In response to the demand for the preparation of wear-resistant and corrosion-resistant coatings on the surfaces of mechanical parts in the fields of ocean and mining, and the problem that the high wear resistance and high corrosion resistance of ceramic particle-reinforced coatings are difficult to be compatible, an ultrasonic-assisted laser cladding test platform was built to prepare tungsten carbide (WC) particle-reinforced coatings with and without ultrasonic action. The influence of ultrasound on the microstructure morphology, element distribution, and thickness of the alloy layer on the WC surface of the composite coating was studied, and further tests on the hardness, friction, wear, and corrosion resistance of the samples with and without ultrasound were carried out. The results show that ultrasonic vibration can refine the grains, reduce the average grain size from 101.0μm to 59.6μum, inhibit segregation, and promote the dissolution of the alloy layer on the WC surface and the uniform distribution of the elements in the cladding layer; under the action of ultrasound, the average microhardness of the sample is increased from 310HVo.1 to 425HVo.1, and the hardness distribution around the WC particles is more uniform under the action of ultrasound; the weight loss of the samples with and without ultrasonic action is 6.5mg and 8.8mg respectively, and the wear rate of the samples is 0.0323mg/m and 0.0438mg/m respectively, and the wear rate of the samples is reduced by 26.2%; under the action of ultrasound, the corrosion current density of the sample is reduced from 5.20μA/cm² to 2.13μA/cm². At the same time, the electrochemical impedance spectrum shows that the surface of the sample has a larger capacitive impedance loop, impedance modulus and phase angle value under the action of ultrasound.

Key components such as blades, picks, and shafts of mechanical equipment in the marine and mining fields have been in a harsh service environment of mechanical wear and electrochemical corrosion for a long time. The interaction of friction and corrosion will accelerate the damage to the surface of the components, resulting in a significant reduction in their service life. In order to extend the service life of the equipment and break through the limitations of the original performance of the substrate, methods such as spray electrochemical deposition, in-situ synthesis, and high-energy beam melt deposition are often used to prepare enhanced coatings with excellent performance on the surface of the components. Among them, laser cladding technology has been widely used in the preparation of surface enhanced coatings for mechanical components in aviation, energy, ocean, mining, metallurgy and other fields due to its advantages such as good metallurgical bonding strength, low dilution rate, fine microstructure, and excellent mechanical properties.

In response to the high wear resistance requirements of the component surface, researchers use hard ceramic particles such as carbides, nitrides, and oxides to mechanically mix with matrix metal powders, and then use laser cladding technology to clad the mixed powder on the surface of the matrix to prepare ceramic particle-reinforced metal-based composite coatings. Among them, tungsten carbide (WC) has become one of the most commonly used ceramic particles for laser cladding enhanced coatings due to its excellent chemical stability, wettability and adhesion. Benarji et al. prepared SS316-WC composite strengthening coating by laser cladding technology and found that the increase of WC volume fraction can improve the microhardness and wear resistance of the coating. Li et al. prepared Inconel625-WC composite coating with different shapes of WC by laser cladding technology and found that WC of different shapes had different decomposition trends under laser irradiation. Spherical WC was less likely to decompose to produce hard and brittle phases such as W.C2.54 and M23C, and had better wear resistance than non-spherical WC. Wang et al. prepared laser cladding strengthening coating with WC particles of different particle sizes and found that WC particles showed different characteristics of dissolution-diffusion controlled thermal damage, shrinkage-dissolution-diffusion controlled thermal damage and dissolution-precipitation controlled thermal damage, which in turn affected the growth process of alloy reaction layer and dendrite on the surface of WC particles, the precipitation of carbides and the uniformity of microstructure. Studies have shown that although the introduction of WC can improve the wear resistance of the coating, the dissolution of WC particles under the action of high-energy laser beams will change the phase composition and microstructure of the coating, thereby affecting the corrosion resistance of the coating. Zhang et al. prepared Inconel625-WC composite coatings with different WC contents and found that with the increase of WC content, the precipitation of carbides will aggravate the local segregation of solid solution strengthening elements, and the wear resistance of the coating will gradually improve, but the corrosion resistance will gradually decrease. Li studied the effects of the content, morphology and distribution of WC particles on the microstructure, mechanical properties, corrosion resistance and wear resistance of laser cladding coatings, and found that the corrosion resistance first increased and then decreased with the increase of WC content. Jayaraj et al. studied the corrosion behavior of reinforced coatings prepared by WC and different bonding phases. The results showed that WC and different bonding phases will produce different microstructures, which greatly affects the formation stability of the passive film of the coating, thereby affecting the corrosion resistance of the coating. In order to solve the problem of microstructure control of laser cladding reinforced coating, domestic and foreign scholars have applied electromagnetic, thermal, acoustic and other external energy fields to assist in shape and property control during laser cladding. Among them, the acoustic cavitation effect, acoustic streaming effect and mechanical effect generated by the ultrasonic energy field in the metal molten pool can inhibit element segregation, refine grains and promote uniform distribution of the reinforcing phase, which is considered to be an effective method to achieve laser cladding microstructure control. However, the current research reports on ultrasonic-assisted laser cladding WC particle reinforced coatings mainly focus on the influence of wear resistance. Wang and Lv prepared Inconel718-WC composite coatings by ultrasonic vibration-assisted laser cladding and found that ultrasonic vibration can reduce particle aggregation, refine grains, and improve average microhardness and wear resistance. Nie et al. studied the effect of ultrasonic amplitude on the flow characteristics of laser cladding WC ceramic particles during the solidification process of the molten pool. By changing the ultrasonic amplitude, the WC ceramic particles were gradually floated in the molten pool, which improved the hardness and high temperature wear resistance of the coating. However, the current research on the effect of ultrasound on the corrosion resistance of WC particle reinforced composite coatings is relatively lacking, and the mechanism of the effect on corrosion resistance is still unclear, which still needs further in-depth research.

Aiming at the problem that the high wear resistance and high corrosion resistance of laser cladding WC particle reinforced coatings are difficult to be compatible, ultrasonic vibration is introduced into the laser cladding process, and WC particle reinforced coatings with or without ultrasonic action are prepared. The influence mechanism of ultrasonic vibration on the microstructure morphology, element distribution, and thickness of WC surface alloy layer of the composite coating is analyzed, and the hardness, friction and wear and corrosion resistance of the sample with or without ultrasonic are further tested. Related research provides a reference for the preparation of WC particle reinforced coatings with high wear resistance and corrosion resistance.

1 Experimental materials and methods

1.1 Experimental materials and equipment

The experimental substrate material is 316L stainless steel plate, and its chemical composition is shown in Table 1. Before the laser cladding test, the plate is polished to remove the surface oxide film, and the surface stains are cleaned with alcohol and dried for standby use. The morphology of the 316L powder and WC ceramic particles used in the experiment is shown in Figure 1 (a) and (b), where the particle size of the 316L powder is 70~100μm, and the particle size of the WC ceramic particles is 50~100μm. Before the experiment, the 316L powder and WC ceramic particles were fully mixed in a mass ratio of 1:4 using a planetary ball mill (QM-3SP4), and the mixed powder was placed in a vacuum drying oven for drying at a holding temperature of 120℃ for 60 min.

The constructed ultrasonic-assisted laser cladding test platform is shown in Figure 1 (d). The laser is a semiconductor laser (Laserline, LDF400-2000), with a maximum output power of 2000W, a wavelength range of 900~1064nm, a focal length of 25mm, and a circular spot with a diameter of 4mm at the focal length. The energy within the spot range is a flat-top distribution; the motion and control system is a 6-axis robot (ABB, IRB2400-16/1.5), with a maximum speed of 150 (°)/s, and a trajectory accuracy and repeatability accuracy of 0.06mm; the ultrasonic vibration device consists of an ultrasonic generator, a transducer, a flange, a horn, and a control panel. Its technical indicators are shown in Table 2. Before the test, the base material is fixed to the upper end face of the ultrasonic horn with bolts to stably transmit the ultrasonic energy to the substrate; the powder conveying and atmosphere protection gases are both argon.

1.2 Test and analysis test method

The laser power used in the test was 1200W, the scanning speed was 8mm/s, the overlap rate was 45%, the powder feeding amount was 20g/min, the protective gas flow rate was 8L/min, the ultrasonic frequency was 20kHz, and the ultrasonic power was 2500W. After the laser cladding test, the cladding layer was sampled using a wire cutting machine with a size of 7mm×3mm×3mm, and then mounted, ground, polished, and etched. Optical microscope (ZEISS, Axioscope) and scanning electron microscope (ZEISS, EVO18) were used. The microstructure of the cladding layer was observed and analyzed by an energy dispersive spectrometer (EDS, BRUKERXFlash6130). The hardness of the cross section of the cladding layer along the depth direction was tested by an automatic microhardness tester (Shimadzu, HMV-2TADWXY), with a loading load of 100N and a holding time of 10s. The friction and wear test was performed by a ball-disc friction and wear tester (MMQ-02), with Si, N4 ceramic balls with a diameter of 4.75mm, a test load of 20N, a wear track radius of 4.0mm, and a motor speed of 200r. /min, wear time is 40min, and the sample is weighed with an electronic scale (Lichen, FA123C) before and after the wear test to obtain the wear mass. A confocal microscope (KEYENCE, VK-X1000) is used to obtain the three-dimensional wear profile of the sample and the microscopic morphology of the wear debris. The corrosion resistance test is performed using an electrochemical workstation (CHI760e). The electrolyte solution is a 3.5% mass fraction NaCl solution, the working electrode is a 1cm² platinum sheet electrode, the reference electrode is a saturated calomel electrode, and the effective area of ​​the sample is 1cm².

2 Experimental results and discussion

2.1 Effect of ultrasound on the microstructure of laser cladding composite coating

Figure 2 shows the metallographic structure around WC particles in laser cladding composite coating with and without ultrasonic vibration. Without ultrasonic vibration, a large number of columnar dendrites and a part of element segregation bands are shown around WC, as shown in Figure 2(a) and (c). Under ultrasonic vibration, no obvious segregation bands are observed in the metallographic structure of the sample, and the grains around WC are significantly refined, as shown in Figure 2(b) and (d). Since laser cladding has the characteristics of rapid temperature rise and cooling, when the metal melt solidifies, the crystals are prone to heterogeneous nucleation on the outer wall of WC and epitaxial growth to form columnar dendrites. At the same time, due to the partial dissolution of WC, the element distribution is uneven during crystallization, and the cladding layer is prone to form segregation bands. The grain distribution diagram of the WC particle-reinforced coating with and without ultrasonic action is shown in Figure 2(e). It can be seen that compared with the grain size without ultrasonic action, the grain size around the WC particles under ultrasonic action is significantly refined. Further statistics on its grain size show that under ultrasonic action, the average grain size of the cladding layer is reduced from 101.0μm to 59.6μm.

Figure 3 shows the scanning electron microscope morphology of the microstructure around the WC particles with and without ultrasonic action and the point scan and surface scan results of its element distribution. Without ultrasound, there is a thick alloy layer on the WC surface. In addition, there are a large number of fine fishbone structures around the WC particles, as shown in Figure 3(a). Under ultrasound, no obvious alloy layer is seen on the WC surface, and a few network-like precipitates are scattered around, as shown in Figure 3(b). EDS results show that in the non-ultrasonic sample, the WC surface alloy layer (P1, P2) contains a large amount of W elements, as well as Fe and Cr elements. P3 is a fishbone eutectic structure with an atomic ratio of Fe: (Ni, W) close to 1:1, and the main component of P4 is Fe. In the ultrasonic sample, the alloy reaction layer becomes thinner, and the point scanning energy spectrum analysis (P5) shows that the enrichment of Fe and Cr elements is weakened. The main elements of P6 and P7 are both Fe, and the W content of P7 is slightly higher than that of P6. The WC radial element line scan is performed. From the strength distribution of the sample without ultrasound (Line 1), it can be seen that in the alloy reaction layer, the Fe, W, Cr and other elements all decrease in the diffusion direction. At the same time, due to the presence of a large number of precipitated phases in the matrix, the element change gradient between WC and the matrix is ​​greater in the sample with ultrasound (Line 2), which indicates that the alloy layer is very thin, and the strength of each element in the matrix does not change much. This is because ultrasound reduces the precipitated phase and evens out the organization.

Due to the high energy of the laser, WC absorbs a large amount of heat in the molten pool and decomposes into free W and C atoms. When the W content (mass fraction) in the molten pool is 35%~52%, that is, around the WC particles, W2C and Fe; W3C (28) will be generated in the molten pool. As the molten pool temperature continues to decrease, M.C is easily generated in the molten pool under equilibrium conditions. Therefore, WC particles are prone to form a surface alloy reaction layer during solidification. P1 is close to WC and contains a higher content of W elements. It is speculated that its main component is WzC; the ratio of Fe to W elements in P2 is close to 1:1, so it is speculated that it is FeWC. At the same time, Cr, Ni and other elements on the WC surface participate in the reaction. The main components of the entire alloy reaction layer should be W, C, FesWsC and M.C (M is Fe, W, Cr and Ni), while the surrounding structure is mainly composed of the matrix phase of 316L>-Fe (P4). The fishbone carbide (P3) included in it is inferred to be the eutectic structure of Fe:WsC iron-rich carbide and y (Fe, Ni) based on its shape and the atomic ratio of Fe:(Ni,W) [19]. In the sample with ultrasound, the cavitation effect of ultrasound causes the alloy layer (P5) to dissolve and thin, approaching the intermediate composition of P1 and P2. By comparing P4 and P6, it is found that their elemental compositions are similar. This is because ultrasound will not affect the organizational composition of the cladding layer phase, but will only reduce the content of the precipitated phase [24-25]. P7 is the precipitated phase of the cladding layer under ultrasound, and no tiny fishbone carbide is formed. This is because the acoustic streaming effect of ultrasound promotes the flow of the molten pool, reduces the W content around the WC particles, and reduces the generation of Fe3W.C. Therefore, the structure around the WC particles after ultrasonic vibration is still mainly -Fe.

The fishbone carbides (P3) precipitated in the non-ultrasonic cladding layer are mostly hard and brittle phases, which are very easy to form a gradient hardness distribution around the WC particles, which is easy to produce stress concentration, so the toughness is poor, and it is more likely to break during friction, affecting the wear resistance. The improvement of the precipitated phase by ultrasound is expected to improve the wear resistance of the cladding layer.

In order to further verify the effect of ultrasound on element distribution, the element analysis around WC was carried out by surface scanning. As shown in Figure 4(a), when there is no ultrasound, the W element around WC is distributed among Fe and Ni elements; while in the sample with ultrasound, as shown in Figure 4(b), the distribution of Fe, Ni, W and other elements is relatively uniform, and there is no obvious inclusion phenomenon.

Under laser irradiation, WC decomposes and releases free W and C atoms. In the subsequent rapid cooling process, due to limited time, W and C atoms cannot diffuse quickly to a larger range, so W-rich carbides are formed around WC particles. Since W carbides have extremely high melting points, they are easier to solidify and precipitate in the molten pool, and the Fe, Ni and W elements are staggered in the surface scanning energy spectrum. The introduction of ultrasound and the acoustic streaming effect accelerate the flow rate of the molten pool, and the main alloying elements such as Fe, Ni, and W are also homogenized. Among them, the W element is also diffused to a farther distance, reducing the W element around the WC particles, and further affecting the formation of the precipitated phase.

The mechanism of ultrasound is shown in Figure 5. During the laser cladding process, the cavitation effect of ultrasound will produce a large number of cavitation bubbles, which will collapse under the combined action of ultrasonic sound pressure and melt drive. The expansion of cavitation bubbles will absorb a large amount of heat from the surrounding liquid, resulting in supercooling of local areas of the molten pool; and microjets will be generated at the moment of cavitation bubble collapse, breaking the primary dendrites growing on the surface of WC particles to form a fine and uniform organizational structure, and dispersing them to various parts of the molten pool to form new nucleation particles. Both factors help to increase the nucleation rate of the molten pool, thereby leading to the refinement of the microstructure. At the same time, the collapse of cavitation bubbles produces local high temperature and high pressure to promote the dissolution of the alloy layer on the surface of WC. Accompanied by the ultrasonic sound flow effect, these dissolved alloy elements are evenly distributed in the entire cladding layer, which can avoid the uneven distribution of elements during the solidification of the molten pool and inhibit segregation. Grain refinement, increased grain boundaries, increased obstacles to dislocation movement, and greater resistance to material deformation are conducive to improving the hardness of the cladding layer. The unevenness of elements will form micro-galvanic cell reactions at the grain boundaries and in the grains, accelerate the corrosion of the coating surface, and greatly reduce the corrosion resistance of the cladding layer. Ultrasound can achieve the homogenization of organization and elements, which is conducive to reducing the potential gap inside the composite coating, reducing the corrosion current, and improving the corrosion resistance of the coating.

2.2 Effect of ultrasound on hardness distribution of laser cladding composite coating

Figure 6(a) shows the microhardness distribution of the cross section of the sample with and without ultrasound along the depth direction. The test direction extends from the top of the cladding layer to the substrate. Compared with the substrate, the microhardness of the cladding layer under ultrasound or without ultrasound is significantly improved, and the microhardness of the heat-affected zone between the cladding layer and the substrate decreases rapidly until it reaches the hardness of the substrate (200 HVo.1). Among them, in the sample without ultrasound, the hardness of the cladding layer is distributed between 310HVo.1, and the fluctuation range of microhardness is 75HVo.1, while the hardness of the cladding layer of the sample with ultrasound is distributed between 425HVo.1, and the fluctuation range of microhardness is 30HVo.1, which is 37% higher than that of the sample without ultrasound. Figure 6(b) shows the hardness distribution around WC with and without ultrasonic action. The hardness test is centered on the WC particle, with an interval of 90° and a step distance of 40μm to make points and calculate the average hardness. Without ultrasonic action, the hardness around WC decreases from 480HVo.1 to 320HVo.1, showing a trend of gradual decrease. Under ultrasonic action, the hardness around WC is 426HVo.1, 417HVo.1 and 413HVo.1, respectively, and the distribution is relatively uniform.

Considering Figures 2 to 5, there are a large number of carbide precipitation phases in the sample without ultrasonic action. These carbides have high hardness, and their uneven distribution leads to large fluctuations in the hardness test. The ultrasonic action causes the WC surface alloy layer to dissolve and diffuse, and the W precipitation phase is evenly distributed in the entire cladding layer, which not only improves the hardness performance, but also reduces the range of hardness fluctuations. In the cladding layer of the sample without ultrasound, the closer to WC, the higher the content of elements such as W and C that have a strong effect on hardness improvement, while the introduction of ultrasound to homogenize the elements reduces the gradient change of the elements, showing a smoother hardness gradient distribution. In summary, the effect of ultrasound in the laser cladding process significantly improves the average hardness of the cladding layer and makes its distribution more uniform by promoting the dissolution and diffusion of the alloy layer on the WC surface and uniform element distribution.

2.3 Analysis of wear resistance of ultrasonic-assisted laser cladding composite coating

Figure 7(a) is a curve of the friction coefficient of the WC particle-reinforced coating changing with time. In the initial stage of friction and wear, the surface of the friction pair is relatively smooth and the friction coefficient is low. With the increase of friction time, the contact area of ​​the friction pair continues to expand, and the friction coefficient rises rapidly. When the friction pair reaches a relatively stable state, the friction coefficient of the sample without ultrasound fluctuates around 0.6, while the friction coefficient of the sample with ultrasound fluctuates around 0.5. Figure 7(b) shows the mass loss of the coating after the friction and wear test. The wear mass of the cladding layer without ultrasound is 8.8 mg, and the wear mass of the cladding layer with ultrasound is 6.5 mg. The wear rate calculation formula is shown in formula (1): Mr=M/πdN (1)
Where: M, is the wear rate; M is the mass loss; d is the diameter of the grinding ring; N is the friction and wear rotation speed.

The addition of ultrasound reduces the wear rate from 0.0438 mg/m to 0.0323 mg/m, a decrease of 26.2%. At the same time, the friction coefficient of the coating without ultrasound shows a trend of first decreasing and then increasing, while the friction coefficient of the coating under ultrasound is relatively stable. This is due to the uneven distribution of the coating structure and hardness without ultrasound. As the wear scar deepens, the hardness of different areas of the coating is different, and its wear resistance is different; under ultrasound, the hardness of the coating structure is evenly distributed, and the coating has a more consistent wear resistance.

Figure 8 shows the three-dimensional morphology of the wear scar after the WC particle reinforced coating is worn. It can be seen that the wear is mainly caused by abrasive wear. By comparing Figures 8(a) to (d), it can be seen that the wear scar of the sample without ultrasound has deeper grooves, with the maximum depth of about 53μm, and the wear scar of the sample with ultrasound has shallower grooves, only about 26μm, but there are still many uneven undulations at the bottom of the groove. Combined with the wear scar morphology, as shown in Figures 8(e) to (f), the sample without ultrasound has deeper grooves, indicating that it has a higher wear rate, which is consistent with the results of Figure 8.

In addition, compared with the sample without ultrasound, there are many fine peeling pits on the wear scar surface. A small amount of wear debris is further taken for morphological photography. It can be seen that the wear debris of the sample under ultrasound is finer than that without ultrasound. During friction and wear, the coating will peel off some of the abrasive particles, producing plowing and micro-cutting effects on the wear surface, leaving groove marks on the material surface, and the detached abrasive particles will press into the friction surface under the action of load, and squeeze out lamellar abrasive particles (Figure 8(g)), while forming furrows in the coating; under the action of ultrasound, the harder coating structure hinders the further wear of the coating by the abrasive particles, resulting in smaller abrasive particles (Figure 8(h)), and the furrows are relatively shallow.

2.4 Analysis of corrosion resistance of ultrasonic-assisted laser cladding composite coating

Figure 9 shows the dynamic potential polarization curve of WC particle-reinforced coating in 3.5% sodium chloride simulated seawater solution. The samples with and without ultrasound show similar curve trends. Corrosion begins at -0.40V and -0.39V, respectively, and enters the passivation zone. Subsequently, the passivation film is broken down and pitting occurs near 1.05V and 1.04V, respectively. The corrosion current density curve of the sample without ultrasound is higher than that of the sample with ultrasound, and is relatively large at each potential value. The corrosion current density is 5.20μA/cm² and 2.13μA/cm, respectively. The electrochemical corrosion parameters of the samples with and without ultrasound are shown in Table 3.

In order to further verify the effect of ultrasound on the corrosion resistance of WC particle reinforced coating, electrochemical impedance spectroscopy (EIS) analysis was carried out. The selected equivalent circuit model is R(Q(R(QR), as shown in Figure 10(c), where R, represents the uncompensated solution resistance, Rct represents the charge transfer resistance, R represents the passivation film resistance, CPE, and CPE2 represent constant phase elements. Figure 10(a) shows the Nyquist curve with and without ultrasound, which consists of two parts: the real part Z’ and the imaginary part Z”. The capacitance arc represents the corrosion reaction at the interface between the cladding layer and the electrolyte. The larger the radius of the capacitance impedance loop, the better its corrosion resistance. The cladding layer shows a larger capacitance arc under ultrasound, that is, better Corrosion resistance. Figure 10(b) is a Bode plot of the sample with and without ultrasound. In the low frequency band, the impedance modulus |Z| of the sample curve under ultrasound is higher and the phase angle value is larger, indicating that there is a denser passivation film at the corrosion interface; in the high frequency range, the sample curve under ultrasound also has a higher impedance modulus |Z|. Generally speaking, the larger the impedance modulus |Z|, the better the corrosion resistance of the material. The amplitude of the phase angle is directly related to the electrolyte penetration resistance of the material. A higher phase angle indicates that the material has enhanced penetration resistance and better corrosion resistance. Comparing the |Z| value and phase angle value of the sample with and without ultrasound, the WC particle reinforced coating under ultrasound has a stronger penetration resistance, that is, better corrosion resistance.

Due to the uneven distribution of tissue and elements in the sample without ultrasound, this uneven coating structure will produce tiny currents to aggravate corrosion and greatly reduce the corrosion resistance of the material. The introduction of ultrasonic vibration will not only evenly distribute the elements in the composite coating, but its refinement effect will also increase the grains and grain boundaries. Generally speaking, the increase in corrosion channels will reduce the corrosion resistance of the cladding layer, but the suppression of element segregation in the cladding layer by ultrasound improves the uniformity of the elements and reduces the number of local micro-galvanic cells. At this time, the higher density of grain boundaries accelerates the growth of the passivation film, making it easier to form a dense passivation film, hindering the anode reaction, and reducing the overall corrosion current. At the same time, the uniform tissue and element distribution increase the penetration resistance of the electrolyte, and ultimately improve its corrosion resistance.

3 Conclusions

In this paper, WC particle reinforced coating was prepared on the surface of 316L stainless steel by ultrasonic assisted laser cladding technology. The microstructure, hardness, wear resistance and corrosion resistance of the coating with and without ultrasonic action were compared and analyzed. The results are as follows:

1) In the cladding layer without ultrasonic, there are a large number of columnar crystals around WC, accompanied by some element segregation bands. Due to the ultrasonic cavitation effect, the average grain size around WC in the cladding layer with ultrasonic decreased from 101.0μm to 59.6μm, and there was no obvious segregation phenomenon;

2) The average microhardness of the WC particle reinforced coating without ultrasonic was 310HVo.1, and the hardness around WC decreased from 480HVo.1 to 320HVo.1. Due to the homogenization of the structure by ultrasound, the average microhardness of the WC particle reinforced coating was 425HVo.1, and the hardness around WC decreased from 426 HVo.1 to 413 HVo.1;

3) The weight loss and wear rate of the sample without ultrasound were 8.8mg and 0.0438mg/m respectively, and the weight loss and wear rate of the sample with ultrasound were 6.5mg and 0.0323mg/m respectively. The maximum depth of the wear scar plowing of the sample without ultrasound was about 53μm, and the wear scar plowing of the sample with ultrasound was only about 26μm. The uniform improvement of hardness by ultrasonic vibration significantly reduced the wear rate;

4) The corrosion current density of the electrochemical samples of the cladding layer with and without ultrasound was 2.13μA/cm² and 5.20μA/cm² respectively. Ultrasonic grain refinement accelerated the formation of passivation film and improved the corrosion resistance of the composite coating.