through rapid cooling, a thin molten film can be formed. Therefore, the molten metal is evenly dispersed across the cut surface with minimal heat accumulation, resulting in a narrow HAZ. In conclusion, for optimal cutting processes, selecting an appropriate laser processing speed to reduce molten metal agglomeration, minimize spatter, and form thin, homogeneous melt films is considered advantageous.
High-speed imaging showing the molten pool merging on the different cutting front
Comparison of the results according to the speed of the schematic diagram of the nozzle-less laser cutting process
4. Conclusion
A nozzle-free laser scanner cutting process for ultra-thin stainless steel 316L foil, potentially applicable for fuel cell separators, was observed and studied to understand the cutting mechanism.
1) Higher cutting speeds produce smaller and more consistent dross sizes, narrower kerf widths, and more uniform cut quality. However, at exceptionally high speeds, uneven melt and spatter removal can occasionally occur.
2) Insufficient laser power results in partial or incomplete cuts, while excessive power widens the kerf, increases variability, and reduces cutting stability. Therefore, laser power should be optimally adjusted in accordance with the cutting speed for consistent results.
3) As cutting speed increases, the flow of molten metal strengthens. However, at speeds above 2,500 mm/s, the amount of molten metal along the cutting surface decreases and is removed primarily as spatter.
4) Low-speed laser cutting of thin sheet materials results in overheating of the material and considerable shape change due to thermal propagation of the molten metal adhered to the cutting surface.
5) With increased cutting speeds, the velocity of scattered spatter also rises. At approximately 3,000 mm/s, spatter often adheres to the base material.
6) In low-speed laser cutting, molten metal adheres to the cutting surface without forming spatter, and adjacent molten droplets merge, creating a heat-affected zone (HAZ) in the surrounding area. In contrast, in high-speed cutting, molten metal disperses as spatter and fume due to the rapid cutting speed and material viscosity. This forms a thin melt film along the cutting surface, enabling cuts with minimal HAZ areas and narrow kerf widths.
In this experimental result, the cutting speed of 2500 mm/s at a laser power of 3.5 kW demonstrated the best cutting quality.
Acknowledgment
This work was supported by “Technology platform for advanced laser beam process of metallic fuel cell plates” (code: P0012884, Eurostars2) of KIAT (Korea Institute for Advancement of Technology) and “Establishment of Demonstration Infrastructure and Technology Advancement for Calandria Segmentation and Heat Transfer System Decontamination in PHWR” (RS2023- 00236918) of KETEP.