High-hardness and wear-resistant coating refers to a thin layer that is metallurgically bonded to the substrate and has strong local resistance to indentation and mechanical wear. Laser cladding technology is a new, green and efficient surface treatment technology with the advantages of fast cooling speed, low dilution rate, small thermal deformation and controllable thickness. It has broad application prospects in the fields of high-end manufacturing equipment such as transportation, mining, petrochemicals, and metallurgy. The research progress of laser cladding technology in preparing high-hardness and wear-resistant coatings was reviewed from four aspects: powder design, laser cladding process, statistical calculation and simulation, and laser cladding auxiliary technology. In terms of powder design, guided by the coating optimization results, the research on the second phase strengthening type, fine grain strengthening type, microstructure optimization type and other types of design in the preparation of high-hardness and wear-resistant coatings was reviewed. In terms of laser cladding process, the influence and mechanism of process parameters on coating performance and quality during cladding process were introduced, and reasonable optimization suggestions were put forward. In terms of statistical calculation and simulation, the role of statistical calculation and simulation in coating preparation, cladding process optimization, coating microstructure performance optimization and cladding theory research was outlined. In terms of laser cladding auxiliary technology, the paper summarizes the auxiliary technologies such as acoustic field, electric field, magnetic field, thermal field, mechanical field and spectral detection, and introduces the influence and mechanism of auxiliary technologies on the microstructure and performance of coatings. Finally, the research on laser cladding to prepare high-hardness and wear-resistant coatings and related technologies is prospected.

According to relevant reports, the annual loss caused by material wear in my country is as high as hundreds of billions of yuan. In the field of high-end manufacturing, the wear of equipment parts often adds extra time and economic costs. In order to reduce the loss, surface strengthening treatment is usually used to improve the wear resistance and service life of the parts. For example, in the field of metallurgy, cold rolling rollers are usually plated with hard chrome on the surface to increase the service life. Due to the problems of environmental pollution and harm to human body, chrome plating will gradually be banned [1]. The use of new technologies and new processes to prepare high-hardness and wear-resistant coatings is particularly important in the current era of increasing emphasis on low-carbon green environmental protection. Laser cladding technology is an environmentally friendly new technology that adds alloy, ceramic and other special powders to the surface of the substrate, uses a laser beam to melt the powder and the surface of the substrate, and self-excites cooling to form a metallurgical bonding coating. This technology has the advantages of high deposition efficiency, controllable thickness, small thermal deformation, fast cooling, small dilution rate and metallurgical bonding. It is often used to prepare high-hardness and wear-resistant coatings with strong bonding and few defects. This paper will review the research progress of the current laser cladding technology for preparing high-hardness and wear-resistant coatings from four aspects: material design, process optimization, statistical calculation and simulation, and auxiliary technology research, including powder design methods to improve the mechanical properties of coatings; the role and results of process optimization in regulating the organization, performance and quality of coatings; prediction of cladding results and optimization of cladding processes through statistical calculation and simulation; laser cladding auxiliary technology and its effects and mechanisms. The relevant research contents covered are shown in Figure 1. At present, the research work on laser cladding high-hardness and wear-resistant coatings mostly starts from a certain aspect to optimize the performance, quality and preparation process of the coating, and has not yet formed a coating preparation and research and development system with multi-path optimization and wide coverage. With the improvement of coating service requirements, the complexity of application scenarios and the development, maturity and connection of laser cladding high-hardness and wear-resistant coating technology in various sub-segments, laser cladding high-hardness and wear-resistant coating technology will gradually form a preparation and R&D system covering material design, process optimization, statistical calculation and simulation and auxiliary technology.

1 Powder design
The type of powder and powder ratio fundamentally determine the performance of the coating, so the rational design of the cladding powder formula is an important part of the experimental research. In addition, the improvement of coating performance is related to its strengthening method, so the powder design guided by the strengthening result is particularly important. Among them, the improvement of coating hardness and wear resistance can be achieved through second phase strengthening, fine grain strengthening, solid solution strengthening, microstructure optimization, coating amorphization, coating structure optimization and other methods. At present, a large number of studies have successfully prepared high-hardness and wear-resistant coatings of excellent quality through composition design, macro design and microstructure optimization. For example, the introduction of rare earth elements, gradient design, in-situ generation of strengthening phases, external strengthening phases, etc.

1.1 Second phase reinforced powder design
Common types of reinforcement phases are carbides, borides and oxides. According to the formation method of the reinforcement phase of the coating, it can be divided into mechanical addition generation and in-situ generation. Zhang et al. prepared TiCx coating on the surface of 40Cr gear steel by mechanical addition generation and in-situ generation of reinforcement phases. The results showed that the in-situ generated reinforcement phase can reduce the number of cracks, but the low content of TiC and FeTix soft phase formed made the hardness and wear resistance of the coating lower than that of the mechanical reinforcement phase coating. Li Rui et al. conducted a laser cladding composite coating study on the surface of Ti811 alloy. The results showed that the intermetallic compound Ti2Ni and the hard reinforcement phase TiC were generated in situ in the coating. The hardness of the composite coating reached 1 303.5HV0.5, and the soft lubricating phase CrxSy was formed. The average friction coefficient was reduced to 0.29, and the wear volume was 8.5% of the base material. Qin Xin et al. melt-coated NiCrCoAlY-Cr3C2 composite coating on the surface of TC4 alloy. Dispersed Cr7C3 and TiC ceramic particles were generated in situ in the coating, making the hardness of the coating higher than 1 200HV, and the wear quality was only 0.9% of that of TC4 alloy. Wang et al. prepared NiCrCoAlY coatings with different B4C contents on the surface of TC4 alloy. The study found that TiC and TiB phases were generated in situ in the coating, and the two phases were used as nucleation points to promote nucleation and effectively refine the grains, so that the hardness of the NiCrCoAlY composite coating reached more than 1 000HV, reducing the friction coefficient and wear rate of the coating. Guo et al. mixed WC-Ni powders of different masses with Ni-based powders and prepared a series of composite coatings with hardness ranging from 850 to 1200HV0.2 on the surface of stainless steel. Li et al. prepared graphene-reinforced Ni60 composite coatings by using graphene powder modified by nano-Ni. The coating hardness reached 920HV0.2, the friction coefficient was reduced by 0.192, and the wear mass was 36% of the Ni60 coating, which effectively improved the hardness and wear resistance of the Ni60 coating. Guo et al. [18] designed a new alloy powder based on Ni60 by adding B, Si, W, CeO2 and other components. The coating showed excellent properties of high hardness, wear resistance and corrosion resistance under the action of ultrafine composite particles. In addition, TiB2 has high hardness and good wear resistance, and its thermal expansion coefficient is similar to that of Ni-based alloys. He et al. used TiB2 and Ni-based alloy powders to prepare coatings with a hardness of 835.5~888.1HV0.5. Wang Ruixue et al.’s research shows that adding nano ZrO2, which undergoes crystal transformation with temperature, to the Ni-Mo coating can effectively reduce the cracks and pores of the coating. The formed Ni3Zr and undecomposed ZrO2 increase the hardness of the coating by 39.7%. In addition, attention should also be paid to the elemental composition of the coating. Savrai et al. [21] prepared a Co-31.8Ni-22.6Cr-4.1W coating with a hardness of 810HV0.1. Since the chromium element in the alloy hinders the formation of tungsten carbide, the content of the strengthening phase decreases, which reduces the wear resistance of the coating. Zhang et al. prepared a 24CrNiMoY alloy coating containing carbon fiber. The research results show that carbon fiber, as a strengthening phase particle, can promote the transformation of the 24CrNiMoY alloy matrix to martensite and form a nanoparticle strengthening phase, which increases the Vickers hardness of the coating to 835HV0.2 and significantly improves the wear resistance of the coating. The schematic diagram of the dissolution of carbon fiber is shown in Figure 2. Introducing a strengthening phase into an alloy coating can increase the nucleation rate of the coating, refine the grains, form a new phase, increase the content of solid solution elements in the matrix, effectively improve the average hardness of the alloy coating, reduce the contact wear between the low-hardness matrix and the friction pair, and use the strengthening phase as a supporting skeleton to resist the invasion of external hard objects, thereby improving the hardness and wear resistance of the coating. At present, many studies often introduce carbides (such as TiC[11], borides and oxides as second phases to prepare high-hardness wear-resistant coatings. The sharp corners of the mechanically added phase are prone to become crack sources for stress concentration. The high-hardness wear-resistant coating prepared by the external strengthening phase is also prone to cracking, warping, strengthening phase precipitation, uneven microstructure and performance, and low surface flatness. In response to the above problems, the following measures can be taken to optimize the coating.
1) Fully consider the thermal physical properties of the matrix phase, strengthening phase and cladding matrix. When designing the composition, the geometric morphology, thermophysical properties, crystal properties and the formation ability of each element phase of the material are comprehensively considered to prepare a cladding coating with low residual stress, few defects, uniform structure, high hardness and excellent wear resistance.
2) Control the energy input to reduce the ablation of the strengthening phase in the molten pool, and avoid excessive growth of the strengthening phase to weaken the coating performance.
3) Design a reasonable cladding strategy to reasonably control the heat accumulation of the coating during the cladding process and reduce the residual stress in the coating.
4) Design a reasonable component ratio to avoid the decrease in the fluidity and thermal conductivity of the molten pool. Auxiliary technology is used to affect the flow of the molten pool and optimize the spacing and distribution of the strengthening phase in the coating.

1.2 Fine-grained reinforced powder design
The refined grains increase the number of grain boundaries in the material, increase the resistance to dislocation movement, and effectively improve the hardness of the coating. Liu Xiaoyu et al.’s research shows that the 17Cr2NiSi iron-based coating doped with trace amounts of nano-yttrium oxide (Y2O3) has a transformation from dendrite to fibrous crystal, reduced oxidation wear by 90%, less blocky plastic peeling, and reduced friction coefficient. Yin et al. prepared a F55 alloy coating containing Y2O3. The results show that the addition of Y2O3 can promote the precipitation of Cr and Si, reduce the content of retained austenite, and reduce the structural stress caused by the transformation of retained austenite to martensite during reciprocating cladding. At the same time, it can refine the grains, relieve stress concentration at the grain boundaries, reduce the precipitation of brittle phases at the grain boundaries, reduce crack sensitivity, and greatly improve the wear resistance and cladding quality of the F55 coating, such as surface flatness, bonding strength, porosity, crack count, etc. Zhang Jiacheng et al. prepared a Ni60A/WC composite coating containing carbon nanotubes. The addition of carbon nanotubes effectively promoted heterogeneous nucleation and grain refinement, and improved the hardness of the coating. In addition, carbon nanotubes have a self-lubricating function, which can significantly reduce the friction coefficient of the coating and effectively enhance the wear resistance of the cladding layer. Liang et al. added a small amount of LaB6 to Ti6Al4V and B4C powders, which effectively avoided powder agglomeration and made the in-situ generated La2O3 more evenly distributed. La2O3 precipitated at the grain boundaries of strengthening phases such as TiB2 can refine the strengthening phase, form a large number of dispersed strengthening phases, and increase the fluidity of the melt. At the same time, the addition of element B also promotes the generation of more TiB2, so that the hardness of the coating reaches up to 1 243.4HV0.2, which has extremely strong wear resistance. The mechanism of action of LaB6 is shown in Figure 3. Wang
Chenglei et al. found that adding appropriate CeO2, Y2O3, La2O3 and other components to Ni60 coating can effectively refine the grains, improve the fluidity of the molten pool, reduce the number of pores and cracks in the cladding layer, and prepare a wear-resistant coating with a hardness of up to 1 300~1 400HV0.1. Xu Huanhuan et al. found that adding 1% CeO2 by mass when preparing Ni-WC coating can increase the nucleation rate, effectively weaken the WC precipitation phenomenon, and improve the microhardness of the coating. In summary, introducing rare earth elements and carbon nanotubes and other nucleation materials into the molten pool as a refinement of the coating structure can improve the cladding quality and reduce cladding defects, which is an effective way to improve the hardness and wear resistance of the coating.

1.3 Powder design with optimized structure
The wear resistance of the coating is related to the phase composition and microstructure, especially the morphology and distribution of the hard support phase in the coating. Common hard support phases usually appear in the form of cellular crystals, dendrites or eutectic morphology, in the form of islands or networks. Since the content and morphology of the strengthening phase have a great influence on the wear resistance of the coating, the stress state of the coating can be optimized and the hardness and wear resistance of the coating can be improved by regulating the phase ratio, morphology and uniformity of the microstructure of the coating.
Yin Guili et al. added 15% Cr3C2 by mass to Fe55, which changed the solidification mode and organizational morphology of the alloy, effectively controlled the precipitation of carbides, changed the content and morphology of the eutectic structure, and improved the wear resistance of the coating. Polak et al. studied 5 different carbide-reinforced laser cladding coatings and studied the relationship between carbide structure and coating by regression analysis. The results show that the average spacing of the strengthening phase has a greater effect on the wear rate than the diameter and content of the strengthening phase. As the particle spacing increases, the wear rate increases, and the relationship between the average particle diameter and content and the wear rate is the opposite. Pereira et al. studied the dry friction and wear properties of (Ni/Co) MCrAlY laser cladding layer based on AISI304 at low and high temperatures. The study showed that the Ni-based coating with high Al content has more β phase and cellular crystal structure, while the Co-based coating is coarse dendrite. In comparison, the Ni-based coating has higher hardness and wear resistance. Shang et al. found that adding B and Si to the W10V5Co4 alloy coating can promote the growth of the network eutectic composed of boron carbide and form more twin martensite. The hardness of the coating increased by 159.7HV0.2 (908HV0.2), and the wear resistance was 2.8 times that of the original. Cao et al. clad Cr3C2-reinforced Ni60A-Ag composite coating on a Cu substrate with NiCr as a transition layer. The design of the NiCr layer effectively reduced the defects at the interface between the coating and the substrate and avoided excessive dilution of the coating components by the substrate. The reinforcement phase Cr7C3 precipitated in the coating and the uniformly distributed Ag particles not only increase the hardness of the coating, but also reduce the friction coefficient of the coating, effectively enhancing the wear resistance of the coating.
It can be seen that taking the reinforcement phase morphology distribution, soft-hard phase ratio and transition layer design as considerations for coating design and preparation can enhance the feasibility of the experiment and promote the development and preparation of high-hardness and wear-resistant coatings.

1.4 Other types of powder design
In addition, there are gradient powder designs that aim to enhance toughness, optimize combined performance and stress distribution, powder designs with solid solution strengthening as the main strengthening mechanism, and powder designs for forming high-hardness and wear-resistant coatings containing amorphous structures. Chen et al. prepared a gradient coating with a maximum hardness of 876HV and good adhesive wear resistance on a Cu substrate by designing a Ni/Co content gradient. The functional schematic diagram of the gradient coating is shown in Figure 4. Subsequently, the team prepared a gradient coating with better strength and toughness on the copper surface, and no defects such as cracks and pores on the surface and inside by changing the content of elements such as Co, Cr, Fe, W, and Mo in the Ni-based alloy. The hardness of the coating is 8.3 times that of the copper substrate (up to 834HV), and the wear resistance is 8.4 times that of the copper substrate. Through reasonable composition gradient design and composition optimization, the difficult-to-combine materials can form a good combination, the stress distribution of the coating is improved, the plasticity and toughness of the material are enhanced, and the coating has better hardness and wear resistance. In addition, there is also a powder design with solid solution strengthening as the main strengthening mechanism. Liu et al. prepared a TiC in-situ generated high entropy alloy coating with a hardness of 1 020HV0.3. The high hardness of the coating is the result of the combined effect of solid solution strengthening, second phase strengthening and fine grain strengthening. Liu et al. mixed cobalt-based amorphous powder with iron powder in different proportions to prepare a series of amorphous/composite coatings with a hardness of 850~1 550HV0.2. Then, the powder was pre-formed on 1045 steel to prepare a Co-based amorphous coating, whose hardness and wear resistance were significantly improved. The microhardness of the coating was greater than 1 500HV0.2, and the wear resistance was 20 times that of the substrate, but the wear mechanism became more complicated. Zhao et al. prepared a FeCo-06 amorphous cobalt-based cladding layer with the help of ultra-high-speed cooling technology. The results showed that the hardness of the coating reached 1 200HV0.2. Affected by the amorphous structure, the wear resistance and high-temperature oxidation resistance of the coating were significantly improved. The introduction of amorphous coatings provides a new method to improve the hardness, wear resistance, corrosion resistance and oxidation resistance of laser cladding coatings. However, the coatings prepared by this method have problems such as easy cracking, poor toughness and excessive residual stress. Gradient design can effectively improve the stress distribution of the coating, improve the plasticity, toughness and bonding performance of the coating. Combining gradient design with the characteristics of amorphous structure, designing the coating composition, and reasonably regulating the content of amorphous structure by gradient, it is expected to prepare high-hardness and wear-resistant coatings with excellent mechanical properties, good plasticity and toughness, and good stress state.

2 Laser cladding process
Laser cladding process parameters include laser power, scanning speed, powder feeding rate, powder-carrying gas flow, overlap rate, cladding path and number of cladding layers. The changes in these process parameters not only indicate that the interaction between the beam, powder, molten pool and substrate has changed, but also indicate that the way energy is used or dissipated during the cladding process has changed. The adjustment of these process parameters often changes the geometric morphology, microstructure and heat balance of the cladding coating, thereby affecting the mechanical properties, surface quality, stress state and cladding efficiency of the cladding coating. Gong Yuling et al. clad Ni60A coating on the surface of TC4. The study found that a high overlap rate can easily lead to dilution of the strengthening phase elements in the coating, a decrease in the content of the strengthening phase, and a decrease in the hardness of the coating. A low overlap rate is not conducive to the accumulation of heat and the precipitation of the strengthening phase. Chen et al. found in the study of multi-layer Fe55 coating that multi-layer cladding can effectively reduce the dilution of the working layer by the substrate, so that the outer surface of the coating has more dispersed strengthening phases and a finer structure, while tempering the previous cladding layer, improving the toughness, hardness and wear resistance of the cladding coating. Liu Shuai et al. studied the effect of the scanning method (Figure 5) on the hardness of the coating. The results showed that the use of the same parallel reciprocating scanning method between layers to prepare the coating can reduce heat accumulation and prepare a coating with higher hardness. Relevant studies on laser power show that excessive laser power will cause serious burning of B and C elements in the coating, severe heat accumulation, increase the dilution rate of the coating, coarsen the microstructure, and significantly reduce the hardness. The increase of dilution rate often leads to the decrease of coating performance. The actual situation is related to the composition of substrate and coating. Sometimes, a higher dilution rate will intensify the flow of molten pool, promote the material exchange of bonding interface, and be conducive to the formation and precipitation of phases, such as precipitation of more carbides to increase the hardness of coating. Xu Nijun et al. studied DL2501 and DL155 gradient coatings with surface hardness of 763HV. The results showed that the hardness of coating decreased with the increase of scanning speed, which was caused by insufficient input energy and less strengthening phase formation. Li Jinbao et al. conducted dimensionless treatment on laser power, scanning speed, powder feeding rate and characteristic energy ratio in laser cladding process, explained the influence and mechanism of different factors on the geometric morphology, molten pool width and grain morphology of single-pass deposition layer, and obtained the reasonable range of characteristic energy ratio E0*, and prepared the ideal single-pass deposition layer. Guo et al. [57] found that when the energy surface density was 72 J/mm2, the wettability of the multilayer 24CrNiMoY coating was good, and no defects such as cracks and unmelted areas appeared. The appropriate energy surface density effectively reduced the precipitation of inclusions and reduced the thermal stress of the coating. Guo et al. designed a new Ni60 alloy powder containing B, Si, W and CeO2. The study found that with the increase of scanning speed and supercooling, the grain size of the coating decreased. With the decrease of energy density, the number of small-angle grain boundaries in the coating increased. After optimizing the process parameters, the coating has excellent wear and corrosion resistance. Lian Guofu et al.’s research results on Ni35A+TiC coating showed that excessive powder feeding rate and gas flow rate would take away too much heat from the molten pool, forming light shielding, thereby reducing the cladding efficiency. Xu et al. studied the bonding strength of two metal coatings, iron and nickel, and found that the scanning speed had the greatest impact on the bonding strength, followed by powder type and laser power. Based on this, the coating hardness and bonding strength were optimized as a whole. When there are many variable levels, process variables, and inspection indicators, orthogonal experimental design can be considered. Wu Jun et al. combined orthogonal experiments with weight matrix analysis to study and prepare Ni60AA coatings with a hardness of up to 1 000HV.
In summary, the cladding process has a significant impact on the coating composition, microstructure, cladding efficiency, geometric morphology, cladding quality, stress distribution, bonding strength and mechanical properties, and also affects the material input, energy input and heat accumulation of the cladding system. Conducting cladding process research can provide a theoretical basis for the efficient preparation of coatings with excellent quality and performance. Although process adjustment can improve the quality and performance of the coating to a certain extent, the physical and chemical properties of different types of materials themselves are different, and conventional cladding processes cannot directly intervene in the changes in the molten pool. The coating prepared by process adjustment may still have problems such as pores, cracks, strengthening particle aggregation and large residual stress, which will reduce the research efficiency of the coating preparation process window. In response to the above problems, improvements can be made through the following methods.
1) Simulate the cladding process, or reasonably simulate, analyze and predict the research object through statistical calculation.
2) When designing the coating composition, consider the substrate, transition layer and coating as an integrated whole, use their thermophysical properties as a reference, and select materials in a targeted manner.
3) Reasonably partition and design the cladding layer, regulate the coating structure distribution and formation process, and reduce the residual stress of the coating.
4) Take appropriate post-processing measures to improve the performance of the coating.
5) Use auxiliary technology to regulate the molten pool state and optimize the distribution of solute elements and strengthening particles in the molten pool.

3 Statistical calculation and simulation
Multivariate influence, dynamic change and nonlinear change of coating performance are typical characteristics of laser cladding. These factors restrict the depth and efficiency of researchers’ understanding of related research. In order to accelerate the research process, reasonable and efficient statistical methods can be used to design and analyze the cladding test, and the performance and quality of the coating can be improved through statistical calculation. Based on relevant knowledge such as fluid dynamics, heat transfer and phase field theory, a finite element model can be established to simulate the cladding strategy, geometric morphology, temperature field, flow field, stress field, phase transformation process, etc., and then study the stress distribution, performance fluctuation, phase distribution and microstructure evolution of high-hardness wear-resistant coatings, so as to provide guidance for the optimization of cladding process, the prediction of coating performance and the theoretical research of cladding process.

3.1 Statistical calculation
Laser cladding high-hardness wear-resistant coatings are affected by many factors. During the cladding process, the molten pool undergoes extremely complex physical, chemical and metallurgical changes. Changes in external factors such as air flow fluctuations will affect the performance and quality of the coating. Universal research and statistical work is conducive to deepening the understanding of the theory of preparation of high-hardness wear-resistant coatings. Appropriate statistical analysis and algorithm research can optimize iterative experimental design work, reduce trial and error cycles, and accelerate the process of product development and product system improvement. Polak et al. found that the effect of carbide particle spacing on wear rate is much greater than the effect of carbide average diameter on wear rate. Zhou et al. studied the relationship between laser scanning speed and the microstructure of 12CrNi2 alloy coating. By measuring the content and size of each microstructure, the functional relationship between the content and size of each microstructure and the scanning speed was fitted (as shown in Figure 6), and the test results of coating hardness, wear and impact toughness were analyzed, among which the microhardness results are shown in Figure 7. Effective mathematical statistics methods can reduce a lot of experimental work. Shi Shuting et al. conducted a four-factor three-level orthogonal design experiment, and optimized the hardness and dilution rate of the cladding single pass by weighted method, which accelerated the design process of coating preparation process. Zhang Shengjiang et al. conducted a cladding test on the surface of ductile iron, and compared the orthogonal test optimization results with the NSGA-Ⅱ algorithm optimization results based on the neural network prediction model. It was found that the optimization of the NSGA-Ⅱ algorithm was faster and more accurate, which effectively improved the hardness and surface quality of the coating, but there was a 7%~8% error between the predicted value and the experimental value. To this end, multiple fitness functions should be constructed and the results should be optimized. In addition, the organic combination of statistical methods and classical theories, such as linear regression and empirical electron theory, helps to establish the relationship between the cladding process and the coating performance and quality, study the complex physical, chemical and metallurgical changes in the cladding process, optimize the coating composition, and promote the development of high-hardness and wear-resistant coating powder systems.

3.2 Simulation
Establishing a predictive model can help researchers understand the research object more objectively and comprehensively. Yang et al. established a finite element model of the temperature field of a thin straight-walled sample. Combining simulation and experimental results, they found that reducing the laser power layer by layer and adding an external water cooling device can promote the thermal balance of the molten pool, reduce the spacing between primary dendrite walls, refine the grains, and improve the hardness of the cladding layer. Wu et al. simulated the temperature field and stress field of 316L coating under different scanning strategies. The study found that the unidirectional parallel cladding method makes the cladding interval time between passes the same, the heat accumulation is small, and the temperature field is more uniform. In the same area, the temperature peak of the unidirectional cladding coating is small, and the thermal stress formed between the coating regions and between the coating and the substrate is low, so a fine and uniform structure can be obtained. Yao et al. established a finite element model of Ni60 cladding coating based on H13 steel based on COMSOL, and obtained the optimal process by combining temperature field simulation. The solidification process of the cladding trajectory and the changes in stress in the xyz direction after solidification were described in detail through Von Mises thermal stress cycle curve and stress field simulation, and the reliability of the simulation results was verified by experiments. The changes in the cladding process and cladding strategy have a significant effect on the thermal behavior of the coating cladding process. The flow condition, heat accumulation and temperature field distribution of the molten pool during the cladding process are affected by the cladding thermal behavior. By changing the cladding process and cladding strategy to affect the molten pool solidification process, the microstructure and surface quality of the coating can be indirectly controlled, which can have a significant impact on the coating performance. Liu et al. established a three-dimensional finite element model of multi-pass and multi-layer cladding process that can implement dynamic preheating, combined the simulated thermal cycle curve with the measured results, and elaborated in detail the effect of dynamic preheating on the thermal behavior of the coating cladding process. By simulating the solidification process of the molten pool, the evolution process of the molten pool grains can be studied at the microscopic scale, providing a theoretical basis for the composition design and coating preparation of high-hardness wear-resistant coatings.
In summary, the use of cladding models and cladding strategy simulations can achieve simulation analysis of cladding thermal behavior, optimize the microstructure, and improve the performance of the coating. By simulating the stress field and temperature field, the temperature change and stress distribution of the coating can be understood, which is of great significance for the stress control and process selection of the coating. At present, there are few studies on the construction of multi-field coupling models for high-hardness wear-resistant coatings. Common simulation studies on temperature and stress fields of coating materials are often highly targeted, and their systematicity and applicability need to be improved. In order to further develop high-hardness wear-resistant coating systems and build a high-hardness wear-resistant coating cladding theoretical system, it is necessary to carry out more comprehensive simulation work around the high-hardness wear-resistant coating system, and in-depth study of the influence of cladding material composition, cladding process, cladding environment and cladding strategy on the molten pool flow field, temperature field, stress field and microstructure evolution.

4 Laser cladding auxiliary technology
Process research and computational simulation can help researchers understand and adjust the complex cladding process, but the optimization of the cladding process cannot directly intervene in the changes of the molten pool. It can only adjust the process of the cladding coating under the requirements of coating quality and performance. Affected by the thermal properties of the cladding material itself, the defects of the cladding system, and the evaporation loss of the cladding material, the solidified cladding coating has problems such as low hardness, low wear resistance, uneven microstructure, large residual stress, and decreased coating surface flatness. The use of auxiliary technology can effectively solve the above problems. At present, the research directions of common auxiliary technologies are mainly divided into ultrasound, electric field, magnetic field, heat source, spectrum and mechanical action, which can be used to improve the quality and performance of the coating; regulate the changes of the molten pool; monitor the coating in real time during the cladding process, and quickly measure the quality and performance of the coating. The comparison of the effects of some auxiliary technologies on the hardness and wear resistance of the coating before and after use is shown in Table 1, and the application scenarios and limitations of several commonly used auxiliary technologies are shown in Table 2. Ultrasonic auxiliary technology has the effects of ultrasonic cavitation and acoustic flow enhancement, which can stir the molten pool to avoid uneven distribution of elements and stress fields. Nie Xuewu et al. found that ultrasound can make WC particles in the coating float up in large quantities, avoiding the problems of high-density particles enriching at the bottom of the molten pool and stress cracking. Zhou et al. used non-contact ultrasonic assisted preparation of 24CrNiMoY alloy coating. The study found that ultrasound effectively promoted the escape of bubbles in the molten pool, reducing the porosity of the single cladding from 5.7% to 0.7%. Under the action of ultrasonic cavitation, the nucleation rate of the molten pool increased, the dendrites were broken, and the grains were significantly refined. When the ultrasonic action angle was 60°, the microhardness and wear resistance of the coating were optimal, and the wear surface of the coating was shown in Figure 8.
Zhang et al. used the acoustic-magnetic composite field to stir the molten pool, strengthened the Marangoni convection, accelerated the discharge of pores, inhibited the growth of columnar dendrites, and effectively improved the hardness of the coating. In addition, ultrasonic impact can form non-contact deformation strengthening on the coating surface. Li et al. applied ultrasonic impact to the cladding coating to form a thin impact layer with obvious plastic deformation, which effectively reduced the surface roughness of the coating and improved the hardness and wear resistance of the coating. In addition, ultrasonic detection data can indirectly and quickly reflect the uniformity of material organization and performance. Zhan Yu et al. calculated and analyzed the ultrasonic data measured by the actual excitation of the laser on the surface of the Al plate and the ultrasonic data generated by the finite element simulation. The study showed that the error between the theoretical value and the calculated value of the elastic modulus was less than 1%. Subsequently, the team used laser ultrasonic technology to monitor the residual stress of the TC4 sample prepared by laser additive manufacturing. The detection results were consistent with the measurement results of the drilling method and the finite element simulation results. The residual stress distribution is shown in Figure 9. In addition, with the help of this technology, the relationship between residual stress and process parameters can be regressed and analyzed to determine the influence relationship between the two. Changes in the distribution of the electromagnetic field will affect the motion trajectory of the charged particles in the molten pool, so that the alternating electromagnetic field has the ability to stir the molten pool and crush the grains. Studies have shown that under the action of the alternating electromagnetic field, the organization of the coating is finer and more uniform, and the hardness and wear resistance are improved. However, the alternating magnetic field cannot completely improve the aggregation of the externally reinforced phase in the coating. For this reason, Huo et al. [83] combined the DC field with the steady magnetic field (ECF). The Ampere force generated by the composite field in the molten pool changed the equivalent buoyancy of the particles and pores in the molten pool, which reduced the porosity of the coating and made the distribution of the reinforced particles more uniform. Zhu Ziming et al. [84] studied the effect of the magnetic field waveform on the coating. The results showed that the microhardness of the coating prepared by applying a pulsed magnetic field was higher and the friction coefficient was reduced by 31.46%. Chen et al. studied the effects of SMF steady magnetic field, AMF alternating magnetic field, and PMF pulsed magnetic field on the main properties of the coating structure and found that pulsed magnetic field, alternating magnetic field and PAMF pulsed alternating magnetic field can better reduce the number of cracks and refine the grains. Among them, the pulsed magnetic field has a more obvious effect on the improvement of hardness, but obvious hard phase segregation bands will appear in the coating. The hardness of the coating prepared under the alternating magnetic field is relatively low and the structure is more uniform. The performance of the coating prepared under the pulsed alternating composite magnetic field is between the first two. Auxiliary cooling can refine the coating grains and even produce amorphous coatings with excellent hardness and wear resistance. Induction heating technology can also promote the formation of strengthening phases in the coating, while avoiding the formation of excessive thermal stress, which is helpful for the preparation of defect-free, high-hardness and wear-resistant coatings. Chen et al. used preheating as an auxiliary means to prepare high TiC content and high hardness coatings on H13 steel plates. The results of Zhou et al.’s induction heating of the cladding substrate showed that the external heat source can expand the cladding process range, and the scanning speed and powder feeding rate were increased to 3500 mm/min and 120 g/min, respectively, which effectively improved the preparation efficiency of the coating and prepared a crack-free Fe-based coating with a WC mass fraction of 20%. Zhou et al.’s research showed that with the increase of the preheating temperature of the substrate, the cracks in the 24CrNiMoY coating were gradually eliminated and the pores were significantly reduced. When the preheating temperature is 200 ℃, the mass ratio of granular bainite to lath pearlite is close to 1:2. At this time, the coating has fine grains, more uniform structure, the highest hardness, and the best toughness. In summary, adding auxiliary heat source during the cladding process can effectively regulate the coating microstructure and reduce coating defects, but excessive heat input can easily cause excessive heat accumulation, increase energy consumption, and coarsen the coating grains, resulting in a decrease in coating hardness and wear resistance. The element content of the cladding layer is often affected by factors such as coating dilution and burning, and the change in the content of each element directly affects the performance of the coating. At present, the composition detection of the cladding coating and the analysis of the causes of fluctuations in the composition and properties of the coating often require destructive sampling of the cladding coating. The application of non-destructive monitoring technology can avoid destructive sampling and reduce research costs. Wang et al. designed several groups of Ni-based coatings with different parameters, and collected the spectra excited by the coatings in real time during the cladding process, corrected the influence of the gas phase spectrum on the liquid phase spectrum, established a calibration curve, and established a real-time monitoring system. The results show that the error of the monitoring results of components with a mass fraction higher than 3% in the molten pool is less than 8%. In addition, sampling is also required to measure the mechanical properties of the coating. Based on the preparation test of AISI 4140 coating, Wang et al. established a real-time monitoring system for the microhardness of the molten zone by collecting spectral data. The maximum relative error between the monitoring result and the actual hardness is 3.09%, and the comparison result is shown in Figure 10. The system energy utilization rate and cladding accuracy have an important influence on the cladding process. Kugler et al. [92] studied the monitoring of the cladding environment and found that during the cladding process, the molten pool showed obvious element burning phenomenon, and a large amount of toxic aerosol composed of nano-scale heavy metal particles evaporated and precipitated near the molten pool. The attenuation rate of the aerosol to the energy of the high-power laser beam reached more than 10%, and a large amount of heat accumulated in the molten pool and the substrate was taken away by the protective airflow and the powder-carrying airflow. It can be seen that the application of non-destructive testing and online monitoring technology is of great significance for coating preparation, quality control, energy saving and environmental protection.
Applying mechanical action to the coating in the solidified state can also improve the performance of the coating and improve the microstructure of the coating. Zhao et al. found that synchronous rolling can break columnar dendrites, improve organizational uniformity, promote the precipitation of strengthening phases, avoid periodic changes in the organization and mechanical properties of the cladding layer, and improve the hardness and wear resistance of the coating. The introduction of external mechanical action is an important supplement to the control of the entire cladding process, and also provides a new way to optimize the performance and organization of the cladding coating.
Laser cladding is a dynamic process of rapid non-equilibrium solidification, from the melting of powder to form a molten pool, to the precipitation of phases and solidification of the coating, this process is accompanied by complex physical, chemical and metallurgical changes. The condensation process is often accompanied by the following problems: the formation of stress and defects; the external strengthening phase is unevenly distributed in the coating; the microstructure and performance of the coating are different; during the cladding process, the molten pool is seriously affected by process fluctuations and the external environment. Auxiliary technology provides a more direct means for coating performance and quality control, cladding process monitoring, performance testing, etc. It has significant advantages in regulating the flow of molten pool solutes, grain evolution, defect reduction, organizational refinement and organizational homogenization, and effectively makes up for the shortcomings of process research and computational simulation in coating quality optimization and preparation. In summary, it can be seen that in the future, auxiliary technology will be of great significance to shorten the research and development time of high-hardness and wear-resistant coating materials, improve the preparation process, enhance the efficiency of coating preparation, and process-based industrial production. The current development and promotion of auxiliary technology still faces the problem of adaptation and running-in with numerical control, electrical and computer, and the research on the coupling of multiple auxiliary technologies and the industrial application of auxiliary technologies are still relatively rare.

5 Conclusion
For the development and application of laser cladding high-hardness and wear-resistant coatings, the four aspects of powder design, laser cladding process, statistical calculation and simulation, and laser cladding auxiliary technology complement and promote each other. In the preparation and material development of high-hardness and wear-resistant coatings, we can try to continue research work from the following aspects in the future.
1) Powder and coating design play a decisive role in coating performance. Reasonable powder design can greatly improve the hardness and wear resistance of the coating. Improper powder and coating design can easily lead to coating performance lower than expected or even failure. In the process of powder design and new powder research, it is not only based on the strengthening effect, but also its thermal physical and chemical properties should be evaluated, measured and recorded, and a relevant database should be established. Combined with relevant data, the expected performance of the coating is analyzed, and the coating structure and cladding strategy are designed and planned in a targeted manner.
2) The laser cladding process is a multi-field coupling process with the participation of multiple factors, which is closely related to powder, substrate, laser, molten pool, airflow and atmosphere. At present, there have been many studies on the temperature field, stress field, flow field and coupling field of the molten pool, while the energy and mass transfer model and microstructure evolution model involved in the cladding process need to be improved. In order to optimize the preparation process of high-hardness and wear-resistant coatings and develop high-hardness and wear-resistant coating material systems, it is necessary to carry out more comprehensive multi-factor intervention cladding process simulation research on high-hardness and wear-resistant coatings. On the basis of accuracy and reliability, the key organizational properties of the cladding layer are simulated, predicted and optimized, and theoretical research and simulation work related to the evolution of the cladding process are carried out to provide a theoretical basis for the research, application and development of laser cladding technology.
3) The quality and performance of the coating are related to the process design and stability of the cladding process. A variety of auxiliary technologies can be integrated to optimize the microstructure and surface quality of the coating. In addition, the corresponding real-time monitoring system is equipped to monitor and adjust the molten pool temperature, molten pool elements, material transportation and power fluctuations through signals such as images, temperature and plasma spectra, so as to achieve the purpose of stabilizing the coating quality and reducing defects. The introduction of auxiliary equipment is of great significance to the industrial development of laser cladding, and it also puts forward higher requirements for the automation, informatization, intelligence and integration of cladding equipment.
4) The traditional iterative material research and development cycle is long. On the basis of innovative design, we can combine the composition and process design with machine learning and high-throughput computing to carry out full-process, parallel and parallel mode research, establish a powder database covering the relationship between composition, process, organization and performance, and accelerate the development of laser cladding high-hardness and wear-resistant coating materials and other functional coating material systems. While ensuring that the coating has high hardness and wear resistance, we will carry out research on coating adhesion, fatigue life, corrosion resistance and high-temperature oxidation resistance according to specific needs.