Abstract: Arc ablation is the main reason for the failure of pure copper switches. In order to improve the electric ablation resistance of pure copper contacts, Cu-W-Ni alloy cladding layers with different W contents were prepared on the surface of pure copper matrix by laser cladding technology. The microstructure, composition distribution, hardness and corrosion resistance of the cladding layer were analyzed. The results show that the cladding layer and the surface of the pure copper substrate are metallurgically bonded, without defects such as holes and cracks. On the premise of ensuring the electrical conductivity of the pure copper matrix in the core, the laser cladding layer significantly improves the surface hardness and corrosion resistance of the contacts. In addition, with the increase of W content, the hardness of the cladding layer increases continuously while the defects increase and the electrical conductivity decreases.

Disconnectors are one of the most widely used and widely used electrical equipment in power systems. Failures often lead to large-scale power outages, seriously affecting the safe operation of power grids. Electrical contacts are key components of high-voltage switchgear. At the moment the switch is closed, a high-temperature, high-energy arc is generated in the contact gap, which burns the contact surface. Every year, disconnectors are damaged and large-scale power outages occur due to defects such as heating and overheating of contacts. This not only causes a large amount of economic losses, but also poses a huge threat to the life safety of substation workers. Contact damage mainly occurs during the arc ablation process, which is mainly composed of the following three parts: First, the volatilization and sputtering of existing attachments on the surface and low-melting-point copper; second, the dynamic balance between the melting and splashing of copper and cooling and solidification; finally, the surface is uneven and strongly ablated. However, the arc breaking will not directly cause the contact welding, because the arc breaking time is relatively long and the energy is large, which will make the contact surface more uneven and the erosion is large, making the contact more prone to high-energy pre-discharge arc, bounce arc and welding when it is closed next time, that is, the contact must not only withstand mechanical impact, but also be corroded by the arc. In addition, there are defects inherent in the manufacturing process.

At present, pure copper contacts are commonly used, which have good electrical conductivity, but they will fail due to strength loss and low deformation resistance during use; the copper surface is prone to generate oxide film, which leads to higher resistance and faster temperature rise, and pure copper has poor performance in corrosion resistance, temperature, strength and other aspects. A lot of research work is being carried out on contact materials at home and abroad. From the perspective of research direction, there are mainly three aspects: first, developing new contact materials; second, finding new preparation processes without changing the materials; third, improving the performance of materials by adding new alloy elements or non-metallic compounds without changing the main materials. This study mainly improves the comprehensive performance of contact materials through the latter two ways.

Laser cladding is a new type of surface technology. Its principle is to use a highly concentrated laser beam to melt and solidify the cladding material and the substrate on the surface of the substrate at the same time, and prepare a laser cladding layer with metallurgical bonding with the substrate and low dilution rate and cladding material characteristics. This cladding layer often has excellent properties that the substrate does not have, such as good wear resistance, corrosion resistance, heat resistance, and oxidation resistance. Laser cladding technology has great application potential in components made of materials with excellent performance that are used under harsh working conditions but cannot be used on a large scale due to economic factors. In addition, according to the research of Wang Yanming and others, under the same current and current breakdown times, the higher the tungsten content in the copper-tungsten alloy, the stronger the arc erosion resistance of the copper-tungsten alloy. At the same time, the higher the tungsten content, the more likely the copper-tungsten alloy will crack under arc erosion. This shows that compared with copper-tungsten alloys with lower tungsten content, the higher the tungsten content, the greater the brittleness of the copper-tungsten alloy, and the easier it is to crack, which corresponds to the hardness change of the copper-tungsten alloy. In this study, the laser cladding technology was used to prepare a copper-based cladding layer with wear resistance and corrosion resistance on the surface of pure copper contacts for protection, and the micromorphology, composition distribution, hardness, conductivity and corrosion resistance of the three prepared copper-based cladding layers were studied.

1 Experimental materials and methods

1.1 Experimental materials
The experimental substrate is a 20 mm × 20 mm × 8 mm pure copper plate (the same as the GW5-35 high-voltage disconnector contact material). The composition of the cladding material is shown in Table 1. The powder particle size is 140 mesh to 325 mesh. Tungsten-copper alloy electrical contacts are generally high tungsten and low copper (the mass fraction of tungsten is 50% to 90%). Cu has good thermal conductivity and electrical conductivity, and W has high density, high strength, high melting point and low expansion coefficient. The two are neither miscible nor can they form intermetallic compounds. However, the increase of copper significantly reduces the porosity of the alloy, but the agglomeration tendency of copper is also more obvious. The W element is unevenly distributed in copper, and the interface between the alloy and the infiltrated copper liquid is loose. Tungsten-copper composite material is a pseudo alloy composed of tungsten with high melting point and high hardness and copper with high electrical conductivity and thermal conductivity. Cu-W material has the advantages of Cu and W at the same time. It has good thermal conductivity and electrical conductivity, high density and small expansion coefficient, so it is widely used as an electrical contact material. At the same time, a small amount of Ni is added to the copper-tungsten alloy to increase the hardness and corrosion resistance of the material.

1.2 Experimental method
Laser cladding is performed on the surface of the pure copper plate substrate by synchronous powder feeding. Before laser cladding, the surface of the pure copper plate needs to be pretreated to avoid affecting the cladding effect. The pretreatment method is: use 200-600 mesh sandpaper to polish the surface of the plate to remove impurities and oxide scale on the surface, then clean it with acetone to remove impurities, and finally fix the plate on the cladding workbench. Laser cladding is performed by fixing the plate and moving the laser head. The alloy powders of the three components in Table 1 were dried at 120 °C for 1 h and then filled into the powder feeding equipment. The laser cladding equipment uses RFL-C3300W laser, and the test parameters of each cladding layer are shown in Table 2.
By adjusting the parameters, the thickness of a single cladding layer can reach 600 μm. Cladding was performed three times in the same way, and the total average thickness of the cladding layer after preparation was about 1200 μm to 1400 μm. The prepared cladding layer was polished and polished to obtain a corrosion-resistant and wear-resistant cladding layer with good morphology. By wire cutting, the test samples required for hardness test, salt spray test (according to GB/T 10125-2012 “Artificial atmosphere corrosion test salt spray test”), and conductivity test were prepared respectively.
The test method for the microhardness of the cross section of the cladding layer is the “S-type dot test method”. Starting from the surface of the cladding layer, dot toward the substrate, take three points in the horizontal direction of the cladding layer, the spacing between each point is 50μm, and the spacing between each point in the vertical direction of the cladding layer is 150μm. The load is 200 g/N, where 0 is the substrate position. The corrosion resistance test is a neutral salt spray test designed according to the test standard (GB/T 10125-2012). A 5% NaCl solution (50 g/L) is prepared to control the pH value of the spray solution collected by the salt spray box between 6.5 and 7.2, and the solution is prepared with new boiling water to reduce the carbon dioxide in the solution and avoid pH changes. The temperature in the salt spray chamber is set to 35 ℃, the temperature of the pressure barrel is set to 45 ℃, the humidity is greater than 95%, the fog drop is 1 mL/(h·cm2), the nozzle pressure is 70 kPa, and the spray is continuous. The sample is taken out every 24 hours for observation and testing. The total test duration is 120 h.

2 Test results and analysis

2.1 Coating layer structure
The surface of the cladding layer was tested, and the thickness was above 1500 μm. Among them, the surface of the CuW10Ni3 cladding layer was smooth and flat, with uniform overall thickness and no obvious holes. As the W content in the cladding layer increased, its surface morphology gradually deteriorated, as shown in Figure 1. Compared with CuW10Ni3, a small number of holes began to appear on the surface of the other two cladding layers, while the surface morphology of CuW40Ni3 was poor and there were more holes. The surfaces of the three cladding layers were non-destructively tested using a colored penetrant flaw detector, and the results showed that there were no cracks on the surfaces of the three cladding layers.
The morphology of the junction between the three cladding layers and the substrate is shown in Figure 2. From Figure 2, it can be seen that the three cladding layers are metallurgically bonded to the substrate, and the two are tightly bonded. The substrate is a single gray phase, and the cladding layer contains white and gray phases. As the W content increases, the white phase content gradually increases.

The composition of different positions of each cladding layer is shown in Table 3. Figures 3 to 5 are the cross-sectional morphology and element distribution diagrams of the three cladding layers, respectively. As can be seen from Figures 3 to 5, the white phase is composed of W elements, and the gray phase is composed of Cu and Ni elements. The Cu element content is much higher than that of Ni. The cladding layer produces W element segregation. This is because W has a high melting point and the W particles in the cladding layer are coated by Cu and Ni elements. The W content in the CuW10Ni3 and CuW20Ni3 cladding layers is relatively low, 10% and 20% respectively; in contrast, the W element distribution in the CuW20Ni3 cladding layer is the most uneven, with both small granular segregation and large agglomerate segregation. In the CuW40Ni3 cladding layer, the W element content is significantly increased and is more evenly distributed in the gray matrix phase. Similar to the CuW20Ni3 cladding layer, there are both small granular segregation and large agglomerate segregation.

2.2 XRD results
Figure 6 shows the XRD results of the three cladding layers. It can be seen from Figure 6 that the diffraction peaks of the three cladding layers are almost the same, and the cladding layers are composed of Cu, W and Cu-Ni solid solution. In addition, no oxides were detected in the cladding layers, which proves that argon plays a full protective role in the laser cladding process, and no oxidation occurs in the prepared cladding layers, which provides a guarantee for the accurate detection of the corrosion resistance and electrical conductivity of the material.

2.3 Hardness analysis
Figure 7 shows the hardness curve from the cladding layer to the substrate. It can be seen from Figure 7 that from the substrate to the cladding layer, there is a hardness growth gradient in the transition area; in the cladding layer area, the overall hardness value is significantly higher than that of the substrate. It increases from 60 HV to 70 HV of the substrate to more than 90 HV. Comparing the cladding layers of the three components, as the W content increases, the hardness of the cladding layer also increases. The average hardness of the CuW40Ni3 cladding layer increases to about 130 HV. Compared with the substrate, the hardness of the cladding layer increases by 1.5 to 2 times. According to the XRD results, due to the presence of Ni, the Cu3.8Ni phase is formed in the cladding layer, which plays a role in the increase of hardness to a certain extent. It is also found that the hardness of CuW10Ni3 and CuW20Ni3 fluctuates greatly. Combined with the scanning electron microscope image, the enrichment and uneven distribution of the W element may lead to this result. In addition, since the laser cladding technology itself has the characteristics of rapid melting, expansion and rapid solidification (the cooling rate usually reaches 102 ℃/s~106 ℃/s), the microstructure characteristics of the cladding layer are generally very fine and dense, which can effectively improve the hardness of the cladding layer. 2.4 Conductivity analysis Table 4 shows the conductivity of each cladding layer. Overall, the conductivity of the cladding layer is lower than that of the pure copper substrate. By comparison, it is found that with the increase of W content, the conductivity gradually decreases. Combined with the cross-sectional scanning electron microscope image and XRD analysis, the presence of a small amount of holes in the cladding layer and the addition of Ni and W elements have a great impact on the surface conductivity while increasing the corrosion resistance and hardness.

2.5 Salt spray test
The corrosion resistance of the three cladding layers was tested through the salt spray test. The relationship between the weight loss of the samples and the corrosion time during the salt spray corrosion test of the three cladding layers is shown in Figure 8.
It can be found from Figure 8 that the weight loss curves of the two cladding layers are approximately parabolic, which shows that during the corrosion process of the cladding layer, corrosion products with protective effects are generated on the surface, thereby reducing the corrosion rate. Compared with CuW10Ni3 and CuW20Ni3 cladding layers, the corrosion resistance decreases with the increase of W element content. The total weight loss is 1.5621 mg/cm2 for CuW10Ni3 cladding layer and 3.7247 mg/cm2 for CuW20Ni3 cladding layer. The weight loss of CuW20Ni3 cladding layer is 2.4 times that of CuW10Ni3 cladding layer.
The weight loss curves of CuW10Ni3 and CuW20Ni3 cladding layers are fitted, and the fitting formulas shown in Table 5 are obtained respectively. It can be seen from Table 5 that after adding Ni element, the weight loss curves of CuW10Ni3 and CuW20Ni3 cladding layers are similar, and both are approximately parabolic relationships. It can be seen from XRD analysis that after adding a small amount of Ni element, in addition to pure Cu and W phases, a new Cu3.8Ni phase appears in the cladding layer. Cu-Ni alloy has good corrosion resistance, so the addition of Ni element greatly improves the corrosion resistance of Cu-W-Ni cladding layer.
Table 6 shows the macroscopic morphology of three cladding layers after salt spray corrosion at different times. Comparing the three cladding layers of CuW10Ni3, CuW20Ni3 and CuW40Ni3 in Table 6, it is obvious that with the increase of W content in the cladding layer, its surface morphology gradually deteriorates, especially the surface of CuW40Ni3 cladding layer is covered by corrosion products, and very obvious holes appear. Combined with Table 5, it can be seen that both CuW20Ni3 and CuW40Ni3 cladding layers lose weight to varying degrees, but the CuW40Ni3 cladding layer has a weight gain phenomenon due to the large number of surface holes and the difficulty in removing corrosion products.
Figure 9 shows the XRD results of CuW40Ni3 cladding layer after 120 h of salt spray corrosion. According to the comparison of XRD cards, the corrosion products on the surface of the Cu-W-Ni system cladding layer are mainly CuCl, Cu2O, Cu2Cl(OH)3, and NiO.
The electrochemical corrosion mechanism analysis of the Cu-W-Ni system cladding layer shows that during the corrosion process, the Ni element in the copper-tungsten-nickel cladding layer will protect the material. In the initial stage of corrosion, in addition to the corrosion and dissolution reaction of copper, reactions (1), (2), and (3) also occur to generate Ni2+. As the corrosion progresses, reactions (4) and (5) occur, and a passivation film is generated on the surface of the sample. Since corrosion products such as NiO, Cu2(OH)3Cl, and Cu2O with certain corrosion resistance are generated on the alloy surface, the corrosion resistance of the alloy is greatly improved.

3. Conclusion and outlook
(1) Laser cladding was used to prepare CuW10Ni3, CuW20Ni3, and CuW40Ni3 cladding layers. In addition to the two phases of Cu and W, the third phase Cu3.8Ni also appeared on the surface of the cladding layer. The cladding layer and the substrate are well integrated and metallurgically bonded. The components of the cladding layer are relatively uniformly distributed, and the W element segregates to a certain extent.
(2) The hardness of the three Cu-W-Ni cladding layers is significantly improved compared to the pure copper matrix. At the same time, as the W element content increases, the hardness value shows an upward trend.
(3) As the W content increases, the electrical conductivity gradually decreases. Combining the cross-sectional scanning electron microscope image and XRD analysis, the existence of a small number of holes in the cladding layer and the addition of Ni and W elements increase the corrosion resistance and hardness. , also has a great impact on surface conductivity.
(4) Overall, the comprehensive performance of the CuW10Ni3 cladding layer is slightly better than the other two Cu-W-Ni system cladding layers.