In order to explore the effect of WC particles on the microstructure and properties of cladding coatings, FeCoCrNiMn-xWC high entropy alloy cladding coatings were prepared on the surface of NM450 steel using a laser power of 1 200 W and a scanning speed of 6 mm/s. The phase, microstructure, mechanical properties and wear resistance of the coatings were studied by X-ray diffractometer (XRD), scanning electron microscope (SEM), Vickers microhardness tester and friction and wear tester. The results show that when WC particles are added to the FeCoCrNiMn high entropy alloy coating, the microstructure of the high entropy alloy composite coating is mainly FCC and BCC phases, containing a small amount of WC, W2C and Cr7C3 phases, and the microstructure is columnar crystal and cellular crystal structure. The composite coating with 10% WC has the best comprehensive performance, with the microhardness reaching a maximum value of 484.5 HV0.3; the friction coefficient is 0.58, and the wear loss and wear rate are the lowest at 0.011 4 g and 0.857×10-5 g/(N·m), respectively. The wear mode of the composite coating is mainly abrasive wear and oxidative wear, accompanied by adhesive wear.

High entropy alloys have become the research focus of new materials due to their advantages of high strength, high hardness, wear resistance, corrosion resistance and high temperature resistance. A notable feature of high entropy alloys is the diversity of their elements. Unlike traditional alloys, which usually have only one or two main metal elements, high entropy alloys have a large number of constituent elements, and the atomic proportion of each element is at a high proportion, usually 5%~35%. Although high entropy alloys contain multiple metal elements, they can form a simple solid solution phase and have better performance than traditional alloys. High entropy alloys have many excellent properties, such as high strength, high hardness, good wear resistance, high temperature resistance, and excellent corrosion and oxidation resistance. These characteristics make high entropy alloys have broad application prospects in aerospace, automobile, petrochemical, electric power, biomedicine and other fields. By laser cladding, high entropy alloy coatings that are well bonded to the substrate are prepared, and the advantages of both are combined to promote the further application of high entropy alloys in industrial production. For example, in the field of aerospace, high entropy alloys can be used to manufacture high-temperature components and corrosion-resistant components; in the field of petrochemicals, they can be used to manufacture corrosion-resistant pipes and equipment; in the field of coal mining machinery, they can be used to manufacture parts with high-strength wear-resistant coatings.

Laser cladding technology can achieve local rapid heating and melting, reducing the waste of raw materials and simplifying the process flow; laser cladding technology has the characteristics of rapid cooling, making the prepared coating grain structure fine and evenly distributed, which helps to improve the density and performance of the coating, such as hardness, wear resistance, corrosion resistance, etc.; during the laser cladding process, a metallurgical bond is formed between the coating and the substrate, which significantly improves the bonding strength between the coating and the substrate, helps to extend the service life of the coating and reduce the phenomenon of coating shedding and cracking; laser cladding technology can repair and modify the surface of failed parts, which helps to reduce resource waste and environmental pollution and achieve sustainable development.

In recent years, strengthening the composite coating of high entropy alloys by adding hard particles has become a hot topic of research. Common hard particles include WC, TiC and SiC. Among them, WC has the advantages of high hardness, good thermal stability and good wetting with metals. WC particles can effectively enhance the strength, hardness and wear resistance of high entropy alloy composite coatings. In this paper, laser cladding technology is used to study FeCoCrNiMn high entropy alloy. The influence of adding different contents of WC on the phase composition, microstructure, microhardness and wear resistance of high entropy alloy coating is studied. By adjusting the amount of WC added, a FeCoCrNiMn-xWC high entropy alloy composite coating with good performance is prepared, and it is applied to the preparation of wear-resistant coating on the surface of the middle trough of coal mine scraper conveyor.

1 Experimental materials and methods
(1) Test substrate The test substrate was NM450 steel. To ensure that the sample surface was free of impurities, the sample surface was first polished with sandpaper, then ultrasonically cleaned, and finally dried before testing.
(2) Powder material The test selected FeCoCrNiMn high entropy alloy powder as the cladding substrate material. The chemical composition is shown in Table 1. The powder particle size is 45~105 μm. WC ceramics were selected as the reinforcement phase particles. In the cladding test, a dual-channel powder feeder was used to adjust the addition amount of WC ceramics in real time to ensure the smooth progress of the test. FeCoCrNiMn-xWC alloys with WC mass fractions of 0, 5%, 10%, 15% and 20% were designed according to the selected powders. The composition is shown in Table 2.
(3) Coating preparation The laser cladding process parameters used in the experiment are: laser power of 1 200 W, defocus of 15 mm, scanning speed of 6 mm/s, 99.99% argon protection during the cladding process, and the argon flow rate of 15 L/min. The experiment is designed to have 5 groups of samples, and the 5 groups of samples are tested separately. The coating thickness of each group of samples is 1 mm.
(4) Coating characterization After the cladding is completed, the test sample is cut perpendicular to the cladding direction using wire cutting. After cutting, the sample surface is lightly polished to remove the oil stains left during cutting, and the sample surface impurities are ultrasonically cleaned in an ultrasonic machine to make the sample completely clean and eliminate interference with subsequent tests. The macroscopic morphology of the coating was observed using a RY-7045 stereo microscope. The sample was corroded with aqua regia (the molar ratio of HCl to HNO3 was 3:1) for 10-20 s. The microstructure of the coating was observed using a JSM-5610LM scanning electron microscope (SEM). The coating phase was analyzed using a D/max2500 X-ray diffractometer (XRD). The scanning angle was 20°-100°, the scanning step was 0.05°, and the scanning speed was 4°/min. The hardness of the sample was tested using a PCHVT-1000Z visual microscopic Vickers hardness tester. The loading load was 300 g and the holding time was 10 s.

The friction and wear characteristics of the coating were measured using a GHT-1000EM friction and wear tester. The friction and wear specimens were ground flat in advance and polished until there were no obvious scratches. The friction pair material was quenched and tempered GCr15 steel. The load was fixed at 300 g, the test time was 1 800 s, the motor speed was 450 r/min, the friction diameter was φ6 mm, and the motor frequency was 17.8 Hz. After the test, the three-dimensional morphology of the wear marks on the surface of the specimen was observed using a stereo microscope.
The coating is characterized by the ratio of the wear amount to the work done by the load, ω = M/FS (1)
Where M is the wear amount, g; F is the test load, N; S is the total friction distance, S = 169 646 mm.

2 Experimental results and analysis
(1) Macromorphology of cladding coating
The macromorphology of the cladding coating surface is shown in Figure 1. The surface morphology of the cladding coating is well formed and the surface is flat. No defects such as cracks and holes are found. With the increase of WC content, powder sticking and agglomeration occur on the surface. Analysis shows that with the increase of WC content, the powder fluidity decreases and the surface cladding temperature decreases. Another part is caused by the splash of the molten pool.
(2) Phase analysis of cladding coating
The XRD spectrum of the cladding coating is shown in Figure 2. As shown in Figure 2, the FeCoCrNiMn-xWC composite coating is mainly composed of FCC phase and BCC phase structure. It can be clearly seen that with the increase of WC addition, the diffraction peak of the FCC phase increases and the diffraction peak of the BCC phase decreases. When the WC addition reaches 10%WC, the diffraction peak of the BCC phase almost disappears completely. WC particles may precipitate from the coating matrix as precipitates. These precipitated WC particles will form additional strengthening phases in the coating, improving the hardness and wear resistance of the coating. Precipitation strengthening will change the composition and distribution of the coating phase structure, thereby affecting the overall performance of the coating. Increasing WC will change the microstructure and phase composition of the heat-affected zone, because the high melting point and thermal stability of WC will affect the formation and evolution of the heat-affected zone. This change in the heat-affected zone will further affect the formation and performance of the coating phase structure. Secondly, WC particles will dissolve in the lattice of the coating matrix to form a solid solution, thereby improving the hardness and strength of the matrix.
(3) Microstructure analysis of cladding coating
The microstructure of the cladding coating is shown in Figure 3. As shown in Figure 3 (a), when WC particles are not added, the coating is mainly equiaxed crystals, the lengths of the crystals in all directions are roughly equal, and the spacing between the crystals is small; as shown in Figures 3 (b) and 3 (c), when 5% WC and 10% WC are added, a small amount of unmelted WC particles begin to appear in the composite coating crystals. When the equiaxed crystals become finer, they transform into columnar dendrites, and the microstructure grains become finer. After adding 10% WC, the composite coating is significantly refined; as shown in Figures 3 (d) and 3 (e), when 15% WC and 20% WC are added, the columnar crystals of the composite coating decrease, and the microstructure is mostly cellular crystals. This shows that the increase in the number of WC particles is conducive to the refinement of the alloy structure, and the interaction between the WC particles and the matrix will also promote fine grain strengthening.
(4) Hardness analysis of cladding coating The cross-sectional microhardness of the cladding coating is shown in Figure 4. The hardness of the FeCoCrNiMn-xWC composite coating has been significantly improved after adding WC particles. When WC particles are not added, the average microhardness of the coating is 393.8 HV0.3; when the WC content is 5%, 10%, 15%, and 20%, the average microhardness of the composite coating is 431.9 HV0.3, 484.5 HV0.3, 450.6 HV0.3, and 430.1 HV0.3. This is because the high hardness of WC itself can effectively improve the hardness of the high entropy alloy composite coating. Secondly, during cladding, some WC particles will generate C elements due to high temperature cracking, and the carbides (Fe3C, Cr7C3, W2C) generated by C elements and Fe, Cr, W and other elements also promote the improvement of the microhardness of the coating.
(5) Tribological analysis of cladding coating The friction coefficient-time curve is shown in Figure 5. When WC is not added to the cladding coating, the average friction coefficient of the composite coating is 0.69; when WC particles are added with a mass fraction of 5%, the friction coefficient of the composite coating is 0.72; when WC particles are added with a mass fraction of 10%, the average friction coefficient of the composite coating is the smallest, which is 0.58; when WC particles are added with a mass fraction of 15%, the average friction coefficient of the composite coating is 0.86; when WC particles are added with a mass fraction of 20%, the average friction coefficient of the composite coating is 0.59.

When WC is added to the coating, it can significantly increase the hardness of the coating. When subjected to external wear, the high-hardness coating can more effectively resist the cutting and scratching of wear particles, thereby improving wear resistance. Adding WC can also refine the grain size of the coating, thereby improving the strength and hardness of the coating. The refined grains can increase the resistance to dislocation sliding and improve the wear resistance of the coating. With the increase of WC content, the friction coefficient tends to rise. This is because too many WC particles may weaken the bonding force between the coating and the substrate. When subjected to external wear, the coating is more likely to peel off from the substrate, thereby reducing the wear resistance.
The wear rate of each cladding layer is calculated according to formula (1), and the wear amount and wear rate bar graph of FeCoCrNiMn-xWC composite coatings with different WC contents are drawn, as shown in Figure 6. The wear rate of the FeCoCrNiMn cladding coating without WC particles is 1.308×10-5 g/(N·m), the wear rate of the 5%WC composite coating is 1.278×10-5 g/(N·m), the wear rate of the 10%WC composite coating is 0.857×10-5 g/(N·m), the wear rate of the 15%WC composite coating is 0.917×10-5 g/(N·m), and the wear rate of the 20%WC composite coating is 0.910×10-5 g/(N·m). Among them, the wear amount and wear rate of the 10%WC composite coating are the lowest, and the wear resistance is the best.
The micromorphology of the coating wear scar after the friction and wear test is shown in Figure 7. Figure 7 (a) shows that without adding WC, the wear scar morphology of the composite coating shows strong adhesion, the surface adhesive material is obvious and mainly adhesion, and the main wear mode is adhesive wear; Figure 7 (b) shows the wear scar morphology of the 5% WC composite coating. The addition of trace WC has an obvious wear-reducing effect on the composite coating, reducing the peeling of the coating, and there are obvious plowing and metal oxides in the wear area; Figure 7 (c) shows the wear morphology of the 10% WC composite coating, in which the plowing is reduced and the peeling is increased; Figure 7 (d) shows the wear morphology of the 15% WC composite coating, in which peeling and friction plowing are visible, and the metal oxide on the surface of the composite coating increases; Figure 7 (e) shows the wear scar morphology of the 20% WC composite coating. When more WC particles are added, the delamination and shedding phenomenon in the wear area of ​​the coating is significantly reduced, and the volume of the pits is also reduced. Cr can form carbides such as Cr7C3 and Fe3C with elements such as Fe and C, and form Cr2O3 with solid lubrication with O. WC will form W2C solid solution after decomposition, which improves the wear resistance of the cladding layer. In summary, combined with the tribological theory analysis, the wear form of the composite coating is mainly abrasive wear and oxidative wear, accompanied by adhesive wear.

3 Application
The results of this paper have been used in the production of the surface coating of the middle groove of the SGZ800/1710 type scraper conveyor for coal mine transportation of Xi’an Heavy Equipment Pubai Coal Mine Machinery Co., Ltd., and the coating thickness reached 3 mm. After an industrial test of 240 days in the coal mine, the wear thickness of the middle groove was 3~5 mm, while the wear thickness of the wear-resistant plate NM450 was 5~10 mm, and its wear resistance was greatly improved.

4 Conclusion
(1) The addition of WC particles significantly changed the microstructure of the coating. The microstructure of the FeCoCrNiMn-xWC cladding coating is mainly composed of equiaxed crystals and columnar dendrites. With the increase of WC content, WC particles and BCC phases also increase, and the microstructure of the coating is significantly refined. The microstructure is mainly FCC phase and BCC phase, and contains a small amount of WC, W2C and Cr7C3 phase.
(2) The amount of WC particles added has a significant effect on the mechanical properties of the coating. With the increase of WC content, the microhardness of the cladding layer increases significantly. The average microhardness of the 10% WC cladding coating is the highest, with the maximum value of 484.5 HV0.3.
(3) The wear loss and wear rate of the 10% WC cladding coating are the lowest, which are 0.011 4 g and 0.857×10-5 g/(N·m) respectively. The wear resistance is the best. The wear modes are mainly abrasive wear and oxidative wear, accompanied by adhesive wear.