This paper introduces the principle of laser cladding technology and the current application status of this technology in the maintenance and remanufacturing of engineering equipment. The application prospects of laser cladding technology in affecting the life of engineering equipment are discussed from the two aspects of surface strengthening and corrosion resistance. It provides guidance for the repair and remanufacturing of engineering equipment and looks forward to the future.
Engineering equipment mainly consists of river crossing bridge equipment, road position machinery, city construction machinery, water supply machinery, engineering reconnaissance and command equipment, etc. When repairing these engineering machinery, many factors are considered, such as short guarantee time limit, difficulty in rear allocation of parts, easy wear and corrosion of equipment, etc. Small parts are mainly replaced by replacement repair, that is, directly replace damaged parts, while large parts are difficult to replace directly and costly. Surface engineering technology is usually used to prepare a coating with better performance on the surface of parts. This method can not only repair the failed parts to their original size, but also improve the performance of the parts, ensuring the reliability and stability of the remanufactured engineering equipment. Compared with traditional replacement repair, it greatly reduces the production cost.
At present, the main surface engineering technologies used in engineering equipment are organic coating, hot dip coating, thermal spraying, electroplating, chemical plating, vapor deposition, surfacing, laser cladding, etc. Compared with other surface engineering technologies, laser cladding technology has the following characteristics: high laser energy density, fast heating speed, small heat-affected zone on the substrate, small thermal deformation; fast cooling rate (102~10°K/s), dense structure on the coating, small grains; low coating dilution, metallurgical bonding between the coating and the substrate, and high bonding strength; wide range of material selection, metal materials, ceramic materials and composite materials can be used as cladding materials; easy to realize automation and no environmental pollution. Laser cladding technology is widely used in shipbuilding, aerospace, automobile remanufacturing, mining machinery, security and other fields [2].
This paper summarizes the principle of laser cladding technology, as well as the research status and development prospects of using this technology to repair and remanufacture engineering equipment parts, and puts forward four research directions for reference.
1 Laser cladding technology
Laser cladding technology (LC) is a surface modification technology that belongs to a multidisciplinary discipline, including laser technology, computer-aided manufacturing technology and control technology. Its principle is to use a high energy density (10’4~10’6W/cm²) laser heat source to melt the coating material and the thin layer on the substrate surface at the same time, and quickly solidify to form a surface cladding layer with extremely low dilution rate and metallurgical bonding with the substrate, thereby significantly improving the wear resistance, corrosion resistance, heat resistance, and oxidation resistance of the substrate surface.
According to the powder feeding method, the most commonly used LC methods are coaxial powder system[3] and pre-set powder system[4]. Coaxial powder feeding is when the powder is ejected from the powder feeding nozzle through the carrier gas, the laser beam irradiates the substrate to form a liquid molten pool[5]. After the powder interacts with the laser, the powder enters the liquid molten pool and forms a cladding layer as the powder feeding nozzle and the laser beam move synchronously. Unlike the coaxial powder system, the cladding material is pre-placed on the substrate in the pre-set powder system. Then, the pre-set powder is melted by laser beam scanning, and the molten pool is rapidly cooled to form a cladding layer. Generally speaking, the pre-set powder system is easy to operate and has good cladding quality, but the penetration depth is difficult to control and the dilution is large. The coaxial powder system has a high laser utilization rate, but it has high requirements on the quality of the cladding equipment.
The application of laser cladding materials mainly includes self-fluxing alloy powders, ceramic materials, metal-ceramic composite materials, and rare earth element modified materials. The performance differences between materials are large, and the appropriate material can be selected according to actual needs.
2 The current status and prospects of laser cladding technology in the maintenance and remanufacturing of engineering equipment parts
At present, the main technologies used for engineering equipment repair include arc spraying, heat treatment, and surfacing. However, with the continuous development of laser cladding technology, the performance of the prepared coating is becoming increasingly excellent, and the efficiency is also constantly increasing, providing new methods and new ideas for the repair and remanufacturing of engineering equipment.
2.1 Application status
Zhang Shiping used laser cladding technology to prepare VC iron-based composite material coatings on the surface of worn bucket teeth of excavators, restoring the original size and improving its wear resistance [7]. Klaierle et al. from Leibniz University of Hannover, Germany, completed the repair of turbine engine blades using a laser cladding and laser remelting composite process [8]. Su Lunchang et al. conducted a comparative study on the remanufacturing of hydraulic support columns, one of the key equipment of coal mine machinery. The study showed that laser cladding remanufacturing is 6 times longer than the service life of electroplating repair technology, and has obvious advantages in economic and environmental benefits [9].
In addition to repairing damaged parts, laser cladding technology can also be used to remanufacture existing engineering machinery parts, thereby improving workpiece performance and extending service life. Jiao Yang et al. used low-carbon high-chromium iron-based cladding powder as the raw material and used laser cladding technology to prepare iron-based cladding layers with different Mo addition amounts, which significantly improved the corrosion resistance of the hydraulic cylinder cladding layer material and could meet the stringent protection requirements of coal mine hydraulic supports [10]. Lu Weiming used laser cladding technology to remanufacture the piston of a diesel engine. He used three categories of self-fluxing alloy powders with a total of 11 different components to perform laser cladding on a sample with 42CrMo as the substrate. Finally, Fe60 was selected as the cladding powder for the piston. Then, a comprehensive analysis of process parameters such as laser incident angle, laser power, and scanning speed was conducted. A total of 12 groups of tests were conducted in two rounds to obtain the optimal process parameters. This study provides strong technical support for the mass production of pistons and the reliability of remanufactured products.
2.2 Application prospects
Compared with high-end manufacturing fields such as aerospace, automobile remanufacturing, and ship remanufacturing, the use of laser cladding to repair and remanufacture engineering equipment has developed slowly and at a low level. In order to ensure the integrity and reliability of engineering equipment, the advanced achievements of laser cladding in other fields were used for reference, and research was conducted from two aspects: surface strengthening and improving corrosion resistance.
2.2.1 Surface strengthening
Engineering equipment has a complex structure and a wide variety of parts. Some parts require not only sufficient rigidity and toughness, but also sufficient hardness and wear resistance on the surface. For example, gear tooth surfaces and transmission shaft mating surfaces all require good wear resistance and high surface hardness.
In terms of improving the wear resistance of parts, Li Yunfeng used ZG42CMoA steel commonly used in large gear rings as the substrate and designed a sandwich composite structure coating including an interface connection layer, a toughening layer and a wear-resistant layer to improve the wear resistance and impact resistance of the tooth surface [12]. Feng Hui et al. used laser cladding technology to prepare an Fe-based cladding layer on the surface of 45 steel. After optimizing the process parameters, a dense and defect-free cladding layer was obtained. Its hardness was significantly higher than that of the substrate, and the problem of excessive wear and cracks on the crankshaft journal was effectively solved [13]. Zhang et al. used laser cladding technology to prepare Ni60A coating on the surface of 45 steel, and systematically studied the high-temperature friction and wear behavior of the substrate and laser cladding coating sliding relative to the GCr15 ball under different loads. With the increase of load, the wear resistance of the substrate and the cladding layer increased, but the wear rate ratio of the coating to the substrate was 1/6.2[14]. When the tank is performing a mission, the engine camshaft will be subjected to the impact of the periodic opening and closing of the valve, which will cause the cam lobe to wear and fail. Dong Shiyun et al. laser clad Fe90 iron-based alloy powder on the surface of a tank camshaft to obtain a coating without cracks, pores and dense structure, and its wear resistance was greatly improved.
In terms of improving the hardness of parts, Zhang Yanli et al. prepared Fe60 alloy and Ni60 alloy laser cladding coatings on the surface of 45 steel respectively, and compared the properties of the two and concluded that although the Ni60 alloy cladding coating has higher hardness, it has more cracks, while the Fe60 alloy cladding coating not only has high hardness in the bonding area, but also has good metallurgical bonding and no obvious defects. It is concluded that Fe60 alloy powder is more suitable for laser cladding treatment on the surface of 45 steel [16]. Niu Xinglin et al. used a certain iron-based alloy powder to prepare a laser cladding layer on the surface of 45 steel. The study showed that due to the extremely fast heating and cooling speed of the cladding process, a large degree of undercooling was obtained, and the alloy elements in the molten pool quickly formed compounds. The grain size and structure of the cladding layer were finer, and a cladding layer with higher hardness was obtained [17]. Wu Si et al. used laser cladding technology to prepare WC-12Co metal ceramic cladding layer on the surface of 45 steel. The hardness of the cladding layer gradually decreased from top to bottom. The surface layer had a higher C content and the highest hardness. It was concluded that WC particles increased the hardness of the cladding layer.
2.2.2 Improving corrosion resistance
Most of the parts of engineering equipment are made of metal. Although their parts have certain anti-rust protection when they are first produced, such as electroplating, painting and other surface engineering technologies, the equipment is generally operated outdoors in harsh open-air environments. The exposed metal is still prone to oxidation and rust. Therefore, laser cladding technology can be used to prepare corrosion-resistant coatings, thereby blocking direct contact between the metal and the atmosphere and moisture, thereby playing an anti-rust protection role [19]
Qiu et al. studied the corrosion resistance of laser cladding AI2CrFeCoxCuNiTiHEA coating on Q235 steel. The results showed that the coating had good corrosion resistance in 3.5% NaCl and 1mol/LNaOH solutions [20]. Bai Yongtao et al. laser clad low-carbon martensite and stainless steel mixed powders in different proportions on the surface of hydraulic props and jacks, which improved their wear resistance and corrosion resistance and extended the service life of hydraulic props [21]. The above research results show that laser cladding technology provides a certain feasibility for the realization of corrosion resistance and wear resistance.
The study found that the corrosion resistance of the cladding layer can be significantly improved by adding and controlling the external field conditions. Zhai Lulu used pure iron as the matrix and Ni60 self-fluxing alloy powder as the cladding material. He used three auxiliary forms, single alternating current, single steady-state magnetic field and electro-magnetic composite field, to investigate in detail the changes in the surface morphology, cross-sectional size, solidification structure, number of cracks and microhardness of the laser cladding coating under electromagnetic field conditions. Through the experiment, it was finally possible to obtain high-quality laser cladding coatings with smaller electro-magnetic composite parameters, which improved the corrosion resistance of the coating. This scheme provides technical guidance for strengthening engineering equipment using laser cladding technology [2]. In summary, laser cladding technology provides a certain feasibility for improving the corrosion resistance of engineering equipment.
3 Conclusion
Since Gnanamuthu obtained a patent for laser cladding a layer of metal on the surface of a metal substrate in 1974, this technology has been studied for nearly half a century. At present, a large amount of experience and experimental data has been accumulated. However, its research and application in the repair and strengthening of engineering equipment is still relatively rare. In order to promote laser cladding in the field of engineering equipment maintenance and remanufacturing, research can be carried out in the following directions in the future:
1) Absorb the results accumulated in the research of laser cladding technology in high-end industries and apply them to engineering equipment, so as to improve the current situation of low level and slow development of engineering equipment maintenance and remanufacturing.
2) The application of laser cladding technology in bridge cranes is still rare. After analyzing the cracking of the steel plate of the bridge crane boom during use, Wuhan Iron and Steel Research Institute found that the equipment was far from reaching its service life. The failure of the steel plate was fatigue fracture of the slag inclusion part of the weld and extended along the weld path. At the same time, there was a brittle martensitic structure in the weld [23]. The use of laser cladding technology to treat the steel plate of the bridge crane boom can not only effectively improve its mechanical properties, but also enhance its corrosion and oxidation resistance, providing new ideas and methods for the maintenance and remanufacturing technology of bridge equipment.
3) The stability of the metallurgical bonding strength between the cladding layer and the substrate needs to be improved. Due to the extremely fast heating and cooling speed of the cladding layer, there are differences in the thermal expansion coefficient, melting point and wettability of the cladding material and the substrate, which makes it easy to crack at the metallurgical bonding, so the metallurgical bonding effect needs to be improved.
4) Optimize and upgrade the process parameters of laser cladding. In laser cladding, how to quickly and efficiently find the optimal solution for a set of process parameter combinations has always been a key topic for scientific researchers. At the same time, suitable process parameters are of great significance for the preparation of high-quality coatings.
