Nickel-based alloys have many advantages such as high strength and strong corrosion resistance, and are widely used in aerospace, energy and power and other fields. Laser cladding technology uses a high-energy laser beam to quickly melt the cladding material on the surface of the substrate to form a good metallurgical bond. Laser cladding of nickel-based alloys has the advantages of high production efficiency and low cost, and has broad application prospects. This article sorts out the research status from three aspects: types of laser cladding nickel-based alloys, research on laser cladding process parameters, and research on cladding layer performance. It summarizes the current research results and prospects the application of laser cladding of nickel-based alloys in reactors, in order to provide a reference for in-depth research and practical application of laser cladding of nickel-based alloys.

1 Introduction
Nickel-based alloys have the advantages of high strength, strong corrosion resistance, excellent processing performance, and good metallurgical stability. They are widely used in various complex service environments such as aerospace, energy and power, and petrochemicals, such as aircraft engine turbine blades and heat transfer tubes of third-generation nuclear power steam generators.

Laser cladding is a surface modification technology with high energy density and high processing accuracy, which is suitable for most metal materials. This technology uses a high-energy laser beam to act on the cladding material (generally metal powder) to quickly melt and solidify the cladding material and the substrate surface material in a local area to form a good metallurgical bond, as shown in Figure 1. Generally, the cladding material has high wear resistance and corrosion resistance, so laser cladding has a surface strengthening effect on the substrate. Laser cladding technology can realize the processing and manufacturing of complex structures and surface strengthening. Compared with traditional subtractive processing methods, the combination of laser cladding technology and nickel-based alloys can save a lot of nickel-based alloys. In addition, the nickel-based alloy and the substrate have good bonding and low dilution rate, and the cladding layer has excellent service performance.

A large number of structural components in the reactor are in a high temperature, high pressure, high wear and strong corrosion environment, which has high requirements for the strength, hardness and corrosion resistance of the materials. Nickel-based alloys have excellent performance in these aspects, such as Inconel 690 alloy has strong stress corrosion resistance and Inconel 52M has low crack sensitivity. At the same time, compared with traditional methods such as cladding, laser cladding technology can realize the preparation of complex structure cladding layers, and the consistency and uniformity of the cladding layer quality are good, which better meets the performance requirements of reactor structural components. Therefore, laser cladding nickel-based alloys also have potential application prospects in nuclear reactors.

2 Types of nickel-based alloys

Due to the excellent performance of nickel-based alloys, some structural parts in important fields such as aerospace and reactors will use nickel-based alloys to improve the hardness, wear resistance, corrosion resistance, etc. of the structure. Commonly used nickel-based alloys and their chemical compositions are shown in Table 1. Among them, the materials that are studied more in laser cladding include Ni60, Inconel 625, Inconel 718, etc.

2.1 Ni60 alloy

Ni60 alloy is a high-hardness nickel-chromium-boron-silicon alloy. Compared with other nickel-based alloys, Ni60 alloy has higher content of B, C, and Si elements, and has excellent hardness, corrosion resistance, and wear resistance. Ni60 alloy powder has good self-melting and wettability, which can improve the fluidity of the molten pool. Therefore, Ni60 alloy is suitable for laser cladding process, but Ni60 alloy laser cladding on some substrates is prone to cracking, which will affect the performance of the cladding layer. It can be improved by optimizing process parameters.

Zhou Hao et al. aimed at the problem that Cr12MoV die steel has poor toughness and is prone to pitting, peeling and other damage. Considering that Ni60 alloy has excellent wear resistance and corrosion resistance, and the melting point and thermal expansion coefficient of Ni60 alloy are similar to those of Cr12MoV steel, it can form a good metallurgical bond. Therefore, they laser clad Ni60 non-crack cladding layer on the surface of Cr12MoV steel, and analyzed the surface cracking, hardness, metallographic structure and wear resistance of the cladding layer. When the cladding layer is longer, the average surface hardness increases, but obvious cracks appear. At the same time, the wear resistance of the Ni60 alloy cladding layer is basically consistent with that of Cr12MoV steel, as shown in Figure 3.

Wang Xuejing et al., in order to improve the problems of low surface hardness, poor wear resistance and poor high temperature oxidation resistance of titanium alloy, considered that nickel-based self-fluxing powder has high hardness and wear resistance, and its thermal expansion coefficient is close to that of titanium alloy, so they laser clad Ni60A and Ni60CuMo composite powder on the surface of TC4 titanium alloy. Under the same process parameters, the thickness of Ni60CuMo cladding layer is about 1.8mm, and the hardness reaches 800-1000HV, while the thickness of Ni60A cladding layer is about 1.2mm, and the hardness is 500-700HV. At the same time, the wear resistance of Ni60CuMo cladding layer is also better than that of Ni60A cladding layer, and the wear rate is 16.42% of Ni60A cladding layer and 4.23% of substrate wear rate.

2.2 Inconel 625 alloy

Inconel 625 alloy has a typical face-centered cubic crystal structure. Its Mo, Nb and other elements are dissolved in the Ni and Cr matrix, and the solid solution strengthening effect is obvious. It has good mechanical properties and fatigue properties at room temperature and high temperature. Therefore, it is widely used in aerospace, petrochemical and nuclear power fields. However, laser cladding Inconel 625 alloy has the problems of low hardness and insufficient wear resistance. At present, it is mainly improved by adding alloy elements or ceramic particles.

Li Jihong et al. constructed a numerical simulation model of the microstructure evolution of Inconel 625 alloy laser cladding based on MATLAB and grain nucleation and growth theory, and conducted experimental verification. In the numerical simulation results, the cladding layer is mainly composed of columnar crystals and subgrains distributed between columnar crystals, and the part near the bonding surface is composed of cellular crystals, which is consistent with the experimental results. At the same time, with the decrease of laser power or the increase of heterogeneous nucleation, the microstructure of the cladding layer is refined and the width of the columnar crystal is reduced.

In order to solve the problems of hot corrosion and high-temperature oxidation of Inconel 738 alloy in gas turbines, Fesharaki et al. prepared an Inconel 625 cladding layer with better corrosion and oxidation resistance on the Inconel 738 substrate. The cladding layer has no pores and cracks and has a good metallurgical bond with the substrate. In addition, a large number of corrosion-resistant phases (NiCr2O3) are formed in the Inconel 625 cladding layer, and the cladding layer has excellent corrosion resistance at 900°C.

2.3 Inconel 718 alloy

Inconel 718 alloy is a precipitation-strengthened nickel-chromium-iron alloy containing Nb and Mo. It still has high strength, toughness and corrosion resistance at a high temperature of 700°C, and is widely used in hot end components in aerospace, nuclear power and other fields. The rapid cooling process of laser cladding will cause the precipitation of Nb elements and form Laves phase. At the same time, the precipitation of strengthening phase is inhibited, which affects the mechanical properties of the cladding layer. At present, it is generally improved by hot isostatic pressing, heat treatment and other methods.

Li Dong et al. studied the effects of different atmospheres on the morphology, structure and performance of laser cladding Inconel 718 cladding layer. The type of powder feeding gas has a certain effect on the morphology and structure of the cladding layer, while the type of shielding gas has no obvious effect on the morphology and structure of the cladding layer. Compared with argon, helium as a powder feeding gas can effectively reduce the segregation of Nb elements in the laser cladding Inconel 718 cladding layer, as shown in Figure 4, while refining the cladding layer structure and improving the microhardness of the cladding layer, but helium as a shielding gas has no obvious effect on the morphology and structure of the cladding layer.

In order to avoid the problems of wear, corrosion, fatigue failure, etc. of Inconel 718 alloy in harsh service environment, Zhang Jie et al. used laser cladding technology to remanufacture aged Inconel 718 alloy, and determined the optimal process based on orthogonal test method. The Inconel 718 cladding layer in the bonding area showed a typical dendrite morphology, the cladding layer and the matrix were metallurgically bonded, the element transition was uniform, and there was no macro segregation, but the microhardness of the cladding layer was lower than that of the aged matrix, and the average shear strength of the cladding layer interface was 608.87MPa, indicating that heat treatment was required after laser cladding to improve the microhardness and shear strength of the cladding layer.

2.4 New nickel-based alloys developed and selected for the nuclear field

In addition to the commonly used nickel-based alloys, researchers have also developed and selected some new nickel-based alloys for typical application scenarios in the nuclear field, such as nuclear valve sealing surfaces and movable parts, and conducted laser cladding process parameter tests and cladding layer performance studies.

In order to meet the requirements of cobalt-free cladding layer materials for nuclear valve sealing surfaces, Fu et al. developed a new nickel-based alloy Ni-3. The Ni-3 alloy cladding layer is mainly equiaxed crystals with fine and uniform grains and good bonding with the stainless steel substrate. The basic phase of the cladding layer is a nickel-based solid solution, and the strengthening phase is carbide (M7C3, M23C6) and metal compounds (Ni3Al, Ni3B, Ni3Si), which effectively improves hardness and wear resistance. The hardness of the Ni-3 alloy cladding layer in the depth direction is about HV500, which is equivalent to Stellite6 alloy, as shown in Figure 5.

Shi et al. developed a new nickel-based alloy Ni-SD by adding some pure metals and carbides to Ni45 alloy as the matrix, and obtained a cladding layer with low dilution rate and uniform bonding through hollow laser cladding. The basic phase of the cladding layer is nickel-based solid solution, and the strengthening phase is metal compounds and carbides. The average hardness of the cross section of the Ni-SD alloy cladding layer is close to 700HV, which is much higher than that of Ni45 alloy and Stellite6 alloy. At the same time, the high-temperature wear resistance of the Ni-SD alloy cladding layer is also better than that of Ni45 alloy and Stellite6 alloy.

Rouillard et al. selected several nickel-based alloys from the perspectives of cobalt-free composition, sodium compatibility, mechanical properties, and radiation resistance for the wear-resistant materials of some parts in the ASTRID sodium-cooled fast reactor, and prepared the cladding layer on the 316L substrate using plasma transfer arc welding and laser cladding. Among them, NiCrBSi alloy has good corrosion resistance, wear resistance, and sodium compatibility, and is a possible substitute for Stellite6 alloy.

Aubry et al. studied cobalt-substitute materials for the main container bracket, steam generator parts, and moving parts in the valve in the MASNA project. According to previous studies, nickel-based alloys Colmonoy52 and Tribaloy700 are two possible cobalt-substitute materials, but it is difficult to prepare a crack-free cladding layer with Colmonoy52 alloy, and its wear resistance is significantly lower than that of Stellite6 alloy. Therefore, Tribaloy700 alloy was selected for laser cladding and performance testing. The Tribaloy700 alloy cladding layer has no cracks and low porosity. There are a large number of uniform Laves phases that are beneficial to improving wear resistance. The hardness of the cladding layer reaches 620HV, and it has good wear resistance.

3 Study on laser cladding process parameters

During the laser cladding process, the process parameters directly affect the temperature gradient and fluidity of the molten pool, and then affect the forming quality of the cladding layer. Commonly used process parameters include laser power, powder feeding amount, scanning speed, etc. Laser power is the most important parameter that determines the heat input of the cladding process. If the laser power is too low, the bonding strength between the cladding layer and the substrate will be reduced. If the laser power is too high, the cladding material will be over-melted and the dilution rate will increase. The scanning speed is related to the existence time of the molten pool. When the scanning speed is too fast, the material cannot be completely melted. When the scanning speed is too slow, a coarse cladding layer is easily formed. The powder feeding rate also has an important influence on the energy distribution. If the powder feeding rate is too low, the substrate material will be over-melted. If the powder feeding rate is too high, the energy supply will be insufficient and a good metallurgical bond cannot be formed. In general, the laser cladding process parameters need to be controlled within a reasonable range to ensure the comprehensive performance of the cladding layer. Therefore, for different nickel-based alloys, process parameter tests need to be carried out, and the optimal process window should be selected through mechanical properties, fatigue properties and other related performance tests.

3.1 Laser power

Deng Dewei et al. used laser cladding technology to clad Ni40 alloy powder on the surface of 304 stainless steel, and studied the effect of laser power on the structure and performance of the cladding layer. It was found that laser power affects the size and element distribution of the cladding layer, but does not cause changes in the physical phase. As the laser power increases, the hardness of the cladding layer decreases, but the wear amount decreases, as shown in Figure 6. The main reason is that the wear mechanism changes from abrasive wear to adhesive wear.

Li Jinhua et al. used a planar continuous heat source and a preset cladding layer method, based on COMSOL simulation software, to numerically simulate the H13 steel laser cladding Ni-based alloy process, and studied the effects of laser power and scanning speed on temperature and stress fields. The results show that the molten pool temperature is proportional to the laser power and inversely proportional to the scanning speed, and the thermal stress is proportional to both the laser power and the scanning speed.

3.2 Scanning speed

In order to determine the optimal production process parameters for laser cladding of Inconel 625 nickel-based alloy powder on ZG06Cr13Ni4Mo stainless steel plates, Li Yanpeng et al. used the response surface analysis method to design and conduct a series of laser cladding parameter experiments. The response of the cladding layer height to the laser power and scanning speed is relatively obvious. To obtain a defect-free, high-hardness, high-quality cladding layer, there is a reasonable limit for both the laser power and the scanning speed.

Zhu Wenxu selected the more typical high-hardness self-fluxing alloy Ni60B as the research material to study the problem of easy cracking of the high-hardness self-fluxing alloy cladding layer prepared by laser cladding, and carried out relevant laser cladding forming experiments. It was found that the cracking sensitivity of the Ni60B cladding layer increased with the increase of the scanning speed, as shown in Figure 7. At the same time, improving the brittleness of the cladding layer and reducing the residual thermal stress can reduce the cracking sensitivity of the cladding layer.

3.3 Powder feeding rate

Xing Deyuan et al. prepared Ni3Al-based alloy cladding layer on the surface of 45 steel by laser cladding, and studied the effect of powder feeding rate on the microstructure characteristics and wear resistance of Ni3Al-based alloy laser cladding layer. The microstructure of Ni3Al-based alloy cladding layer is mainly Ni3Al and fine dispersed in-situ self-generated M7C3. When the powder feeding rate increases to 1.45kg/h, coarse Cr3C2 appears in the cladding layer. At the same time, with the increase of powder feeding rate, the wear resistance of Ni3Al-based alloy cladding layer is significantly improved, which is much better than the wear resistance of vermicular graphite cast iron, as shown in Figure 8.

Shi Qiang et al. used a high-speed laser cladding system to prepare a Ni60 wear-resistant cladding layer on the surface of high manganese steel, a common material in crusher hammer heads. Through orthogonal experimental range analysis and optimization, it was found that the parameters affecting the hardness of the cladding layer were scanning speed, laser power, and powder feeding rate, from primary to secondary. The best process parameter combination was laser power 1200W, scanning speed 4mm/s, and powder feeding rate 7.5g/min. The surface hardness of the cladding layer could reach 811.41HV.

4 Study on the performance of cladding layer

4.1 Microstructure

The microstructure of laser cladding nickel-based alloy cladding layer is mainly affected by temperature gradient G and solidification rate R, while G and R are affected by process parameters. Generally speaking, when the ratio of G to R is large, the grains are mainly columnar crystals, and when the ratio of G to R is small, the grains are mainly equiaxed crystals. In addition, recent studies have shown that the addition of rare earth elements can reduce grain size and inhibit crack growth.

Jelvani et al. studied the effects of laser power, scanning speed and powder feeding rate on the microstructure of laser cladding Inconel 718 alloy cladding layer. The microstructure of the cladding layer is mainly composed of columnar crystals and equiaxed crystals. When the laser power is low, the cladding layer is mainly composed of columnar crystals, while when the laser power is high, the cladding layer is mainly composed of equiaxed crystals. In theory, as the scanning speed increases, the energy density of the molten pool decreases, the ratio of G and R increases, and the microstructure growth characteristics will change from equiaxed crystals to columnar crystals, but this is not the case in the study. Jelvani et al. believed that the particles and pores in the molten pool played a role in nucleation. As the powder feeding rate increases, the powder melted per unit time increases, G and R both increase, but the ratio of G and R decreases, and the microstructure is mainly composed of equiaxed crystals.

Zhang Zhenyu et al. laser clad a Ni60-WC-Cu composite cladding layer on the surface of B18 white copper and studied the effects of Ni60 and WC content on the microstructure of the cladding layer. The main phases of the cladding layer include NiW, Cr7Ni3, Cr3C2, Cu0.8Si0.2, WC, γ-(Cu, Ni). The hard phases NiW and Cr7Ni3 are the main components of the cladding layer, which makes the cladding layer have good mechanical properties. The formation of cellular crystals and dendrites was observed in the microstructure from the bonding zone to the top of the cladding layer, and the fishbone-shaped dendrites accounted for a large proportion, which played a role in fine grain strengthening. Agglomerated reinforcement phases were observed in the cladding layer with a WC mass fraction of 20% and 25%, which also improved the hardness and wear resistance of the cladding layer.

4.2 Hardness and wear resistance

The hardness and wear resistance of the laser cladding nickel-based alloy cladding layer are mainly affected by the elemental composition. The hardness and wear resistance of the cladding layer can be significantly improved by mechanically adding hard phases such as carbides, nitrides, oxides, or in-situ generating hard phases such as carbides and borides. In addition, the hardness and wear resistance of the cladding layer can be improved to a certain extent by optimizing process parameters and heat treatment.

Wang Wei et al. laser clad Ni35, Ni45, Ni60, and Ni62 nickel-based alloys on the surface of 45 steel and 316L stainless steel, analyzed the element distribution, and tested the hardness and wear resistance of the cladding layer and the substrate. The hardness and wear resistance of the Ni60 and Ni62 cladding layers are significantly better than those of the Ni35 and Ni45 cladding layers, indicating that with the increase in the content of elements such as B, C, and Si in the nickel-based alloy, the amount of hard phases in the cladding layer increases, resulting in an increase in the hardness and wear resistance of the cladding layer. In the depth direction, the hardness along the cladding layer to the substrate shows a decreasing trend, and the effect of material dilution on the hardness of the heat-affected zone can be clearly observed.

Meng et al. studied the effect of laser incident angle on the hardness and wear resistance of Inconel 718 cladding layer. As the laser incident angle increases, the element segregation of the cladding layer becomes more serious, and the Laves phase decreases, resulting in a decrease in hardness and wear resistance, as shown in Figure 10. When the incident angle is small, the temperature gradient of the molten pool is larger, and the solubility of the Nb element is higher, which can play a role in solid solution strengthening and significantly improve the hardness and wear resistance of the cladding layer.

4.3 Tensile properties

The tensile properties of laser cladding nickel-based alloy cladding layers are mainly affected by heat treatment. The rapid heating and cooling process of laser cladding will lead to large residual stress in the cladding layer. At the same time, there are differences in the organization and composition of different parts, which may form defects, all of which will affect the tensile properties of the cladding layer. Heat treatment after laser cladding can improve the organization of the cladding layer and enhance the tensile properties of the cladding layer.

Zhang Yaocheng studied the room temperature and high temperature mechanical properties of laser cladding Inconel 718 alloy. At room temperature, the tensile strength of laser-clad Inconel 718 alloy is slightly higher than that of cast and forged Inconel 718 alloy, reaching 953MPa, and the fracture mode is plastic fracture. After standard heat treatment, the tensile strength of laser-clad Inconel 718 alloy is increased to 1334MPa, mainly because the strengthening phase is dispersed and uniformly precipitated during the heat treatment process. At 650℃, the strength of the standard heat-treated sample decreases, and the strength and fracture mode show obvious differences in different directions.

Zhang Sheng also conducted relevant research, using process parameters with dense and uniform microstructure to prepare Inconel 718 cladding layer and heat treatment. The yield strength of the cladding layer sample at room temperature is 1016MPa, the tensile strength is 1110MPa, and the elongation at break is 8.5%, of which the tensile strength
reaches 89.5% of the original material, with good performance.

5 Application Prospects of Laser Cladding of Nickel-Based Alloys in Reactors

5.1 Nuclear Valve Sealing Surface

The sealing surface is the most important working part of nuclear valve equipment. At present, laser cladding of nuclear valve sealing surface mainly uses cobalt-based alloys, but cobalt elements will form Co60 isotopes when irradiated, which has strong radioactivity. Therefore, it is an inevitable trend to use cobalt-substitute alloys as nuclear valve sealing surface materials. At present, the research on cobalt-substitute materials for nuclear valve sealing surfaces is mainly divided into three types: nickel-based alloys, iron-based alloys, and high-entropy (medium-entropy) alloys. Among them, nickel-based alloys are used in a variety of complex service environments. At the same time, they are included in the wear-resistant cladding layer materials in the French standard RCC-MS8000, and they occupy an important position in the research of cobalt-substitute alloys.

5.2 Movable Parts

There are certain movable parts in the reactor, such as control rods and assembly brackets in sodium-cooled fast reactors. These parts need to have good wear resistance, sodium compatibility, and radiation resistance. Nickel can form a typical solid solution (γ phase), and nickel can form an A3B type intermetallic compound strengthening phase (γ′ phase) with a coherent ordered structure when combined with other metal elements. Nickel-based alloys with γ and γ′ phase structures have high strength at both room temperature and high temperature. At the same time, some nickel-based alloys have high Cr content, strong resistance to intergranular stress corrosion, and can serve for a long time in a nuclear radiation environment. Therefore, nickel-based alloys are candidate materials for surface strengthening of movable parts in reactors.

5.3 Dissimilar joints

In reactors, dissimilar joints are widely used to connect ferritic steel and austenitic stainless steel pipes of reactor pressure vessels, steam generators, and boosters. In order to connect different materials, nickel-based alloy coatings are usually prepared at both ends of the pipe to prevent the diffusion of carbon elements in the joints. Nickel-based alloy coatings play a key role in dissimilar joints, but the types and elemental composition of nickel-based alloys need further study. For example, Inconel 52M alloy contains appropriate amounts of Nb and Cr elements, which can reduce crack sensitivity and has been used in dissimilar joints. However, Li et al. found that the interface of Inconel 52M has Nb-rich precipitates, which reduce the tensile strength and toughness of the joint at room temperature and may also lead to reduced corrosion resistance.

5.4 Steam generator heat transfer tube

The steam generator heat transfer tube is an important component of the pressure boundary of the reactor coolant system. The early heat transfer tubes mainly used Inconel 600 alloy. Because it is prone to stress corrosion cracking, it was later replaced by Inconel 690 alloy with better stress corrosion resistance. However, surface damage to the heat transfer tube can also induce stress corrosion cracking. For aged Inconel 600 alloy heat transfer tubes and surface damaged Inconel 690 alloy heat transfer tubes, laser cladding process can be used for repair. Baldridge et al. prepared an Inconel 690 alloy cladding layer on an Inconel 600 alloy substrate. The hardness of the cladding layer was 40% higher than that of the substrate, the dilution zone was small and the bonding was good, which verified the feasibility of laser cladding to repair the heat transfer tube of the steam generator.

5.5 Outlook

Laser cladding of nickel-based alloys has broad prospects in reactors, but there are still some problems that need to be improved and solved before the mature application of this method. First, the application requirements of different scenarios in the reactor and the technical difficulty of defect regulation have brought challenges to the powder composition design and process parameter optimization of laser cladding, and a lot of basic research work needs to be carried out; secondly, the complexity and diversity of key structures in the reactor have put forward more urgent needs for intelligent planning of laser cladding paths, online monitoring of the manufacturing process, and dynamic adjustment of process parameters; finally, how to comprehensively verify and evaluate laser cladding materials in different scenarios and ensure the availability of this technology in the reactor environment is the top priority, and a complete laser cladding material performance evaluation system needs to be established as soon as possible.

6 Conclusion

Based on the material properties of nickel-based alloys, this paper briefly describes the research status of laser cladding of nickel-based alloys such as Ni60, Inconel 625, and Inconel 718. Based on the comprehensive performance of the cladding layer, this paper briefly describes the research progress of process parameters such as laser power, scanning speed, and powder feeding rate. Based on the main influencing factors, this paper briefly describes the research progress of cladding layer properties such as microstructure, wear resistance, and tensile properties.

This paper summarizes the application of nickel-based alloy laser cladding in nuclear valve sealing surfaces, movable parts, dissimilar joints, and steam generator heat transfer tubes in reactors, and looks forward to the current problems of powder composition design, process parameter optimization, manufacturing process monitoring, and evaluation system establishment, in order to provide a reference for in-depth research and practical application of nickel-based alloy laser cladding.