1. Introduction
Aluminum alloys are widely adopted across various industries, including aerospace, shipbuilding, and automotive sectors, owing to their lightweight, corrosion resistance, and outstanding thermal conductivity. In particular, A3003, A5083, and A6061 alloys are commonly used in cooling systems such as heat exchangers1-4). However, heat exchange systems should be operated stably even under extreme thermal and pressure conditions, in which their performance is largely influenced by surface cleanliness and corrosion resistance. During processing and assembly, various contaminants such as cutting oil, lubricants, oxide films, and fine metal particles can remain on the surface, which often leads to reduced thermal conductivity, poor weldability, and surface defects, generally degrading the quality and durability of subsequent processes5,6).
Previously, these contaminants were removed chemically (alkaline or solvent cleaning) or mechanically (grinding, sandblasting); however, these approaches are increasingly limited due to the use of harmful chemical substances, rising costs, limited compliance with environmental regulations, and difficulties in precise local control7-9).
Accordingly, a pre-cleaning process using a quick-dry lubricant is commonly applied in fields currently. This particular lubricant evaporates quickly due to its high volatility and leaves almost no residue after evaporation due to low viscosity, thus being high compatible with subsequent processes. However, the cleaning efficiency decreases over time, and the lubricant needs to be replaced regularly, which can cause financial burden and environmental pollution.
Against this backdrop, laser cleaning technology is gaining recognition as a high-precision, non-contact, non-consumable, and eco-friendly cleaning alternative, with its applications rapidly expanding across industries as a next-generation surface-treatment technology. In particular, the finding the lubricant removal mechanism varies depending on the laser wavelength and the optical absorption characteristics of the contaminants has important implications. For example, optically transparent mineral oils show penetrative characteristics for Nd:YAG(1064 nm) laser, and exhibit high removal efficiency from the explosive vaporization mechanism at the oil-substrate interface due to local overheating. This mechanism is not greatly affected by oil thickness, and is capable of effectively removing films of up to 150 μm thick. On the other hand, ultraviolet (UV, excimer) lasers operate primarily through a surface ablation mechanism due to the high absorption rate of commercial lubricants, resulting in a relatively lower removal efficiency10). Thus, this study comparatively analyzed the removal efficiency and changes in surface characteristics according to key laser process parameters-such as non-focal distance, overlap rate, average power, pulse width, and the number of scans-for the quick-dry lubricant applied to the surface of an A3003 aluminum alloy. Accordingly, optimal laser cleaning conditions for practical industrial applications were identified, and the feasibility of a laser-based, non-contact cleaning process was experimentally analyzed.
2. Study Material and Experimental Method
2.1 Study Material
The specimen used in this study is a non-heat treatable aluminum alloy A3003 of the 3,000 series, containing approximately 1% manganese (Mn). This alloy exhibits excellent corrosion resistance, economic feasibility, and formability, thus being widely applied in various sectors, including shipbuilding, aerospace, household appliances, automotive, and heat exchangers11,12). Notably, A3003 demonstrates outstanding machinability and brazing suitability for reliably implementing thin, complex fin configurations in the heat exchange field, thereby playing a critical role in both performance and the manufacturing process2,13). Table 1 presents the chemical composition of the aluminum alloy (A3003). The experiment was conducted using a quick-dry lubricant (vaporizing press oil) with a viscosity of 1.5 cSt; its chemical properties are shown in Table 2.
Table 1
Chemical composition of substrate (wt.%)
| Element Substrate | Si | Fe | Cu | Mn | Mg | Cr | Zn | V | Ti | Al |
|---|---|---|---|---|---|---|---|---|---|---|
| A3003 | 0.167 | 0.557 | 0.167 | 1.192 | - | - | - | - | 0.018 | 97.884 |
Table 2
Analysis of the chemical properties according to Vaporizing press oil14)
| Category | Vaporizing press oil |
|---|---|
| Color | Colorless |
| Flash Point | ≥ @51°C |
| Specific Gravity | 0.76 ± 0.02 |
| Viscosity | 1.5 ± 0.15 (@40°C) |
2.2 Experimental Method
The laser cleaning system used in this study is a high-power device operated using a Q-switched fiber laser with an average power of 300 W. This equipment consists of a laser control panel, X-Y-Z stage, fiber laser source, optical head, and dust collector. Fig. 1 illustrates the entire system configuration and its components in detail. In particular, process parameters (power, frequency, and scanning speed) can be precisely adjusted using the laser control panel, thus creating a wide range of experimental conditions.
To ensure even application of the lubricant on the specimen surface, a 1 mg dropper was used to apply a consistent amount. Applying the lubricant in a fixed amount is intended to ensure the repeatability and reliability of the experiment. Among various process parameters of laser cleaning, the characteristics and efficiency of laser cleaning were compared based on the number of scans (Ns), pulse overlap rate (R), line overlap rate (Ry), average power (Pa), and pulse width (τp). Fig. 2 illustrates a schematic diagram of the laser cleaning experiment. A laser beam transmitted through an optical fiber is deflected by two galvanometer mirrors inside the optical head according to the scanning location. The F-theta lens ensures that the laser beam is directed onto the focal point on the specimen surface. The laser beam with a diameter of 94μm scans the area of 30 mm × 30 mm on the specimen surface. Further- more, the effectiveness and characteristics of laser cleaning were comprehensively evaluated by comparing the oil removal conditions and changes in microstructure, based on observations of the surface of the specimen treated under optimal conditions.
Fig. 3 shows the conceptual diagram of the overlap rate in laser cleaning, along with the maximum energy intensity at the center of the laser beam. Considering the nature of the Gaussian beam, a low overlap rate of the laser beam can lead to varying energy absorption rates across the material surface, resulting in uneven cleaning. Therefore, surface was evenly cleaned by applying the overlap rate (R, Ry) between laser beams. The overlap rate in X and Y directions is defined by the pulse overlap rate (R) and line overlap rate (Ry), respectively. In the experiment, the pulse overlap rate (R) is controlled by the scanning speed, while the line overlap rate (Ry) is set as the line pitch length defined as the distance between those lines.
3. Experimental Results
3.1 Effects of Non-Focal Distance and Overlap Rate
Fig. 4 is an image of surface of the cleaning part in relation to the laser beam’s non-focal distance and overlap rate. The surface image shows that the lubricant was not sufficiently removed at d = 0 mm, even at high overlap rates. On the other hand, when the non-focal distance was adjusted from -1 mm to -3 mm and the overlap rate was increased, the lubricant was effectively removed from the specimen surface. However, since it is difficult to determine visually whether the lubricant has been fully removed from the surface, considering the nature of the quick-dry lubricant, Fourier transform infrared spectroscopy (FTIR) was utilized to analyze more precisely whether any lubricant remained on the surface, depending on the non-focal distance and overlap rate. As shown in Fig. 5, lubricant removal efficiency due to the overlap rate could not be verified for a non-focal distance from 0 mm to -2 mm; however, in the wavenumber range above 1,500 cm-1 and below 3,000 cm-1, the remaining oil peaks could be identified through absorption rate. In contrast, the lubricant was most effectively removed at a non-focal distance of -3 mm and an overlap rate of 75%, where the intensity of the oil peak decreased further under other overlap rates. Consequently, the best cleaning efficiency was achieved at a non-focal distance of -3 mm, average power of 100 W, and overlap rate of 75%; an additional experiment was conducted by fixing the non-focal distance and overlap rate.
3.2 Average Power of Laser Beam and Effect of Pulse Width
Fig. 6 shows the surface image of a laser beam cleaning part with respect to average power. While the non-focal distance is fixed at -3 mm and the overlap rate at 75% as derived from previous experiments, the average power was adjusted from 100 W to 300 W in 50 W increments to examine the changes in the surface and cleaning effectiveness. In the results, the lubricant was visually removed under all conditions, but surface damage became progressively more severe as the average power increased from 100 W in 50 W increments. Notably, the lubricant began to show irregular burning when the average power exceeded 200 W, and surface damage also increased sharply and irregularly. It is attributed to the large amount of fume generated during burning, which exhibits irregular and dynamic patterns. As a result, the penetration of the laser beam is temporarily reduced, and surface damage occurs irregularly. Therefore, for efficient lubricant removal, it is important to set an optimal laser power range that minimizes surface damage to the greatest extent possible.
Fig. 7 illustrates the different surface conditions when the laser beam was scanned under various pulse width conditions. While the power is fixed at 100 W, the pulse width was varied from 20 ns to 500 ns to examine the changes in the surface and cleaning effectiveness. The lubricant was not removed at all from 20 ns to 30 ns due to the extremely short scanning time. The removal became effective starting at 60 ns, and the highest laser scanning efficiency was shown at 120 ns. Efficiency was reduced due to a decrease in peak power at 224 ns and 500 ns with longer scanning times. Consequently, the best removal efficiency was achieved with an average power of 100 W and a pulse width of 120 ns.
3.3 Optimal Overlap Rate of Laser Beam and Effects of the Number of Scans
Based on the threshold values derived from the basic experiment, this study analyzed the lubricant removal efficiency according to overlap rate and the number of scans to obtain the optimal conditions. The effect of the overlap rate was evaluated by adjusting it from 73% to 77% in 1% increments, while the number of scans was increased sequentially from one to three times to compare the lubricant removal performance under each condition. Fig. 8 shows magnified surface images at 30x and 100x, according to the overlap rate and number of scans. As the number of scans increased to three times, the almost no lubricant residue was observed visually under the most conditions. In contrast, as the overlap rate increased from 73% to 77% due to heat accumulation and the overlap effect from repeated scanning, vaporization became more noticeable on the surface. Local surface damage from the laser beam was also observed.
FTIR analysis was performed to more precisely verify the oil residue, and the results are shown in Fig. 9. FTIR analysis results showed that the lubricant was not completely removed as certain peaks partially remained after scanning was performed one and two times. When the number of scans was increased to three times, the oil peak disappeared and was completely removed at the overlap rate of 75%.
Meanwhile, the removal efficiency decreased when the overlap rate exceeded 75%, which is attributed to excessive overlap negatively impacting lubricant removal.
Therefore, this study experimentally demonstrated that an overlap rate of 75% and three scans are the most effective conditions for removing the lubricant while minimizing surface damage. Figs. 10 and 11 present the surface treatment results (SEM and EDS) of the specimen before laser cleaning and after treatment with one to three scans at the optimal overlap rate, respectively. In the specimen before laser cleaning, aluminum and oxygen, the major components of the A3003 aluminum alloy, were detected. The presence of carbon is assumed to originate from the carbon coating applied to ensure conductivity during the analysis.
Fig. 11
EDS spot analysis results of laser cleaned lubricant surface in Rx = 75 %, Ry = 75 % according to number of scans (Ns)
Under the condition of a single scan, the aluminum component is unevenly distributed, while the carbon component is spread over a broad area with a high concentration, as observed in the EDS analysis. On the other hand, under the condition of three scans, the distribution of aluminum on the surface was more uniform, and only a small amount of carbon was detected over a very limited area. This implies that repeated scanning effectively removed the residual contaminants on the surface.
3.4 Laser Cleaning Mechanism of Quick-Dry Lubricant
Fig. 12 illustrates the removal mechanism of the quick-dry lubricant in steps. In step (a), the laser beam was scanned across the specimen surface Since the quick-dry lubricant has low viscosity (1.5 cSt) and high transparency, the laser beam is minimally hindered by the oil layer and penetrates it to reach the metal surface of the specimen. In step (b), the laser beam is applied to the specimen surface as a heat source, thus increasing the temperature of the metal surface. In step (c), the specimen surface temperature exceeds the evaporation point of the lubricant, causing the oil layer to begin heating indirectly. In step (d), indirect heating causes the lubricant to begin evaporating, thereby removing it from the surface. This sequential mechanism enables the quick-dry lubricant to be effectively removed using laser.
4. Conclusion
This study systematically evaluated the effects of laser cleaning process conditions on the removal efficiency and surface changes of the quick-dry lubricant (viscosity 1.5 cSt) applied to the A3003 aluminum alloy surface.
Removal characteristics were compared according to the combination of key parameters such as non-focal distance, overlap rate, average power, and the number of scans; physical and chemical changes were also examined through FTIR as well as SEM-EDS analysis. The conclusions are as follows:
1) When the non-focal distance is 0 mm, the laser focus is positioned on the upper part of the oil layer, preventing effective energy delivery. However, when the non-focal distance is -3 mm, the laser focus is near the base metal, increasing the absorption rate and significantly improving removal efficiency. FTIR analysis results showed that the C-H peak below 3000 cm-1 disappeared when the non-focal distance was -3 mm, indicating the complete removal of the lubricant.
2) At an average power of 100 W, uniform scanning and stable control were achievable; however, at average powers of 200 W or higher, burning occurred due to excessive heat accumulation, which also caused surface damage to increase sharply and irregularly. Therefore, the optimal power in this process is considered 100 W.
3) In terms of pulse width, the removal efficiency was minimal between 20 ns and 60 ns due to low heat input, while it began to decrease at 224 ns and 500 ns due to reduced peak power. A pulse width of 120 ns was determined to be the optimal condition, as the lubricant removal efficiency was the highest.
4) At overlap rates of 35-65%, the lubricant was only partially removed; however, at 75%, the lubricant peak disappeared, indicating complete removal. When the number of scans was one or two, FTIR peak was partially remained, which was completely removed after three scans. The removal efficiency tended to decrease when the overlap rate exceeded 75%, indicating that excessive overlap had a negative impact on lubricant removal.
5) In the SEM-EDS analysis, the aluminum distribution was uneven and carbon was detected over a wide area after one scan, whereas after three scans, the aluminum distribution became more uniform and only a minimal amount of carbon was detected. This indicates that repeated scanning sequentially removes residual contaminants and improves surface cleanliness.
6) Consequently, base material damage is minimized while the lubricant can be completely removed under the following conditions: a non-focal distance of -3 mm, an average power of 100 W, a pulse width of 10 ns, an overlap rate of 75%, and three scans. Due to the low viscosity and high transparency of the quick-dry lubricant, the laser beam penetrated the oil layer and increased the surface temperature through indirect heating. As a result, the lubricant began to evaporate and was eventually removed from the surface. This sequential mechanism is identified as the primary operating principle.
The optimal conditions suggested in this study can be applied to eco-friendly laser cleaning designs for precision components, such as aluminum heat exchangers, and can serve as foundational data for future research on the removal of high-viscosity lubricants or multi-layer oxide films.
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