Laser Cladding Alloying and Composite of steel materials: Comparison
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Laser cladding technology is a reliable and efficient surface modification technology, which has been widely used in surface alloying and composite processing of steel materials. Laser cladding alloying and compounding can enhance the wear resistance and corrosion resistance of steel materials.

  • steel materials
  • laser cladding
  • alloying
  • compound

1. Research Background

Due to the unique processing and mechanical properties of steel materials, they are widely used in the fields of vehicles, ships, ocean engineering, chemical equipment, and aerospace [1][2][3]. In many industries with poor working conditions, mechanical equipment is often subjected to periodic or continuous abrasive wear, erosion wear, and composite wear, which causes surface wear failure of parts and components of mechanical equipment and reduces the service life of mechanical equipment. Wear, corrosion, and fatigue are the three main failure modes of materials [4][5][6]. It has been reported that 70–80% of mechanical equipment failures are due to wear and tear of all kinds [7][8]. In addition, with the proposed implementation of the “Made in China 2025” development strategy, higher requirements have been put forward for the quality and efficiency of mechanical equipment manufacturing.
In 1976, Gnamuthu D.S obtained a patent for the laser cladding of one metal coating to another metal substrate. Laser cladding technology has been important to the field of material surface strengthening since its discovery. Laser cladding is a method of adding powder materials or wire materials on the surface of the substrate and employing a high-energy density laser beam to melt materials together with the thin layer on the surface of the substrate, forming a metallurgical composite cladding coating. If the cladding material is wire, wire feeding equipment is usually used to transport the wire to the surface for cladding. Theoretically, the wire cladding material’s utilization rate can reach 100%. If the cladding material is powder, laser cladding can be divided into two types: powder laying and powder feeding, according to the different ways of material supply. Powder laying laser cladding is easy to operate and does not need special powder feeding equipment, but the powder utilization rate and production efficiency are low. Powder feeding laser cladding needs special powder feeding equipment to transport the powder to the surface to be fused, and the powder utilization rate and production efficiency are higher.
Laser cladding has been a widely used surface modification technology. The cladding coating prepared by this technology has excellent performance, and the thickness of the cladding coating can be changed from a few microns to a few millimeters. The thickness of the cladding coating can be flexibly adjusted according to the actual user demand, which also considerably saves the cladding materials. Therefore, laser cladding has become an important means to improve parts’ surface wear and corrosion resistance. In addition, laser cladding technology can save rare and precious metals, reduce energy consumption, in line with the national energy conservation, emission reduction, green and low-carbon strategic development goals.

2. Laser Cladding Process of Steel

Laser cladding is an instantaneous processing process. Although the cladding process is very short, it contains rich interactions of the laser with the cladding material and substrate material [9][10], the complex metallurgical physicochemical reactions after the formation of the molten pool, and the rapid solidification process of the molten pool [11][12]. The completion of each stage of the cladding process is closely related to the technological parameters of laser cladding. The main technical parameters of laser cladding include laser power, scanning speed, and laser spot diameter. The dilution rate is one of the important parameters to characterize the quality of the cladding coating. The dilution of the cladding coating by the substrate material is inevitable [13], and the substrate’s surface must be melted to form good metallurgical bonding at the interface between the cladding coating and the substrate. However, to maintain the properties of the substrate material and cladding coating and avoid the influence of substrate dilution, the dilution rate should be controlled in an appropriate range. Each process parameter affects the surface formability, dilution rate, microstructure, and phase composition of the cladding coating [14]. It is necessary to optimize the process parameters and control them within a reasonable range to obtain the cladding coating with good surface molding and a dense internal microstructure.
The laser is the heat source of the cladding process. Laser power is directly related to the heat input in the cladding process. When the laser power is too large, the surface roughness of the cladding coating increases due to the increase of disturbances in the molten pool, and the molten pool easily absorbs the gas. The excessive heat transfer to the substrate leads to a rise in the dilution rate, which affects the cladding coating’s surface shape and chemical composition [15][16][17]. On the contrary, when the laser power is too small, the cladding material cannot completely melt, and the wettability between the molten cladding materials and the substrate becomes worse, which leads to poor molding of the cladding coating. The laser energy absorbed by the substrate surface is insufficient, and the substrate cannot form a good metallurgical combination with the cladding coating.

3. Laser Cladding Materials of Steel

Laser cladding materials directly affect the chemical composition of the coating. Therefore, the choice of cladding materials has an important influence on the quality and performance of the coating. Currently, most commercial alloy powders are designed and produced according to the technological characteristics of flame spraying, plasma spray welding, and powder metallurgy, which cannot meet the specialized needs of laser cladding. Although laser cladding materials apply to a wide range of substrates, there are some more suitable for a certain cladding substrate in a certain working environment. Selecting appropriate cladding materials is a prerequisite for obtaining good surface-forming quality, a smooth surface, and an internal defect-free coating for the determined substrate material. According to the technological characteristics of laser cladding, generally, the selection of laser cladding materials follows the following principles [18].
(1)
The close thermal expansion coefficient principle between cladding materials and the substrate material is provided as follows. Laser cladding is a process in which the cladding materials melt rapidly under laser irradiation and then cool sharply and solidify. Due to the rapid heat and cold, there will be a certain thermal stress between the cladding coating and the substrate. Suppose the thermal expansion coefficient of the cladding materials and the substrate is different. In that case, the thermal cracks between the cladding coating and the substrate are easy to occur under thermal stress, which will affect the performance of the cladding coating. On the contrary, if the thermal expansion coefficient of the cladding materials and the substrate is close, it is beneficial to reduce the thermal stress between the cladding coating and the substrate and reduce the tendency of cracks in the cladding coating.
The matching principle of the linear expansion coefficient between laser cladding materials and the substrate material is as follows:
− σ 2 ( 1 − γ ) / ( E Δ T ) 〈 Δ α 〈 σ 1 ( 1 − γ ) / ( E Δ T )
where σ1 is the tensile strength of the cladding coating (MPa), σ2 is the tensile strength of the substrate (MPa), Δα is the linear expansion coefficient difference between the cladding coating and the substrate, ΔT is the difference between the cladding temperature and room temperature, E is the elastic modulus of the cladding coating, and γ is Poisson’s ratio. After laser cladding, the main reason for cracking is residual stress in the cladding coating. Thermal stress, phase transition stress, and constrained stress are the main components of residual stress, among which thermal stress accounts for the heaviest proportion compared with the other two. The thermal stress of laser cladding is determined as follows [19]:
σ th = E Δ α Δ T / ( 1 − γ )
where σth is thermal stress, E is the elastic modulus of the cladding coating, γ is Poisson’s ratio, Δα is the difference of the linear expansion coefficient between the cladding coating and the substrate, and ΔT is the difference between the cladding temperature and room temperature. From Equation (2), it can be seen that when the difference between the linear expansion coefficient of the cladding coating and the base material is smaller (in other words, when the linear expansion coefficients of the cladding coating and the base material are closer), the thermal stress of σth becomes smaller. Then, the cracking tendency of the cladding coating is the smallest.
(2)
The second principle is based on similar melting points between cladding materials and substrate materials. After melting the cladding materials and the surface materials of the substrate, the molten pool is formed, and the molten pool solidifies to form the cladding coating. The melting point of the cladding materials and the base material is similar, which can ensure that the cladding materials and the surface material of the substrate melt simultaneously and form a metallurgical combination to reduce the dilution rate of the cladding coating. When the melting point of the cladding materials is much lower than the melting point of the base material, the surface material of the substrate cannot reach the melting point. It cannot melt after the laser energy melting of the cladding materials, and the substrate cannot form a metallurgical combination with the cladding coating. After increasing the laser heat input, the surface material of the substrate melts. At this time, the melting temperature of the cladding materials to form the molten pool exceeds the melting point of the cladding materials. The cladding materials will be vaporized, resulting in the loss of the cladding materials and easy-to-produce pores, shrinkage holes, and other defects in the cladding coating. When the melting point of the cladding materials is much higher than the melting point of the base material, the cladding materials have not been melted when the base material is melted. This process will cause the burning of the base material, and hence it cannot form a good cladding coating.
(3)
The third principle is applying the wettability of cladding materials to substrate materials. After the molten pool is formed, the cladding materials need to spread on the surface of the substrate to form the cladding coating. The good wettability between the cladding materials and the substrate is the prerequisite for the molten pool to spread on the surface of the substrate and is also the key to the good molding performance of the cladding coating [20]. The wettability of cladding materials on the surface of the substrate is directly related to the surface tension of the molten pool. Increasing the temperature of the molten pool is an effective way to reduce the surface tension of the molten pool. Still, a too-high temperature of the molten pool will also cause the burning of alloying elements.
At present, the commonly used cladding materials for laser cladding are metal (alloy) powder [21], ceramic powder [22], and metal-ceramic composite powder [23]. In addition to following the above three principles, selection of cladding materials is made according to requirements such as actual needs and expected performances.

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