Abstract Al-Si-Ni-WC alloy powder was used to prepare a coating with excellent anti-wear properties on the surface of ZL108 aluminum alloy brake disc through laser cladding technology. The microstructural properties, hardness value, wear volume, and corrosion resistance of the coating were analyzed. The research results show that a smooth and uniform thickness cladding surface is obtained by using Al-18%Si-30%Ni-5%WC composite coating. The physical phases in the coating mainly include cemented carbide phases such as AlNi, AlNi3, WC, and SiC, and the cemented carbide phases generated in situ are evenly distributed in the cladding layer. The substrate and coating are closely combined, and the grains inside the cladding coating are refined. The average hardness value of the cladding coating surface is 272 HV, which is 3.1 times the average hardness value of the ZL108 substrate surface. Compared with the substrate, the coating has better wear resistance, and the wear of the coating is smaller in high temperature environments. The coating wear mechanism is abrasive wear, adhesive wear and a small amount of oxidation wear. The wear resistance of the aluminum alloy brake disc is significantly increased.
Keywords laser technology; aluminum alloy; laser cladding; brake disc; microstructure; friction and wear

1 Introduction
With the increasing demand for automobiles in my country, the demand for brake discs on automobile wheels is increasing. Most brake discs on the market are made of gray cast iron [1]. However, the weight of a gray cast iron brake disc is 4 to 5 times that of an aluminum alloy, which greatly increases the fuel consumption of the car during driving and leads to energy waste. Lightweight car design not only reduces energy consumption, but also enables the car to reach a faster speed [2]. At present, more and more high-end cars use aluminum alloy brake discs instead of integrally cast gray cast iron brake discs. The main reason is that aluminum alloy is a structural alloy with low density and easy processing. However, compared with the existing gray cast iron brake discs, aluminum alloy brake discs have low hardness and poor wear resistance. Plastic deformation and other problems will occur during the driving process of the car, which limits the application of aluminum alloy in automobile brake discs [3]. When the surface of the brake disc is excessively worn, the braking distance of the car is affected, which poses a threat to the driver’s life safety [4]. When a car is braking for a long time, the continuous friction between the brake disc surface and the brake pad will generate a lot of heat. This heat is distributed on the brake disc surface, causing the brake disc surface to soften, which in turn causes the wear resistance of the brake disc to decrease. Therefore, it is of great significance to improve the performance of aluminum alloy surfaces using appropriate processes [5-6].

Laser cladding technology is a new type of surface modification technology. This technology can be used to clad a coating with stronger mechanical properties and wear resistance on the surface of aluminum alloys to meet the needs of specific working conditions [7-8]. Li Yu [9] used laser cladding technology to prepare Al-70Si gradient coatings on 1050 aluminum alloys with a hardness of 245 HV. By continuously adjusting equipment parameters and adding a small amount of CeO2, the coating has better wear resistance. Peng Shixin [10] used laser cladding technology to clad Fe-Ni alloy powder on the surface of ZL114 aluminum alloy. When using CO2 laser as cladding equipment, the laser power and spot diameter will affect the internal structure of the cladding layer. Chen Zixin et al. [11] effectively reduced the cracks during laser cladding by optimizing laser process parameters and powder composition, and proposed a method to solve the cracking of the cladding layer. Shao Haiquan [12] used laser cladding technology to clad Al2O3/NiCrAl composite coating on the surface of ZL205A aluminum alloy substrate. It was found that the addition of ceramic materials can refine the grain structure inside the cladding coating, and the addition of ceramic materials greatly increases the hardness of the coating surface and significantly enhances the wear resistance. Lei Linping [13] used laser cladding technology to clad NiAl alloy on the surface of 6065 aluminum alloy. The newly generated NiAl, Ni3Al, NiAl3 and other new intermetallic compounds in NiAl alloy have excellent wear resistance at high temperature. These hard reinforcement phases are dispersed in the cladding coating, and at the same time form finer cellular crystals and columnar dendrites, with good overall uniformity. The intermetallic compounds formed between nickel and aluminum alloys are relatively brittle, which easily causes pores and cracks in the coating. Compared with metals, the intermetallic compounds generated in situ in nickel and aluminum alloys have greater brittleness, and compared with ceramics, they show greater plasticity [14-15].

This paper designs and proportions the composition to ensure that the surface of the laser cladding aluminum alloy brake disc has excellent wear resistance at room temperature, and has good wear resistance and excellent corrosion resistance in high temperature environment. Through orthogonal experiments, it is found that when the Ni mass fraction is 30%, the performance of the cladding layer is optimal.

2 Experimental methods and material selection
2.1 Experimental materials
Surface energy spectrum analysis of aluminum alloy brake discs revealed that its composition was close to that of ZL108 materials. Therefore, the experiment used ZL108 aluminum alloy as the cladding substrate, and its chemical composition is shown in Table 1. After the ZL108 aluminum alloy sheet was processed into a plate with a size of 100 mm×100 mm×10 mm, its surface was polished with 240# sandpaper to destroy the oxide film on the surface of the aluminum alloy and obtain a uniform rough surface; the substrate was immersed in a container containing acetone solution for ultrasonic cleaning to remove impurities such as oil on the surface of the substrate [16]; the substrate surface was cleaned with a 0.5% mass fraction of HCl solution to make the surface grains or grain boundaries corroded, thereby enhancing the aluminum alloy surface’s absorption effect on lasers; it was placed in a dryer for drying to ensure that all surface impurities were removed [17-18].

The chemical composition of the cladding material is shown in Table 2, and there are 4 cladding design schemes. Since the wear resistance of hypereutectic Al-Si alloy is 25%~40% higher than that of eutectic or hypoeutectic Al-Si alloy, the content of Si element is increased to the hypereutectic composition point of Al-Si alloy. Cr element can adjust the toughness of the alloy, but excessive Cr element will hinder the recrystallization and growth of nucleation in the aluminum alloy substrate, so the addition amount (mass fraction) of Cr element in the aluminum alloy generally does not exceed 0.35% [19]. Excessive content of WC ceramic alloy powder will make the difference between the hardness of the coating and the hardness of the substrate surface too large, resulting in coating shedding at the joint, but the WC ceramic phase generated inside the coating can enhance the friction and wear resistance of the coating, and the small amount of ceramic material added can play a role in refining the structure [12]. The addition of Cu element is mainly to improve the bonding force inside the coating. The addition of a large amount of Ni alloy powder makes the thermal expansion coefficient of the cladding coating lower, thereby ensuring the friction and wear performance of the coating at high temperature. The intermetallic compound formed inside the nickel-aluminum alloy has a low density and strong oxidation resistance, and has strong corrosion resistance and wear resistance under high temperature conditions.

The prepared powder is placed in a planetary ball mill for mixing. The ball mill speed is 200 r·min-1, and 6 operating cycles are set, each operating cycle is 60 min. After the ball milling is completed, the mixed powder is filtered using a sieve. Finally, it is dried in a drying oven at 100 ℃ for 5 h to remove moisture. The maximum laser power of the fiber laser in the cladding equipment is 2 kW, and the cladding process parameters are shown in Table 3. The environment in the laser cladding chamber is a vacuum environment to ensure that no redox reaction occurs in the experiment. The laser cladding equipment and its processing principle diagram are shown in Figure 1.

2.2 Experimental method
The prepared coating was cut along the direction perpendicular to the laser scanning direction to obtain the sample, and the surface was ground and polished with different types of SiC sandpaper; the polished sample was placed in the prepared Keller reagent for surface corrosion, and the surface microstructure was observed at the same time; the microscopic morphology of the sample surface was characterized by scanning electron microscopy (SEM), the phase composition of the material was measured by X-ray diffraction (XRD) technology, and the hardness value of the cladding coating surface was measured by Vickers hardness tester; the friction and wear performance of the sample was tested by friction and wear tester, and GCr15 steel ball with hardness of 240 HV and diameter of 4.6 mm was selected as the friction pair during the test, the applied load (F) was 20 N, and the wear time was 20 min; the substrate and coating materials were electrochemically tested at room temperature using a 5% mass fraction NaCl solution to analyze their corrosion resistance.

3 Results and discussion
3.1 Phase characterization and analysis
The phase composition of the cladding coating was characterized and analyzed by XRD. The scanning spectrum is shown in Figure 2. The phase of the coating is mainly composed of α-A1 supersaturated solid solution and some ceramic phases (WC, SiC, WSi2) and NiAl intermetallic compounds. NiAl grains belong to the B2 type ordered body-centered cubic structure [20], with neat atomic arrangement, regular distribution, strong bonding force, good thermal conductivity and high temperature oxidation resistance, strong wear resistance, excellent thermal stability, melting point of 1638 ℃, thermal conductivity of 70~80 W·mK-1 (20~1100 ℃), which makes the temperature gradient and thermal stress value of the material low, and thus improves its thermal fatigue performance [21-22]. As can be seen from Figure 2, the first peak corresponding to the α-Al phase decreases with the increase of Ni content. The main reason is that with the increase of Ni content, the Al content in the molten pool decreases continuously. At the same time, the peak value corresponding to the generated Ni3Al material increases slowly, and hard ceramic phases such as WSi2 and SiC mainly appear in the side peaks.

3.2 Microstructure
In order to ensure that the designed cladding coating has a strong bonding force with the substrate, and to explore the content of specific element components in the coating to ensure that the coating has strong wear resistance, this paper adds different contents of Ni alloy powder and verifies the wear resistance of the brake disc surface through orthogonal experiments. Figure 3 shows the SEM images of the coating under different Ni contents, where points A and B are typical NiAl intermetallic compound structures. It can be seen from the Ni Al crystallization process that with the increase of Ni content, NiAl compounds increase, which directly affects the content of Ni3Al compounds.
Ni+Al→NiAl, (1)
Ni+Al+NiAl→Ni3Al. (2) As shown in Figure 3, the main crystal forms of the Ni-Al intermetallic compounds in the coating are equiaxed crystal structure (point A) and cellular crystal structure (point B). When scanning points A and B, it is found that the atomic ratio of Ni and Al at point A is about 1:1, and the atomic ratio at point B is about 3:1. Combined with the XRD spectrum analysis in Figure 2, it is inferred that the phase structures at points A and B are NiAl and Ni3Al. Compared with Figure 3, it can be seen that the continuous addition of Ni leads to the gradual increase of Ni3Al cellular crystal structure. When the Ni mass fraction reaches 35%, the coating locally breaks at point B. This is mainly because with the continuous addition of Ni elements, the in-situ generated Ni-Al intermetallic compounds in the cladding coating gradually increase, and the excessive Ni3Al phase causes the brittleness of the coating to continue to increase [23-24]. Under the rapid solidification of laser cladding, the excessive brittleness will cause the coating to break. This will cause the coating to fall off in actual working conditions, thus affecting the life of the brake disc. Therefore, the advantages and disadvantages of the coating with a Ni mass fraction of 35% will not be discussed in subsequent experiments.

Figure 4 is an energy dispersive spectrometer (EDS) spectrum of the coating when the Ni mass fraction is 30%. At points B′ and A′, the mass fraction of the WC element is 98%. Analysis shows that the WC after thermal decomposition gradually enters various areas of the coating; the main elements at point C′ are Al and Ni; the main elements at point D′ are Si and C. EDS analysis of the coating shows that: Ni and Al elements are well combined inside the coating, and new intermetallic compounds AlNi phase and Ni3Al are generated in situ. They are evenly distributed inside the coating in the form of columnar crystals and cellular crystals; and the primary Si phase and ceramic phase WC are embedded in the middle of the coating. These hard and wear-resistant phases play a supporting role in the coating and have the function of resisting friction and wear.

3.3 Hardness analysis
Figure 5 shows the microhardness of the cross-section of the coating and the SEM image of the bonding surface. It can be seen that the cladding coating and the aluminum alloy substrate are mutually infiltrated and no pores are generated. Compared with the hardness near the heat affected zone (HAZ) and the hardness of the substrate, the surface hardness of the coating has been greatly improved. With the continuous addition of Ni, the hardness of the coating gradually increases, but there is no phenomenon of too high or too low hardness, indicating that the internal structure of the coating is evenly distributed. Laser cladding has a high scanning speed, which causes the molten pool to solidify too quickly [25], and the grains have no time to grow inside the coating, so the internal grains are refined. At the same time, during the too fast solidification process, the metal compounds precipitated inside the molten pool indirectly enhance the performance of the coating and greatly improve the hardness of the coating. The hardness test was carried out at intervals of 0.5 mm in the substrate and coating. The width of the bonding area between the substrate and the coating was about 0.75 mm. Since the width of the bonding area was relatively small, the hardness test was carried out at intervals of 0.25 mm. It can be seen that the hardness value of the bonding area gradually increased. The average hardness of the ZL108 aluminum alloy surface is 90 HV. When the Ni mass fraction is 30%, the average hardness of the coating bonding area is about 214 HV, and the average hardness of the coating surface is about 272 HV, which is about 3 times the average hardness of the ZL108 substrate. The main reason is that under the action of laser cladding, the ability of the grain boundary to hinder deformation gradually increases, so it is not easy to cause plastic deformation inside the coating, and the coating has a higher hardness. At the same time, a large amount of NiAl intermetallic compounds and some hard phases are formed inside the cladding coating, which will hinder the dislocation slip in the crystal [26]. Under the effect of fine grain strengthening and intermetallic compounds, the internal hardness of the coating is greatly improved compared with that of the substrate, which prevents the coating surface from being damaged due to the hardness difference during the friction and wear process.

3.4 Friction and wear performance
Figure 6 shows the friction coefficient curves of the coating and the substrate at room temperature (RT) and high temperature when the Ni mass fraction is 30%. As can be seen from Figure 6 (a), the friction coefficient of the cladding coating does not appear to be too low or too high as the temperature increases. At the beginning of friction and wear, the coating surface is in point contact with the GCr15 steel ball on the experimental table, and the surface roughness of the polished coating and the steel ball is very small [27]. However, as the friction and wear experiment continues, the coating slowly enters the running-in stage, and the friction coefficient between the surfaces fluctuates slightly. During the friction and wear process, the contact area between the GCr15 steel ball and the substrate gradually increases, the contact mode gradually changes to surface contact, the change of the surface friction coefficient begins to decrease, and the coating enters the stable wear stage.

As shown in Figure 6 (b), the average friction coefficient of ZL108 aluminum alloy is 0.31 at room temperature, and the average friction coefficient of the coating is lower than that of the substrate when the Ni mass fraction is 30%, which is about 0.29. When the ambient temperature reaches 100 °C, the fluctuation of the average friction coefficient of the coating surface is smaller than that of the substrate surface, and the fluctuation of the average friction coefficient of the substrate surface is larger than that of the coating in the initial stage. At the same time, due to continuous wear, the fluctuation of the friction coefficient of the aluminum alloy substrate gradually decreases, while the coating has good adaptability to temperature and the friction coefficient is relatively stable. Under stable wear, the friction coefficients at different temperatures are close, which may be because the debris generated during the friction and wear process undergoes abrasive wear on the surface. At different temperatures, the friction coefficient of the coating is more stable than that of the substrate.

Figure 7 (a) shows the wear volume of the substrate and the coating at different Ni contents. The wear depth and wear volume of all cladding coatings are lower than those of the substrate material, which intuitively reflects the wear resistance of the coating. At the same time, the increase in Ni in the cladding coating causes the wear volume to gradually decrease. When the mass fraction of Ni in the cladding coating is 30%, the wear volume of the coating is only 1/4.47 of that of the substrate.

Figure 7 (b) shows the wear volume of the substrate and the coating at different temperatures when the mass fraction of Ni is 30%. As the temperature increases, the wear volume of the ZL108 aluminum alloy surface gradually increases. The main reason is that as the temperature increases, the surface of the aluminum alloy material “softens”. During the high-temperature wear process, part of the adhesive wear causes more friction loss. Compared with the aluminum alloy substrate, the coating is added with high-temperature resistant Ni elements, and the softening degree of the material surface is low. At the same time, a certain amount of oxidation wear occurs during the friction and wear process under high temperature conditions [26]. The oxide film generated on the substrate surface is thinner, while the coating has better antioxidant properties than the ZL108 aluminum alloy. In the process of surface oxide film damage, a new oxide film is generated, which can reduce friction and resist wear on the coating surface [28]. Therefore, when the temperature rises, the wear volume of the coating decreases instead.

The wear scar surface was observed by white light interferometer, and the two-dimensional and three-dimensional morphologies of the wear scar were recorded. At the same time, the two-dimensional wear depth curve was integrated using Gwyddion software to calculate the average wear depth, as shown in Figure 8. It can be obtained that the average wear depth of the substrate is 65.31 μm, the average wear depth of the coating is 20.23 μm when the Ni mass fraction is 20%, the average wear depth of the coating is 17.8 μm when the Ni mass fraction is 25%, and the average wear depth of the coating is 15.96 μm when the Ni mass fraction is 30%. The increase in the Ni content in the coating makes the wear depth value of the coating smaller and smaller. This also shows that with the continuous addition of Ni, the wear resistance of the substrate is gradually enhanced. Combined with the coating EDS image in Figure 4, the main reasons are as follows: after adding Ni, the wear-resistant enhancement phase in the coating material is dispersed inside the coating. These primary Si and in-situ generated ceramic phases SiC, WSi2, etc. have high hardness and strong wear resistance in the coating, which hinders the wear of the material. The generation of hard phase reduces the friction loss in the coating. This is also the reason for the uneven three-dimensional morphology.

3.5 Wear scar morphology
Figure 9 shows the wear morphology of ZL108 aluminum alloy and cladding coating at different temperatures. It can be seen that the aluminum alloy substrate has plowing and fatigue shedding of varying degrees at different temperatures, and the fatigue shedding is more serious at a temperature of 100 ℃. This is mainly because the pure aluminum alloy has a serious softening phenomenon at high temperature, and the substrate is prone to severe thermal oxidation [29]. The wear mechanism of the aluminum alloy substrate is abrasive wear at room temperature, and adhesive wear and a small amount of oxidative wear at 100 ℃. However, for the coating material, no surface material fell off, whether at room temperature or at high temperature. The wear mechanism of the aluminum alloy substrate and coating is mainly abrasive wear. As shown in Figure 10 (a), during the wear process, due to the lower strength of the coating surface than GCr15, slight peeling will occur. Part of the peeled fragments become particles and are continuously squeezed on the contact surface [Figure 10 (b)], part of them are squeezed onto the wear surface by the load force to form an adhesion layer [Figure 10 (c)], and part of them become an oxide layer due to the heat generated by friction [Figure 10 (d)]. The hardness of the oxide layer is not high, and cracks will be generated under continuous wear, and then it will peel off into oxide fragments, and a new oxide layer will form on the surface [30]. As the temperature increases, the oxidation wear and adhesive wear slowly increase. Under the environment of 100 ℃, the coating surface is worn smoother. This is because during the wear process, the oxide film of the coating has a larger hardness and grows faster, and can replenish its own friction loss in time while wearing.

3.6 Electrochemical corrosion
Figure 11 shows the electrochemical polarization curves of the substrate and the alloy coating, where ΔEp is the passivation zone potential, Epit is the pitting potential, and Ecorr is the corrosion potential. Table 4 shows the electrochemical calculation results of the substrate and the coating, where Icorr is the corrosion current density. It can be seen that with the addition of Ni alloy powder, the corrosion potential of the alloy coating gradually increases. The higher the self-corrosion potential, the stronger the corrosion resistance of the material. The experimental equipment uses a relatively stable three-body electrode. According to Faraday’s law of electrolysis, the corrosion current density in the Tafel curve can directly characterize the corrosion rate of the material [31]. With the increase of Ni content, the corrosion current density gradually decreases, which indicates that the corrosion resistance of the material is enhanced. When the mass fraction of Ni is 35%, the polarization curve changes greatly. The main reason may be that more cracks appear on the surface of the cladding layer, which affects it. However, both the self-corrosion potential and the current density indicate that its corrosion resistance is enhanced. At the same time, both the Tafel polarization curve of the substrate and the Tafel polarization curve of the coating generate a smooth and continuous passivation interval during the corrosion process, which is directly manifested as a corrosion product film on the coating surface. Therefore, the addition of Ni reduces the Al element content and gradually shortens the passivation interval. As the corrosion process continues, this corrosion product film is decomposed, the corrosion rate gradually increases, and finally returns to the original level.

4 Conclusion
The Al-18%Si-30%Ni-5%WC composite coating was prepared on the surface of the ZL108 aluminum alloy substrate by laser cladding. Its phase was characterized and analyzed, the internal microstructure was observed and analyzed, friction and wear experiments were carried out at room temperature and high temperature, and finally its corrosion resistance was explored.

The hardness of the cladding coating has been improved compared with that of the ZL108 aluminum alloy, and the wear resistance has also been improved. The in-situ generated NiAl, Ni3Al, SiC and other hard phases are evenly distributed inside the coating, which supports and strengthens the coating. The wear mechanism of the substrate and the cladding coating at room temperature is mainly abrasive wear. The wear mechanism of the substrate in a high temperature environment is adhesive wear and a small amount of oxidative wear, while the wear mechanism of the coating in a high temperature environment is mainly composed of abrasive wear, oxidative wear and adhesive wear. The wear resistance of the coating material is significantly better than that of the substrate material. As the ambient temperature increases, the thermal oxidation reaction of the coating intensifies, and a composite oxide film is produced on the coating, which alleviates the friction loss caused by adhesive wear and abrasive wear, which makes the wear volume of the coating at high temperature smaller than that at room temperature. In a saturated NaCl solution, the corrosion resistance of the coating is significantly higher than that of the substrate, and the improvement of corrosion resistance greatly extends the service life of the brake disc.