Investigation on the Surface Properties of 5A12 Aluminum Alloy after Nd: YAG Laser Cleaning

Abstract

The surface of the aluminum alloy is prone to oxidation, which in turn affects the quality of the weld. The 5A12 aluminum alloy was cleaned by acousto-optic Q-switched diode-pumped Nd:YAG laser and the effects of different laser powers and different cleaning speeds on the surface roughness, microstructure, element content, microhardness, residual stress and corrosion resistance of aluminum alloy were investigated. The results show that when the power is 98W and the cleaning speed is 4.1 mm/s, the effect of Nd: YAG laser on the removal of oxide film on 5A12 aluminum alloy surface is the most effective. After laser cleaning, the smoothness and strength of aluminum alloy surface can be effectively improved. However, as a major element in 5A12 aluminum alloy, the content of magnesium decreased. At the same time, the residual tensile stress was generated on the surface of the aluminum alloy after cleaning, and the corrosion resistance slightly decreased.

1. Introduction

Aluminum alloy is one of the most widely used materials in industrial production [1]. Given its low density, high strength, good plasticity, good electrical conductivity, high thermal conductivity, and corrosion resistance, it is widely applied in the automobile manufacturing, aerospace, shipbuilding, and chemical industries [2,3,4]. However, aluminum is an active metal and easily oxidizes in the natural environment, with an oxide film forming on the surface. These oxide films absorb moisture and other impurities [5] and increase the hydrogen source during the welding of aluminum alloys, resulting in hydrogen-induced pore formation, which reduces the welding quality [6]. So, cleaning the aluminum alloy before welding is particularly important [7]. Traditional metal surface cleaning methods mainly include mechanical friction, chemical corrosion, liquid solid impact, and ultrasonic treatment [8]. Although these methods can successfully achieve cleaning, they have some shortcomings: Cleaning may be incomplete, the matrix can be damaged, and the environmental polluted. Therefore, an effective method to remove the oxide layer on the surface of aluminum alloy needs to be found.

In the global vigorously developing green environmental protection industry, laser cleaning has begun to be applied in industrial manufacturing as a green cleaning technology. Since the 1980s, laser cleaning technology has been used remove unwanted layers and encrustation on artworks to enhance their appearance and ensure their longevity [9]. In the following decades, laser cleaning technology was increasingly perfected and gradually applied to various metal materials. Laser cleaning is characterized by cleaning without changing the surface morphology, non-contact, high efficiency and suitable for various materials, and considered the most reliable and effective solution in many fields. Niroomand et al. [10] used CO2 lasers to surface-treat tire molds and tire cords, improving the bonding performance and friction coefficient of tires, thereby reducing the risk of puncture. Takahashi et al. [11] proposed a new concept of laser-assisted material removal that uses laser-assisted ultraviolet radiation to achieve nanoscale correction of microscopic three-dimensional (3D) objects. Palomar et al. [12] used a nanosecond Q-switched Nd: YAG laser to remove dirt from pure silver products, and then determined the optimal cleaning threshold of pure silver cultural relics. Delaporte et al. [13] used a Nd: YAG laser (6 ns) and a xenon flash lamp (200 ms) for dust removal of radioactive compounds. With the rapid development of laser technology, laser cleaning has become more automated, effective, and cheaper, and it has been widely used in paint and rust removal, tire mold cleaning, cultural relics protection, and nuclear purification [14,15,16].

The laser cleaning process is the interaction between light and the substrate. By irradiating the substrate surface with a laser, the binding force between the substrate and the pollutants attached to the substrate surface is disrupted, and the pollutants on the substrate surface are separated from the substrate surface by evaporation, breakage, and vibration [17]. To achieve the best cleaning effect, the first step is to find the best cleaning parameters, including laser wavelength, power, cleaning speed, etc. Since different materials have different absorption rates for lasers, different materials have different optimal cleaning thresholds. The absorption rate of common metals for lasers is shown in Figure 1 [18,19,20]. For aluminum alloy, when the laser wavelength is 1064 nm, the absorption rate of laser to the substrate is higher. Therefore, an acousto-optic Q-switched diode-pumped Nd: YAG laser is used to clean aluminum alloy. In this study, we discussed the effects of different cleaning processes on the surface properties of aluminum alloys and determined the best cleaning parameters for removing oxide film to provide reference for the large-scale application of laser cleaning in metal surface treatment.

2. Scope of Research

In this paper, a naturally placed high-magnesium aluminum alloy was selected as a reference sample, and the oxide film on its surface was cleaned using a laser for pre-welding treatment. We examined whether laser cleaning can effectively remove the oxide film on the surface of a high magnesium aluminum alloy and compared the surface properties of aluminum alloy after laser cleaning to provide a reference for laser cleaning to replace traditional surface treatment methods. Previous studies have shown that a low energy density Nd: YAG laser can effectively remove oxides from metal surfaces without causing thermal oxidation [21]. Other studies have shown that when the energy density is greater than 1 J/cm², thermal oxidation occurs on the surface of Mg-Al alloy to form a new oxide layer [22]. Therefore, we aimed to control the laser energy density by changing the laser power to determine optimum laser process parameters for removing the oxide film on the surface of high-magnesium aluminum alloy. The efficacy of oxide film removal is mainly measured by the content of oxygen on the surface of the aluminum alloy before and after cleaning. The changes in the other main elements and the smoothness, strength, residual stress, and corrosion resistance of the surface after cleaning also need to be considered simultaneously. We wanted to examine the effect of changing the scanning speed of the laser on the cleaning effect when the laser energy density is constant. Therefore, we addressed two variables of laser energy density and scanning speed: Power and cleaning speed. The surface roughness, micro-morphology, element distribution, microhardness, residual stress and corrosion resistance of aluminum alloy cleaned under different laser parameters were tested, and the optimum parameters were obtained through analysis and comparison.

3. Materials and Methods

In the experiment, 5A12 aluminum alloy was used as the sample. Its composition is outlined in

Table 1. Chemical composition of the aluminum alloys 5A12.

Alloy	Si	Cu	Mg	Zn	Mn	Ti	Ni	Fe	Al
Mass fraction/%	≤0.3	≤0.05	8.3–9.6	≤0.2	0.4–0.8	0.05–0.15	≤0.1	≤0.3	allowance

and the sample size was 10 mm × 10 mm × 4 mm. In the experiment, we used medium-power and high-energy laser-diode-pumped pulsed solid-state laser cleaning equipment (SC200W–350 KW, Laser Institute, Shandong Academy of Sciences, Jinan, China). The schematic diagram of the laser cleaning system is shown in Figure 2. The continuously optimized resonator design for high-energy solid-state lasers, combined with the flexible conduction advantages of fiber coupled output, enable the model to have much higher ultra-high peak power (over 350 KW) and wider usability than the market equivalent laser equipment. The main parameters of the lasers are shown in

Table 2. Main parameters of the lasers.

Characteristic	Symbol	Value	Units
Source	−	Pulse laser adopting Nd:YAG	−
Wavelength	λ	1064	nm
Maximum average power	P	200	W
Pulse frequency	F	7–15	KHz
Maximum pulse energy	Pe	25	mJ
Max pulse power	Pp	350	KW
Output spot size	d	0.5–2.5	mm
Scan line width	l	1–10	cm
Producer	Laser Institute, Shandong Academy of Sciences, China

. The laser energy density is controlled by changing the power of the laser. The power can be controlled at 50–110 W, the fixed repetition frequency is 11 KHz, and the corresponding energy density is 0.6–1.6 J/cm². The average power of the laser is proportional to the loading current of the laser module, so the power of the laser can be changed by controlling the current of the laser. The current can be controlled at 22, 24, 26, 28 and 29 A, with corresponding laser power of 50, 88, 98, 104 and 110 W, respectively. The cleaning speed of the laser is related to the number of scanning passes, the spot overlap ratio, the galvanometer frequency, the scanning line width, the laser frequency, and the spot area. In this study, the cleaning speed was controlled by changing the scanning variables of the laser in the same position. The number of scans of the laser in the same position were set to 5, 10, 15, 20, 25, 30, 40, 60, 80 and 100. The cleaning speed was determined to be 20.7, 10.4, 6.9, 5.2, 4.1, 3.5, 3.0, 2.6, 1.7 and 1.0 mm/s, respectively.

The roughness of aluminum alloy specimens was measured using a white light interferometer (MFT-4000, Lanzhou Huahui Instrument Technology Co., Ltd., Lanzhou, China). We adopted the surface sweep method, the test area size was 2 mm × 2 mm, we selected 3 areas to test and used the average value of the three and record the 3D shape with a super depth of field digital microscope (VH-2500R, KEYENCE, Osaka, Japan). Scanning electron microscopy (JSM-7610F, JEOL, Tokyo, Japan) was used to perform microscopic characterization and elemental analysis of the aluminum alloy samples. The cleaning effect was determined by comparing the oxygen content under different cleaning parameters. Elemental stratification was conducted to compare the samples that were not cleaned and those that were completely cleaned to prove the cleaning effect. The surface microhardness of aluminum alloy was tested using a micro-Vickers hardness tester (402-MVD, Wilson, Norwood, USA). The test method involved measurement at 4 points with a loading force of 300 g. The residual stress was measured using an X-ray residual stress analyzer (iXRDCOMBO, Proro, Canada), the radiation type was Cr-Kα, the diffraction Bragg angle was 139.0°, and the wavelength was 0.2291 nm. The aluminum alloy samples were subjected to electrochemical polarization testing using a potentiostat (CHI604E, Huachen Instrument Ltd., Shanghai, China) in 3.5 wt.% NaCl solution. The traditional three electrodes were immersed in the solution for electrochemical measurement and the reference electrode was Ag-AgCl (in saturated KCl) and the counter electrode was a platinum sheet. Then, the Tafel polarization curve was obtained, which shows the relationship between current density and overpotential.

4. Results and Discussion

4.1. Surface Topography

4.1.1. Macroscopic Morphology

To discuss the effect of laser cleaning on the surface morphology of 5A12 aluminum alloy, the roughness of the samples under different power and different cleaning speeds was tested to illustrate the effect of laser cleaning on the macroscopic morphology of aluminum alloy. The surface roughness of the aluminum alloy after laser cleaning at different powers and speeds is shown in Figure 3. The 3D topography of the cleaned aluminum alloy surface is shown in Figure 4 and Figure 5.

Firstly, a median velocity, 4.1 mm/s was fixed. Based on this, we tested the roughness of the aluminum alloy surface under different laser powers (Figure 3a) and recorded their surface 3D topography (Figure 4). At a laser power of 50 W, the texture and scratches of the aluminum alloy surface disappeared, and the surface roughness rapidly increased from the 0.98 to 1.51 μm. With the increasing laser power, the surface roughness of aluminum alloy gradually decreased. Under 98W laser power, the surface roughness reduced to the lowest value recorded: 0.87 μm. At this time, the surface of the aluminum alloy was the flattest, and we hypothesized that the removal effect of the oxide film was improved. Then, as the power increased, the surface of the aluminum alloy became rough again until the power was increased to 110 W. Whether this is caused by thermal oxidation of the aluminum alloy surface was examined in the following experiments. Next, the laser power was fixed to 98 W, the laser cleaning speed was changed, and then the roughness was measured (Figure 3b) and the 3D surface topography was recorded (Figure 5). The effects of reducing the cleaning speed and increasing the power on the surface roughness of the aluminum alloy seem to be similar. When the cleaning speed was higher than 10.4 mm/s, the surface was significantly rougher than that of the original sample. When the cleaning speed dropped below 10.4 mm/s, the roughness began to decrease and reached a minimum at 4.1 mm/s. As the cleaning speed was further reduced, the roughness value increased to 1.7 mm/s.

Through the above analysis and discussion, selecting certain laser process parameters can improve the flatness of the aluminum alloy surface, but whether this represents the effect of removing the oxide film required further experimental verification. Next, we selected aluminum alloy samples with laser powers of 50, 88, 98, 104 and 110 W for SEM. We selected three representative aluminum alloy samples with cleaning speeds of 10.4, 4.1 and 1.0 mm/s for SEM to investigate the effect of laser cleaning on the microstructure of 5A12 aluminum alloy.

4.1.2. Microscopic Morphology

From the SEM, the surface of the uncleaned aluminum alloy sample had obvious visible light streaks (Figure 6a), which are strip defects caused by uneven microstructure and composition of the extruded profile during production. These strip defects are often rich in oxygen and moisture [23]. Aluminum alloys are usually treated with anodic oxidation before they are manufactured. They easily oxidize as active metals under natural conditions, thus forming a dense oxide film on the surface of aluminum alloys (Figure 6a1). When the laser power was 50 W, the bright stripes on the surface of aluminum alloy disappeared, and many annular structures and holes were produced (Figure 6b). As the laser acts on the surface of aluminum alloy, the light energy is transformed into heat energy, which expands the oxide film on the surface of aluminum alloy via heat and break down. The impact force produced during the expansion process and the recoil force of the aluminum alloy itself make the surface of aluminum alloy form an annular structure (Figure 6b1) [24]. The hydrogen in the stripes on the surface of aluminum alloy is the main reason for the formation of holes [25]. Under the thermal effect of laser irradiation, the surface of the substrate melts and the precipitation of hydrogen causes more voids on the surface of the aluminum alloy. Due to the lack of laser energy, the molten metal liquid can be quickly solidified when it is not injected into the vacancy, thereby forming a hole (Figure 6b1). When the laser power reached 88 W, the annular structure and the hole on the surface of the aluminum alloy considerably reduced (Figure 6c). When the laser power was 98 W (Figure 6d), the energy generated by the laser was enough to fill the cavity with molten metal liquid. The surface smoothness of aluminum alloy was much better than before, the annular structure almost disappeared, and the number of holes greatly decreased. As described above, we speculate that the removal of the oxide film on the surface of the aluminum alloy was the most effective under these conditions, which was confirmed by the energy dispersive spectrometer (EDS). As the power continued to increase, when the laser power was 104 W (Figure 6e), the surface smoothness of the aluminum alloy began to decrease, and the obvious visible spot action marks on the surface formed meteorite craters [26]. At this time, the excessive melting of the aluminum alloy itself occurred via thermal oxidation, which again increased surface roughness. When the laser power was 110 W (Figure 6f), more craters formed on the surface of the aluminum alloy, and as the laser power increased, the depth of the crater marks caused by the spot effect gradually increased. At this time, the high energy density of the laser remelted the thermal oxidation site of the substrate, so that the surface of the cleaned aluminum alloy was slightly smoother than before.

Next, the effect of cleaning speed on the micro-morphology of aluminum alloy was analyzed (Figure 7). SEM analysis showed that that increasing the irradiation time on the surface of aluminum alloy did not seem to have much effect on the cleaning effect when maintaining the energy density of the laser. When the cleaning speed was 10.4 mm/s, the surface of the aluminum alloy appeared rougher, and the surface of aluminum alloy still showed fewer annular structures and holes. When the cleaning speed was dropped to 4.1 mm/s, the surface of the aluminum alloy was basically flat with only a few holes. As the cleaning speed continued to drop to 1.0 mm/s, the number of holes increased obviously.

By analyzing and discussing the effects of laser power and cleaning speed on the macroscopic morphology and microstructure of aluminum alloy, we concluded that when the surface power of Nd:YAG laser cleaning aluminum alloy is about 98 W and the cleaning speed is about 4.1 mm/s, the removal of the oxide film seems to be the most effective. To confirm this point, EDS was performed on these samples.

4.2. Element Content

To further study the effect of laser cleaning on oxide film removal, EDS was performed on these aluminum alloy samples. The macroscopic picture of the cleaning effect is shown in Figure 8. To more clearly demonstrate the cleaning effect, the mechanically ground aluminum alloy sample was added for comparison. The element composition of aluminum alloy after laser cleaning at different powers and speeds is shown in

Table 3. Element content of aluminum alloy samples under different powers by an energy dispersive spectrometer EDS).

Element (Wt)	Al	O	C	Mg	Mn	Cr	Fe	Cl	Ca
Reference sample	59.9	14.4	13.0	11.7	0.4	–	0.2	0.2	0.1
50W	77.9	10.2	6.6	4.5	0.8
88W	78.1	9.2	6.1	6.0	0.6
98W	77.2	8.3	7.3	6.7	0.5
104W	77.0	9.6	6.4	6.3	0.7
110W	77.6	9.2	6.2	6.4	0.7

and

Table 4. Element content of aluminum alloy samples at different speeds by an energy dispersive spectrometer (EDS).

Element (Wt)	Al	O	C	Mg	Mn	Cr	Fe	Cl	Ca
Reference sample	59.9	14.4	13.0	11.7	0.4	–	0.2	0.2	0.1
10.4mm/s	76.5	10.0	6.3	5.7	0.9	0.1	0.6
4.1mm/s	77.2	8.3	7.3	6.7	0.5
1.0mm/s	72.6	9.3	6.9	10.4	0.6		0.2

.

Figure 7 shows that a dense oxide film adhered to the surface of the aluminum alloy before cleaning, and the surface was dark black. When the laser power was 50 W, the surface of the aluminum alloy was much brighter, but the remaining oxide film was clearly visible on the surface. As the laser power continued to increase, the brightness of the surface of the aluminum alloy also increased. When the power reached 98 W, the surface of the sample was the brightest, and the actual color of the aluminum alloy surface substrate at this time was determined by comparison with the sample subjected to the mechanical grinding treatment (Figure 8c). When the laser power was higher than 98W, the surface brightness of the aluminum alloy gradually decreased, which may have been caused by the ablation and thermal oxidation of the aluminum alloy due to the high laser energy. Similarly, when the laser cleaning speed was 10.4 mm/s, the surface of the aluminum alloy had a layer of oxide that was not completely cleaned. When the cleaning speed was reduced to 4.1 mm/s, the aluminum alloy surface was the brightest and seems to be the best cleaning speed. As the cleaning speed is further reduced, the surface of the aluminum alloy dimmed.

The primary purpose of laser cleaning is to remove oxide film on the surface. Therefore, the cleaning effect of the oxide film is mainly determined by the oxygen content. The oxygen content of aluminum alloy after laser cleaning at different powers and speeds is shown in Figure 9, which shows that when the laser power is 98 W and the cleaning speed is 4.1 mm/s, the oxygen content is the lowest and the cleaning effect is the most effective, which proves that the conclusion we inferred before is correct. When the power is lower than 98 W, the oxide film removal on the surface of the aluminum alloy is not complete; when the power is higher than 98 W, the surface of the aluminum alloy is thermally oxidized, again increasing the oxygen content again. Similarly, when the cleaning speed is higher than 4.1 mm/s, the surface is under-cleaned, and when the cleaning speed is lower than 4.1 mm/s, the surface is over-cleaned. The results show that laser cleaning can effectively remove the oxide film and impurities on the surface of aluminum alloy. Figure 10 shows the layered image of the elements of aluminum alloys that are both not cleaned and completely cleaned. Before cleaning, the oxygen content on the surface of aluminum alloy was 14.4%, and the oxygen element distribution was dense. After cleaning, the oxygen element content decreased significantly, the oxygen content was 8.3%, and the oxygen element distribution was uniform and dispersed. This shows that laser cleaning can effectively remove the oxide layer on the surface of aluminum alloy. Laser cleaning causes the oxide film on the surface of aluminum alloy to peel off from the substrate and reveal the intrinsic color of the substrate, which significantly increases the aluminum content on the surface. Since the ionization energy of magnesium ions is much lower than the ionization energy of aluminum ions, magnesium is more likely to melt and evaporate than aluminum [27]. As an important alloy element in 5A12 aluminum alloy, the content of Magnesium decreases obviously after cleaning. After laser cleaning, the surface microstructure and element content of aluminum alloy have change, which may affect the physical and chemical properties of aluminum alloy. So, the effects of laser cleaning on the microhardness, residual stress and corrosion resistance of aluminum alloy are discussed below.

4.3. Physical Property

4.3.1. Microhardness

The microhardness value of the surface of the aluminum alloy under different laser parameters is shown in Figure 11. The experimental results show that when laser power is less than 98 W, the surface microhardness of the aluminum alloy after cleaning increases slightly, and when the laser power is greater than 98 W, the surface microhardness of aluminum alloy decreases. When the energy impact produced by a pulsed laser is higher than the dynamic yield point of the aluminum alloy, yield and plastic deformation occur on the surface of the matrix, and grain dislocation and slip become difficult, resulting in the formation of a hardening layer on the surface of the aluminum alloy, thus improving the surface hardness value [28]. However, when the laser power is too high, the high-energy laser causes the hardened layer formed on the surface of the aluminum alloy to melt. As an important element improving the strength of 5A12 aluminum alloy, the loss of magnesium, eventually leads to a decrease in the surface hardness of the aluminum alloy. The experimental results show that changing the cleaning speed of the laser does not seem to have any effect on the microhardness of the surface of the 5A12 aluminum alloy. We conclude that the main method to improve the hardness of the 5A12 aluminum alloy is to change the energy density of the laser. The surface of the aluminum alloy can be strengthened by selecting appropriate laser parameters.

4.3.2. Residual Stress

The residual stress values of aluminum alloy surface under different laser parameters are shown in Figure 12. The results show that the residual stress before and cleaning are compressive and tensile stress, respectively. The residual stress on the surface of the aluminum alloy after cleaning is increased and the sample tends to stretch. The strengthening effect of magnesium on aluminum alloys is obvious. The tensile strength of aluminum alloys increases by about 30 MPa with a 1% increase in magnesium. During the cleaning process, the high energy and heat of the laser cause magnesium to evaporate from the surface of the aluminum alloy, and the tensile and creep properties decrease rapidly [29]. The surface of high-strength and high-toughness aluminum alloy obtained by solid solution strengthening produces fracture and reorganization, and the residual stress is released and redistributed. The annular structure and holes on the surface of the matrix may also be an important reason for the increase in the residual stress in aluminum alloy. Therefore, the surface of the 5A12 aluminum alloy is in a plastic state after laser cleaning. This cannot be ignored for components that require high dimensional accuracy and stability.

4.4. Corrosion Resistance

The 5000 series of high magnesium aluminum alloys are often used as marine equipment materials due to their high strength and good corrosion resistance. Therefore, we needed to conduct electrochemical experiments on the cleaned samples to study their corrosion resistance after laser cleaning. We prepared a NaCl solution with a mass fraction of 3.5% with pure NaCl and distilled water to simulate the seawater environment. The potential kinetic curves of aluminum alloy samples under different laser parameters are shown in Figure 13. The corrosion potential (Ecorr) characterizes the thermodynamic stability of the tested samples under electrochemical corrosion conditions [30]. Corrosion current density (Icorr) denotes corrosion rate and breakdown potential have the lowest potential for pitting corrosion [31]. The electrochemical parameters of the tested aluminum alloy samples are listed in

Table 5. Electrochemical corrosion parameters of aluminum alloy after cleaning under different laser powers.

Power(W)	Ecorr (V vs.Ag/AgCl)	Icorr (A/cm−2)	Ba (mVdec−1)	Bc (mVdec−1)
Reference sample	−1.452 ± 0.06	9.153×10−6	6.066	6.573
50	−0.708 ± 0.06	2.173 × 10−4	4.823	11.470
88	−1.085 ± 0.06	2.162 × 10−5	7.497	2.869
98	−1.252 ± 0.06	1.941 × 10−5	6.981	0.825
104	1.242 ± 0.06	2.673 × 10−4	7.893	1.682
110	−0.994 ± 0.06	2.346 × 10−8	9.806	1.639

.

Figure 13 and Table 5 show that the polarization potential and corrosion current density of aluminum alloy without cleaning are −1.452 V and 9.153 × 10⁻⁶ A/cm⁻², respectively. When the laser power was 50 W, the polarization potential of aluminum alloy after cleaning rose to −0.708 V, and the corrosion current density rose to 2.173 × 10⁻⁴ A/cm⁻². When the oxide film falls off, the corrosion rate of aluminum alloy increased and the corrosion resistance began to decrease. With further increases in laser power, at 88 W, the polarization potential of aluminum alloy decreased to −1.085 V and the corrosion current density decreased to 2.162 × 10⁻⁵ A/cm⁻². At this time, the corrosion rate of aluminum alloy began to decrease, which indicates that the laser power is enough to refine the surface grains of the aluminum alloy [32], and the corrosion resistance of aluminum alloy was better than before. When laser power was 98 W, the polarization potential of aluminum alloy decreased to −1.252 V and the corrosion current density decreased to 1.941 × 10⁻⁵A/cm⁻². After that, the corrosion current density increased and the corrosion resistance of the aluminum alloy decreased. Continuously increasing the laser power to 110 W, the oxide layer composed of MgO + MgAl₂O₄ was removed, and a new Al₂O₃ + MgO oxide film formed on the clean surface via thermal oxidation [33], which considerably improved the corrosion resistance. However, the laser caused serious melting and ablation of the substrate surface.

Laser cleaning removes the oxide layer on the substrate surface, which inevitably reduces the corrosion resistance of aluminum alloy. Although high power laser can considerably improve the corrosion resistance of aluminum alloy, it damages the matrix and is not suitable for engineering applications. Combined with the laser processing parameters discussed above, we concluded that the optimum power of Nd:YAG cleaning 5A12 aluminum alloy is about 98W. Based on the above results, the corrosion resistance of aluminum alloys at different cleaning speeds is similar to that of power, which is not discussed here.

5. Conclusions

In this study, a 5A12 aluminum alloy was cleaned using an acousto-optic Q-switched diode-pumped Nd:YAG laser. The properties of the aluminum alloy surface after cleaning with different cleaning parameters were studied. The following conclusions were drawn:

(1) Laser cleaning can effectively remove the oxide film and other impurities on the surface of 5A12. When the laser power is about 98 W and the cleaning speed is about 4.1 mm/s, the removal of the oxide film is the most effective, and the strength and smoothness of the aluminum alloy are improved;

(2) When the laser power is lower than 98 W, the oxide film on the surface of the aluminum alloy is incompletely cleaned, and many ring structures and holes appear; when the laser power is higher than 98 W, the thermal oxidation of the surface of the aluminum alloy causes the oxygen content to rise again, and some craters caused by a high-energy laser can be observed on the surface;

(3) Magnesium, an important element in 5A12 aluminum alloy, is considerably lost after cleaning. The residual stress on the surface of aluminum alloy before and after laser cleaning is compressive and tensile stress, respectively;

(4) After laser cleaning, the corrosion resistance of 5A12 aluminum alloy decreases. However, at high energy density, the corrosion resistance of aluminum alloy is higher than that before cleaning, but, at this time, a new oxide layer forms on the surface of the aluminum alloy, which is not suitable for practical applications. When the laser power is 98 W, the corrosion resistance of the sample is the second best after cleaning, so 98 W is considered as the optimal technological power;

(5) To summarize, the optimum parameters for Nd: YAG laser cleaning of 5A12 aluminum alloy are a power of 98 W and a cleaning speed of 4.1 mm/s.