A universal laser marking approach for treating aluminum alloy surfaces with enhanced anticorrosion, hardness and reduced friction

Abstract

Lined and grating grooves have been fabricated on a large scale engineering Al alloy plate via a simple, convenient, efficient, and low cost laser marking method. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize and analyze the microstructure and composition of the as-prepared samples. The results demonstrate that regular and tailored roughness structures were successfully constructed on the surface. In addition, the roughness of the structures could be easily modulated. Furthermore, the corrosion resistances to seawater and tribological properties have been investigated. The corrosion resistance, hardness and friction resistance of Al alloy surfaces treated by the laser marking approach were remarkably enhanced, which presents a great opportunity for applications in the field of engineering.

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Introduction

Aluminum (Al), especially its alloys, has been widely applied in various fields, including the automotive, medicine, electrical, electronics and aerospace industries because of its high strength to weight ratio, unique conductivity of heat and electricity, non-ferromagnetic property, nontoxicity, good formability and easy recyclability.^1–5 However, it is also accompanied with some intrinsic characteristics, such as low hardness, poor wear resistance and poor corrosion resistance, which greatly limit further applications.^1,6–8 Therefore, it is very important to overcome the aforementioned disadvantages of Al and its alloy in the mechanical engineering field.

Aiming to solve the above mentioned issues, tremendous efforts have been made and various approaches have been developed over the past several decades. The typical strategy is to change the fine composition of the bulk and surface for Al alloy. Some metals, including manganese (Mn), tin (Sn), molybdenum (Mo), lead (Pb), chromium (Cr), niobium (Nb), tungsten (W) and zirconium (Zr), have been employed as modifiers to improve the wear and corrosion resistance properties of Al alloy in some specific working conditions.^9–13 For example, Hukovic et al. have investigated the electrochemical properties of the Al–Mo alloy system and found that the content of Mo can affect the corrosion resistance of the alloy.¹⁴ Investigation of the mechanical properties of Al–Pb alloys and Al–Sn alloys indicated that the content of Sn and Pb could both change the friction coefficient of alloys.¹⁵ However, these improvements are still limited owing to the low solid solubility (<1 at.%) between the doped metals and Al.¹ Surface modification as an alternative approach is a comprehensive concept to change the chemical and mechanical properties of the surface layer of metal materials.^16–21 Through depositing a coating on their surface, their corrosion and wear resistance could be enhanced.^22–25 For example, Fix et al. have deposited a silica–zirconium based hybrid film with nanocontainers on an Al alloy (AA2024) to improve its corrosion resistance.²⁶ By electrical deposition, functionally graded (FG) nano-structured Ni–Co/SiC was coated on an Al substrate and improved the wear resistance of the surface.²⁷ Furthermore, the corrosion and wear resistance of Al alloy can also be improved by changing the surface composition. For example, a composite layer is formed on the Al 6061-T651 alloy surface by stirring and mixing sub-micron sized Al₂O₃ and SiC particles into the surface. Compared with a non-processed Al surface, the composite surface exhibited substantial friction and wear reductions.²⁸ By forming compact Al–O–Si and Si–O–Si covalent bond network bridges with sulphide phases in the interface zone, Guo et al. have produced a bilayer silanisation film layer on AA2024-T3 alloy and remarkably improved the corrosion resistance of the alloy.²⁹ Although various surface modifications have been successfully carried out, there still are some challenges, including the current methodology requiring rigorous and time consuming fabrication processes. More importantly, the processes cannot be performed on the large scale and most of the fabricated surface films, which are reported to date, are easily destroyed. Restricted by the aforementioned shortcomings, it is still desirable and necessary to develop simple, high efficient approaches with processing on the large scale to improve the wear and corrosion resistance of Al alloys.

Due to the fast adaptability, high precision and it being environmentally friendly, laser technology has been widely applied in various fields. Especially for surface treatment, it can tailor the superficial properties and simultaneously maintain the bulk properties.^30,31 For example, by laser gas assisted melting, the metallurgical structure and microhardness of a tungsten carbide surface had been changed and improved, respectively.³² Furthermore, previous reports have demonstrated that a specific surface texture into a sliding contact surface can influence the coefficient of friction of the surface.^33–35 For instance, Steinhoff et al. have investigated sheet-surface structure and found that different types of surface structures are able to affect the tribological properties of a surface.³⁶ Initiated from this view, herein a rapid and one-step method laser marking approach has been employed to construct specific microstructures on an Al alloy surface to improve its corrosion resistance, hardness and wear resistance simultaneously. This method is low cost, highly efficient and facile to operate on a large scale. First, two typical patterns of lined and grating grooves are fabricated on the Al alloy surface via the laser marking approach. Through optimizing the working parameters of the laser marking machine, roughness structures on the Al alloy surface can be accurately modulated. The corrosion resistance to seawater and the tribological properties of the fabricated surfaces were carefully investigated. It was demonstrated that the corrosion resistance, hardness and friction resistance of Al alloy surface treated by the laser marking approach could be simultaneously enhanced, which shows great promise for applications in the field of engineering.

Experimental section

Materials

The commercially available Al alloy (AA7075) plates were purchased from the Alcan Alloy Products Company. Ltd (the chemical composition in mass percent: 0.4 Si, 1.2 Cu, 0.3 Mn, 2.1 Mg, 0.20 Cr, 5.1 Zn and the balance was Al). The plate was cut by wire cut electrical discharge and machined in the form of a 20 mm × 20 mm square pin. The surface of the cut samples were then ground to a surface roughness less than 0.05 μm and rinsed with ethanol and acetone.

Laser marking treatment

The surface of the samples was treated under different processing parameters by a portable laser marking machine (YAG laser with a wavelength of 1064 nm, output power of 20 W). Different processing parameters included laser frequency, laser scanning speed and laser scanning times. The lined and grating grooves were made on the sample and the angle of the grating pattern was 90°. After treatment, all the samples were cleaned twice in an ultrasonic bath filled with alcohol for 10 minutes each time.

Characterization

The textured surfaces were observed by a Quanta 200 FEG Environmental scanning electron microscope (ESEM). X-ray photoelectron spectroscopy (XPS) analyses of the samples were conducted on a VG ESCALAB MKII spectrometer using an Mg Kα X-ray source (1253.6 eV, 120 W) operating in constant analyzer energy mode. The Vickers hardness of the textured surface was measured using a HVS-1000 Vickers hardness instrument with a load of 1 kg and a dwell time of 8 s. Five tests were conducted and the mean value was obtained. The density of the sintered specimens was determined by Archimedes' principle.

The tribological properties of the textured samples were measured with a HT-1000 ball-on-disk high temperature tribometer. A disk was made of the samples, and the counterpart ball was a commercial C45E4 steel ball with a diameter of 6 mm (600 HV, Ra 0.02 μm). Detailed experimental conditions of the test are as follows: rotation speed at 500 rpm, load of 1.65 N, at room temperature and a time of 3 min.

Results and discussion

The entire process of the laser marking approach is described in Fig. 1a. During the marking process, the laser beam travels along a track that has been set up. As shown in Fig. 1b, the polished Al alloy surface was rapidly heated and melted along the laser track. Then, it was vaporized and removed from the track, leading to the formation of grooves. Simultaneously, some melted part was squeezed out from the grooves due to the laser impact force, and then piled up together and cooled down rapidly, leading to the formation of many humps on the surface. This approach is not complicated nor does it use sophisticated equipment. Additionally, the process area just depends on the area of plate of laser marking machine and there is no requirement for the thickness of the materials. Therefore, it is quite easy to realize the large-scale treatment of the surface of Al alloy by expanding the working area of the laser marking machine.

Fig. 1 (a) Schematic process of the fabrication of the roughened structure on the Al alloy surface by the laser marking machine; (b) schematic process of the formation of the roughened structure.

Surface topography

As shown in Fig. 2a, a typical Al alloy plate with a size of 230 mm × 230 mm × 60 mm was treated by the laser marking machine. Fig. 2b presents a SEM image of the untreated surface and Fig. 2c and d present SEM images of the as-prepared textured surface. For the untreated sample presented in Fig. 2b, its surface is relatively smooth. Through the laser marking approach, it can be seen that the designed groove patterns have been fabricated on the surface. Fig. 2c and d present the typical regular lined and grating patterns, respectively. Clearly, the spacing of their texture is uniform at about 50 μm for both lined and grating grooves, which is in good agreement with the designed pattern. However, due to the melted fusion deposition, some of lined textures flocked together. From the SEM image shown in Fig. 2c, it is evident that many irregular micro-humps were formed along the lined distribution on the surface. For the grating pattern, the cross angle of the lined micro-humps was 90°, this is presented in Fig. 2d.

Fig. 2 (a) The treated Al plate; (b) SEM image of untreated Al alloy surface morphology; (c) SEM image of the lined groove surface morphology (texture spacing of 50 μm, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹); (d) SEM image of the grating groove surface morphology (texture spacing of 50 μm frequency of 2500 Hz, scanning speed of 50 mm s⁻¹).

To better understand the effect of processing parameters, such as frequency, scanning speed and the number of laser scans, the morphology of the treated grating groove surface was investigated. SEM images of the as-prepared samples are shown in Fig. 3. At the laser frequency of 5000 Hz, the degree of melt in the material decreased due to the decrease in laser stay time on the surface and laser displacement. As shown in Fig. 3a, the melted material individually formed microstructures instead of flocking together, which is different from that shown in Fig. 2c that was acquired at a frequency of 2500 Hz. From the magnified SEM image shown in the inset of Fig. 3a, it can be seen that the shape of the milky protrusions are not uniform. When the grating groove surface was fabricated under a higher scanning speed of 150 mm s⁻¹, the distance between the two beams of the laser increased, consequently resulting in an increase of the distance among the melted spots. It is difficult to flock together for the melted material around each melted spot. As clearly shown in the inset of Fig. 3b, the milky protrusions turned out to be more compact and uniform. When the number of scans was increased to 2, from the high magnified SEM image inset in Fig. 3c it can be easily seen that the micro-humps become bigger and more regular than before. This phenomenon can be ascribed to the material of the textured surface being melted and repeatedly cooled down with no effect to the grating groove pattern. When the number of scans was increased to 4 (Fig. 3d), the micro-humps became more regular, while the diameter deceased. Based on the above results, the frequency and scanning speed of the laser in contrast with the scanning times had a great influence on the laser state (laser stay time, laser displacement and distance of each beam of laser) on the surface, which has a great effect on the construction of microstructures. Therefore, roughened structures on the surface can be accurately modulated by changing the laser frequency and scanning speed.

Fig. 3 Low-magnification SEM image of the grating groove surface (texture spacing of 50 μm) morphology treated by different laser processing parameters: (a) laser frequency of 5000 Hz, scanning speed of 50 mm s⁻¹ and 1 scan; (b) frequency of 2500 Hz, scanning speed of 150 mm s⁻¹ and 1 scan; (c) frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 2 scans; (d) frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 4 scans. Inset: high-magnification SEM image of the surfaces.

In addition to the change of microstructure of the treated surface, the composition of the surface layer may also have changed under the instantaneously high temperature generated by the laser in an air environment. In order to demonstrate this point, X-ray photoelectron spectroscopy (XPS) was performed to investigate the composition of the Al alloy surfaces. As shown in Fig. 4a, it can be observed that oxygen, carbon, aluminum, magnesium and silicon are detected on the untreated surface. As shown in Fig. 4e and f, the elements of zinc and copper emerged after laser treatment, indicating that they are exposed from the bulk of the Al alloy to form a precipitated phase on the surface. From the XPS spectra of Al 2p as shown in Fig. 4c, both the peaks at 72.4 and 74.7 eV can be assigned to the untreated surface. They are from the pure aluminum and the oxidized aluminum, respectively. After the laser treatment, the peak at 72.4 eV disappeared, indicating that the pure aluminum was completely oxidized to Al₂O₃ on the surface under the high temperature induced by the laser. Interestingly, silicon was not detected on the surface, as shown in Fig. 4d, which may be attributed to the surface region being completely covered with the oxide film and the new precipitated phase. All the C1s peaks were located at the same position (284.8 eV), which indicate that they come from the same source. Carbon is a type of extremely sensitive element, therefore it is easy to detect by analyzing the XPS spectra. The carbon on the surface may come from the accumulated pollution during pre- and post-laser treatment. As shown in Table 1, it is apparent that the carbon content decreased significantly after laser treatment. For oxygen and aluminum, their contents dramatically increased. Additionally, because of the differences in the microstructures on the surface, the magnesium content of the textured surface decreased slowly when the number of laser scans was increased from 1 to 2 and stayed at the same level when a total of four scans were applied. The copper content slightly increased when the number of laser scans was increased to two but then the copper content decreased with further scanning. Based on the above XPS results, we infer that the laser marking treatment, which can change the composition of the surface layer, will affect the properties of material. The processing parameters (frequency, scanning speed and number of scans) of laser marking have little effect on the composition of the surface. The thickness of the surface oxidized film (Al₂O₃) will increase after laser treatment, which may enhance the corrosion resistance of the surface. Furthermore, due to the aging strengthening process induced by laser treatment, zinc, copper and magnesium will be separated, leading to the formation of the precipitated phase on the surface, which may improve the hardness of the Al alloy.

Fig. 4 XPS spectra of untreated and textured surfaces (grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1, 2 and 4 scans) of (a) survey; (b) Mg 2p; (c) Al 2p; (d) Si 2p; (e) Zn 2p3; (f) Cu 2p.

Table 1 Surface element contents calculated from XPS survey spectra

	C (at.%)	O (at.%)	Al (at.%)	Mg (at.%)	Si (at.%)	Zn (at.%)	Cu (at.%)
Untreated	59.24	29.64	7.56	0.89	2.67	None	None
Scanning 1 times	11.60	64.34	20.16	2.30	None	1.28	0.32
Scanning 2 times	11.30	64.97	20.39	1.55	None	1.42	0.36
Scanning 4 times	12.17	63.34	21.16	1.65	None	1.28	0.40

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Corrosion resistance to seawater

Due to the wide applications of Al alloys on shipping and deep-diving submersible vehicles, the corrosion resistance to seawater of these Al alloys must be excellent.^37,38 When the Al alloy contacts with seawater, the Al alloy surface will directly suffer from corrosion.¹ In order to investigate the corrosion resistance to seawater of the textured surface, a corrosion experiment was conducted by immersing the textured sample into seawater to simulate real conditions. The seawater was obtained from the coastal waters of China. The textured sample and the polished sample were immersed into seawater at room temperature.

Compared with the polished surface shown in Fig. 5a, there was a large area black mark on the surface after immersing it into seawater for 20 days and the surface with the black mark was uneven, which indicates that the oxidized film of the surface lost efficacy and the material surface reacted with seawater. In Fig. 5c, the region of the large area black mark is irregularly observed, meaning that there are some defects on the surface of the polished sample and the integrity degree of the alloy oxidation film was affected by the defects. On the contrary, there was no black mark on the surface of textured sample. Notably, the surface immersed into seawater for 20 days was almost the same as the previous surface, shown in Fig. 5b. Only some of the black spots appeared on the edge of the surface (Fig. 5d). It can be implied that the black spots appeared on the edge of the surface because the edges and sides of the surface were not treated by the laser. In order to gain further insight into the surface after immersion into the seawater, the surface of the two samples was observed by SEM. As shown in Fig. 5e, it can be easily found that there are some blisters on the surface of the polished sample that are randomly distributed on the surface. Due to the reaction between the Al alloy and seawater, atomic hydrogen will diffuse into the material of the surface. As the atomic hydrogen aggregates into the material, the hydrogen atoms become hydrogen molecules. With the increase in hydrogen pressure, the material of the surface will deform and blistering will be induced on the surface. The hydrogen blistering indicates that the surface suffers from the corrosion of seawater and the alloy oxidation film loses efficacy. The hydrogen blistering will weaken the performance of the material. Both hydrogen blistering (Fig. 5e) and a large area black mark (Fig. 5c) were observed on the polished surface and indicate that the surface was corroded seriously by seawater after immersing into seawater for 20 days. Compared with the former, the textured surface had no change after immersion into seawater for 20 days (Fig. 5f), which indicates that the surface was not affected by the corrosive conditions. The corrosion experiment proves that the corrosion resistance of the textured surface treated by the laser marking system was better than that of the polished surface. More importantly, compared with the polished surface, the surface treated by laser marking had a long-term stability performance in seawater, which will have extensive applications. Combined with the XPS results, it can be concluded that the performance of corrosion resistance to seawater totally depends on the integrity degree and thickness of the alloy oxidation film on the alloy surface.

Fig. 5 (a) Smooth surface corresponding to the sample; (b) textured surface corresponding to the sample; (c) smooth surface corresponding to the sample soaked in seawater after 20 days; (d) textured surface corresponding to the sample soaked in seawater after 20 days; (e) SEM image of the smooth surface; (g) SEM image of the textured surface.

Friction and wear properties of the textured Surface

In order to investigate the friction resistance of the textured surfaces, tribological tests were conducted. Before the tribological tests, the Vickers hardness of the textured surfaces treated by different processing parameters were measured (Fig. 6). The hardness is an important parameter, which can affect the friction resistance of the surface. The hardness of a smooth surface is about 185 HV. Therefore, it can be easily found that the hardness of the textured surface was higher than the smooth surface after laser treatment, which indicates that the laser treatment was able to improve the hardness of the surface. Due to the quench hardening of the surface material induced by laser marking, precipitated phases were formed on the surface and the hardness of the surface was enhanced directly. The variation of hardness of the textured surfaces with different laser process parameters also shows the effect of the microstructure on the surface hardness.

Fig. 6 Hardness distributions of the sample surface treated by different laser processing parameters: (a) lined groove surface and grating groove surface, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1 scan; (b) grating groove surface, texture spacing of 50 μm, scanning speed of 50 mm s⁻¹, and 1 scan; (c) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz and 1 scan; (d) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1, 2 and 4 scans. Inset: SEM image of the surface corresponding to the samples. Scale bar: 50 μm.

As shown in Fig. 6a, it is apparent that the hardness of the grating groove surface with low texture spacing (≤50 μm) is higher than that of lined groove surface. However, the hardness of lined groove surface and grating groove surface both decreased with an increase in texture spacing. This is because the number of microstructures per unit area of the surface decreases with the increase in texture spacing, which will decrease the load carrying capacity of the textured surface per unit area and decrease the hardness of the surface directly. With the textures spacing of the surface increases from 75 μm to 100 μm, which was due to the decrease in the number of microstructures per unit area of the surface being further reduced, the load carrying capacity of the textured surface per unit area decreased. Therefore, the hardness of the textured surface (grating groove surface and lined groove surface) is a little lower than that of a smooth surface. The variation of hardness of the textured surface indicates that the microstructure on the surface has a great effect on the hardness of the fabricated surface. Moreover, it is proven that the load carrying capacity of the grating groove surface with a low texture spacing (<50 μm) is better than the lined groove surface. With the increase in texture spacing, the number of microstructures per unit area of the grating groove surface became smaller than that of the lined groove surface. Thus, the grating groove surface showed a lower hardness than the lined groove surface.

From the SEM image presented in the inset of Fig. 6b, it is obvious that the laser stay time on the surface and laser displacement gradually decreases as the laser frequency is increased to 5000 Hz, leading to a decrease in the degree of melted material. The irregular milky protrusion microfeatures were produced on this surface. However, at higher laser frequency, the melted degree of material was extremely low such that the melted material was unable to flock together. The low melted degree of material also leads to the difficulty in forming individually constructing microstructures of melted material. Therefore, a micro-pore pattern was produced on the surface by the laser impacting force. Moreover, due to the extremely low melted degree of material, the integrity of the textured surface was bad. Due to the modification of the surface topography, the hardness of the surface gradually decreased with increasing laser frequency. Fig. 6c shows the variations of hardness of the grating groove surface with laser scanning speed. With an increase in scanning speed (≥100 mm s⁻¹), the distance between two laser beams was gradually increased, which lead to an increase in the distance between each melted spot. It is hard for the melted material around each melted spot to flock together. Thus, irregular milky protrusion microstructures are formed on the surface. Due to the change of the microfeatures of the textured surface, the hardness of the grating groove surface decreased when the scanning speed was increased from 50 to 100 mm s⁻¹ and reached a minimum value of 178 HV at the scanning speed of 100 mm s⁻¹. As the scanning speed was continuously increased, the milky protrusion microstructures became more uniform and the number of microstructures per unit area of the surface increased, which could increase the load carrying capacity of textured surface per unit area. Therefore, the hardness of the textured surface increased rapidly with increasing scanning speed. As shown in Fig. 6d, the laser scanning times cannot change the degree of quench hardening induced by the laser marking but the microstructures on the surface. Furthermore, the microstructures of the surfaces treated by different laser scanning time were similar. Thus, the hardness of the textured surface did not fluctuate much when the number of scans was increased.

Increasing the number of laser scans causes the microstructures on the surface to become bigger and more regular (Fig. 3c), which increased the load carrying capacity of the microstructures and the number of microstructures per unit area on the surface. Thus, the hardness of the surface was increased. Increasing the number of scans to 4 (Fig. 3d) caused the microstructures to be more regular and the diameter to decrease. Although the number of microstructures per unit area of the surface slightly increased, the load carrying capacity of microstructures decreased, leading to a decrease in the hardness of the surface.

The hardness test not only proves that the hardness of the surface can be improved after the laser marking treatment, but it also indicates that the microstructures on the surface have a great effect on the hardness of the surface. The shape of the microstructures and the number of microstructures per unit area of the surface are crucial factors. The shape of the microstructure with a high load carrying capacity and a great number of microstructures per unit area on the treated surface would lead to a higher hardness of the surface. On the contrary, the hardness of the textured surface would be low and even lower than that of the untreated surface. Thus, accurately modulating the roughened structures on the surface is important and essential for practical applications. When the number of microstructures per unit area of the surface is the same, it is considered that the load carrying capacity of the grating structure would be better than the lined groove and milky protrusion structures. Through the laser marking treatment, i.e., fabricating proper roughened structures on the surface of Al alloy, the hardness of Al alloy was able to be further enhanced. The hardness of the textured surface treated under different processing parameters is in the range of 170 to 260 HV. Thus, compared with the smooth surface (185 HV), the maximum increase in the hardness of these textured surfaces treated by this method reaches 40%.

Fig. 7a and b show the curves of the dynamic friction coefficients of the lined groove and grating groove surfaces with texture spacing against the steel ball friction pair at a constant sliding speed of 16 mm s⁻¹ and load of 1.65 N at room temperature. Clearly, the friction coefficient of the smooth surface sharply increases to about 0.55. Then, it has fluctuations. The textured surfaces (both lined groove and grating groove surfaces) show low and stable friction coefficients at the beginning. The reason is that the microstructure on the surface can increase the load carrying capacity of the surface and reduce the friction force between the surface and steel ball. Therefore, the friction coefficient of the surface can be reduced by constructing a roughened structure on the surface. However, as shown in Fig. 7a and b, the friction coefficient of texture surfaces was increased after some sliding time. It is considered that the roughness structure of the textured surface was gradually worn down during sliding friction and finally could not effectively decrease the friction between the two surfaces. The sliding time before the friction coefficient increased sharply, and this is seen as the wear life. Thus, it is easily seen that the wear life of textured surfaces treated under different processing parameters were different. The wear life of the textured surface will affect the friction resistance of the surface. As shown in Fig. 7a, it can be easily found that the wear life of the lined groove surface with a texture spacing of 100 μm was the highest of all, and the lined groove surface with texture spacing of 50 and 150 μm had the same wear life. The wear life of the lined groove surface with a texture spacing of 75 μm was the lowest. Compared with lined groove surface, the grating groove surface had a higher wear life (Fig. 7b), indicating that the friction resistance of the grating groove surface was better than that of the lined groove surface. The wear life of the surface decreased with increased texture spacing, showing more regular variation of wear life of the grating groove surface than that of the lined groove surface. Furthermore, the stable friction coefficient of the surface increased by increasing the texture spacing. It is considered that the microstructures on the surface were not worn out after the test.

Fig. 7 Dynamic friction coefficient of the sample surface treated by different laser processing parameters (load of 165 N, sliding speed of 16 mm s⁻¹): (a) lined groove surface, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1 scan (A: untreated; texture spacing of B: 25 μm; C: 50 μm; D: 75 μm; E: 100 μm; F: 150 μm); (b) grating groove surface, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1 scan (A: untreated; texture spacing of B: 25 μm; C:50 μm; D: 75 μm; E: 100 μm; F: 150 μm); (c) grating groove surface, texture spacing of 50 μm, scanning speed of 50 mm s⁻¹ and 1 scan; (d) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz, and 1 scan; (e) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz, and a scanning speed of 50 mm s⁻¹.

When the grating groove surface was treated at a laser frequency of 5000 Hz, the irregular milky protrusion microfeatures were produced on the surface. The wear life of the surface dramatically decreased (Fig. 7c), which indicates that it is easy for the milky protrusion to be worn out and the wear resistance of the surface was poor. However, the wear life of the surface increased with a higher laser frequency. That is because, compared with the convex structure, it is difficult for the concave structure on the Al alloy surface to be destroyed. Though the wear life of the surface was the highest of all, the integrity of the surface was very bad and uncontrolled. In other words, it is hard for facile operation in large scale and engineering applications. As shown in Fig. 7d, the wear life of the grating surface decreased when the laser scanning speed was increased. The variation of the wear life of the grating groove surface with laser frequency and scanning speed also indicates that the wear life of the milky protrusion structure was lower than that of grating groove structure. Furthermore, when the number of scans was increased to 2, the roughened structure of the grating groove surface was more easily worn out. As the number of scans was increased to 4, the wear life of the textured surface increased (Fig. 7e). Therefore, the regular microstructures on the surface would have a long wear life.

The friction test showed that the friction coefficient of the surface can be remarkably reduced by constructing a roughened structure on the surface. Compared with the smooth surface (0.562), the decrease in the friction coefficient of the textured surface could reach 80% of that value. Thus, this is an efficient method to reduce the friction coefficient and improve the friction resistance of the surface. However, to the textured surface, the wear life of microstructures on the surface is considerably more important than the friction coefficient and should be paid attention. Different microstructure on the textured surface had different wear lifetimes, which affects the friction resistance of the Al alloy. Compared with the lined groove structure and the milky protrusion structure, the grating groove structure on the Al alloy surface had a longer wear life, which indicates that the friction resistance of the grating groove surfaces of Al alloy was better than others. Moreover, due to the regular variation of wear life of the grating groove surface with texture spacing, it is easy for the grating groove surface to be applied in engineering.

Fig. 8 shows the variations of the wear volume of the textured surface treated by different processing parameters. It can be easily found that the wear volume of the textured surfaces is lower than that of the smooth surface. This indicates that the friction resistance of the textured surface can be enhanced by the laser marking treatment. As shown in Fig. 8a, the variations of the wear volume of the lined groove surface decreased with increases in the texture spacing from 25 to 50 μm. Then, it dropped very slowly with increases in the texture spacing. Clearly the wear volume of the surface was not only affected by the hardness of the surface, but it was also influenced by the wear life of microstructure on the surface. On the contrary, the variations of wear volume of the grating groove surface increased as the texture spacing was increased. With the increase in the laser frequency, the wear volume of the grating groove surface decreased (Fig. 8b). From the variation of wear volume shown in Fig. 8c, it can be observed that the wear volume of the textured surface had increased when the laser scanning speed was increased from 50 to 100 mm s⁻¹ and fell dramatically when the laser scanning speed was increased from 100 to 200 mm s⁻¹. The variations of wear volume of the textured surface with scanning times are shown in Fig. 8d. It can be easily found that the wear volume decreased when the number of scans was increased.

Fig. 8 Wearing volume of sample surface treated by different laser processing parameters (load of 165 N, sliding speed of 16 mm s⁻¹): (a) lined groove surface and grating groove surface, frequency of 2500 Hz and scanning speed of 50 mm s⁻¹ and 1 scan; (b) grating groove surface, texture spacing of 50 μm, scanning speed of 50 mm s⁻¹ and 1 scan; (c) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz and 1 scan; (d) grating groove surface, texture spacing of 50 μm, frequency of 2500 Hz, scanning speed of 50 mm s⁻¹ and 1, 2 and 4 scans.

The wear volume of textured surfaces after the friction experiment indicates that the friction resistance of the textured surface treated by this method was considerably better than that of the smooth surface. The wear volume of smooth surface was 0.57 mg, but the minimum of the wear volume of the fabricated surface was 0.14 mg. In other words, the wear volume of the fabricated surface could be one fifth lower than that of smooth surface. Moreover, the variation of the wear volume of the textured surface treated by different processing parameters proved that the hardness of the surface and the wear life of microstructure on the textured surface had a combined effect on the friction resistance of the textured surface. The textured surface with high hardness and long wear life microstructures would have excellent friction resistance. Therefore, the friction resistance of the textured surface can be further improved by constructing a proper roughness structure on the surface. Compared with other textured surfaces, the friction resistance of the grating groove surface with low texture spacing was considerably better. The grating groove surface of Al alloy will have extensive applications in engineering.

In the following, the antifriction mechanism of the textured surface was further investigated. SEM images of the worn surfaces of the smooth surface and textured surfaces are shown in Fig. 9. As shown in Fig. 9a and b, it can be easily observed that the microstructures on the lined groove surface were worn out after friction test, showing a short wear life of the microstructures. The smooth surface seems as smooth as the textured surface after the friction test. Indeed, the smoothness of smooth surface and lined groove surface were different after the test. From the high-magnification SEM image shown in Fig. 9d and e, it is obvious that the smooth surface was very uneven and many scratches appeared on the surface, while on the contrary, the lined groove surface was comparatively smooth after the test. The phenomenon shows that the wear debris of the textured surface treated by laser marking can behave as spacers and form a lubrication film to reduce friction between the steel ball and the surface. The wear debris of the smooth surface will increase the friction between the two surfaces. Therefore, it has been proven that the friction resistance of the Al alloy can be enhanced though laser marking constructing microstructures on the surface. As shown in Fig. 9c and f, the microstructures on the grating groove surface still remained after the test, which is in good agreement with the curves of the dynamic friction coefficients of the surface and it also proved that the grating groove structure has a longer wear life. Thus, it is considered that the textured surface of Al alloy with the grating groove structure has an excellent friction resistance.

Fig. 9 SEM images of the middle of the worn surfaces under dry sliding for Al alloy: (a and d) untreated surface; (b and e) lined groove surface; (c and f) grating groove surface.

Conclusions

Lined and grating groove textures were produced on an AA7075 Al alloy surface by a simple, convenient, low cost and scalable method. The effect of processing parameters on microstructure, corrosion resistance to seawater, hardness and friction resistance of the textured surfaces has been studied. Through adjusting the processing parameters (laser frequency, laser scanning speed, laser scanning times, texture spacing), roughened structures on the surface can be accurately modulated. Compared with untreated Al alloy, the corrosion resistance to seawater of the textured surface treated by this method could be significantly enhanced. Moreover, it is proven that the hardness of the surface can be improved after the treatment. More importantly, tribological measurements illustrated that the friction resistance of the surface can be enhanced by this method, and showed that the microstructures on the surface will affect the friction resistance of the textured surface. Therefore, through this convenient method constructing proper microstructures (the textured surface with a grating groove structure and low texture spacing) on the surface, the corrosion resistance to seawater, hardness and friction resistance of Al alloy could be dramatically improved simultaneously, which would extend the applications of Al alloy in the field of engineering.

Acknowledgements

This work was supported by the National Key Scientific Program-Nanoscience and Nanotechnology (2011CB933700), and the National Natural Science Foundation of China (21073197). X.J.H. acknowledges the CAS Institute of Physical Science, University of Science and Technology of China (2012FXCX008), for financial support.

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