Compared with high-end manufacturing fields such as industrial machinery, aerospace, and automobiles, agricultural machinery manufacturing always lags behind them. 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 advanced technical research results 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, research can be conducted from the following four aspects: in-situ repair; improve wear resistance; improve corrosion resistance; improve hardness.
In-situ repair: Agricultural machinery has high use intensity and poor working environment conditions. Many agricultural machinery parts are in an overloaded state during long-term use, so it is easy to have plastic deformation, wear, cracks, corrosion and other problems. 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 deformation, fast cooling speed, high precision, and excellent performance, they have been widely used in the field of agricultural machinery repair. For example, during the operation of agricultural machinery, gear parts will be affected by strong alternating stress, which is easy to cause flash, tooth gnawing, deformation and other problems. The use of laser cladding technology can restore the defective gear to its original size. Through experimental verification, the gears repaired by laser cladding can not only operate normally, but also the gears’ impact resistance, hardness, wear resistance and other properties have been greatly improved.
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 parts are also affected by friction and wear, and the impact of friction and wear is more significant, which is also the main 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 penetrate into it to form wear, resulting in regional deep scratches. The scratches enhance the effect of abrasive particles, which in turn aggravates the damage process and forms a vicious circle. The application of laser cladding technology to in-situ repair of bearings can fill scratches and restore the surface morphology of the shaft. The coating prepared by laser cladding 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.
Improve wear resistance: 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. Soil or sand directly in contact during farming will cause 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 the base material of agricultural machinery parts, good metallurgical bonding, and high wear resistance and low cost of the coating. Single 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 wear resistance. In addition, adding rare earth elements to the cladding material can improve the wear resistance of the cladding coating. The cladding alloy powder is mechanically mixed according to the chemical composition CeO2 ratio, and the cladding coating is prepared on the surface of the steel substrate. The obtained cladding layer reacts in situ to generate a ceramic hard phase with dense structure, no pores and cracks, and significantly improved wear resistance. Rare earth elements can significantly improve the microstructure of the cladding layer and refine the dendrite structure. Among them, grasping the reasonable addition amount of rare earth elements is the key. The appropriate addition of rare earth elements can ensure the effective improvement of the wear resistance of the cladding layer.
Improve corrosion resistance: Agricultural machinery farming parts often work in humid and corrosive environments such as pesticides, fertilizers, and organic fertilizers, which accelerates the damage of agricultural machinery. The composition of laser cladding powder will directly affect the corrosion resistance of the cladding layer. In the research and exploration of corrosion resistance, nickel-based self-fluxing alloy powder is the most prominent in laser cladding materials, and is widely used in the repair of local corrosion-resistant components. 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. Fe60 alloy coating was prepared by laser cladding on the surface of 45 steel, and a composite field device of mechanical vibration and magnetic field was added to assist. Under the action of the composite field, the Fe-Cr solid solution content in the coating was effectively increased, which significantly improved the corrosion resistance of the coating, and provided guidance for further optimizing the process of laser cladding to strengthen agricultural machinery.
Improve hardness: Because there are large stones and plant 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, which places higher requirements on the hardness of agricultural machinery. Some scholars used 45 steel as the base material to compare the cladding layers of Fe60 alloy and Ni60 alloy in terms of hardness. 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 more ideal comprehensive performance and are more suitable for laser cladding surface treatment of 45 steel. Later, the microstructure and properties of the Fe60 alloy cladding layer were further studied. The single-pass cladding Fe60 alloy cladding layer mainly consists of equiaxed crystals, dendrites and columnar crystals rich in Ni, Cr, and Si. Appropriate laser cladding process control achieves rapid melting and solidification of the cladding layer, forming a non-equilibrium, sub-crystalline dendritic eutectic structure. The solid solution strengthening of Si atoms and the fine grain strengthening of the laser-treated structure form a smooth, dense, heat-affected, high-quality cladding layer, which significantly improves the hardness of the coating.
Laser cladding of hard phase particles has attracted 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 substrate. Ma et al. used laser cladding technology to prepare Ni60/WC composite coatings, which have eutectic structure characteristics and high hardness. Hard-reinforced metal-based composite coatings have been widely used on the surfaces of various mechanical parts with wear conditions due to their high hardness and certain plastic strain capacity.
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 itself and the complexity of the 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, and 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 three directions in the future:
- Learn from the research results accumulated in the high-end manufacturing industry, combined with the actual production conditions of agricultural machinery repair and strengthening, to change the current situation of low level and slow development of agricultural machinery design and manufacturing.
- Research and develop new composite material powders suitable for agricultural machinery repair and strengthening. Based on the existing material system, combined with the characteristics of agricultural machinery, a high-quality cladding material powder system suitable for different working conditions is designed.
- Laser cladding is the result of the joint action of multiple process parameters. Only by selecting a suitable combination of process parameters can the cladding coating with the best performance be obtained. Therefore, the research on the optimization 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. (And Huirui-Laser has currently used laser cladding technology for batch repair of agricultural machinery tools, and the effect is very good)
