Abstract In order to improve the wear resistance and corrosion performance of heat exchanger tube components for solar thermal power generation, this paper carried out surface strengthening treatment on 321 austenitic stainless steel, which is widely used in heat exchanger components, to solve the problem of component failure caused by severe wear and corrosion. Ni60/WC alloy coating was prepared on the surface of 321 stainless steel substrate using high-speed laser cladding technology. Laser power, scanning speed and powder feeding rate were used as influencing factors, and hardness and dilution rate were used as characterization variables. The process parameters were optimized using single-factor experiments. The coating prepared with the optimal process parameters was selected, and its phase composition was analyzed by X-ray diffractometer, and its coating morphology and element distribution were studied by scanning electron microscopy. Finally, a set of optimal process parameter combinations were obtained by joint analysis of macroscopic morphology, XRD and SEM. Conclusion: The optimal process parameters are: laser power 1000W, scanning speed 10mm/s, powder feeding rate 3.5g·min-1. The coating phase is mainly composed of solid solution (γ-Fe, Ni), carbides M7C3 and M23C6, etc. The wear resistance of the coating is greatly improved. The average friction coefficient of the cladding layer is about 0.4, which is lower than the average friction coefficient of the substrate 0.8; the wear loss of the coating is 2.75mg, which is about 64% of the wear loss of the substrate 4.24mg. The self-corrosion potential of the coating is -0.674V, which is greater than the self-corrosion potential of the substrate -0.754V. The arc radius of the coating is significantly higher than that of the substrate, indicating that the coating can reduce the corrosion rate of the substrate; the maximum hardness value of the coating reaches 608HV0.2, which is about 1.91 times the hardness of the substrate, which is significantly higher than that of the substrate, indicating that the Ni60/WC alloy coating has good corrosion resistance. In summary, the Ni60/WC alloy coating significantly improves the wear resistance and corrosion resistance of the 321 stainless steel surface.
Keywords Laser cladding; Ni60/WC; microstructure; microhardness; friction and wear; electrochemical corrosion

1 Introduction

321 austenitic stainless steel is widely used in heat exchanger tube components for solar thermal power generation due to its titanium stabilization properties, excellent corrosion resistance and high strength [1][2]. 321 austenitic stainless steel is an alloy particularly suitable for the manufacture of automobiles, aircraft, chemical plants and nuclear power plants. In the manufacturing industry, the material selection for special structural applications is in many cases based on strength effects, but in some cases it is based on corrosion resistance effects. The deformation behavior of alloys, such as 321 austenitic stainless steel (ASS), used in applications requiring excellent corrosion resistance, under various mechanical loading conditions has not been well studied. Most of the research on these alloys focuses more on their behavior in corrosive and high temperature environments [3][4].

Currently, traditional surface cladding processes include flame thermal spraying, plasma cladding, deposition welding, etc. When thermal spraying technology is used to prepare stainless steel surface coatings, the surface pretreatment process of the substrate is cumbersome; the deposition welding technology will cause serious overburning on the surface of 321 stainless steel, resulting in certain changes in the substrate structure. These traditional cladding processes have certain limitations, such as large heat-affected zone and large deformation, low geometric accuracy, and poor surface quality. Traditional cladding processes can no longer meet the requirements of part surfaces [5].

Laser cladding technology is a surface modification technology that uses a high-energy-density laser beam to melt the coating material and fuse it with the substrate surface, thereby forming a cladding layer with specific properties on the substrate surface [6]. The high-energy-density laser beam is focused on the surface of the substrate material, causing the local area to heat up rapidly. The substrate surface and the coating material reach the melting point at the same time to make them evenly distributed, forming a coating with high wear resistance and corrosion resistance, ensuring good bonding strength with the substrate. This technology has shown significant advantages in improving part performance and extending service life.

In recent years, LI et al. [7] used laser cladding technology to generate a cladding layer with wear resistance, corrosion resistance, and oxidation resistance on the surface of the substrate material. Studies have shown that the wear rate of Ni-based composite coatings is low and the wear resistance is significantly improved. QI et al. [8] prepared a high wear-resistant alloy coating by laser under the assistance of a magnetic field, proving that laser cladding can significantly improve the performance of the substrate. At present, the commonly used cladding powders in laser cladding technology include Fe alloys [9], Nickel alloys [10] and Cobalt alloys [11] [12]. Zhou Jianbo et al. [13] showed that laser process parameters have a significant effect on the internal cracks of the Ni60/WC cladding layer, and the grain size increases with the increase of laser power. Huang Haibo et al. [14] found that through the planning and analysis of orthogonal experiments, the results showed that the maximum hardness of the Ni60 coating reached 3.3 times that of 45 steel. The uniform and dense organizational structure effectively inhibits the formation of cracks and enhances the strengthening effect of the coating on the substrate. Zhao et al. [15] found that the microstructure of the coating was optimized with the increase of laser power and the control of low heat source input. This regulation not only helps to improve the density of the coating, but also reduces the formation of cracks and improves the strengthening effect of the coating on the substrate. In this paper, a five-axis laser deposition additive/high-speed subtractive composite manufacturing equipment produced by Shandong Leishi Company is used to high-speed laser clad Ni60 and WC powders on a 321 stainless steel substrate to analyze the microstructure, properties, wear rate and corrosion of the cladding layer.

2 Experiments

2.1 Experimental materials and equipment

Alloy 321 (UNS S32100) is a titanium-stabilized austenitic stainless steel that is particularly suitable for high-temperature environments. Its corrosion resistance is excellent in the range of 800-1500°F (427-816°C), and it can effectively resist intergranular corrosion caused by chromium carbide precipitation. Compared with 304 and 304L stainless steels, 321 still maintains good oxidation resistance up to 1500°F (816°C) and exhibits better creep and stress rupture properties. In addition, the low-temperature toughness of 321 stainless steel is also quite good. The addition of titanium significantly enhances its ability to resist intergranular corrosion and helps to inhibit the formation of chromium carbide, so it is more suitable for use under high temperature and high pressure conditions. The physical properties of 321 stainless steel are shown in Table 1.

The experimental substrate uses a heat exchanger tube assembly widely used in solar energy. Its material is 321 stainless steel and its size is 170mm×10mm×70mm. The chemical composition is shown in Table 2. In order to avoid the influence of rust and surface impurities on the experimental results, 600, 800, and 1000 purpose Sci sandpaper is used for grinding and aqua regia is used to clean the surface of the substrate to keep its surface smooth and minimize the influence factor. The prepared cladding coating materials are mainly Ni60 (particle size: 50~150μm) and WC (particle size: 100-150μm) powders, and their chemical composition elements are shown in Table 3. The coating powders used are Ni60 and WC, and a mixed powder experiment is required first.

2.2 Powder mixing experimental method

Use a horizontal mixer to place the powder in a horizontal container, and mix the Ni60 powder with a mass fraction of 0.75 and the WC powder with a mass fraction of 0.25, and mix the powders with a stirring blade. After the powder treatment is completed, the drying oven temperature is set to 120℃ for 1.5-3 hours until the powder is dried. The purpose is to avoid uneven distribution of the powder and pores caused by liquid vaporization, which interfere with the quality of the coating. After mixing, the powder is separated from the stirring blade to obtain a mixed powder of 0.75 Ni60 and 0.25 WC. The morphology of the Ni60+25%WC composite powder is shown in Figure 1.

2.3 Laser cladding experimental method

Figure 2 shows the HMC-320A equipment for the additive and subtractive laser cladding experiment. It is a five-axis laser deposition additive/high-speed subtractive composite manufacturing equipment produced by Shandong Leishi Intelligent Manufacturing Co., Ltd., which is an advanced manufacturing technology. The main parameters of the equipment are shown in Table 4. It combines laser cladding additive manufacturing with high-speed cutting material manufacturing, which can realize efficient and high-precision manufacturing of complex parts, and can also perform high-speed laser cladding technology. The equipment consists of a fiber laser, a double-barrel gas-carrying powder feeder, a control cabinet, a chiller, a cladding head, etc.

Ni60/WC powder coating cladding experiment was carried out. The laser power changed from A1 to A5, the scanning speed changed from B1 to B5, and the powder feeding rate changed from C1 to C5. The influence of macroscopic morphology and dilution rate was analyzed to select the optimal experimental process parameters. The specific experimental parameters are shown in Figure 5 below.

After the cladding experiment, the sample was cut into 10mm×10mm×10mm cubes along the vertical scanning direction using an electric spark wire cutting device. Subsequently, the phase composition of the cladding layer was analyzed by an X-ray diffractometer [16]. The microstructure and elemental composition were analyzed by scanning electron microscopy and energy dispersive spectrometer. The hardness test of the polished sample cross section was carried out using a TMHV-1000Z micro Vickers hardness tester under the conditions of 0.2N load and 10 seconds holding time. Finally, the friction and wear test was carried out using an MFT-5000 wear tester. The experimental parameters included a load of 60N, a test time of 25 minutes, a frequency of 2 times/second, and a silicon nitride ball as the friction pair.

The purpose of the electrochemical corrosion experiment is to evaluate the corrosion resistance of metals under specific environmental conditions[17]. In this study, a multi-channel electrochemical workstation device was used for the experiment. First, a metal sample with a size of 10mm×10mm×10mm was cold-embedded in epoxy resin to expose an area of ​​1cm² to be tested. The sample was connected to the electrochemical workstation via a copper wire. The sample was used as the working electrode, the saturated calomel electrode was used as the reference electrode, and the platinum electrode was used as the counter electrode. A 3.5% NaCl solution was used as the electrolyte in the experiment.

The AC impedance test was set to an AC amplitude of 10mV, and the test frequency range was 100Hz to 100kHz. The scanning speed of the dynamic potentiodynamic polarization curve test was set to 4mV/s, and the potential range was -0.5V to -1.5V. Through these experiments, the impedance frequency and dynamic potentiodynamic scanning data of 321 stainless steel were measured and analyzed to simulate the complex working conditions of mechanical parts in a corrosive environment, providing a scientific basis for the corrosion resistance of 321 stainless steel.

3 Analysis of single-pass cladding layer

3.1 Macromorphology of single-pass laser cladding

As shown in Figure 3, the specific fifteen-pass single-pass cladding characterization of the experiment is as follows. The coating surface is well formed as a whole. The characterization can clearly show the smoothness of the surface, without particularly prominent cracks and pores, indicating that the laser power used for the substrate and powder is appropriate, the powder feeding rate is uniform, and the scanning speed is moderate.

The selection of laser cladding process parameters is crucial to the final coating quality, and directly affects the microstructure, mechanical properties and surface characteristics of the cladding layer. Appropriate process parameters can ensure the uniformity, density and metallurgical bonding of the coating with the substrate, and avoid the generation of defects such as cracks and pores. Laser power, scanning speed and powder feeding rate are three critical parameters in the cladding process, and there is a certain relationship between these parameters and energy. As shown in the following formula (1): See formula (1) in the figure. In formula (1), ε represents specific energy, Q represents laser power (W), D represents spot diameter (mm); v represents scanning speed (mm.s-1). It can be seen that the specific energy is proportional to the laser power, while the spot diameter and scanning speed are inversely proportional to it.
The cladding layer is calibrated with the help of a rangefinder such as a vernier caliper to calibrate the cladding layer’s melt width Q, melt height L, and melt depth l, and the dilution rate value is analyzed according to the dilution rate formula. Formula (2) is the calculation formula for the dilution rate, where δ is the dilution rate, l is the cladding depth, and L is the cladding height.
The calibration method is shown in Figure 4. A high dilution rate usually leads to a decrease in the hardness and strength of the cladding layer, because the components of the base material are incorporated into the coating, which may weaken the wear resistance and corrosion resistance of the coating. On the contrary, an appropriate dilution rate helps to maintain the excellent performance of the coating.

3.2 Effect of single factor three-level experiment on the quality of laser cladding coating

Figures 5(a) and 5(b) show the effect of the change of laser power from A1 to A5 on the coating range and dilution rate. The effect of laser power on the dilution rate is very important in the laser cladding process. The increase of laser power increases the temperature of the molten pool, resulting in a deeper melting of the base material and the cladding material. This usually causes more base material to be incorporated into the cladding layer, thereby increasing the dilution rate. Controlling the laser power is crucial to obtaining an ideal dilution rate. The power setting needs to be optimized in the experiment to control the dilution rate within a reasonable range while ensuring the coating quality [18]. Figure 5 (b) shows that the dilution rate reaches the lowest value when the laser power is set to 1000W.

Figures 6(a) and 6(b) show the effect of the change in scanning speed from B1 to B5 on the coating range and dilution rate. When the scanning speed is reduced, the laser beam stays at the same position for a longer time, which will cause the molten pool heating time to be extended, so that the cladding material melts more thoroughly and forms a wider coating area. On the contrary, when the scanning speed is increased, the laser beam stays at each point for a shorter time, the molten pool temperature is insufficient, resulting in a lower melting degree of the cladding material, which makes the coating range smaller. Lower scanning speeds will cause the molten pool temperature to increase and the amount of substrate material melted to increase, which may increase the dilution rate of the coating. In this case, the proportion of substrate material in the cladding layer is higher. Increasing the scanning speed usually reduces the molten pool temperature, reduces the melting and mixing degree of the substrate material, and thus reduces the dilution rate, making the coating composition closer to the cladding material itself [19]. Figure 6(b) shows that the dilution rate reaches the minimum value when the scanning speed is 10 mm/s.

Figures 7(a) and 7(b) show the effect of changing the powder feeding rate from C1 to C5 on the coating size and dilution rate. An increase in the powder feeding rate results in a decrease in the molten pool depth and an increase in the cladding layer height. When the powder feeding rate increases, more coating material is fed into the molten pool. This may lead to an increase in coating thickness and thus an increase in coating size. A high powder feeding rate helps to form a thicker coating, especially in the laser cladding process, where the increased amount of material can effectively fill the molten pool. In contrast, a lower powder feeding rate results in an insufficient supply of cladding material, resulting in a reduced coating thickness and a relatively small coating size. Increasing the powder feeding rate results in the cladding material occupying a larger proportion in the molten pool, which can reduce the mixing degree of the substrate material and thus generally reduces the coating dilution rate. In this way, the composition of the coating will be closer to the cladding material itself, improving the coating performance. A lower powder feeding rate may result in the substrate material occupying a larger proportion in the molten pool, thereby increasing the dilution rate. This may result in a decrease in coating performance because the composition of the substrate material may weaken the characteristics of the cladding layer. Figure 7(b) shows that the dilution rate reaches the minimum when the powder feeding rate is 3.5 g·min-1.

In summary, combined with the three factors of laser power, scanning speed and powder feeding rate that affect the dilution rate, it can be concluded that A2, B3, and C4 are the optimal process parameters for the fifteen high-speed laser cladding processes A1-C5, with a laser power of 1000w, a scanning speed of 10mm/s, and a powder feeding rate of 3.5 g·min-1.

3.3 Microhardness analysis

Figure 8 shows the microhardness distribution along the layer depth direction of the substrate section under the optimal process parameters of A2. To avoid the influence of defects, the distance between the two indentations is set to 100μm. The hardness of the heat affected zone (HAZ) is usually lower than that of the cladding layer because the substrate material undergoes phase transformation or grain coarsening at high temperature, resulting in a decrease in its microhardness. The hardness of the heat affected zone is related to the composition, heat treatment state, and cooling rate of the substrate material. The microstructure of the heat-affected zone may change due to high temperature, such as grain growth, phase transformation, and formation of precipitates. These changes will directly affect the mechanical properties of the material, including hardness. Thermal cycles during laser cladding may cause residual stress in the heat-affected zone, which may also have an indirect effect on the microhardness of the material. High residual stress may cause hardness changes in the material at the microscale. The highest hardness value of the coating reached 608HV0.2, which is about 1.91 times the hardness of the substrate[20].

3.4 Microstructure

The microstructure of laser cladding coatings is usually composed of multiple phases and exhibits a fine dendritic crystal structure. During the cladding process, the rapid cooling rate leads to the formation of fine grains, which are evenly distributed during the cladding process. Different phases may also appear in the coating, such as reinforcement phase and matrix phase, which enhance the mechanical properties of the coating. In addition, the microstructure of the interface area usually shows good bonding, reducing defects between the coating and the substrate, thereby improving wear resistance and corrosion resistance. The cladding layer in Figure 9 shows a typical dendrite structure. Due to the large contact surface of the laser head at the bottom, the crystal grows preferentially along the heat flow direction, forming a coarse and ordered structure. In the middle area, the dendrites are more obvious and the morphology is more significant, which indicates that the undercooling in the middle is low and the temperature gradient is moderate, which is conducive to the good growth of dendrites. The smaller temperature gradient and larger undercooling in the top area limit the further growth of dendrites, but still maintain a uniform and compact morphology [21].

3.5 Phase analysis

Figure 10 shows the XRD diffraction pattern of the Ni60+25% cladding layer. It can be seen that WC melts and decomposes into W and C elements during the laser cladding process. The changes in the dissolution and carbonization reactions in the molten pool are as follows: See formulas (3) and (4) in the figure.

In the XRD spectrum, the characteristic diffraction peak of WC should appear at a specific 2θ angle, usually around 35° to 40°. The diffraction peaks of WC are marked in the figure (indicated by ▲), which are displayed near 40° and 70°. The ▲ in the figure marks the diffraction peaks of WC, but some diffraction peaks may change in position or weaken in intensity, which is due to the decomposition and recrystallization of WC to form new tungsten and carbon-based compounds.

The melting and carburization reaction of WC in the molten pool generates hard phases such as WC, W2C, W2B5, M7C3, and M23C6, which become the main source of matrix performance enhancement. Each cladding layer is mainly composed of symbol λ (Fe, Ni) solid solution, and its face-centered cubic structure helps to improve corrosion resistance. WC and W2C, as particle reinforcement phases, can effectively improve the microstructure of the cladding layer, inhibit dislocation slip and grain boundary movement, and thus achieve fine grain strengthening. The precipitation of M7C3 and M23C6 carbides acts as an obstacle to limit the activity of grain boundaries and reduce the grain boundary energy, thereby effectively reducing the risk of grain boundary corrosion and improving the hardness and corrosion resistance of the cladding layer [22].

Figure 11 shows the EDS element surface scan results of the coating under the optimal parameters of the cladding layer. It can be clearly seen from the figure that the four elements Fe, Co, Mo, and Cr are evenly distributed without obvious segregation. This is because the laser cladding energy density is high and the elements do not have enough time to disperse during the cladding process; Ni, Si, Mn, Al, and Wc have obvious segregation. On the one hand, this may be because during the laser cladding process, the high energy density causes the elements to melt and solidify rapidly, and the low content of the elements makes it difficult for them to diffuse effectively in a short time, and the effect of uniform distribution cannot be achieved. On the other hand, the mixing entropy of Ni, Si, Mn, Al, and Wc is quite different and they are easy to react at high temperatures, making them more likely to segregate. A large number of literatures have also reached this conclusion [23][24].

4 Analysis of wear resistance and corrosion performance of cladding coating

4.1 Analysis of friction and wear performance

In industry and manufacturing, the wear resistance of parts is extremely important. Friction and wear performance analysis mainly involves the ability of materials to resist wear during friction. It is usually evaluated by parameters such as friction coefficient, wear volume and wear mechanism. The size of the friction coefficient directly affects the resistance of the material on the friction contact surface. A lower friction coefficient usually means better lubrication and less energy loss. As can be seen from Figure 12(a), the difference in friction and wear performance between the 321 stainless steel substrate and the Ni60/WC coating changes with time. For 321 stainless steel, it shows significant fluctuations within 400 seconds, indicating that the abrasive wear is more serious during the running-in period, resulting in an uneven and rough surface and constantly changing resistance. After entering the stable wear stage, the friction coefficient stabilizes at about 0.7~0.8, which indicates that the wear of the stainless steel substrate is more serious and the surface is difficult to keep flat.
The friction coefficient of the Ni60/WC coating slowly increased from about 0.2 to about 0.3~0.4 in the first 200 seconds, indicating that its surface is easier to reach a relatively stable state during the running-in period, reducing the initial severe wear. After entering the stable stage, the friction coefficient of the coating remained at about 0.4 with a small fluctuation range, indicating that the wear resistance of the Ni60/WC coating is significantly better than that of 321 stainless steel, and can more effectively reduce friction resistance and wear. This phenomenon is mainly attributed to the excellent metallurgical bonding of the coating, fewer defects in the microstructure, and the uniform distribution of hard phase components, which makes it difficult for the friction coefficient to stabilize quickly in the early stage. Figure 12(b) shows that after 30 minutes of wear test, the mass loss of the Ni60/WC coating is 2.75mg, which is only 64% of the mass loss of the substrate (4.24mg), indicating that the coating has better wear resistance.

4.2 Corrosion performance analysis

In Figure 13(a), the open circuit potential (OCP) curves of the substrate and coating in 3.5wt% NaCl solution are shown. During the 1200-second experiment, the OCP values ​​of the substrate and coating tended to be stable. The larger the negative value of OCP, the easier it is for the coating to lose electrons and corrode; relatively speaking, the smaller the negative value, the lower the corrosion tendency. The positive movement of OCP indicates that a protective film has formed on the surface of the sample, while the rupture of the protective film corresponds to the negative movement of OCP. The OCP of the substrate is -0.47V, while the OCP of the coating is -0.38V. The higher OCP value of the coating indicates that a tighter protective film has formed on its surface. Figure 13(b) shows the polarization curves of the substrate and the coating. The anodic polarization area shows a significant improvement in corrosion resistance. This is mainly because the active metal ions react with the ambient gas through adsorption to form a dense protective film, which significantly reduces the corrosion rate.

Table 6 shows the intrinsic corrosion potential (Ecorr) and intrinsic corrosion current density (Icorr) data obtained by Tafel linearization electrochemical data analysis. The intrinsic corrosion potential of 321 stainless steel substrate is -0.754V, and the intrinsic corrosion current density is 6.77×10-7A.cm-2; the intrinsic corrosion potential of Ni60/WC coating is -0.674V, and the intrinsic corrosion current density is 4.76×10-8 A.cm-2. It can be seen that the corrosion resistance of the coating is significantly better than that of the substrate, and its corrosion rate is also significantly lower than that of the substrate. This shows that the Ni60/WC coating can effectively improve the corrosion resistance of the substrate. Figure 13(c) is the impedance arc diagram of the substrate and the cladding layer. It can be seen that the larger the impedance arc radius, the better the corrosion resistance of the material. In the frequency range shown, the impedance arc radius of the cladding layer is much larger than that of the substrate, which indicates that the cladding layer has better corrosion resistance.

Figure 13(d) shows the impedance radiation frequency diagram of the substrate and the cladding layer. In the impedance frequency diagram, there is an approximately linear functional relationship between lg|Z| and lg (Frequency). The larger |Z| is, the less the cladding layer is corroded and the better the pitting resistance is. It can be seen from Bode that the impedance modulus of the cladding layer is greater than that of the substrate, which indicates that the coating has better corrosion resistance. The phase angle of the cladding layer reaches its maximum value in the mid-frequency range of 100-102, and the phase angle of the cladding layer is significantly greater than that of the substrate, which indicates that the corrosion rate of the cladding layer is reduced. This is because the Fe and Ni content is high, and the Fe and Ni elements have strong corrosion resistance, which can prevent the ions in the corrosive liquid from penetrating into the coating. In this frequency range, the impedance modulus of the coating is always higher than that of the substrate, which means that the corrosion rate is significantly reduced, and the corrosive ions in the corrosive liquid are difficult to penetrate the passivation film into the coating.

In order to better describe the corrosion performance of the coating and the substrate, a similar model was used to analyze the corrosion behavior and impedance response characteristics of the substrate and the cladding layer. The equivalent circuit structure of the cladding layer and the substrate is shown in Figure 14. In this model, R1 represents the resistance of the NaCl solution, CPE1 represents the constant phase element of the capacitance between the protective film and the solution, and R2 represents the resistance of the protective layer on the substrate surface. The size of R1 is related to the conductivity of the electrolyte, and the density or porosity of the cladding layer also affects the impedance of this part. The change of R2 indicates the protective effect of the cladding layer on the substrate. The larger the R2, the greater the charge transfer resistance and the better the corrosion resistance of the material. The parameters of CPE1 can reflect the roughness of the cladding surface and its microstructural characteristics. The CPE index obtained by fitting can be used to describe the degree of non-ideal capacitance behavior of the electrode surface.

5 Conclusion

In this paper, Ni60/WC alloy coating was prepared by high-speed laser cladding technology, and it was analyzed in detail by characterization methods such as XRD, SEM and EDS. At the same time, the hardness, wear resistance and corrosion resistance of the coating were tested. The main conclusions are as follows:
1) A single-factor experiment was carried out using the control variable method to study the single-pass cladding layer. The results showed that the surface of the cladding layer showed uniform molten ripples without defects such as cracks or pores. Taking the dilution rate and hardness as the response indicators, combined with the macroscopic morphology of the cladding layer, the optimal process parameters were determined: laser power of 1000W, powder feeding rate of 3.5g·min-1, and scanning speed of 10mm/s.

2) The Ni60/WC alloy coating has a typical dendritic structure. The grain size at the top of the cladding layer is small and uniformly dense; the grains in the middle have undergone secondary growth, are of moderate size, and are evenly distributed; the grains at the bottom are relatively large, cellular, and arranged in an orderly manner. The hardness test results show that the maximum hardness of the cladding layer is 608HV0.2, which is about 1.91 times the hardness of the substrate, and the overall hardness of the coating is significantly improved. XRD analysis shows that hard phases such as WC, W2C, W2B5, M7C3, and M23C6 are generated in the cladding layer. The presence of these compounds effectively improves the hardness of the coating.

3) Friction and wear performance tests show that the friction coefficient of the substrate is about 0.8, while the friction coefficient of the cladding layer is about 0.4. The wear amount of the cladding layer is 2.75mg, which is about 64% of the wear amount of the substrate 4.24mg, indicating that the wear resistance of the cladding layer is significantly improved. This is mainly attributed to the dense dendrite structure and hard phase components in the cladding layer, which effectively reduce wear.

4) The electrochemical test results show that the open circuit potential (OCP) of the cladding layer is -0.38V, while the OCP of the substrate is -0.47V, which is slightly higher than that of the substrate. The self-corrosion current density of the cladding layer is 4.76×10⁻⁸ A.cm², which is significantly lower than that of the substrate (6.77×10⁻⁷ A.cm²). The self-corrosion potential of the cladding layer is -0.674V, which is higher than that of the substrate (-0.754V). In addition, the capacitance ring radius of the cladding layer is significantly larger than that of the substrate, and the impedance value of the cladding layer is also higher than that of the substrate at the same frequency, indicating that the corrosion resistance of the Ni60/WC alloy coating is better than that of the substrate, which effectively improves the corrosion resistance of 321 stainless steel.