My country is a major agricultural country in the world. For example, it ranks first in cotton production, second in rice planting area, and first in rice production. However, the current development of agricultural machinery is far from matching its scale. Since the end of the 19th century, my country has begun to promote the development of agricultural machinery towards mechanization and automation on a large scale. According to statistics, the degree of agricultural mechanization in my country has reached 48% in 2000. However, due to the low manufacturing process and level of agricultural machinery in my country, there is still a large gap between product reliability, safety, durability, and adaptability and those of developed countries. Among them, the quality of key parts of agricultural machinery directly affects the service life and working performance of agricultural machinery. At present, there is a big gap between the service life of agricultural machinery in my country and that in foreign countries. The failure of key wear-resistant parts of cutting tools such as rotary tillers, harvester blades, and furrow openers has greatly increased the maintenance cost and seriously affected the working efficiency of agricultural machinery. Most of the current maintenance of agricultural machinery is just simple disassembly and replacement of parts, which not only increases the operating costs of agricultural machinery enterprises, but also hinders the promotion and popularization of agricultural mechanization in my country. Therefore, exploring advanced repair technologies and performance improvement methods for key parts of agricultural machinery is of great practical significance for improving the service performance of agricultural machinery and extending its service life.

Since its advent, laser technology has been widely used in agriculture, industry, science and technology, national defense and other fields. The application of advanced laser technology in the agricultural field has played a significant role in promoting the relatively slow development of traditional agriculture. Lasers are widely used in agriculture, among which laser cladding technology based on agricultural machinery repair and strengthening is the most eye-catching. Laser cladding repair of damaged parts of agricultural machinery with alloy materials with special properties can not only restore the original size of the defective parts, but also improve the quality of the parts to improve service performance. It is of great significance to extend the trouble-free working time of agricultural machinery, improve the production efficiency of agricultural machinery, and promote the improvement of my country’s agricultural machinery level. The research status and application of laser cladding in the field of agricultural machinery repair and strengthening are explained, and the future development trend is prospected.

2 Laser cladding technology
Laser cladding is a surface modification technology for substrates. By presetting the cladding path, laser irradiation is used to make the cladding material and the surface of the substrate quickly melt into a molten pool where the laser passes, and then quickly solidify into a cladding layer with metallurgical bonding and low dilution, so that mechanical parts can be repaired in situ, or the wear resistance, corrosion resistance, heat resistance, and oxidation resistance of the original substrate surface can be improved. Compared with traditional processing technology, laser cladding technology has the following characteristics: the cladding layer can form a metallurgical bond with the substrate; the substrate is less affected by heat and is not easy to deform, the dilution rate of the cladding layer is low, and expensive cladding materials are saved; the thickness of the cladding layer can be controlled; specific parts of the parts and parts that are difficult to be treated by other methods can be repaired.

Laser cladding technology can be used to repair and enhance the surface performance of agricultural machinery parts. Before cladding and repairing agricultural machinery parts, the surface of agricultural machinery parts needs to be pretreated. Due to the harsh working environment of agricultural machinery, the surface of agricultural machinery parts to be laser clad often carries dirt, oil, moisture, fertilizers and pesticides, rust, oxide layer, etc. Therefore, the pretreatment means are more complicated than the repair of parts in other fields. For agricultural machinery parts, the general operation is to wash away the surface dirt of the soil-touching tillage parts with water first. If the dirt on the damaged surface is relatively firm, it can be cleaned by mechanical sandblasting. For parts such as engines and shafts, the oil is first cleaned with a detergent heated to a certain temperature. After preliminary cleaning, the surface to be clad is sanded with sandpaper and cleaned with acetone to obtain a smooth and smooth surface, so as to avoid surface stains entering the cladding layer during laser cladding, causing cladding defects and affecting the quality and performance of the cladding layer.

3 Application status of laser cladding technology in agricultural machinery repair and strengthening in my country
The main technologies for traditional agricultural machinery repair are heat treatment, infiltration sawing, arc spraying, etc., but they are difficult to meet the performance requirements of wear resistance, corrosion resistance, and pollution-free required by agricultural machinery. With the continuous breakthroughs in laser cladding technology, the performance of the prepared cladding coating is becoming more and more excellent, and the cost is becoming more and more economical. Therefore, scholars have begun to study the application of laser cladding technology in the repair and strengthening of agricultural machinery. my country’s research on the repair and strengthening of agricultural machinery by laser cladding started in the early 21st century, 10 years behind foreign advanced technology. Ma Yulei used laser cladding technology to repair the engine crankshaft of imported high-horsepower tractors. The repaired crankshaft not only extended its service life, but also reached or exceeded the quality of new products after repair. Zheng Xu et al. established a three-dimensional model of the missing parts of the tractor engine cylinder, designed the cladding path, and used CNC programming to automate the operation. They performed laser cladding repair on the tractor engine cylinder and verified its feasibility. Wang Jiasheng et al. used laser cladding technology to remanufacture and repair the shaft surface of the tractor main shaft made of 40Cr, and prepared a N160A alloy coating with high wear resistance. When the defocus amount was 8 mm, the surface of the obtained cladding layer was smooth and flat, and there were no defects such as pores, cracks and slag on the bonding surface.

In addition to repairing damaged parts, laser cladding technology can also be used to enhance the performance of existing agricultural machinery parts. As early as 2009, Chen Zhuojun and others explored the laser treatment of the surface of rotary tiller blades. By designing orthogonal experiments and performing range analysis, they found that the main factors affecting the surface strengthening effect of the substrate were laser power > scanning speed > spot diameter. After the material was melted by laser surface, the matrix structure was significantly refined, and many fine high-carbon martensite was formed in the cladding area, which increased the microhardness and improved the wear resistance of the rotary tiller blade. Tian Yongcai conducted further research on this and optimized the combination of process parameters. The hardness of the cladding coating can reach up to 820HV., and the abrasive wear resistance of the rotary tiller blade is significantly improved. Yan Yong used laser cladding technology to clad high-performance wear-resistant sickle-based alloy materials on the surface of the deep tillage shovel tip to increase its service life, and field tests showed that the friction and wear performance of the deep tillage shovel tip clad with optimized orthogonal experimental parameters was significantly improved. Ye Pengyun et al. designed an optimization experiment to compare the effects of ordinary heat treatment, laser heat treatment and laser cladding on extending the service life of lawn mower blades, and found that laser cladding was better than the first two. In order to improve the wear resistance of 65 Mn steel hammer of citrus branch crusher, Meng Liang et al. used laser quenching, water quenching-medium temperature tempering and laser cladding technology to test. The three methods were compared and the microhardness of the laser cladding treatment was the highest and the average friction coefficient was the lowest, indicating that laser cladding treatment of 65 Mn can effectively improve the wear resistance, the process is simple, and it meets the performance requirements of the hammer surface.

4 Prospects of laser cladding technology applied to the repair and strengthening of agricultural machinery in my country
Compared with high-end manufacturing fields such as industrial machinery, aerospace, and automobiles, agricultural machinery manufacturing always lags behind other fields, with low manufacturing level and slow development. In order to promote the development of agricultural modernization, it is necessary to strengthen the application of laser cladding technology in the repair and strengthening of agricultural machinery. Learn from the research results of advanced laser cladding technology in other fields to provide research directions for the repair and strengthening of agricultural machinery. Therefore, in order to improve the reliability of agricultural machinery in complex soil environments, the transformation and application of laser cladding technology in the repair and strengthening of agricultural machinery can be studied from the following four aspects: in-situ repair; improve wear resistance; improve corrosion resistance; improve hardness.

4. 1 In-situ repair
Agricultural machinery has high use intensity and poor working environment conditions. Many agricultural machinery parts are in a state of overload operation for a long time during repeated cyclic use, so it is easy to have defects such as plastic deformation, wear, cracks, and corrosion. In-situ repair refers to the specific treatment of defective parts to restore their original size, and laser cladding is one of the main in-situ repair technologies. Because the repaired parts have the advantages of not easy to deform, fast cooling speed, high precision, and excellent performance, they have been widely used in the field of agricultural machinery repair. For example, gear parts are under high-intensity cycle operation during the operation of agricultural machinery, and will be affected by strong alternating stress, which is easy to cause flash, tooth gnawing, deformation and other problems. The application of laser cladding in-situ repair technology can restore the original size of the defective gear. Through experimental verification, the gears repaired by laser cladding in-situ can not only complete normal operation, but also greatly improve the impact resistance, hardness, wear resistance and other properties of the gears. Wang Zhijian from South China University of Technology solved the problem of three-dimensional forming of broken gears by laser cladding, and proposed key process measures for high shape precision forming in the example, thus providing a feasible solution for the rapid and high-precision repair of the defective parts of the three-dimensional structure of gears in agricultural machinery. In addition, shaft parts are also one of the parts that often need to be repaired in agricultural machinery. In addition to being affected by alternating stress, shaft components are also affected by friction and wear, and the impact of friction and wear will be more significant, which is also an important reason for their damage. The working environment of agricultural machinery is relatively harsh. During the long-term rotation of the internal shaft, high-hardness sand particles will infiltrate and cause wear. The result of wear is regional deep scratches, which enhance the effect of abrasive particles, thereby aggravating the damage process and forming a vicious circle. The application of laser cladding technology to in-situ repair of bearings can fill the scratches and restore the surface morphology of the shaft. The thickness of the coating prepared by the laser cladding in-situ repair technology is relatively thin, and the operator can effectively control the thickness of the cladding coating, thereby ensuring the geometric tolerance and dimensional accuracy of the repaired parts. Luo Xingxing used ANSYS simulation software to determine the feasibility of the laser cladding repair process for shaft parts, and experimentally realized the laser cladding repair of 45 steel-based shaft parts. The various indicators were evaluated to meet the repair needs. In summary, the use of laser cladding technology to repair defective parts in-situ can not only meet the repair needs and achieve the normal working performance of the original parts, but also have a surface strengthening effect.

4.2 Wear resistance
The wear in agricultural machinery is generally divided into adhesive wear and abrasive wear, among which abrasive wear is the most common. Abrasive wear is the wear caused by friction between the surface of parts and relatively hard abrasive particles. Direct contact with soil and sand during farming causes serious wear. There are many cladding materials to improve wear resistance, among which iron-based cladding materials are the most widely used in the field of agricultural machinery. Iron-based alloys have been widely used because of their similar composition to agricultural machinery substrates, good metallurgical bonding, and high wear resistance and low cost. Wang Hongli used laser cladding technology to prepare an iron-based alloy cladding layer on the surface of 65 Mn, and obtained a good metallurgical bonding between the cladding layer and the substrate. The microhardness of the cladding layer is 2.5 times that of the substrate, and the wear resistance is 2.6 times that of the substrate. However, iron-based alloys also have disadvantages such as high melting point, poor alloy self-fluxing, poor oxidation resistance, poor fluidity, and more pores and slag inclusions. Later, in order to improve iron-based alloys, Fe-based self-fluxing alloys with B and Si elements were added to further improve the wear resistance of the cladding layer. Fe-based self-fluxing alloy powder is suitable for parts that require local wear resistance and are easy to deform. The matrix is ​​mostly cast iron and low-carbon steel, which is compatible with the materials of agricultural machinery. He Jianqun et al. laser clad Fe55 iron-based self-fluxing alloy powder on the surface of 45 steel, and the hardness of the cladding layer can reach 11 times the original, and the wear resistance is 11 times higher than the original.
In addition, adding rare earth elements to the cladding material can also improve the wear resistance of the cladding coating. Li Qingtang et al. proposed a rare earth element-containing NbC particle reinforced iron-based wear-resistant laser cladding coating and preparation method. The cladding alloy powder is mechanically mixed with CeO2 according to the chemical composition ratio, and the cladding coating is prepared on the surface of the steel matrix. The obtained cladding layer reacts in situ to generate a ceramic hard phase with dense structure, no pores and cracks, and the wear resistance is effectively improved. Rare earth elements can significantly improve the microstructure of the cladding layer and refine the dendrite structure. Among them, grasping the reasonable amount of rare earth element addition is the key. The appropriate addition of rare earth elements can ensure the effective improvement of the wear resistance of the cladding layer.
In recent years, amorphous alloys have gradually become the focus of research on agricultural machinery repair and strengthening due to their excellent wear resistance, magnetism and toughness, high resistivity and electromechanical coupling performance, and simple process. Zhang Luan et al. laser clad amorphous alloy powder and crystalline powder with the composition of Fe79.73B8.86Si11.41 on the surface of 45 steel. The results showed that under the same cladding process conditions, the cladding performance of amorphous alloy powder was better than that of crystalline powder. Tang Cuiyong et al. used laser cladding to carry out Fe60Nb13Ti13Ta13 amorphous alloy powder cladding tests with different process parameters on the surface of 45 steel, and obtained good metallurgical bonding between the coating and the base material, without obvious cracks and other defects; the coating structure was fine α-Fe supersaturated solid solution equiaxed crystals and a small amount of amorphous phase, the average hardness of the coating was 873 HV, about 4 times that of the base material, and the wear resistance of the coating was significantly better than that of the base material.

4.3 Corrosion resistance
The tillage parts of agricultural machinery often work in a humid and corrosive environment such as pesticides, fertilizers, and organic fertilizers, which accelerates the damage of agricultural machinery. The composition of laser cladding powder directly affects the corrosion resistance of the cladding layer. Many scholars have studied the corrosion resistance of different cladding materials. Xu Sicheng and Li Zengquan modified low-carbon steel with Fe-Cr alloy powder to obtain a cladding layer with good corrosion resistance. After electrochemical testing, it was found that it has corrosion resistance equivalent to 316 stainless steel. Wang Enting et al. added trace rare earth elements to the laser cladding corrosion-resistant coating material to strengthen the grain size of the grain boundary surface layer and further improve the corrosion resistance of the matrix. In the research and exploration of corrosion resistance, the research on sickle-based self-fluxing alloy powder in laser cladding materials is the most prominent, and it is widely used in the repair of local corrosion-resistant components. Ye Lujun proposed a laser cladding coating preparation process for sickle-saw borosilicate Ni-based self-fluxing alloy. Through the combination of powder composition, particle size and laser cladding process parameters, a cladding coating with excellent corrosion resistance was obtained. Yu Yueguang et al. invented a method for preparing a corrosion-resistant coating corresponding to sickle-based alloy powder and its specific composition. After the nickel-based alloy powder is clad on a stainless steel and low-carbon steel substrate, the cladding layer has a dense surface, good macroscopic morphology, no defects such as pores and cracks, high hardness, and good corrosion resistance.
With the deepening of research, scholars have found that the addition and control of external field conditions during the cladding process have a significant effect on the corrosion resistance of the cladding layer. Shi Hai prepared Fe60 alloy coating on the surface of 45 steel by laser cladding, and assisted by a composite field device of mechanical vibration and magnetic field. Under the action of the composite field, the Fe-Cr solid solution content in the coating was effectively improved, which significantly improved the corrosion resistance of the coating, and provided guidance for further optimizing the process of laser cladding to strengthen agricultural machinery.

4.4 Hardness
Because there are large stones and large tree roots under the soil, tillage parts such as rotary knives and disc harrows may encounter large impacts and be damaged during the process of plowing and plowing, which places higher requirements on the hardness of agricultural machinery. Zhang Yanli et al. used 45 steel as the base material and conducted a comparative study on the hardness of the cladding layers of Fe60 alloy and Ni60 alloy. Under the conditions of the same laser power and powder feeding amount, the hardness of the Ni60 alloy cladding coating is higher, but there are more crack defects, while the Fe60 alloy has a high hardness in the bonding area, a flat overall hardness distribution, a good metallurgical bond, and no obvious defects. Compared with nickel-based alloys, iron-based alloy powders have ideal comprehensive performance and are more suitable for laser cladding surface treatment of 45 steel. Later, Yi Xiangbin et al. conducted further research on the microstructure and properties of the Fe60 alloy cladding layer. The single-pass cladding Fe60 alloy cladding layer is mainly composed of equiaxed crystals, dendrites rich in Ni, Cr, and Si, and columnar crystals. Appropriate laser cladding process control achieves rapid melting and solidification of the cladding layer, forming a non-equilibrium, sub-crystalline dendritic eutectic structure, solid solution strengthening of Si atoms and fine grain strengthening of the structure after laser treatment, forming a smooth, dense, heat-affected, high-quality cladding layer, which significantly improves the hardness of the coating.
Laser cladding hard phase particles have received widespread attention in recent years. Hard phase particles include WC, NbC, TiC, TaC and VC. The addition of WC particles has a positive effect on improving the microhardness of the new matrix. Ma et al. used laser cladding technology to prepare Ni60/WC composite coatings, which have eutectic structure characteristics and high hardness. Liu Feng et al. conducted experiments and tests and found that the improvement of the hardness performance of the cladding layer is because the ceramic hard phase particles are evenly distributed in the Ni60 matrix, and there are few interface products between the reaction ceramic phase and the metal phase. The hard phase-enhanced metal-based composite coating has been widely used on the surfaces of various mechanical parts with wear conditions because of its high hardness and certain plastic strain capacity.

5 Summary
After nearly half a century of research, laser cladding technology has accumulated a large amount of experimental data and basic theories in the field of high-end manufacturing. However, due to the characteristics of agricultural machinery and the complexity of its working objects, its manufacturing, repair and strengthening methods lag behind the high-end manufacturing field. Therefore, the research and application of laser cladding technology in the repair and strengthening of agricultural machinery started late. It is basically in the primary research stage of coating material preparation process and wear and corrosion protection performance. In order to realize the promotion and application of laser cladding technology in the field of agricultural machinery repair and strengthening, research can be carried out in the following directions in the future:
(1) Absorb the accumulated research results of the high-end manufacturing industry and combine them with the actual production conditions of agricultural machinery repair and strengthening, so as to change the current situation of low level and slow development of agricultural machinery design and manufacturing.
(2) Develop new composite materials or pioneer materials suitable for agricultural machinery repair and strengthening, such as metal ceramics, rare earths, amorphous alloys, multi-principal high entropy alloys, etc. In the early stage, scholars have developed a large number of material systems, but agricultural machinery is restricted by working conditions, and the existing material systems cannot meet its repair and strengthening needs. Therefore, it is an urgent goal to design a high-quality cladding material system suitable for different working conditions based on the existing material system and in combination with the characteristics of agricultural machinery.
(3) Optimal design of laser cladding process parameters. Laser cladding is the result of the combined effect of multiple process parameters. Only by selecting a suitable combination of process parameters can the cladding coating with the best performance be obtained. How to quickly and conveniently find this optimal combination has always been a difficult problem that has troubled scientific and technological workers. Therefore, the study of the optimal design of process parameters has very important practical significance. Although there are still some difficulties in the application of laser cladding technology in the repair and strengthening of agricultural machinery, it is believed that with the joint efforts of scientific researchers, in the near future, laser cladding technology can go out of the laboratory and be widely used in the field of agricultural machinery repair and strengthening, providing a strong guarantee for my country’s agricultural mechanization process.