Since its invention in 1960, laser technology has developed rapidly, and the achievable laser power has been steadily increasing . The most powerful industrial fiber laser produced to date generates a power of 100 kW , and it is known that fiber lasers with powers higher than 100 kW can be also produced . Since these lasers can be delivered through optical fibers, convenient processing is possible with only a small head located several tens or hundreds of meters away from the laser body. In addition, the high price of a laser itself, which was a disadvantage in its use, has been continuously falling in recent years . Therefore, laser processing has been widely used industrially, and its demand will increase further.
In the field of nuclear decommissioning, the laser is also receiving much attention as a next-generation cutting tool with many advantages over other cutting methods [5,6]. Compared to mechanical cutting, laser cutting enables faster cutting for thick objects. In addition, while mechanical cutting generates a large reaction force, laser cutting has almost no reaction force as it is non-contact thermal cutting so that a manipulator with a high payload is not required. Furthermore, mechanical cutting requires periodic replacement of saw blades, but laser cutting requires no special maintenance. Moreover, laser cutting is material independent and the amount of secondary waste generated is less due to the relatively narrow kerf width. These properties make laser cutting a better method than other thermal cutting, such as plasma arc cutting and oxy-fuel cutting. Laser cutting can also be done underwater. For these reasons, the use of laser cutting can prevent radiation exposure to workers and reduce the cost of dismantling waste in the dismantling of nuclear facilities.
Due to its many advantages, the development of laser cutting technology for application to nuclear decommissioning has been actively progressing. Various research institutes in countries with nuclear power plants have been developing their own technologies for effectively cutting thick metal and using underwater cutting technologies with lasers. K. Tamura et al. conducted studies to cut steel plates with a thickness of more than 100 mm using a 30- kW fiber laser [7-10]. They cut specimens of stainless steel (SUS304) and carbon steel (SM490A) up to 300 mm thick, as well as various types of simulated steel components. C. Chagnot et al. conducted a cutting study on stainless steel using an 8-kW Nd:YAG laser . They succeeded in cutting up to 100 mm thick and showed that the cutting capability was about 10 mm per kW. P.A. Hilton and A. Khan performed an underwater laser cutting study with a 5-kW fiber laser . They cut 32 mm thick stainless steel underwater at a speed of 100 mm∙min−1. A.B. Lopez et al. cut carbon-manganese steel bars with a thickness up to 70 mm by parameter optimization using a 10-kW fiber laser . A. Choubey et al. performed cutting studies on thick stainless- steel (AISI SS304) sheets with a thickness of 4–20 mm in dry air and an underwater environment using a pulsed Nd:YAG laser .
Our group has also carried out cutting studies to cut thick steel plates in air and underwater using 6-kW and 10-kW fiber lasers [15-23]. J. S. Shin et al. cut stainlesssteel (SUS304L) plates up to a thickness of 150 mm with a 10-kW fiber laser , and also cut up to a thickness of 100 mm underwater with the same laser . Using a 6-kW fiber laser, they cut stainless-steel (SUS304L) and carbonsteel (SA508 grade 3 class 1) plates up to a thickness of ~70 mm underwater . These studies focused on cutting thick steels at high speed as efficiently as possible and the specimens were all cut only in the same direction. However, at an actual dismantling site, cutting is not performed in only one specific direction, but should be applicable in any direction. For cutting in air, the cutting direction will hardly affect the performance. However, for underwater cutting, the cutting performance may change depending on the direction by rising of the assist gas due to buoyancy.
In this work, we conducted studies on underwater laser cutting of thick stainless steels in various cutting directions. The performance dependence on the cutting direction was evaluated through the cutting tests. We also performed a cutting test in which the cutting direction was changed by 90 degrees. In addition, the cutting performance was also evaluated for vertically downward laser irradiation. We analyzed and summarized all the cutting test results in this paper.
2. Experimental Setup and Procedure
A 6-kW ytterbium-doped fiber laser system (IPG Photonics, Model#: YLS-6000) was used as a light source for the underwater cutting experiment. The generated beam from the laser system was delivered to the workspace though a 20-m long process fiber. The process fiber had a core diameter of 100 μm and was connected to the underwater cutting head in the workspace. Fig. 1(a) shows the internal scheme and 1(b) a photo of the underwater laser cutting head used in the study. This cutting head was waterproof and designed to work completely submerged in water.
Looking at the optical structure as shown in Fig. 1(a), the cutting head consisted of a collimation lens with a focal length of 160 mm and a focusing lens with a focal length of 600 mm. Both lenses were designed with aspherical surfaces to minimize aberrations. In our previous studies, a water- cooled parabolic mirror with the same focal length was used instead of a focusing lens [15-23]. For compactness, we replaced it with a focusing lens having the same focal length in this study. Prior to the cutting experiment, the properties of the focused laser beam with this cutting head were measured by a scanning diagnostics system (PRIMES, FocusMonitor). To prevent damage to the sensor, measurement was performed at a laser power of 4 kW rather than full power. The measured value of the beam waist diameter was 370 μm, which was similar to the geometrically calculated value of 375 μm from the core diameter of the process fiber and the focal lengths of the applied lenses. The BPP (beam parameter product) value indicated the beam quality was 3.791 mm∙mrad, and the beam divergence angle was 41.038 mrad.
While passing through the optical elements of the cutting head, the laser beam is focused and irradiates the cutting object. The object material is then melted while absorbing the laser power. The cutting proceeds by strongly blowing the resulting melt with a high pressure assist gas jet. Effective cutting of thick metal requires a nozzle capable of ejecting a uniform assist gas jet over long distances. Especially for underwater cutting, not only must the assist gas jet be able to blow out the melt by overcoming the resistance of water, but it must also form a local-dry-zone so that the laser power can be transferred to the object surface without loss due to water absorption. In our previous studies, the minimum length nozzle (MLN) was evaluated as a supersonic nozzle capable of this role and it showed excellent underwater cutting results for thick steel plates [19-21]. Therefore, the same MLN was employed as the underwater cutting head in this study. It was optimally designed for a supply pressure of 1.5 MPa and has a throat diameter of 3 mm and an exit diameter of 4.28 mm.
We conducted several cutting tests to determine the underwater laser cutting characteristics according to the cutting direction. First, the cutting tests were performed for cutting with horizontal laser irradiation. For this purpose, the cutting head was horizontally mounted at the X-Y-Z stage. The movement of the head was driven by computerized numerical control (CNC). Fig. 2 shows the experimental setup for underwater laser cutting with horizontal laser irradiation.
With horizontal laser irradiation, the cutting tests was performed as follows.
1. Cutting along the horizontal direction: The cutting test was performed for the case of cutting along the horizontal direction. The cutting process was as follows. The specimen was placed in a water bath and the bath was filled with water to the extent that the specimen was completely immersed. At this time, the cutting head was located outside the water. When the cutting process started, the assist gas was supplied and ejected through the nozzle of the head. At the same time, the head went down into the water. When the cutting head reached the proper position, the head stopped going down and the laser irradiated the specimen. Then, the cutting was performed as the head moved horizontally from the outside to the inside of the specimen. After cutting to the appropriate extent, the laser irradiation was turned off and the head moved out of the water. All of these operations were controlled remotely via a CNC program. In this test, a 48 mm thick stainless-steel plate was used as the specimen. For each cut, the 2-step cutting method consisting of the initial cutting section at low speed and the remaining cutting section at high speed was applied. This method showed excellent cutting performance in our previous studies [15-23]. In this test, the initial section of 15 mm was cut at a low speed of 10 mm∙min−1 and the subsequent section of 30 mm was cut by varying the speed for each cut line to find the maximum cutting speed.
2. Cutting along the vertical direction: The cutting test was performed for the case of cutting along the vertical direction. In this case, the cutting process was the same as cutting along the horizontal direction, but the difference was that the cutting direction was applied vertically. Since the assist gas rose in the upward direction due to the buoyancy of water, the performance of cutting in the upward direction and cutting in the downward direction could differ from vertical cutting. Therefore, both the vertical upward and downward cutting tests were performed. In the test, a 48 mm thick stainless-steel plate was used as the specimen. As with the cutting along the horizontal direction, the total cutting length was 45 mm and the initial section of 15 mm was cut at a low speed of 10 mm∙min−1. After that, a test was additionally performed to completely cut the specimen from the bottom to the top in the upward direction. In this test, a 47 mm thick stainless-steel plate was used as the specimen.
3. Cutting with 90° change of direction: The cutting test was performed for the case where the cutting direction was changed by 90 degrees. In this test, a 46 mm thick stainless-steel plate was used as the specimen. The cutting process was as follows. The specimen was vertically cut for a length of 50 mm from the bottom to the top at a location 50 mm inward from the right end. At this time, the initial section of 15 mm was cut at a low speed of 10 mm∙min−1. After that, the cutting head stopped moving for 3 seconds and the laser continued to irradiate the specimen as it was. This prevented a cutting delay between the front and rear surfaces. After that, the specimen was cut horizontally to the right end of the specimen. If the cutting was successful, the part with a width of 50 mm and a height of 50 mm located in the lower right corner would be completely separated from the specimen.
Following the horizontal laser irradiation, cutting tests were performed for cutting with vertical downward laser irradiation. For this purpose, the cutting head was mounted vertically downward at the X-Y-Z stage. Fig. 3 shows the experimental setup for underwater laser cutting with vertically downward laser irradiation. In this test, a 48 mm thick stainless-steel plate was used as the specimen. In the same way the as horizontal laser irradiation, the initial section of 15 mm was cut at a low speed of 10 mm∙min−1, and the subsequent 30 mm section was cut by varying the speed for each cut line to find the maximum cutting speed.
Several conditions were applied equally to all tests. The laser power was fixed as 6 kW. The stand-off distance, which means the distance between the nozzle exit and the front surface of the specimen, was maintained at 10 mm. The focal position, which means the beam waist of the focused laser, was positioned 16 mm inward from the front surface of the specimen. Dry compressed air was used as the assist gas and the supply pressure for the assist gas jet was ~1.5 MPa. At this time, the flow rate ejected to the nozzle exit was 1,310 L·min−1 under Atmosphère Normale de Référence (ANR, French) conditions (20°C, 101.3 kPa, and 65% relative humidity). SUS304L stainless-steel plates were used as cutting specimens. They were made in the form of blocks with a width of 120 mm and a height of 120 mm.
3. Experimental Results
3.1 Cutting With Horizontal Laser Irradiation
3.1.1 Cutting Along the Horizontal Direction
Fig. 4 shows the front, rear and side views of the 48 mm thick stainless-steel plate after underwater cutting xalong the horizontal direction with horizontal laser irradiation. The cutting results are summarized in Table 1. The results of the experiment confirmed that cutting was performed well at a speed of 110 mm∙min−1 or less, and a slight cutting delay was observed at a speed of 120 mm∙min−1. Therefore, our study confirmed that the maximum cutting speed for the 48 mm thick stainless-steel plate was 110 mm∙min−1. Considering that the 48 mm thick underwater maximum cutting speed obtained using a cutting head with a parabolic mirror in our previous study was 110 mm∙min−1 , the results obtained in this work showed the same performance as before. Thus, we confirmed that the new cutting head composed of the focusing lens worked well.
3.1.2 Cutting Along the Vertical Direction
Fig. 5 shows the front, rear and side views of the 48 mm thick stainless-steel plate after underwater cutting along the vertical direction with horizontal laser irradiation, and the cutting results are summarized in Table 2. For the vertically upward direction, the cuttings were done well at all applied speeds, 100, 110 and 120 mm∙min−1. The cutting results along the horizontal direction for the same thickness confirmed that vertical upward cutting was also possible at the maximum speed of the horizontal cutting. In addition, cutting up to 120 mm∙min−1 was possible so we confirmed that the cutting performance was slightly better. However, it is difficult to conclude that the difference was due to buoyancy because it may arise from the fluctuation from a slight variation in cutting conditions. For vertically downward direction, the cutting was performed at a speed of 120 mm∙min−1. In this case, the cutting was also done well at high speed.
Fig. 6 shows the views of the cut specimen for 47 mm thick stainless-steel plate after underwater cutting from the bottom to the top in the upward direction. The initial section of 15 mm was cut at a low speed of 10 mm∙min−1 and the remaining section was cut at a speed of 100 mm∙min−1 (~90% of the maximum cutting speed). The test results also demonstrated that the cutting was done well even when the specimen was completely cut from the bottom to the top. Looking at the striation pattern as shown in image of the kerf plane (Fig. 6), we see that it was straight from the front to a depth of ~12 mm but was meandering thereafter. This means that if the assist gas penetrates more than 12 mm into the specimen, the gas flow is turbulent. Nevertheless, the specimen was completely separated along the cutting line. Since the cut quality is not important in the dismantling work, a complete separation alone is sufficient to be acceptable.
3.1.3 Cutting With 90° Change of Direction
Fig. 7 shows the views of the cut specimen for 46 mm thick stainless-steel plate after underwater cutting with a 90° change of direction. The specimen was first cut in the vertically upward direction from the bottom at a speed of 100 mm∙min−1 after initial cutting. The upward cutting was done well to a length of 50 mm. Then, cutting was done at the same speed for the remaining 50 mm section in the horizontal direction to the side edge. After the cutting was finished, the 50 mm × 50 mm part was completely separated from the specimen and fell off. The results again confirmed that like cutting in one direction, it was possible to cut the 46 mm thick stainless-steel plate at high speed even when cutting with a 90° change of direction. The striation pattern was similar to the result obtained by cutting from the bottom to the top in the upward direction as shown in Fig. 7.
3.2 Cutting With Vertically Downward Laser Irradiation
Fig. 8 shows the front, rear and side views of the 48 mm thick stainless-steel plate after underwater cutting with vertically downward laser irradiation. The cutting results are summarized in Table 3. Even though the assist gas was ejected vertically downward, the cutting was done well up to a speed of 120 mm∙min−1 without performance degradation due to buoyancy. The specimen was not cut at speeds exceeding 120 mm∙min−1. Thus, the maximum cutting speed was determined to be 120 mm∙min−1. As a result, the cutting performance was not different from that of horizontal laser irradiation.
In summary, we conducted underwater laser cutting studies on thick stainless-steel plates in various cutting directions. When cutting with horizontal laser irradiation, stainless-steel plates were cut well at the maximum cutting speed regardless of the horizontal or vertical directions. Even when the direction was changed 90 degrees, the cutting was also done well at high speed without a degradation of performance. When the cutting was vertically downward laser irradiation, the stainless-steel plates were also cut well at high speed. The maximum cutting speed was almost the same as that when cutting with horizontal laser irradiation. In conclusion, it was possible to cut thick stainless steels regardless of the laser irradiation direction and the cutting direction even though the assist gas rose up due with the buoyancy. This shows that the dependence on the cutting direction can be excluded among the things that the operator should consider. These observations are expected to benefit laser cutting procedures during the actual dismantling of nuclear facilities.