High entropy alloys (HEA) are super solid solution alloys made of 5 or more equal or approximately equal metals. They have excellent comprehensive properties, such as ultra-high strength, ultra-high fracture toughness, high wear and corrosion resistance, low elastic modulus and high high temperature oxidation resistance. Compared with other alloys, high entropy alloys show high entropy effect in thermodynamics, slow diffusion effect in kinetics, and lattice distortion effect in structure. They are widely used in aerospace, military industry and nuclear power industry. At present, the commonly used processes for preparing high entropy alloy coatings include physical vapor deposition, chemical vapor deposition, laser surface alloying and laser cladding. The characteristics of laser cladding process are high energy density, high efficiency, metallurgical bonding with the substrate and fast solidification rate. At present, there are many studies on it. This paper reviews the process parameters, alloy composition, The influence of annealing and other treatment methods on the microstructure and properties of laser cladding high entropy alloy coatings and the latest research results.

1. The influence of process parameters on the microstructure and properties of coatings Process parameters are crucial to the performance of laser cladding high entropy alloy coatings, including laser power, scanning rate, laser energy density, powder feeding voltage and oxygen flow rate.
LI et al. prepared CoCrFeNiMo high entropy alloy coatings with good high temperature wear resistance. The study found that: with the increase of laser power, the precipitation phase decreased, and due to the flow and dilution of the molten pool, the matrix structure changed from a network to a dendrite, and the oxide glaze layer of the coating could significantly reduce the friction and wear rate of the coating.
QUE et al. prepared AlCoCrFeNiTi high entropy alloy coatings on the surface of AISI 1045 steel using synchronous powder feeding laser cladding technology. The relationship between laser power, scanning rate, powder feeding voltage and microhardness and wear volume was established using response surface methodology. The results show that the coating is composed of body-centered cubic lattice unit cells and TiC phase, and a good metallurgical bond is formed between the substrate and the coating; the microhardness increases with the increase of laser power and powder feeding voltage, and decreases with the increase of scanning rate; the wear volume increases with the increase of scanning rate, and decreases with the increase of laser power and powder feeding voltage. The errors between the predicted value and the actual value of the microhardness and wear volume of the coating are 0.43% and 2.48%, respectively, which verifies the accuracy of the established model. SAMOOILOVA et al. studied the microstructure, composition and properties of Al0.25CoCrFeNiV high entropy alloy coating on austenitic stainless steel by laser cladding. Laser cladding was carried out in argon atmosphere, and the laser power was 1 The laser power was 400 W, the spot diameter was 3 mm, the trajectory displacement was 1.2 mm, and the scanning rate was 10 mm/s. Compared with the cast high entropy alloy Al0.25CoCrFeNiV, the microstructure of the laser cladding Al0.25CoCrFeNiV coating changed, that is, the size of the vanadium precipitate phase was significantly reduced from 20 ~ 40 μm in the cast state to 1 ~ 3 μm; the hardness of the coating surface was 370 HV0.3, while that of the interface with the substrate was 270 HV0.3, which may be due to the diffusion of Fe in the stainless steel into the coating during the laser cladding process. Nevertheless, the coating is tightly bonded to the substrate without defects such as cracks. WANG et al. studied the effect of oxygen content in VA1TiCrW high entropy alloy coating on its room temperature mechanical properties and 700 ℃ tribological properties. When the oxygen flow rate increased from 0 ml/min to 10 ml/min, the hardness of the coating increased from 3.3 GPa to 11.9 GPa, and the elastic modulus increased from 79.92 GPa to 181.7 GPa. The friction coefficient was very small, 0.17, and the wear rate was very low, 3.38×10-5 mm3/Nm. The V2O5 phase in the coating was the key to reducing its friction coefficient, and the hard oxide phases WO3 and AlVO4 led to a decrease in the wear rate of the coating. Xu Jiale et al. [11] studied the effect of laser energy density on the microstructure and properties of AlCoCrFeNi high entropy alloy coatings on H13 steel. The results showed that the AlCoCrFeNi high entropy alloy coatings prepared with lower or higher laser energy density were prone to metallurgical defects such as pores and cracks, and the coating structure was coarse; the high entropy alloy coating prepared with a laser energy density of 7.23 kJ/cm-2 had a smoother surface, fewer metallurgical defects and a significantly refined structure. The microhardness of the high entropy alloy coating was significantly higher than that of the substrate; the coating prepared with a laser energy density of 7.23 kJ/cm-2 had the highest hardness of 540.7 HV, while the substrate was 225 HV, which was significantly higher than that of the substrate. The microhardness Weibull modulus (a parameter in the Weibull distribution function in statistical fracture mechanics, which is often used to reflect the discreteness of material strength. The higher the Weibull modulus, the smaller the discreteness, indicating that the strength distribution is more uniform) of the coating is about twice that of the high entropy alloy coating prepared by other laser energy densities, indicating that the hardness discreteness is smaller and the microstructure is more uniform.
MA et al. prepared CoCrFeNiMo0.2 high entropy alloy coating with optimized process parameters (laser power of 880 W, scanning rate of 18 m/min, overlap rate of 60%, powder feeding rate of 3 r/min). The surface roughness, microstructure, phase composition and other properties of the coating were tested respectively. The composition and element distribution of the alloy were investigated, and electrochemical tests were carried out in a 3.5 % NaCl (mass fraction, the same below) solution using a three-electrode system to test the corrosion resistance of the CoCrFeNiMo0.2 high entropy alloy coatings prepared by high-speed laser cladding and conventional laser cladding. The test results show that the surface roughness of the CoCrFeNiMo0.2 high entropy alloy coating prepared by high-speed laser cladding technology is 15.53 μm, the coating is a single FCC phase, and it is metallurgically bonded to the substrate. The microstructures are planar crystals, cellular structures, columnar dendrites and equiaxed dendrites, as shown in Figure 1. The electrochemical test results show that compared with the CoCrFeNiMo0.2 high entropy alloy coating prepared by conventional laser cladding technology, the The corrosion resistance of CoCrFeNiMo0.2 high entropy alloy coating is relatively high. The self-corrosion current density of the high-speed laser cladding coating calculated by Tafeel extrapolation method is 5.441 1 × 10′-6 A-cm’-2 and the self-corrosion potential is -0.744 5 V. The self-corrosion current density and self-corrosion potential of the traditional laser cladding coating are 2.708 3 × 10′-5 A-cm’-2 and -0.968 5 V, respectively. The excellent corrosion resistance of CoCrFeNiMo0.2 high entropy alloy coating prepared by high-speed laser cladding technology is mainly related to its uniform and fine surface structure. High-speed laser cladding technology can be used for the repair and remanufacturing of workpieces used in various complex environments.

Li Li et al. optimized the laser cladding process parameters by orthogonal experiment. They clad the AlCoCrFeNiCu high entropy alloy coating on the surface of Q235 steel by pulsed Nd:YAG solid laser. They used X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer and microhardness tester to detect the phase composition, microstructure, elements and hardness distribution of the coating. They used three-electrode system to detect the polarization property and electrochemical impedance spectroscopy (EIS) of the high entropy alloy coating. They studied the corrosion behavior of the high entropy alloy coating in 3.5% (volume fraction) NaCl solution. The optimal process parameters finally obtained were: powder thickness of 1.25 mm, scanning rate of 2.00 nm, and corrosion rate of 1.50 nm. The high entropy coating is composed of Cu-rich FCC phase and (Al, Ni)-rich BCC phase. The surface microstructure is fine and uniform equiaxed crystals, the middle is coarse columnar dendrites, and there are obvious plane crystals at the junction of the coating and the substrate. Cu element is slightly segregated between dendrites. The maximum hardness of the coating is 521 HV0.2, which is 2.7 times that of the substrate. In 3.5% (volume fraction) NaCl solution, the AlCoCrFeNiCu high entropy alloy coating has a more positive self-corrosion potential, a smaller self-corrosion current density, and a larger capacitance than the substrate. Arc radius and impedance modulus value, showing good corrosion resistance.

2 Effect of alloy composition on coating structure and performance
The alloy elements that have been used as additives for high entropy alloy coatings include Cu, Nb, Co, Cr, Mo, Ti, W, etc., which have refined the coating structure and improved the mechanical properties to a certain extent, but the precipitation of some elements has reduced the wear resistance of the coating.

JIANG et al. used laser cladding technology to prepare (CoCrNi)72-xB18 Si10Nbx (x = 0, 2, 4, 6, 8) high entropy alloy composite coatings. The (CoCrNi)72B18 Si10 coating is composed of a dual-phase FCC phase rich in Co, Ni, and Si and a Cr, B-rich phase. The addition of Nb leads to the formation of FCC ceramic phase g￣ phase, which changes the morphology and properties of the coating. Due to solid solution strengthening and second phase strengthening, the mechanical properties of the coating are significantly improved, thereby improving the wear resistance and corrosion resistance. ZHU et al. prepared FeCoCrNiCux and FeCoCrNiSix high entropy alloy coatings by laser cladding technology, detected the microstructure of the coating, and studied the tribological properties of the coating when grinding with Si3N4 ball at room temperature and 600 ℃. The results show that after adding Cu or Si, the room temperature tribological properties of the coating do not change much, but the high temperature tribological properties are significantly improved. The coating has the best tribological properties at 600 ℃, with an average friction coefficient and wear rate of 0.19 and 0.677 [×10′-4 mm’3 / (N􀅰m)], respectively. In summary, the addition of Cu improves the thermal conductivity, toughness and bonding strength of the coating; the addition of Si refines the grain size of the coating and improves the wear resistance of the coating.
JIANG et al. prepared CoCrNiMoCB high entropy alloy composite coating by laser cladding technology and studied the effect of Co content on the corrosion resistance of the coating. The results show that the addition of Co can reduce the composition difference between the FCC matrix and the strengthening phase, thereby reducing the potential difference and reducing the driving force of galvanic corrosion. Through first-principles calculations, it is found that CoCrNiMoCB high entropy alloy composite coating has a great influence on the corrosion resistance of the coating. The addition of Cr can reduce the interface bonding energy, thereby improving the corrosion resistance and increasing the interface bonding strength.
WANG et al. prepared a defect-free CoCrFeNiMo0.2 high entropy alloy coating on Inconel 718 alloy using laser cladding technology. The average thickness of the coating was 990 μm. The thermal erosion test showed that Cr and Mo elements can form a passivation film to improve the thermal erosion resistance of the coating; the average microhardness of the coating is about 328.9 HV10, which is 1.28 times that of the substrate; due to the formation of σ phase and fine grain strengthening, the wear resistance of the coating is excellent; the average residual stress on the coating surface is -723.3 MPa, and cracks are difficult to generate.
Zhang Li et al. studied The effect of Ti element on the microstructure, microhardness and wear resistance of AlCoCrFeNi high entropy alloy coating was studied. The results show that the coating phase consists of Co3Ti, AlFe and Al80Cr13Co7 phases, and a typical uniform network spinodal decomposition structure is formed, and the microstructure is better than that of AlCoCrFeNi high entropy alloy coating. In addition, as shown in Figure 2, the average microhardness of AlCoCrFeNiTi0.5 high entropy alloy coating reaches 989 HV0.5, which is 32% higher than the average microhardness, and the wear loss is reduced by 85%.
Liu Hao et al. prepared AlCoCrFeNiSix(x = 0.044mm) on the surface of AISI 304 stainless steel by laser cladding technology. 0.1, 0.2, 0.3, 0.4, 0.5) high entropy alloy coatings were prepared and the effect of Si element on the microstructure and properties of the coatings was analyzed. The results show that the microstructure of the AlCoCrFeNiSix high entropy alloy coating is a body-centered cubic solid solution (as shown in Figure 3). With the increase of Si content, the Si element substitution solid solution causes the lattice to shrink, the grains to refine, the nanoscale spherical AlNi phase to dissolve from the crystal, and a small amount of Cr23C6 carbides to precipitate along the grain boundaries. The evolution of the microstructure leads to an increase in the microhardness of the coating.
Ma Shizhong et al. used RFL-C1000 fiber laser to prepare the AlCoCrFeNiSix high entropy alloy coating on the surface of 45 steel. CoCrFeNiWx (x = 0, 0.2, 0.4, 0.6, 0.8) high entropy alloy coatings were prepared. The effect of W content on the microstructure and properties of the coatings was studied. The results show that the addition of W element reduces the wetting angle between the coating and the substrate and improves the wettability. With the increase of W content, the coating transforms from a single FCC phase to FCC phase and μ phase (Fe7W6), the microstructure transforms from cellular crystals to dendrites, the grain size decreases, and the CoCrFeNiWx coating with x = 0.8 has obvious eutectic structure and a large amount of μ phase. The microhardness of the coating increases with the increase of W content. The coating has the highest microhardness, reaching 432.02 HV0.3, which is about 2.1 times the hardness of the substrate and 2.2 times the hardness of the CoCrFeNiWx coating. The CoCrFeNiWx coating with x = 0.6 has the smallest wear volume, which is only 30.85% of the CoCrFeNiWx coating, and the average friction coefficient is the smallest, about 0.311. With the increase of W content, the wear mechanism of the coating changes from adhesive wear and abrasive wear to abrasive wear, indicating that the addition of W element can improve the wettability between the coating and the substrate, promote the formation of μ phase, increase the hardness of the coating, and improve the wear resistance of the coating. The strengthening mechanism is solid solution strengthening, grain refinement and second phase strengthening.
Existing studies have shown that the mixing enthalpy of boron and alloying elements is small, as shown in Table 1. Adding B element to the Fe-Cr-Ni alloy system can generate hard second phases such as Fe2B, Cr2B or CrB, which significantly improves the mechanical properties of the coating. Gu Mi et al. studied the effect of ferroboron powder content on the microstructure and properties of laser cladding AlCoCrFeNi high entropy alloy coatings. The results show that adding an appropriate amount of ferroboron powder can improve the surface quality and refine the microstructure of AlCoCrFeNi coatings. When 3% to 4% (mass fraction, the same below) ferroboron powder is added, the surface quality of the coating is improved; when the ferroboron powder content is 5% to 6%, the coating formability decreases and the cracking tendency increases. The hardness and wear resistance of AlCoCrFeNi high entropy alloy coating with appropriate amount of ferroboron powder are significantly improved compared with the coating without ferroboron powder. The coating with 4% ferroboron powder has the best surface quality and no obvious cracks. Yang Changzheng et al. used 45Cr steel for internal combustion engine as the substrate and prepared CoCrFeNiMo/SiC high entropy alloy coating by laser cladding technology. The microstructures of S1 sample (CoCrFeNiMo), S2 sample (10% SiC added to CoCrFeNiMo), and S3 sample (20% SiC added to CoCrFeNiMo) were analyzed. The research results show that: in addition to the S2 and S3 samples, the In addition to the FCC phase, M7C3 (M represents Cr, Mn, Fe) is also formed, which significantly improves the wear resistance of the coating.
Zhou Yong et al. prepared AlCoCrFeNi high entropy alloy coating and AlCoCrFeNi￣TiC composite coating on the surface of Q235 steel using laser cladding technology, and studied the effect of TiC on the wear resistance of the coating. The results show that the AlCoCrFeNi coating is composed of BCC phase, and the AlCoCrFeNi￣TiC composite coating is composed of BCC phase and TiC particles. Compared with the AlCoCrFeNi coating, the hardness of the AlCoCrFeNi￣TiC composite coating is better than that of the AlCoCrFeNi coating. The hardness of the steel sheet increased by nearly 1 times (from 443 HＶ０.３ to 862 HＶ０.３), the friction coefficient decreased by 33%, the volume wear rate decreased by 64%, and the wear resistance increased by nearly 200%.
Huang Yuansheng et al. laser clad a high entropy alloy coating of AlxTixNixCoCrCu0.5FeMo (x = 4, 5, 6, 7, 8, 9, molar ratio) on the surface of 40Cr steel, and annealed it at 300 ~ 900 ℃. The microstructure and properties of the annealed coating were tested. The results show that the high entropy alloy coatings without annealing and annealing at 300 ℃ are both single body-centered cubic (BCC) phase, a new intermetallic compound similar to the Al2Ti3 structure is generated in the coating after annealing at 500 ~ 900 ℃; the corrosion resistance of the coating decreases with the increase of annealing temperature; with the increase of annealing temperature, the hardness of the coating first decreases, then increases, and then decreases. The Al4Ti4Ni4CoCrCu0.5FeMo high entropy alloy coating annealed at 700 ℃ has the highest hardness of 1 019 HV.
Zhang Lianwang et al. prepared CoCrFeNiSi2.0 high entropy alloy coating by laser cladding technology and kept it at different temperatures (600 ℃, 800 ℃, 1 000 ℃) for 30 min The coating was annealed by furnace cooling. The microhardness, friction and wear properties, microstructure and phase composition of the coating were detected by using microhardness tester, vacuum friction and wear tester, Leica DVM6 optical microscope, scanning electron microscope, X-ray diffractometer and other equipment. The results showed that the phases of the coating without annealing were mainly BCC phase and a small amount of (Fe, Mn)2SiO4 phase. After annealing at different temperatures, (Fe, Mn)2SiO4 decomposed, and (Fe, Mn)2SiO4 completely disappeared in the coating annealed at 1000℃. Cr2Si and Ni4Si phases precipitated, and their contents increased with the increase of annealing temperature within a certain range. The diffraction peak height of BCC phase first increased and then decreased with the increase of annealing temperature. This shows that the annealing temperature affects the comprehensive performance of the coating. The comprehensive performance of the coating annealed at 800 °C is better than that of the unannealed coating. The structure is more uniform, and the precipitated phases such as Cr2Si and Ni4Si are the most and evenly distributed. The coating has a maximum microhardness of 1 347.4 HV0.3, and the friction coefficient is basically stable at 0.19. The CoCrFeNiSi2.0 high entropy alloy coating annealed at 1 000 °C is a single BCC phase, which will inevitably affect its performance.

4 Effects of other treatment methods on coating structure and properties
In addition to the cladding process parameters, alloy composition and annealing treatment, some other treatment methods will also affect the structure and properties of the coating, such as ultrasonic surface rolling (USRE) technology, ultrasonic impact strengthening, etc.
Liu Hao et al. used laser cladding technology to prepare a reinforced CoCrFeMnNiM (M＝Ti, Mo) high entropy alloy cladding layer, and used ultrasonic surface rolling technology to strengthen the surface of the cladding layer. X-ray diffraction, field emission scanning electron microscopy, energy spectrometer, atomic force microscope, microhardness tester and friction wear tester were used. studied the effect of USRE treatment on the microstructure, surface morphology, mechanical properties and tribological properties of high entropy alloy laser cladding layers. The results showed that the CoCrFeMnNiTi high entropy alloy laser cladding layer was composed of a face-centered cubic (FCC) solid solution and TiC in-situ precipitation phase, while the CoCrFeMnNiMo high entropy alloy laser cladding layer was still composed of a single FCC solid solution. The phase composition of the coatings did not change after USRE treatment, but the grains of the CoCrFeMnNiMo coating were significantly refined. USRE treatment reduced the surface roughness of the high entropy alloy laser cladding coating and increased the residual compressive stress and microhardness. After USRE treatment with the same process parameters, the microstructure of the high entropy alloy laser cladding coating was significantly improved. After treatment, the volume wear rate of the CoCrFeMnNiTi high entropy alloy laser cladding coating decreased from 1.90×10′-4 mm’3 / (N-m) to 0.71×10′-4 mm’3 / (N-m); however, due to the change of wear mechanism and the increase of surface brittleness, the wear rate of the CoCrFeMnNiMo high entropy alloy laser cladding layer increased instead.
Zhang et al. prepared AlCoCrCuFeNi high entropy alloy coating by laser cladding technology in order to study the effect of ultrasonic shock on the microstructure and properties of laser cladding high entropy alloy coating, and strengthened the surface of the coating by ultrasonic shock. The laser cladding AlCoCrCuFeNi high entropy alloy coating is composed of FCC phase and BCC phase. After ultrasonic shock strengthening, the AlCoCrCuFeNi high entropy alloy coating is composed of FCC phase and BCC phase. No new phase is formed in the AlCoCrCuFeNi high entropy alloy coating, but the volume fraction of the BCC phase increases, and a hardened layer with a depth of about 15 μm is formed on the surface. The average grain size decreases from 34.4 μm to 18.6 μm; the surface hardness of the coating is 715 HV0.2, and then gradually decreases to the hardness of the coating without ultrasonic impact (530 HV0.2). The ultrasonic impact strengthened AlCoCrCuFeNi high entropy alloy coating has high corrosion resistance in 3.5% (volume fraction) NaCl solution, which is related to its low surface roughness and fewer structural defects.

5 Conclusion
This paper introduces the latest research results from four aspects: laser cladding process parameters, alloy composition, annealing and other treatment methods. It can be seen that adjusting the laser cladding process parameters (laser power, scanning Adjusting the scanning rate, laser energy density, powder feeding voltage, oxygen flow rate, etc.) and increasing the content of alloying elements (Cu, Nb, Co, Cr, Mo, Ti, W, etc.) are common methods to refine the microstructure and improve the performance. The performance of high entropy alloy coatings can also be improved by annealing treatment, ultrasonic surface rolling, etc. At present, the research on high entropy alloys is not sufficient, and the research on the composition system of high entropy alloys should be strengthened in the future. In addition, new processes and technologies for preparing high entropy alloy coatings should be further developed.