1. Introduction
When the coated paints and oxide layers on the steel surfaces inside and outside the ships are removed during shipbuilding and repairing work of used vessels, a lot of wastes are generated, requiring significant amounts of disposal costs. Furthermore, because there is no ventilation system inside vessels, the wastes generated during work often contaminate the surrounding work areas. This causes work delays, reducing work efficiency. Therefore, to resolve the problem caused by conventional technology, it is required to develop laser cleaning technology that can significantly reduce the generation of secondary waste during painting treatment work1-4).
Because of the benefits of laser cleaning technology, its application areas have been continually increasing, including cultural asset restoration5,6), semiconductors7), electric/electronic devices, aircraft8-10), and railway industry11). Particularly in the shipbuilding and marine industry, research has been actively conducted to increase the applicability of laser cleaning technology12-14). Through fundamental technology research, our research team has confirmed that laser cleaning technology can be applied to the removal of epoxy paints, shop primers, and oxide layers on steel surfaces in ships1-4). D’Addona et al. removed primers on steel surfaces by adopting laser ablation technology as a pretreatment process of welding for the prevention of weld defects; as a result, they increased the welding efficiency15). It is also important to remove marine microorganisms periodically, which inhabit the surface of ships and offshore structures, to increase ship operational efficiency. Tian et al. reported that they removed the marine microorganisms on the aluminum alloy surface effectively by using nanosecond pulsed fiber laser16). However, there is no previous study has been reported on the cleaning performance according to the thickness of paint surface and various types of anti-corrosive paints that are commonly used in shipyards. Furthermore, there is virtually no study on the scan mode control according to the shape of the laser cleaning-applied part, such as a square and a circle, based on the excellent controllability of the laser.
In this study, therefore, we used the hand-held type laser cleaning equipment that our research team developed to increase the ease of work in narrow areas of the vessel and investigated the cleaning performance improvement for different types of anti-corrosive paints used in shipbuilding. We analyzed the cleaning characteristics according to the scan mode by setting the laser beam’s scan mode to the square and circle to increase the ease of work according to the shape of the cleaning-applied part.
2. Experimental Materials and Method
In this study, we used the following experimental materials: steel for shipbuilding coated with epoxy and anti-fouling paints used for anti-corrosion in ships; urethane paints used for decorative effects such as color and lusters; and zinc primer paints used for temporary anti-corrosion. Table 1 shows the thickness of the coating, boiling point, and main components for each paint type.
Table 1
Properties of experimental materials
The laser used in the experiment was a Q-switching pulsed fiber laser with an average power of 100 W. Using the low-power laser, we developed air-cooled hand-held laser cleaning equipment and applied it to this study. The laser cleaning experiment was conducted by changing the process parameters such as scan mode, scan line distance, and energy density. Fig. 1 shows the schematic diagram of the experimental method according to the scan mode. Using the 2D scanner inside the laser optical head, the circular laser beam scans square and circular regions. Here, the hand-held laser cleaning equipment’s laser head and stage were fixed to obtain quantitative experimental data. When applied on-site, an indicating bar corresponding to the distance from the laser optical head to the focal position was designed to be mounted to maintain the focal length. The scan area was 50mm×50mm for square areas and 50mm in diameter for circular areas. Table 2 shows the laser irradiation conditions according to the main process parameters used in the experiment.
3. Experimental Results and Discussion
3.1 Coating Removal Characteristics According to Scan Mode
We conducted an experiment according to the scan mode to increase the ease of work according to the shape of the cleaning-applied part in industrial sites. Fig. 2 shows the scan path diagram of the circular laser beam according to the scan mode and the movement of the laser beam captured by a high-speed camera at certain time intervals. The laser moves repeatedly from top to bottom and from bottom to top when scanning a square area, and draws circles while spiring out from the center when scanning a circular area. Here, overlaps are placed between the laser beams to clean the entire area uniformly.
Fig. 3 shows surface images of the laser-cleaned areas according to the scan mode and the number of scans (Ns) of the specimen coated with the anti-fouling paint, as the representative of the four experimental material types. The anti-fouling paint that had a thickness of 170㎛ was completely removed in five scans. When we examined the coating removal characteristics according to the scan mode, we found that the coating was removed uniformly in the entire area when the laser beam scanned the square area. This was because the laser energy was irradiated uniformly with constant overlaps in the entire area. In contrast, when the circular area was scanned, the thermal energy was concentrated at the center, removing the coating at the center first.
To analyze the damage and thermal effect on the base metal by the laser heat source during laser cleaning, we magnified and analyzed the surfaces of the base metal and the laser-cleaned specimens. Fig. 4 shows a micrography and its 3D image for the surfaces at the center position cleaned in square and circular areas under the condition of performing the scan five times. On the base metal, a roughness of about 35㎛ had been formed to improve the adhesion of the paint, and a silver surface was observed. When the specimens were examined after laser cleaning, we observed the surface shape and roughness value similar to those of the base metal regardless of the scan mode and did not observe the thermal effects such as oxidation and discoloration.
Fig. 4
Micrography and 3D images of base metal and laser cleaned surface when the number of scans(Ns) is 5
Hence, we observed excellent cleaning performance without mechanical and thermal damages of the base metal when scanning not only the square area but also the circular area where the energy was concentrated at the center. Since the laser cleaning technology enables selective cleaning of target materials, which was not possible with traditional mechanical/chemical cleaning technology, we expect that significant improvements will be achieved in terms of product quality and work time.
3.2 Effect of Scan Line Overlap Rate
The scan line overlap rate is determined by the scan line distance, which is the distance between two lines, as shown in the schematic diagram in Fig. 5. In this study, we changed the scan line overlap rate to 20%, 50%, and 70%.
Fig. 6 shows the paint removal conditions for the four paint types according to the scan line overlap rate. In the case of epoxy paint, the removal was successful when the laser scanning was performed 10 times at an overlap rate condition of 20%, 7 times at a condition of 50%, and 5 times at a condition of 70%. In other words, as the scan line overlap rate increased, the paint removal condition decreased. The same trend was observed in other specimens as well. When the scan line overlap rate increased from 20% to 70%, the paint removal condition decreased from 7 times to 3 times in the case of the anti-fouling paint; from 3 times to 2 times in the case of the urethane paint; and 4 times to once in the case of the zinc primer paint. This is because as the line overlap rate increases, the number of laser lines irradiated on a scan area of 50mm×50mm increases, which increases the thickness of the paint removed with one laser scan due to the heat build-up in the material. Furthermore, through the fundamental research, our research team has confirmed that uniform cleaning is difficult under the condition of a low overlap rate because the transfer of thermal energy is insufficient in the edge areas due to the laser beam characteristics of Gaussian distribution1,2). Therefore, we determined that an overlap condition of 20% is not suitable when cleaning paints.
As the overlap rate increases, the number of scans for the paint removal decreases, but the scan time increases. Therefore, we need to consider the total work time calculated by using the time per scan and the number of scans. Under an energy density condition of 13.6 J/cm2, the time per scan is 7.96s when the overlap rate is 50%, and it increases by twice to 13.25s when the overlap rate increases to 70%. In short, when the total work time was compared, we found that the work time was shortened significantly under an overlap rate condition of 50% compared to a condition of 70%, confirming that efficient work is feasible. Based on this, we have found that selection of the optimal overlap rate is important because if the overlap rate increases more than necessary, it will increase the work time and can have a thermal impact on the base metal.
3.3 Effect of Energy Density
The experimental results according to the scan line overlap rate showed that in the case of the urethane and zinc primer paints, there was no difference in the number of scans when the scan line overlap rate increased from 50% to 70%. Therefore, we conducted an additional experiment by reducing the energy density to reduce the work time for these specimens. As the energy density decreased, the scanning speed increased, reducing the time per scan. Here, the energy density is defined as the pulsed energy irradiated per unit area.
Fig. 7 shows the removal conditions of the urethane and zinc primer paints according to the energy density at scan line overlap conditions of 50% and 70%, respectively. In the case of the urethane paint, the number of scans for the paint removal decreased when the energy density increased. However, the zinc primer paint was removed through one laser scan, even when the energy density changed. In the case of the energy density increasing from 9.4J/cm2 to 13.6J/cm2, the time per scan is 5.46s and 7.96s, respectively, when the overlap rate is 50%; 8.2s and 13.25s, respectively, when the overlap rate is 70%. When the total work time was compared for each process parameter, we found that the conditions for the most efficient removal of each paint film were: an overlap rate of 50% and an energy density of 13.6J/cm2 for the urethane paint; an overlap rate of 50% and an energy density of 9.4J/cm2 for the zinc primer paint.
Fig. 8 shows surface and cross-sectional images of the laser-cleaned surface of the specimens coated with the zinc primer according to the energy density. We can observe that the zinc primer paint was removed in just one laser scan in every condition. The XRD component analysis results of the cleaned surfaces shown in Fig. 9 also confirm that the peak of Zn, the main component of the zinc primer paint, is not detected.
In sum, the work time can be reduced when removing a thick paint film such as urethane by increasing the amount removed per scan at a high energy density, which decreases the number of scans. On the other hand, in the case of thin paint film such as zinc primer paint, the energy density has no significant effect because the paint is removed in just one laser scan. Therefore, by selecting the minimal threshold conditions of removing paints, cleaning work can be performed efficiently.
4. Conclusions
In this study, we used the developed hand-held laser cleaning equipment and analyzed the effects of major cleaning process parameters including the scan modes of square and circle for four types of specimens coated with different paints and thicknesses. As a result, we reached the following conclusions.
1) Using a circular laser beam, square and circular areas were completely cleaned, and we observed excellent cleaning performance without mechanical and thermal damages of the base metal caused by the laser heat source. In the cleaning process, the paint was uniformly removed when a square area was scanned, and the paint at the center was removed first when a circular area was scanned.
2) According to the results of analyzing the effects of the scan line overlap rate, the cleaning work can be finished at the earliest time when the overlap rate is 50%. Based on this, we have found that selection of the optimal overlap rate is important because if the overlap rate increases more than necessary, it will increase the work time and can have a thermal impact on the base metal.
3) Efficient cleaning work was possible at a condition of a high energy density for the removal of a thick paint such as urethane and at a condition of relatively low energy density for the removal of a thin paint such as zinc primer. The optimal cleaning conditions for each paint type were as follows: a scan line overlap rate of 50% and an energy density of 13.6J/cm2 in the case of the epoxy, anti-fouling, and urethan specimens; a scan line overlap rate of 50% and an energy density of 9.4J/cm2 in the case of the zinc primer specimens.
4) In sum, by using the hand-held cleaning equipment with the average power of 100 W, we completely removed different anti-corrosion paints on the steel surfaces for shipbuilding. Furthermore, the cleaning performance can be improved and the work time can be shortened compared to conventional technology by deriving the optimal process parameters according to the paint type, the thickness of the paint, and the scan mode suitable for the shape of the laser cleaning-applied surface.
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