The Study of Patterns and Mechanisms of Continuous Laser Ablation of Carbon Steel Rust Layers in Multi-Medium Environments

Abstract

A new multi-scenario, low-cost, high-efficiency, medium-assisted continuous laser cleaning of corrosion layers was developed. By comparing the roughness and cleaning depth of rust layers cleaned under conditions of liquid-assisted, solid-assisted, and mixed solid–liquid-assisted laser cleaning, simultaneously establishing a three-dimensional finite element model to study the variations during the cleaning process, and conducting a comparative analysis of the results of both, the cleaning mechanism is elucidated. The experimental results indicate that under conditions of water-assisted cleaning, the depth of rust layer increases initially and then decreases with varying water layer heights. The maximum cleaning depth is achieved at a water layer height of 0.1 mm, while the optimal surface roughness occurs at a water layer height of 0.2 mm, indicating a change in cleaning mechanism. The cleaning pattern with SiO₂ activator assistance follows a similar trend to a water medium, reaching maximum cleaning depth at 0.1 mm height, with a slight improvement in surface roughness compared to water-assisted cleaning. Finally, solid–liquid mixing can achieve cleaning completion and improve surface roughness under conditions where water-assisted cleaning alone fails to reach a clean state. Therefore, the active agent can be used for laser cleaning to promote the cleaning process, and solid–liquid mixing to assist the laser cleaning can be a theoretical guide for the field of laser cleaning.

1. Introduction

Rust alters the surface of metals, compromises their internal operations, and hastens deterioration. Data released by the U.S. Steel Construction Industry Association in the previous year indicate that the demand for structural steel has leveled off at approximately 30 million metric tons in the current market. Rust poses an issue that must be addressed in conjunction with the use of structural steel, and the substantial demand implies that addressing rust prevention and removal is essential. Consequently, innovative cleaning techniques that are versatile, cost-effective, and efficient across multiple scenarios must be created.

Laser cleaning, recognized for its high efficiency and environmentally friendly attributes, has been extensively researched as a novel cleaning technique. The adoption of continuous wave laser cleaning has become widespread due to its combination of affordability and robust performance [1,2,3]. Among them, the input power is the parameter with the most significant influence on the cleaning results. Sun et al. [4] found that the cleaning power is positively correlated with the cleaning depth. In the realm of numerical computation, stress alone does not fully explain the patterns of depth variation in cleaning [2,5,6,7,8,9]; however, deformable meshing can mitigate this limitation. Continuous, direct laser irradiation produces heat buildup, and prolonged irradiation leads to surface roughness degradation and performance degradation [5]. Under certain conditions, laser power may not effectively process the surface, and some medium is required for assistance. At this time, the choice of laser-assisted cleaning will solve this problem; for laser cleaning under the auxiliary medium, the surface morphology is entirely different from the results of the air medium, and the liquid medium on the surface of the degree of cleanliness has the role of enhancing [10]; selecting some different processing environment applied to the machining work will achieve some unexpected results [11]. Among the findings, studies have shown that the depth of laser processing depends on several conditions, including the nature of the material itself and the medium used in the processing [12]. In previous studies, the main focus has been on aqueous environments [13,14,15], ethanol and aqueous environments [16], and liquid isopropanol environments [17], but there are also drawbacks such as excessively high heights that prevent processing or lower heights that exacerbate surface burning.

On the issue of heat accumulation, Yu et al. [14] investigated the effect of the underwater environment on the laser ablation of SiC surfaces. It was found that underwater ablation would reduce heat accumulation. However, it would reduce the ablation depth, and the relationship between the depth of the water layer and the removal depth still needs to be investigated. For the morphological results after surface cleaning, Sharma et al. [15] investigated the laser ablation of aluminum alloy oxides in air, water, and ionic liquids. The study found that the cleaned surface was cleaner after water and ionic liquid ablation. Wang et al. [18] investigated continuous laser descaling under soapy water as a liquid film medium, which resulted in a reduction in thermal effects and an increase in surface roughness compared to direct irradiation. However, it was observed that when the water layer height is too small during liquid film cleaning, the ablation intensifies. Later on, for the study of cleaning efficiency, Kanitz et al. [17] carried out the laser ablation of steel surfaces in a variety of media: methanol, ethanol, toluene, acetone, and water. It was found that the medium was water at low power, with the most excellent ablation rate for all, and acetone had the worst results. At high power input, the average effect of the scavenging rate was best when the lower auxiliary medium was water. After Zhu et al. [3] studied the mechanism of cleaning efficiency enhancement, they found liquid water is in the irradiation process, where ionized free electrons cannot escape, and a large number of ions or nuclei constantly collide to produce a sufficient sizeable kinetic energy, so that the cleaning efficiency is improved. In summary, the selection of laser processing medium is still mainly concentrated in the liquid; the water layer has a good cleaning effect, but there needs to be more relevant results of in-depth study. In addition, SiO₂ possesses a high melting point, which enables it to maintain its stability. When utilized in mechanical processing for the removal of surface films, it exhibits good efficacy [19]. Moreover, when in a colloidal state, its removal efficiency is significantly enhanced [20]. Based on the aforementioned research, SiO₂ is utilized as an auxiliary medium in laser cleaning to investigate its influence on the cleaning efficiency.

Research within the realm of assisted laser cleaning is currently somewhat constrained, with the primary focus being on the fine-tuning of parameters that affect surface morphology during the cleaning procedure and the examination of how different cleaning media impact efficiency. However, there is a notable gap in the understanding of the fundamental cleaning mechanisms and principles when lasers interact with various media, as well as the challenges posed by limitations within individual media. To address this, the present study delves into the intricacies of continuous laser cleaning across a spectrum of media settings. The objective is to elucidate the outcomes of laser cleaning when enhanced by liquid, solid, and solid–liquid mixtures on aspects such as surface morphology, cleaning depth, and surface roughness. The research also endeavors to enhance the efficacy of continuous laser cleaning techniques. Furthermore, this paper synthesizes the variations in cleaning dynamics for rust layers in diverse media contexts, with the goal of offering theoretical guidance for the advancement of laser cleaning technologies.

2. Experimental

2.1. Experimental Materials

The material used in this experiment was Q345 steel, which produced a corrosion layer after five months of placement(Figure 1a). The mean value of the corrosion layer was observed to be 33 μm (Figure 1b). The corrosion layer was placed in a graduated container, and the height of the addition was calculated according to the density–mass formula.

First, the sample was placed in a container beneath the laser. For the aqueous medium added, the dimensions of the container were measured and the volume of water was calculate using the density–mass formula based on the height of the water layer added. Subtracting the volume of the sample yielded the height of the added water layer. The liquid medium was H₂O and the solid medium was hydrophilic, spherical SiO₂ (20 nanometers in diameter, sourced from Suzhou Yoyan New Materials Co., Ltd., Suzhou, China). After the solid–liquid mixture was stirred to achieve homogeneity, the suspension was poured into a container and lateral vibrations applied to prevent particle aggregation, thereby maintaining the uniformity of the suspension. A schematic diagram of the setup is shown in Figure 2. Equipment specifications and experimental parameters are provided in

Table 1. Main specification parameters of RFL-C1500 laser.

Laser Model	RFL-C1500
Center wavelength	1080 ± 5 nm
Output power	1 KW
Modulation frequency	1024 Hz
Beam diameter	0.5 mm
Speed	10 mm/s

. Subsequently, the obtained results underwent morphological analysis and panoramic scanning using optical microscopy (ZEISS, Oberkochen, Germany)and laser confocal microscopy(Olympus, Tokyo, Japan) to compare cleaning efficiency. Finally, microscopic hardness testing (200 g, 10 s) was conducted on the surface to explore the effect of laser cleaning in different media on surface hardness.

2.2. Experimental Method

Due to the differences between numerical calculations and actual experiments and the primary consideration of the removal of the rust layer, the following assumptions were used in the model: (1) All rust layers and the substrate were considered uniformly distributed solid models, and the thermo-physical properties did not change with the temperature; (2) the occurrence of the phase transition is the basic process of laser cleaning, and only the effect of the temperature is considered in the model. The temperature of the corrosion layer is considered to be the temperature at which the corrosion layer is removed from the solid state. Calculate the sublimation temperature setting above which the corrosion layer changes directly from solid to gaseous. The heat of the sublimation setting is the energy required for the object to reach the gaseous state.

Table 2. Thermophysical properties of materials [20].

	Rust	Matrix	Water	SiO2
Density/(kg/m3)	5200	7860	1000	227
Heat capacity/J/(kg·K)	900	600	431.4	966
Thermal conductivity/W/(m·K)	4.3	44.5	0.6	1.4
Latent heat of vaporization/(kJ/kg)	4300	6071	x	x
Sublimation temperature/K	2046	2633	100	2530

lists the settings for the main parameters of this:

• The rust and the substrate are all considered solid models with uniform distribution, and the thermophysical properties do not change with temperature.
• The occurrence of phase transition is an important process of laser cleaning; only the effect of temperature on the model is considered. Calculation sets the sublimation temperature; over this temperature, the corrosion layer directly moves from the solid state into a gaseous state.
• Adding media plays an auxiliary role in using heat transfer coefficient instead of various conditions corresponding to different heat transfer coefficients. The heat of the sublimation setting is the energy required for an object to reach the gaseous state.
• The density of SiO₂ is set to the value of density under normal placement conditions, and this value does not change over time.
• In the numerical calculation, the design of the water layer is uniform and constant, neglecting water evaporation and other losses.

The results of numerical calculation are shown in Figure 3. The topmost layer of this model represents the height of the added medium layer, which is the primary parameter that is altered.

This study considers laser power density as a heat source with Gaussian distribution characteristics.

In the selection of modules, the laser beam acting on the surface of the object is simplified to the heat conduction equation with the heat source fixed on the surface, i.e., the solid heat transfer stage:

$$\mathsf{\rho }{\mathrm{C}}_{\mathrm{p}}\frac{\partial \mathrm{T}}{\partial \mathrm{t}}+\mathsf{\rho }{\mathrm{C}}_{\mathrm{p}}\mathrm{u}\nabla \mathrm{T}+\nabla \cdot \mathrm{q}=\mathrm{Q}$$

(1)

$$\mathrm{q}=-\mathrm{k}\nabla \mathrm{T}$$

(2)

where “ρ” is the density of the solid; “C_p” is the heat capacity of the solid at constant pressure; “k” is the thermal conductivity of the solid; “u” is the velocity field defined by the translational motion sub-node; and “Q” represents the heat source. However, under the assumption that the absorption and reflection of light are not taken into account, the laser energy irradiated on the material’s surface is converted into thermal energy on the material’s surface, and therefore “Q = 0”. That is, Equation (1) can be summarized as follows:

$$\mathsf{\rho }{\mathrm{C}}_{\mathrm{p}}\frac{\partial \mathrm{T}}{\partial \mathrm{t}}+\mathsf{\rho }{\mathrm{C}}_{\mathrm{p}}\mathrm{u}\nabla \mathrm{T}=-\nabla \cdot \mathrm{q}$$

(3)

The following equations represent the average mesh velocities of the standard mesh velocity nodes of the deformed geometry as they change with the depth of the heat input:

$$\frac{\partial \mathrm{X}}{\partial \mathrm{t}}·\mathrm{n}={\mathrm{v}}_0+{\mathrm{v}}_{\mathrm{mbs}}$$

(4)

where “v₀” is the desired normal mesh velocity and “v_mbs” is the smoothing velocity. The relationship between heat and depth is the change in volume per unit time of the grid through constant heat accumulation.

Furthermore, the subsequent addition of underwater laser cleaning, which uses laminar flow and later ensures the constant flow of the water layer, the fluid uses incompressible flow, which is controlled by the following equations:

$$\mathsf{\rho }\frac{\partial \mathrm{u}}{\partial \mathrm{t}}+\mathsf{\rho }\left(\mathrm{u}·\nabla \right)\mathrm{u}=\nabla ·\left[-\mathsf{\rho }\mathrm{I}+\mathsf{\mu }\left(\nabla \mathrm{u}+{\nabla \mathrm{u}}^{\mathrm{T}}\right)-\frac{2}{3}\mathsf{\mu }\left(\nabla ·\mathrm{u}\right)\mathrm{I}\right]+\mathrm{F}$$

(5)

Boundary conditions are established where, to ensure that the heat source is closer to reality, a continuous surface heat source is used instead of a heat source, and the equations are as follows:

Heat source equation:

$$\mathrm{Q}={\mathrm{e}}^{\frac{\sqrt{{({\mathrm{x}}^2+{\mathrm{y}}^2)}^2}}{2\times {\mathrm{sigma}}^2}}\times \frac{4\mathrm{P}1}{\mathrm{pi}\times {\mathrm{r}}^2}$$

(6)

where “r” is the spot radius, “P1” is the input power, and “sigma” is the standard deviation with a value of r/6.

Initial condition temperature: “T_t=0” = 293.15 [K], all set with thermal insulation except the upper boundary, and thermal convection applied to the surface by the heat source:

$${\mathrm{q}}_{\mathrm{a}}=\mathrm{h}({\mathrm{T}}_{\mathrm{a}}-\mathrm{T})$$

(7)

where “q_a” is the heat flux of material ablation, “T_a” is the ablation temperature, and h = h(T) denotes the temperature-dependent heat transfer coefficient, which is 0 for T < Ta and increases linearly for T > Ta, with h = 1× 10⁶.

The ablation rate of the solid boundary is as follows:

$${\mathrm{v}}_{\mathrm{a}}=\frac{{\mathrm{q}}_{\mathrm{a}}}{\mathrm{p}{\mathrm{H}}_{\mathrm{s}}}$$

(8)

where “v_a” denotes the rate of ablation, “p” denotes the density of the material, and “H_s” denotes the heat of sublimation. Only the Z-axis velocity is added, while the X-axis and Y-axis are set to 0.

3. Results and Discussion

3.1. Numerical Calculation Result

In order to explore the effect of different environments on laser cleaning, laser cleaning in a water environment was further researched. Around a variety of water layer thicknesses (the same power and speed) on the cleaning effect of research, for the four conditions for analysis of 0.3 mm, 0.5 mm, 1 mm, and 2 mm, the results are shown in Figure 4.

The findings indicate that when the water layer height was 0.3 mm, the cleaning depth achieved was 102 µm, which was slightly greater than the depth of the water layer by approximately 2 µm. At a water layer height of 0.5 mm, the cleaning depth increased to 110 µm, exceeding the depth of the water layer by about 10 µm, and significantly impacting the substratum. With a water layer height of 1 mm, the cleaning depth was 100 µm, which was sufficient to achieve the desired cleaning effect, corresponding to the full removal of contaminants at the substratum level. However, there was a noticeable decrease in cleaning depth compared to the previous condition of a 0.3 mm water layer height and a cleaning depth of 102 µm. As the water layer height increased from 0.5 mm to 1 mm, the cleaning depth decreased to 100 µm, suggesting that the optimal cleaning range lies between 0.3 and 1 mm. When the water layer height reached 2 mm, the cleaning depth was reduced to 56 µm, which was inadequate to achieve the desired level of cleanliness. These changes are reflected in Figure 5.

The results of the research surface, laser cleaning in the conditions of the water environment, in a water layer height of less than 1 mm has the effect of promoting the cleaning, which is manifested in the increase in the cleaning depth. When the height of the water layer exceeds 1 mm, the height of the water layer has an inhibiting effect on the cleaning process. The optimal water layer height is between 0.3 and 1 mm.

The efficacy of laser cleaning was investigated across four scenarios with SiO₂ layers of 0.3 mm, 0.5 mm, 1 mm, and 1.5 mm thickness. The corresponding graph is presented in Figure 6 below. The analysis reveals that when a 0.3 mm layer of SiO₂ is added, the cleaning depth increases to 109 µm, surpassing the initial 100 µm, indicating an enhanced cleaning effect. With a 0.5 mm layer of SiO₂, the cleaning depth remains at 100 µm, equivalent to the initial condition, suggesting that the cleaning effect starts to diminish at this point. As the SiO₂ layer thickness is increased to 1.0 mm, the cleaning depth is reduced to 86 µm, which is less than the initial 100 µm, signifying a suppression of the cleaning process. Finally, when the SiO₂ layer is increased to 1.5 mm, the cleaning depth further decreases to 65 µm, indicating that the cleaning is not fully accomplished.

The results depicted in Figure 7 indicate that when adding the SiO₂ active agent, the cleaning effect is best before 0.5 mm; at this time, the addition of the SiO₂ active agent promotes laser cleaning; when the height of SiO₂ active agent is more than 0.5 mm, it has already inhibited the cleaning effect.

3.2. Influence of Liquid Medium on the Cleaning Effect of Corrosion Layer

In the context of enhancing laser cleaning by incorporating a water layer, the facilitative role of the water medium during laser treatment was investigated. Experimental comparisons were conducted as a preliminary step to analyze the effects of various water layer heights on the cleaning process. The experimental images following laser cleaning, used for comparison (each group of air cleaning, control group is Figure 8a including all subsequent experiments), for the results of media-assisted cleaning with water; microscopic observation revealed that when the initial addition of water height was 0.1 mm, the laser-induced intensification of the surface resulted in a flat surface appearing with laser tracks (the cleaning mechanism is illustrated on the left side of Figure 9). When the water height was doubled, the surface and the dry cleaning showed distinct differences; the laser tracks were mostly absent, with only a few shallow traces remaining (as indicated by the arrow in Figure 8c. At this point, the surface became flat. However, with the further increase in water layer height, small melted and solidified lumps appeared (in Figure 8d (the cleaning mechanism is presented on the right side of Figure 9). Subsequently, as the water layer height continued to rise, the lumps increased, and when the height reached 2 mm (as shown in Figure 8f, substances resembling the state of corroded material appeared on the surface. This indicated that the water layer height had reached 2 mm, which posed an even greater obstacle to the cleaning process.

The three-dimensional topography is shown in Figure 10. Utilizing the findings from micro-metallography, the sample was laser-scanned to generate a three-dimensional morphology map. The resulting figure is depicted below: at a water layer height of 0.1 mm, the surface’s overall heterogeneity is heightened, and the height discrepancy reaches 14 μm, which confirms the intensification of ablation from the three-dimensional topography. For the more pronounced change (Figure 10c), the surface topography is not noticeably smoother at this stage, with a height difference of approximately 8 μm. Despite this, the overall color is similar, with the green area comprising 95% of the surface area, and the overall plane is not inclined, as evidenced by the subsequent roughness, which is smoother at this point in the planar surface. As for subsequent changes, from the three-dimensional morphology of the map, it is evident that the number of protrusions increases; the overall height difference also reaches about 23 μm. At this juncture, it can be observed that the water layer seems to inhibit the cleaning effect that would otherwise be observed.

Figure 11 and Figure 12 depict the variations in surface roughness and the depth of rust layer removal. Figure 11 illustrates the disparity in roughness with and without water-assisted cleaning. Compared to the control group without water assistance, surfaces cleaned with water exhibit greater smoothness, achieving optimal roughness between 0.8 and 1.0 μm without surface damage. However, an increase to 0.5 mm results in a significant rise in roughness. From Figure 12, it is evident that complete removal of the rust layer is achievable up to 0.2 mm, beyond which complete cleaning efficacy diminishes. At this point, cleaning efficacy is inhibited rather than enhanced. In contrast to the approximately 3 μm optimal roughness, Sa, achieved by Wang et al. [18] using continuous laser rust removal, our method achieves a roughness range of 0.8 to 1.0 μm, thus demonstrating a significant improvement.

The simulation results of cleaning depth and experimental results were compared and analyzed, and the results are shown in Figure 13.

Figure 13 presents a comparison chart of the simulation results of laser cleaning in an aqueous media environment and the experimental results. The research findings indicate that complete cleaning can be achieved at approximately 0.3 mm, with the cleaning depth showing a trend of first increasing and then decreasing. The actual experiment revealed that in an aqueous media environment of about 0.2 mm, complete cleaning can be achieved without damaging the substrate. It was observed that the cleaning depth varies with the height of the water layer in a manner similar to the simulation results, with the two heights closely matching, thus enhancing the reliability of the data.

3.3. Effect of Solid Media on Cleaning Effect

The processing of aqueous media can render the surface smoother with a lower roughness. Now, we will investigate the influence of solid surfactants on their cleaning efficacy.

The microscopic diagram of laser cleaning after applying silica is shown in Figure 14. It is observed that from the onset of SiO₂ addition, the surface begins to exhibit molten material and intensified laser tracks, indicating that the object’s surface becomes more efficient in absorbing laser energy after the introduction of SiO₂, akin to the previous instances of laser cleaning. As seen in Figure 14, the thickness of SiO₂ increases with the height of SiO₂ up to 0.05 mm; the laser tracks are more concentrated and accompanied by the formation of some remelts compared to when no SiO₂ is added. At 0.1 mm, the laser tracks disperse, the surface becomes flatter, and remelts gradually increase. Beyond 0.1 mm, some incomplete ablation marks appear on the surface, and the laser marks gradually decrease. When the height reaches 0.8 mm, there are only a few cleaning traces, most of which are still in the corroded state, and the active agent at this height significantly hinders the cleaning process.

The three-dimensional morphology of laser cleaning after applying silica is shown in Figure 15. The three-dimensional morphology map is now being analyzed, and it is observed that when SiO₂ is first added, the surface exhibits significant fluctuations and evidence of laser action, resembling mountain ranges when the height is doubled to 0.1 mm. The surface tends to be flat, primarily green and blue, and there is a tendency for the surface to be in a flatter and smoother condition after excluding error points. As the subsequent height increases further, the unevenness of the plane increases, the non-planar area increases, and the overall roughness value rises. When the height reaches 0.8 mm, the surface has only a few protrusions produced by the ablation behavior, and the remaining part of the surface is in an uncleaned state, with poor flatness, which can also be seen in the surface morphology corresponding to the metallographic state. The addition of SiO₂ in the right conditions will exacerbate the surface ablation behavior, and the impact on the surface morphology is more significant, resulting in an increase in the overall unevenness; when the height of the addition of SiO₂ is too high, the cleaning process is inhibited; the best effect occurs at a height of 0.1 mm.

The cleaning results of the study are shown in Figure 16 and Figure 17. The study results are as follows: after the addition of SiO₂, the range of changes in the surface roughness values under the heights of 0.05 mm and 0.1 mm is small, and can be regarded as basically unchanged under the consideration of the error. When the height of added SiO₂ reaches 0.2 mm, the surface roughness undergoes a significant change, increasing from about 1.0 µm to about 2.2 µm, and subsequently, with the increase in SiO₂ height, the surface roughness value gradually increases to about 3.5 µm when it reaches 0.8 mm in height. From the cleaning depth, when adding 0.05 mm height of SiO₂, the cleaning depth has a slight increase, about 1.0 µm; when the height continues to double, the cleaning depth continues to increase, reaching a maximum value of 37.5 µm or so. At this time, compared with the non-addition of SiO₂, the depth of the cleaning is increased by about 3.5 µm. Subsequently, as the height of SiO₂ increases, the removal depth decreases sharply from about 25 µm to about 13 µm, at which time the addition of SiO₂ plays an inhibitory role in laser cleaning.

Figure 18 shows the comparison between the numerical calculation and the actual cleaning results of SiO₂ environmental media. It was found that the simulation conditions give results at <0.3 mm, according to the estimated value at the height of 0.15 mm located within extreme values; at the same time, the actual results are within the 0.1 mm or so, and the results of the two yield some error, which can be accepted.

3.4. Solid–Liquid Mixture of Media on the Cleaning Effect of

Building upon the results obtained from aqueous media and SiO₂ processing, we contemplated the outcomes of combining the two. Given that the water layer did not yield the desired cleaning effect at a height of 0.5 mm, a unique surface emerged, to which SiO₂ was subsequently added for comparative analysis to investigate the efficacy of laser cleaning following their mixture. Due to the inherent physical properties of SiO₂, it is not readily soluble in water and can only be suspended in water. Consequently, varying proportions of the mixture were employed as an auxiliary medium for laser cleaning, with different amounts of SiO₂ added to the suspension. These proportions are expressed as percentages and solely indicate the ratio of SiO₂ within the mixture.

The microscopic morphology of laser cleaning after solid–liquid mixing is shown in Figure 19. Upon further examination of the above results with the aqueous medium as an auxiliary, it was observed that the addition of SiO₂ enhanced the cleaning effect. Specifically, when the proportion of SiO₂ was set at 0.5% (Figure 19b), the surface exhibited pits after ablation, indicating that SiO₂’s addition promoted the cleaning process. As the proportion of SiO₂ increased to 1% (Figure 19c), the surface morphology underwent a transformation, with the region becoming smoother but also with the emergence of some pits. Additionally, the shadow traces still hinted at the presence of the rust layer, which had not been fully removed. At a SiO₂ proportion of 2% (Figure 19d), the surface appeared flat, with regions of over-ablation, and some rust remained. As the proportion of SiO₂ continued to rise, the cleaning effect deteriorated, as seen in Figure 19f, where at 10% SiO₂, there was a more pronounced presence of rust that had not been removed. The mixture of SiO₂ and H₂O, at certain proportions, can enhance laser cleaning and is conducive to the cleaning process.

The three-dimensional morphology of laser cleaning after solid–liquid mixing is shown in Figure 20. Under cleaning in the mixture, the three-dimensional morphology did not show significant noticeable changes; from the addition of 0.5% of SiO₂, some surface bulging appeared, and the overall height difference increased from 15 µm to 25 µm. The bulging part of the surface corresponds to the ablation area, which also corresponds to the addition of SiO₂ to promote the cleaning of the underwater medium; when reaching the addition of SiO₂ to 1%, the surface begins to flatten again, compared with the maximum value of 20 µm, which is shown in Figure 20c. That is, in Figure 20c, the surface begins to be flat again, and the maximum height difference comes to 20 µm, which is 5 µm less than that in Figure 20b, and the protruding area is reduced; furthermore, with the addition of SiO₂, when it comes to Figure 20d, the maximum height difference of the surface reaches 12 µm, which is the most minor height difference of the surface in this stage; subsequently, with the increase in the added amount of SiO₂, the height difference of the surface increases, and the height difference of the surface reaches 10%, which corresponds to the ablation area, and it also corresponds to the promotion effect of adding SiO₂ on underwater media cleaning; at this time, it also corresponds to the promotion effect of adding SiO₂ to the underwater media cleaning. When the proportion of SiO₂ is 10%, the height difference reaches 45 µm. The addition of SiO₂ to the aqueous solution first decreases the difference and then increases it.

Surface roughness and removal depth are analyzed to investigate the effect of SiO₂ percentage size on both. The results are shown in Figure 21 and Figure 22. From the surface roughness, with the increase in SiO₂ percentage content, the surface roughness does not change much, but before the percentage of 1% and then a slightly increasing trend, from about 1 µm to about 1.2 µm, when the percentage of SiO₂ reaches 2%, there is a minimum value of about 0.8 µm, with a more uniform distribution. Subsequently, the surface roughness increases gradually with the increase in SiO₂ percentage and rises to a maximum value of 5.7 µm at 10%.

From a removal depth perspective, despite the initial dataset not achieving complete cleaning (26 µm), as the proportion of SiO₂ increases, the removal depth steadily rises. Upon reaching a SiO₂ proportion of 2%, the cleaning depth reaches approximately 34 µm, effectively removing the rust layer. This suggests that, at this juncture, the SiO₂ proportion positively influences the cleaning efficacy. Subsequently, as the SiO₂ proportion continues to increase, the cleaning depth gradually decreases from 34 µm to 21 µm, reaching approximately 5 µm at a SiO₂ proportion of 10%.

From the perspective of machining oxide removal, the use of the mixed suspension improved efficiency by 19.3% [21]. In terms of laser cleaning for rust removal, under the same parameter conditions, there was a 25% increase in the removal of rust layers, indicating that the efficiency of a solid–liquid mixture is greater than that of a single medium assistance.

The cleaning mechanism is now analyzed, and the results are shown in Figure 23. The rust layer, when subjected to a solid–liquid mixed dielectric, undergoes vaporization after being irradiated by the laser, with the liquid medium absorbing heat. This vaporization serves to ablate surface contaminants. Subsequently, solid particles collide with each other and continuously impact the rust layer through mechanical motion. The cleaning mechanism of the solid–liquid mixture is a hybrid mechanism, where the ions of the removed rust layer are dispersed in the liquid and do not remain on the cleaned surface.

Solid–liquid mixture-assisted laser cleaning technology is an advanced cleaning method that combines solid particles and liquid media. It enhances the absorption and transmission of laser energy by adding solid particles during the cleaning process, thereby improving cleaning efficiency and quality. Cleaning efficiency may be influenced by various factors, including laser parameters, particle characteristics, compositions of cleaning solutions, etc., which require further research for optimization.

3.5. Surface Performance Testing

The hardness test was carried out on the surface of the cleaned specimen and the results are shown in Figure 24. Compared to the microhardness value of 150 HV obtained after direct laser action on the surface, microhardness of the original substrate of 102 HV was observed at a 0.2 mm height of water and 0.1 mm height of SiO₂. SiO₂ accounted for a ratio of 2% of the mixture of the three measured microhardness of the surface of 119 HV, 135 HV, and 131 HV, compared to the microhardness of the original substrate. Media-assisted cleaning played a role in surface strengthening, but compared to the 150 HV obtained by direct laser treatment, there is still a large gap, the most significant gap, being about 15 HV. However, compared to the untreated surface, SiO₂-assisted cleaning has the most significant impact—the microhardness has been strengthened to a certain extent—improving the HV to 32 or so, an increase of 30%. The effect of water was the smallest, with an enhancement of only about 20 HV, or about 20%. Description of the laser cleaning process: the laser’s direct action on the surface of the microhardness obtained is higher than the microhardness of the added active agent (SiO₂), and the water–dry cleaning laser strengthening effect is the best.

Although solid–liquid mixture-assisted laser cleaning can reduce the thermal impact on the substrate, there are still certain risks involved, especially in high-power laser applications, where a low water layer height can exacerbate surface burning. Further optimization of laser parameters, the types and sizes of solid particles, and liquid selection can enhance cleaning efficiency and surface quality while reducing damage to the substrate.

4. Conclusions

This experiment mainly compares and investigates the effects of different solid active agents, liquid active agents, and solid–liquid mixing on the laser cleaning effect, ensuring that the laser parameters remain unchanged and only changing the height of the additive amount to explore the effect of the size of the additive amount on the cleaning result and the subsequent surface roughness as well as the depth of removal is analyzed. The conclusions are as follows:

• A 0.1 mm water layer height achieves the maximum removal depth of 37 μm, and when the water layer is at a height of 0.2 mm, the laser cleaning mechanism under water medium is ablative, with a surface roughness of approximately 0.8 μm.
• SiO₂ active agent at a height of 0.1 mm obtains a cleaning depth of 37 μm and surface roughness of 1.1 μm, and SiO₂ plays an auxiliary role in the laser cleaning process.
• The combination of solid and liquid mixtures can integrate the advantages of both, resulting in an improved surface roughness and increased depth of removal.
• Under medium conditions, the microhardness values obtained from laser cleaning are lower than those from direct laser irradiation, with the surface hardness after SiO₂-assisted cleaning being greater than that after water-assisted laser cleaning.