Research and Progress of Laser Cladding: Process, Materials and Applications

Laser cladding (LC) is a process in which materials with certain properties are added to the surface of a substrate [1,2,3]. Under a high-energy-density laser beam, the materials are melted and then solidified, forming a cladding layer which is metallurgically bonded with the substrate [4]. LC has the advantages of high energy density, small deformation, metallurgical bonding with the matrix and so on [5]. The addition of cladding materials mainly consists of the coaxial powder feeding method and the preset method; the coaxial powder feeding method can be divided into coaxial powder feeding and off-axis powder feeding [6]. The advantage of preset method is that the cladding material is not limited by powder size and fluidity; however, it is not suitable for large-scale industrial applications. The powder of the coaxial powder feeding method is sent out synchronously during the LC process, which is more conducive to industrial promotion [7]. In terms of the coaxial powder feeding method, the main process parameters of LC include laser power, laser scanning speed, powder feeding rate, spot diameter, defocus amount, overlap rate, shielding gas flow rate and so on. Process parameters have great effects on the macroscopic size, microstructures, defects and properties of the cladding layer. A variety of methods have been adopted to optimize the parameters of the LC process, such as the single variable method, the orthogonal test method and machine learning [8,9,10,11,12]. In order to avoid defects and reduce elemental segregation of the cladding layer, in recent years, some researchers have combined LC with other technologies and developed ultrasonic-assisted LC, electromagnetic-assisted LC and induction heating LC [13,14]. To further improve the production efficiency of LC, extreme high-speed LC technology has been developed, and its efficiency is three to five times higher than that of conventional LC. Extreme high-speed LC meets the requirements of green and pollution-free development and is expected to replace electroplating [15,16,17].

The LC process involves the interaction between the laser, powder and substrate. Analyzing the molten pool flow field, temperature field and stress field during the LC process is of great help in understanding the macroscopic morphology, microstructure evolution and properties control of LC. Many scholars simulated the molten pool flow behavior, temperature field, stress field and microstructure based on fluid dynamics and physical phase field process [18,19,20,21,22].

The cladding material exerts great influence on the performance of the cladding layers. Since the birth of LC technology, the research and development of cladding material has been of high concern for researchers. By selecting a specific cladding material, the wear resistance, corrosion resistance, high-temperature oxidation resistance and other properties of the substrate surface can be improved, and the consumption of precious metals can be reduced at the same time. To meet different components’ working conditions, the flexible selection and design of cladding materials is an important issue. According to different material compositions, the cladding materials can be divided into self-fluxing alloy powders, ceramic powders, rare earth powders, amorphous alloys, high-entropy alloys and so on. The self-fluxing alloy powders mainly consist of iron-based, nickel-based and cobalt-based self-fluxing alloy powders, which are mixed with boron and silicon elements with deoxidation and self-fluxing properties [23,24,25,26]. Compared with metal alloy powders, ceramic powders exhibit the characteristics of high hardness, high melting point and low toughness and can be used as reinforcing phases in the LC layers. At present, the most commonly used ceramic powders are carbide powders, oxide powders and nitride powders, and the carbide powders include WC, TiC, VC, SiC, NbC, ZrC, and so on [27,28,29,30,31,32]. Rare earths and their oxides are mainly used as modified materials in LC, and the addition of less than 2% can significantly improve the microstructures and properties of the LC layers. At present, Ce, Y, La and their oxides are widely studied. The addition of rare earth to the cladding layers can increase the nucleation rate, refine the grains, and improve the high-temperature oxidation resistance and corrosion resistance of the cladding layers [33,34]. Due to the disordered structure of amorphous alloys, there are no grains or grain boundaries in amorphous alloys, and they generally possess high hardness and excellent wear resistance, as well as good corrosion resistance. The rapid heating and cooling of the LC process is very beneficial to the formation of amorphous alloys [35,36,37]. How to improve the amorphous ratio in LC layers is an important research topic in this field. Due to their multi-component and simple solid solution structures, such as FCC and BCC, high-entropy alloys display high hardness and excellent wear resistance, and have decent development prospects. At present, the most frequently studied LC high-entropy alloy coating systems include CoCrFeMnNi, CoCrCuFeNiTi, TiZrNbWMo, CoCrBFeNiSi and so on [38,39,40,41,42].

LC layers generally exhibit good wear resistance, corrosion resistance and high-temperature oxidation resistance. At present, the research on the properties of the LC layer is mainly focused on wear resistance, corrosion resistance, high-temperature oxidation resistance, self-lubrication and biocompatibility. One of the main methods to improve the wear resistance of the cladding layer is to directly add or in situ synthesize the hard phase, and the other is to improve the wear resistance of the coating by adding lubricants [43,44,45,46,47,48,49].

LC technology is mainly used in repairing and surface strengthening in the fields of the aerospace, automobile and petrochemical industries, as well as metallurgy and rail transit. It provides a new method for the repair of important damaged parts, thereby greatly reducing the cost and enhancing work efficiency. Using LC technology to strengthen the surface of important parts can improve the properties of the parts, thereby prolonging their service life and reducing costs [50,51,52].

Due to its unique processing characteristics, LC technology has great development potential in repairing and surface strengthening of engineering components. With the progress of computer hardware and software, the decline in laser price and the improvement in automation, LC technology has received extensive attention in recent years. However, the quality of the LC layer is affected by many factors, and there are still inevitable defects in the cladding layer that affect the performance of the components. It is necessary to conduct basic research to have a deeper understanding of the deposition process, reduce the formation of defects such as cracks and pores in the cladding layer, and improve the performance of the cladding layer, so that LC can achieve large-scale industrial production as soon as possible.