Abstract: Objective To study the effect of Mo content on laser cladding CoCrFeNiW0.6 high entropy alloy coating. Methods CoCrFeNiW0.6Mox (x=0, 0.2, 0.4, 0.6, 0.8) high entropy alloy coating was prepared on the surface of 45 steel substrate using RFL-C1000 fiber laser. The macroscopic morphology and dilution rate, phase structure, microstructure, hardness and corrosion resistance of the cladding layer were tested and analyzed using Leica DVM6 optical microscope, scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spectrometer, microhardness tester and electrochemical workstation. Results After adding Mo element, the bonding state and surface morphology were good. When x=0~0.4, the microstructure of the coating was mainly in the form of dendrites, and the grains gradually became finer. When x≥0.6, cracks began to appear on the coating surface. With the addition of Mo element, the coating gradually precipitated σ phase and the grain size gradually decreased. When x=0.8, a eutectic structure is formed. The microhardness of the coating increases with the increase of Mo element, but due to the presence of more cracks at x=0.8, the appearance of cracks affects the hardness of the coating, resulting in a decrease in the hardness at x=0.8. When x=0.6, the average microhardness of the coating is the highest, reaching 959.69HV0.3, which is about 20.32% of the average hardness of the CoCrFeNiW0.6 coating. When x=0~0.6, the corrosion resistance of the coating gradually improves with the increase of Mo element content. When x=0.8, the corrosion resistance deteriorates, because the appearance of cracks and the formation of σ phase make the corrosion resistance of the coating worse. When x=0.6, the corrosion resistance of the coating is the best. Conclusion The addition of Mo element causes the σ phase to appear in the microstructure of the coating, and at the same time has the effect of refining the grains, which can significantly improve the hardness and corrosion resistance of the coating.

The concept of high entropy alloy was first proposed by Professor Ye Junwei and others in 2004. It generally refers to an alloy composed of 5 or more main elements (the molar ratio is 5%~35%) in equal or nearly equal molar ratios. High entropy alloys have four main characteristics: high entropy effect, slow diffusion effect, lattice distortion effect and “cocktail” effect. Under the action of these four characteristics, high entropy alloys have the advantages of high hardness, high strength, good corrosion resistance and high temperature oxidation resistance, which can meet the needs of many harsh working environments and special equipment. Therefore, since the high entropy alloy was proposed, it has been at the forefront of research in the field of materials. Since the failure of parts such as fatigue wear and corrosion mostly occurs on the surface of parts, the preparation of high entropy alloy coatings can not only effectively improve the mechanical properties and service life of parts, but also the cost of alloy coatings is low compared to the preparation of high-performance parts. Ma Shizhong et al. prepared CoCrFeNiWx (x=0, 0.2, 0.4, 0.6) entropy alloy coatings by laser cladding technology. The study showed that with the addition of W element, the formation of μ phase was promoted, the grains became finer, and the hardness and wear resistance of the coating were significantly improved. Liu et al. prepared CoCrFeNiMox (x=0~1.5) high entropy alloy by vacuum arc furnace melting method. The study showed that with the addition of Mo element, the hardness and yield strength of the alloy increased significantly. Manzoni et al. used vacuum melting technology to study the effects of Mo, W and Ti elements on the microstructure and mechanical properties of Al8Co17Cr17Cu8Fe17Ni33 alloy. The study showed that the addition of Mo, W and Ti elements promoted the formation of hard phase in the alloy, thereby significantly improving the microhardness. At present, there are many process methods for preparing high entropy alloy coatings. Among them, laser cladding technology has developed rapidly in recent years. It has the advantages of fast heating and cooling, low dilution rate (≤5%), and small heat-affected zone, and is widely used in coating preparation and other fields. Laser cladding technology is to quickly melt the powder and the substrate under the action of a high-energy density laser beam. After rapid solidification, a high-entropy alloy coating that is metallurgically bonded to the substrate is formed on the surface of the substrate, thereby significantly improving the wear resistance, corrosion resistance, hardness and other properties of the substrate surface. Therefore, this paper will use laser cladding technology to prepare high-entropy alloy coatings.

CoCrFeNi series high-entropy alloys are easy to form simple face-centered cubic or body-centered cubic structures due to the similar atomic radius and electronegativity of their constituent elements and the small mixing enthalpy between elements. They also show relatively excellent mechanical properties in all aspects, but the mechanical properties of this series of high-entropy alloys are poor and are not enough to meet the surface working requirements of parts. Therefore, other elements are added to improve the hardness, wear resistance, corrosion resistance and other properties of the high-entropy alloy coating. Research has found that due to the large atomic radius of Mo, adding an appropriate amount of Mo to a high-entropy alloy will intensify the lattice distortion effect, thereby effectively improving the performance of the alloy. It is widely used in solid solution strengthened high-temperature alloys, and Mo is beneficial to improving the mechanical and chemical properties of materials in most cases. At the same time, the previous research results of the author’s research group show that adding an appropriate amount of W to CoCrFeNi high-entropy alloys can improve the various properties of the alloy. In order to further improve the performance of the alloy, this study used laser cladding technology to prepare CoCrFeNiW0.6Mo (x x=0, 0.2, 0.4, 0.6, 0.8) high-entropy alloy coatings on the surface of 45 steel, and studied the effects of different Mo contents on the structure and properties of the cladding layer.

1 Experiment

1.1 Materials and preliminary preparation
Use JA2003 electronic precision balance (accuracy ±0.1 mg) to weigh CoCrFeNi alloy powder (purity 99.9%, particle size 45~105 μm), pure W powder (purity 99.9%, particle size 8.47 μm), and pure Mo powder (purity 99.9%, particle size 8.47 μm) prepared according to the molar ratio of the substances. The molar ratio of the relevant powders is shown in Table 1. Use MSK-SFM-1 horizontal planetary ball mill to mix the alloy powders evenly, the speed is 300 r/min, the mixing time is 120 min, the grinding ball material is cemented carbide material with a diameter of 3~5 mm, and the ball-to-material ratio is 3:1. The evenly mixed alloy powder is placed in a drying oven for 2 h, and then put into a sealed bag for use. The substrate used in the experiment is 45 steel, which is cut into 50 mm×50 mm×10 mm and 25 mm×50 mm×10 mm specimens using an electric spark cutting machine, which are used for multi-pass and single-pass cladding, respectively. Then, the specimens are polished in sequence using 240-1 200 grit sandpaper to remove the oxide scale until the surface is smooth. The substrate is cleaned with anhydrous ethanol in an ultrasonic cleaner to remove surface impurities, dried with cold air, and placed in a drying dish for later use.

1.2 Preparation of laser cladding coatings
This experiment uses the pre-coating laser cladding method. Before laser cladding, the prepared metal powder is placed in a beaker, and an appropriate amount of alcohol is added to stir into a paste, and then evenly applied on the surface of the substrate. Use 2 1 mm precision feeler gauges to modify the coating thickness and shape (preset coating thickness is 1 mm). After the alcohol is naturally dried, use RFL C1000 fiber laser to perform single-pass and multi-pass overlap laser cladding experiments on the surface of 50 mm × 25 mm × 10 mm and 50 mm × 50 mm × 10 mm substrates. Argon gas protection was introduced throughout the experiment to prevent the material from being oxidized by high temperature. In multiple groups of experiments, the optimal process parameters were selected: laser power P = 1 000 W, scanning speed v = 12 mm/s, laser spot diameter D = 2 mm, multi-pass overlap rate of 50%, laser distance from substrate surface 40 mm, argon gas flow rate of 10 L/min.

1.3 Organization and performance test
After successful cladding, the sample block was cut into samples of different sizes using an electric spark cutting machine for subsequent experiments, and the cut samples were polished with sandpaper of 240, 400, 600, 800, 1 000, and 1 200 grains in turn. The surface roughness Ra after polishing was 0.05 μm. The macroscopic morphology and dilution rate of the cladding layer were observed and analyzed using a Leica DVM6 optical microscope. The microstructure of the cladding layer was observed using a HITACHI TM3030 scanning electron microscope (SEM), and the element distribution of the cladding layer was analyzed by combining an energy dispersive spectrometer (EDS). The phase structure of the cladding layer was analyzed using a D8 X-ray diffractometer. The instrument used a CuKα target, a diffraction angle of 30°~100°, and a diffraction speed of 8 (°)/min. The microhardness of a single-pass cladding layer was tested using an HV1000Z automatic turret microhardness tester with a load of 0.3 kg and a loading time of 10 s. A point was punched every 0.1 mm in the longitudinal direction, and three points were tested at an equal interval of 0.1 mm in the same horizontal direction. The average value was taken, and a hardness curve and a bar graph of the average hardness of the cladding layer were plotted to compare the hardness of different contents. The coating was electrochemically corroded using an Lk2010 electrochemical workstation, and its corrosion resistance was analyzed by the obtained Tafel curve.

2 Results and analysis
2.1 Macromorphology of cladding layer and nondestructive testing analysis
The cross-sectional macromorphology of a single-pass cladding layer of CoCrFeNiW0.6Mox high entropy alloy is shown in Figure 1. The coating is divided into three parts: cladding zone (CZ), bonding zone (BZ), and heat affected zone (HAZ). The calculation formula of the dilution rate is as follows: see Figure area (1)
Where: S1 is the area of ​​the cladding layer above the substrate; S2 is the area of ​​the substrate melting area.
The dilution rate of the cladding layer can be obtained by calculation, and the results are shown in Table 2. The dilution rate is between 9.29% and 24.09%, indicating that during laser cladding, the substrate surface has a good metallurgical bond with the cladding material.
The macroscopic morphology of the multi-pass cladding layer of CoCrFeNiW0.6Mox high entropy alloy is shown in Figure 2, and its flaw detection morphology is shown in Figure 3. It can be seen from Figure 2 that when x=0, 0.2, and 0.4, the surface morphology of the cladding layer is relatively flat, without cracks and pores, and the forming quality is good; when x=0.6, cracks begin to appear on the surface; when x=0.8, the cracks increase further, and all cracks are perpendicular to the laser scanning direction. This shows that excessive addition of Mo element will cause cracks on the surface of the cladding layer because the addition of Mo element causes the precipitation of σ phase in the coating, which can effectively prevent the movement of dislocations and the formation of stacking faults in the matrix, resulting in the accumulation of dislocations at the interface, which causes stress concentration and cracking during compression. As the Mo content increases, the cracking phenomenon becomes more obvious.

2.2 Phase analysis of coating
According to the electron hole, when the average number of electron holes is greater than 2.5, the alloy is prone to form σ phase, and near the melting point, the number of holes can reach 1% of the total number of atoms. It is calculated that the average number of electron holes in the high entropy alloy coatings with x=0, 0.2, 0.4, 0.6, and 0.8 are 3.35, 3.55, 3.74, 3.93, and 4.13, respectively. Therefore, it can be inferred that σ phase is likely to be produced in CoCrFeNiW0.6Mox (x=0, 0.2, 0.4, 0.6, 0.8) alloys. The XRD spectrum of the CoCrFeNiW0.6Mo (xx=0, 0.2, 0.4, 0.6, 0.8) high entropy alloy cladding layer is shown in Figure 4. It can be seen that each group of coatings forms obvious characteristic peaks near 43°, 50°, 74°, and 90°. After comparison with the standard PDF card, it can be seen that it is Cr0.19Fe0.7Ni0.11, and the main phase of the coating is composed of FCC phase. Diffraction peaks are formed near 36°. After comparison with the standard PDF card, it can be seen that this phase is Fe7W6 phase (μ phase). When x=0, 0.2, the coating is composed of FCC+μ phase (Fe7W6), indicating that when the atomic number fraction of Mo is ≤0.2, the alloy structure has not undergone phase change; when x＞0.2, a new diffraction peak appears near 42°. After comparison with the standard PDF card, it can be seen that the phase is Cr0.549Co1.539Mo0.912 phase (σ phase). The fine σ phase helps to improve the mechanical properties of the alloy, and the addition of Mo content causes the diffraction peak to shift to a small angle, indicating that the addition of Mo element causes the change of lattice parameters. Due to the large atomic radius of Mo and W, when Mo and W atoms occupy the lattice position during the substitution or solid solution process, the interatomic spacing will change and the lattice will expand. With the addition of Mo and W atoms, the lattice distortion effect becomes stronger. The degree of lattice distortion of high entropy alloys is generally expressed by the atomic radius difference δ. The calculation formula of the atomic radius difference is as follows: See the image area (2)
The atomic radius difference δ cannot accurately show the actual lattice distortion, but its size can express the degree of lattice distortion of the alloy. It is calculated that when x=0, 0.2, 0.4, 0.6, 0.8, the atomic radius difference of CoCrFeNiW0.6Mox high entropy alloy is 4.08%, 4.37%, 4.61%, 4.80%, 4.94% respectively. It can be seen that the addition of Mo element enhances the lattice distortion of the alloy.

2.3 Microstructure morphology of coating
The microstructure morphology of the CoCrFeNiW0.6Mox high entropy alloy cladding layer obtained by crystal phase corrosion is shown in Figure 5. It can be seen that the microstructures of different Mo element contents are typical dendrite structures, but the microstructure morphology has changed significantly. The white bright area is the dendrite structure, the gray area is the interdendritic structure, and with the increase of Mo content, the distance between dendrites gradually decreases, the grains gradually refine, and the secondary crystal axis gradually forms. When x=0, the coating microstructure is mainly in the form of dendrites, the grain size of the cladding layer is large, the average size is about 5 μm, and due to the hysteresis diffusion effect of the high entropy alloy coating, the grains are arranged uniformly and compactly; when x=0.2, the coating microstructure begins to form a secondary crystal axis, and the average grain size is about 2 μm; when x=0.4, the coating microstructure shows that the dendrites continue to grow; when x=0.6, precipitates are precipitated between dendrites; when x=0.8, the coating microstructure has a σ phase, and a eutectic structure appears between dendrites.

The EDS test results of different positions of each parameter are shown in Table 3. It can be seen that the Fe content is much greater than the theoretical value. This is because during the laser cladding process, the laser energy is relatively concentrated and the heating speed is fast, which will cause the substrate to partially melt. The Fe on the surface of the 45 steel substrate diffuses into the cladding layer, resulting in a high Fe content in the cladding layer. When x=0~0.4, the content of Cr, W, and Mo elements in the dendrite is relatively high; when x=0.6, the content of Co, Cr, and Ni elements between dendrites is relatively high, and the atomic ratio is about 1:1:1; when x=0.8, a eutectic structure is formed in the white bright area. Mo and W elements are mainly distributed in the dendrite. This is because during the cooling process, Mo and W elements have larger atomic radius, electronegativity, and larger negative mixing enthalpy, so Mo and W elements will not be dissolved in the FCC phase rich in Co, Cr, Fe, and Ni. Combined with the XRD diagram, it can be seen that these elements diffuse into the dendrites to form Cr0.549Co1.539Mo0.912 and Fe7W6 intermetallic compounds, and finally form a eutectic structure of Cr0.549Co1.539Mo0.912 phase and Fe7W6 phase in the dendrites. At the same time, with the addition of Mo element, the segregation of W element becomes weaker, indicating that the addition of Mo element can inhibit the dendrite segregation of W element.

2.4 Microhardness of coating
The hardness curve of the surface of CoCrFeNiW0.6Mox coating and the average hardness of the cladding zone are shown in Figure 6. As shown in Figure 2b, the cladding layer contains three parts: cladding zone, heat-affected zone, and substrate. Due to the high temperature and rapid cooling of the heat-affected zone, martensite is formed, so the hardness in this area changes significantly. As can be seen from Figure 6, when no Mo element is added, the average hardness of the cladding layer is the lowest, which is 472.23HV0.3. With the increase of Mo element content, the hardness of the cladding layer increases first and then decreases. When x=0.6, the average hardness of the coating is the highest, which is 959.69HV0.3. When x=0.8, the microhardness of the cladding layer has decreased significantly, because the appearance of cracks reduces the hardness. It can be seen that adding an appropriate amount of Mo element can effectively improve the microhardness of the coating. The main reasons are as follows: 1) Mo elements with larger atomic radius can act as solute atoms, causing higher local distortion of the matrix structure, aggravating the degree of lattice distortion and hindering dislocation movement, playing a role of solid solution strengthening; 2) As shown in Figure 5, the addition of Mo element reduces the dendrite spacing, increases the grain boundary area, increases the resistance to dislocation movement, and makes it difficult for the alloy material to deform, thereby achieving the effect of strengthening the alloy. For most alloys, the reduction of dendrite spacing can significantly improve its hardness; 3) Combining the XRD graph and EDS results, it can be seen that with the addition of Mo content, the coating microstructure changes from FCC phase + μ phase to FCC phase + μ phase + σ phase, and the appearance of high-strength σ phase significantly improves the microhardness of the coating.

2.5 Electrochemical corrosion performance of coatings
At room temperature, the polarization curve of CoCrFeNiW0.6Mox high entropy alloy cladding layer in 3.5% NaCl solution is shown in Figure 7. It can be seen that no obvious passivation zone occurs in the cladding layer in the NaCl solution, indicating that a stable passivation film is formed on the surface of the coating at the corrosion potential.
The electrochemical parameters of CoCrFeNiW0.6Mox high entropy alloy coating in 3.5% NaCl solution are shown in Table 4. Among them, the corrosion potential Ecorr represents the corrosion tendency of the coating. The smaller the corrosion potential, the more stable the corrosion of the coating. The corrosion current density Jcorr is obtained by extrapolation. In the same corrosion environment, the corrosion rate of the material is calculated by Faraday’s formula: see the picture part (3)
Where: v is the corrosion rate of the material, g/(c㎡·s); M is the atomic weight of the corroded material; n is the number of electrons gained or lost by the metal ion; F is the Faraday constant. Therefore, in the same corrosion environment, the smaller the corrosion current density Jcorr, the slower the corrosion rate and the better the corrosion performance. As shown in Table 4, with the increase of Mo content, the corrosion potential Ecorr decreases, indicating that the addition of Mo element makes the coating more prone to corrosion; the corrosion current density shows a trend of decreasing first and then increasing. When x=0~0.6, the corrosion current density gradually decreases. When x=0.6, the corrosion current density is the lowest, which is only 0.154 7×10–6 A/cm2, and the corrosion resistance is significantly improved. This is because the Mo element itself has good corrosion resistance. Due to the “cocktail” effect of high entropy alloy, the corrosion resistance of its cladding layer will also be improved. At the same time, after adding Mo element, under the action of rapid heating and solidification of laser cladding, the nucleation rate is improved, thereby obtaining a uniform and dense organizational structure. As shown in Figure 4, with the addition of Mo element, the grain size of the coating microstructure decreases, resulting in an increase in the grain boundary length, thereby reducing the corrosion current[30] and improving the corrosion resistance. As the Mo content increases further, the corrosion current density increases and the corrosion resistance of the alloy deteriorates. This is because the atomic radius of the Mo element is large. In the process of forming a substitutional solid solution, it causes internal segregation of the cladding layer, strengthens the lattice distortion effect, and makes the structure uneven. At the same time, the precipitation of Cr and Mo-rich σ phases and the appearance of cracks between dendrites lead to the corrosion of the alloy. In summary, adding an appropriate amount of Mo element can effectively reduce the corrosion rate of the coating and improve the corrosion resistance of the alloy.

3 Conclusion
In this paper, CoCrFeNiW0.6Mox high entropy alloy coating was prepared on the surface of 45 steel by laser cladding technology, and the macroscopic morphology, phase, microstructure and mechanical properties of the coating were observed and analyzed. When x=0, 0.2, the coating is composed of FCC phase and μ phase; when x=0.4, σ phase begins to appear in the coating, and the main structure of the coating is dendrite. The addition of Mo element promotes the formation of σ phase; when x=0.6, cracks begin to form. At the same time, with the addition of Mo element, the grains gradually refine and the microhardness of the coating gradually increases. Due to the formation of cracks when x=0.8, the hardness value decreases. When x=0.6, the microhardness is the highest. The addition of Mo element effectively changes the corrosion resistance of the coating, but due to the generation of cracks and the formation of σ phase, the corrosion performance deteriorates when x=0.8. The strengthening mechanisms are grain refinement, solid solution strengthening and second phase (σ phase) strengthening.