In order to improve the comprehensive performance of the axle surface, different contents of WC particles were added during the laser cladding of Ni-C to obtain Ni-xWC-C coatings. The optimization effect of WC addition on the performance of Ni-xWC-C coatings was analyzed, and its surface hardness was measured. The research results show that a relatively smooth Ni-C coating is formed by laser cladding on the axle surface, and most areas present regular polygonal states. After adding WC particles, some irregular shaped structures and a large number of pores are formed on the coating surface. After increasing the content of WC particles, smaller pores are formed on the coating surface. WC particles are added to prepare a harder Ni-xWC-C layer, and the hardness of the cladding layer increases with the increase of WC content. When 80g/L of WC particles are added, the outer diameter of the Ni grain obtained is 29.2nm, and the hardness HV reaches 551. This study helps to improve the service life of axles under variable loads and has good application value.

During the operation of the axle, the surface is impacted by mechanical forces in the external environment, which increases the risk of damage to the axle. In addition to the conventional coating to prevent atmospheric corrosion, the axle protective coating should also consider the use of appropriate anti-stone impact coating. The axle is subjected to variable loads, and the coating of protective coating on its surface plays an important role in improving the service life. Ni-based composite coatings have many advantages such as excellent wear resistance, low high temperature oxidation resistance, and chemical medium corrosion resistance, and can withstand the use requirements of the axle working environment [1-3].

Metal particles have excellent electrical conductivity and can form a composite structure coating with Ni metal by co-deposition [4-6]. For example, Ni-based coatings can be prepared by adding WC particles. CAI et al. 7 found that the Ni matrix grains can be made smaller by adding WC particles. When the WC surface contacts the corrosive chemical medium, it is easier to react to form WC oxides, thereby effectively controlling the local corrosion of the Ni-based coating and making the coating more corrosion resistant. Zhao et al. [8] found that the addition of WC particles helps to form smaller grains in the Ni matrix structure, making the Ni-based coating have higher hardness and significantly improving the surface wear resistance. Xu Huanhuan et al. [9] heated the WC particles to 900℃ for oxidation, making the Ni-based coating have stronger resistance to high-temperature oxidation. By adding WC particles, Ni-based coatings with better structures can be obtained, and various performances can be comprehensively improved [10]. The WC particles are used to modify the Ni-C coating. The Ni-xWC-C coating actually obtained has many characteristics such as high surface hardness and excellent corrosion resistance, and can withstand harsh working conditions.

This paper mainly studies the structural changes of laser cladding Ni-xWC-C coatings with different contents of WC particles. The optimization effect of WC particles on the performance of Ni-xWC-C coatings is analyzed in detail.

1 Experiment

1.1 Sample preparation

The purity of the C and WC particles used in this experiment is analytically pure. Figure 1 shows the microstructure images of C and WC particles. C and WC particles were added to Ni particles and mixed thoroughly. 5 g/L of C was added, and the mass concentration of WC particles was controlled to be 0 g/L, 10 g/L, 20 g/L, 40 g/L, and 80 g/L.

Nickel-aluminum bronze alloy was used as the cathode substrate material, and it was processed into an outer dimension of 20 mm × 8 mm × 6 mm. The surface of the sample was polished with sandpaper of different mesh numbers.

The uniformly mixed powder was pre-laid on the substrate surface as cladding powder to form a powder layer of 1 mm thick and 3 mm wide. Table 1 shows the specific process parameters of laser cladding in this experiment. Two samples with a size of 15 mm × 15 mm × 8 mm were cut from the middle part of the cladding sample, and then the surface was cleaned with deionized water to fully remove impurities, and then sealed and stored.

1.2 Test method

The coating morphology was observed by HitachiTM3030 scanning electron microscope, and the elemental composition of the coating was tested by OxfordSwift3000 energy dispersive X-ray spectrometer and LabRAMH Raman spectrometer. The crystal structure and phase composition of Ni-xWC-C coating were tested by RigakuUltimaIV X-ray diffractometer. The XRD diffraction spectrum of the coating was fitted by the Rietveld full spectrum method of Maud software, and the Ni grain size was calculated at the same time.

2 Results and discussion

2.1 Coating morphology analysis

Figure 2 shows the surface micromorphology results of laser cladding Ni-xWC-C coating with different contents of WC particles. At this time, a relatively smooth Ni-C coating was formed, and most areas showed regular polygonal organizational morphology. After adding WC particles, it can be observed that some irregular nodular structures were formed in the surface area of ​​the coating, and a large number of pores were generated. During the laser cladding treatment, WC particles are adsorbed to the surface area of ​​the cathode and are mainly concentrated in the protruding area of ​​the surface. Therefore, the laser cladding resistance of the particle surface is reduced, Ni grains are formed preferentially, and Ni grains with dendritic characteristics are obtained. According to the test results in Figure 2, after gradually increasing the content of WC particles, smaller pores are formed on the coating surface. This is because after adding WC particles, the pores generated by c particles can be filled, thereby obtaining a coating structure with a smaller porosity. Figure 3 shows the change in the content of WC particles in the laser cladding Ni-xWC-C coating under the condition of adding different contents of WC particles. According to the results in Figure 3, it can be found that when p(WC) increases from 10g/L to 80g/L, the mass fraction of WC in the coating increases from the initial 2.6% to 23.8%. This is mainly because as the amount of WC added gradually increases, more WC particles help reach the cathode surface.

2.2 Coating hardness analysis

Figure 4 shows the surface hardness distribution of the laser cladding i-xWC-C coating. The test shows that the hardness (HV) of the cladding layer composed of Ni-C is 326. Relatively speaking, the Ni-xWC-C layer has a higher hardness. When the content of WC particles is gradually increased, the Ni-xWC-C layer obtains a higher hardness (HV), which increases to 551. The higher hardness of the Ni-xWC-C layer is due to the structural transformation process in the layer structure, which results in finer grains and achieves the effect of fine grain strengthening. The Ni crystal structure contains the {101}<110> slip system. When the (101) crystal plane is stressed in a vertical direction, the shear stress formed in the slip plane is 0, so the hardness of the Ni structure can be significantly improved. In this experiment, after adding WC particles to the matrix, smaller Ni grains are formed, which significantly reduces the [110 peak. The above results show that a Ni-xWC-C layer with higher hardness can be prepared by adding WC particles, and the hardness of the cladding layer increases with the increase of WC content.

3 Conclusions

1) Laser cladding forms a smoother Ni-C coating, and most areas present regular polygonal morphology. After adding WC particles, some irregular nodular structures are formed in the surface area, resulting in a large number of pores. After increasing the content of WC particles, smaller pores are formed on the surface of the coating. When p(WC) increases from 10g to 80g, w(WC) in the coating increases from 2.6% to 23.8%.

2) A Ni-xWC-C layer with higher hardness can be prepared by adding WC particles, and the hardness of the cladding layer increases with the increase of WC content. With a higher content of WC particles, the Ni-xWC-C layer obtains a higher hardness.