As one of the advanced representatives of 3D printing technology, laser cladding provides a new direction for the repair of large castings and forgings, and effectively solves the problems of traditional repair technology, such as long repair time and high cost. By introducing the principles and characteristics of laser cladding repair technology, combined with international needs and actual demands of the industry, this paper reviews and summarizes the progress made by laser cladding technology in repairing related large castings and forgings, and looks forward to its future development trend.

The development of the large casting and forging industry is a very important part of the industrial infrastructure construction of various countries. Its development level undoubtedly greatly demonstrates the country’s heavy industrial strength. Its particularity and importance are related to national security and the lifeline of the national economy. my country has been committed to the research and development and manufacturing of large casting and forging products. Especially since the reform and opening up, through various forms of technological upgrades, from technology introduction, technological transformation, imported equipment, cooperative production to later independent technological innovation, equipment development, after more than 40 years of development, my country’s large castings and forgings have been developed. The production scale and capacity have been significantly improved, and a relatively complete domestic R&D and manufacturing system has gradually been formed. Combined with the characteristics of my country’s heavy industry, the large casting and forging industry has provided a large number of high-quality products, equipment and services for the rapid development of aerospace, petrochemical, steel metallurgy, railway transportation, nuclear power and water conservancy and other fields, laying the foundation for the major equipment manufacturing industry and stimulating the industry demand for domestic high-end power equipment.

However, major equipment in the field of high-end power still faces a common industry technical bottleneck, that is, the required large castings and forgings under long-term alternating loads and extreme working conditions. The first overhaul of the Three Gorges unit in 2018 found serious cavitation problems, and the loss of each unit per day due to maintenance was as high as 5 million yuan.

The traditional repair of large castings and forgings is not only difficult to disassemble and transport, but also has high costs for replacing old ones with new ones, and it takes a long time to return to the factory for repair. In addition, it may The repair of the equipment components cannot meet the use requirements, which has certain risks. In order to solve the above problems, we urgently need new repair technologies with high efficiency, high performance, strong adaptability and high intelligence to meet the repair needs of high-end power equipment from multiple aspects such as reducing maintenance/downtime, extending the service life of parts, adapting to complex environments, high forming accuracy and strong controllability. As one of the advanced representatives of 3D printing technology, laser cladding provides a new direction for the repair of large castings and forgings, and to a certain extent solves the problems of difficulty, long time and high cost of traditional repair technology. This paper briefly introduces the working principle and characteristics of laser cladding technology, combines the national strategic needs and the actual demands of the relevant industries, reviews and summarizes the progress of laser additive repair technology in the large casting and forging industry, and finally looks forward to its future development trend.

1 Introduction to laser cladding technology

1.1 Principle and characteristics of laser cladding technology

With the rapid development of laser technology, in the past few decades, the use of laser technology to carry out material processing has gradually become a new research hotspot, with broad application prospects and application range. Laser processing technology can be simply summarized as the use of high-energy-density laser beams to interact with selected materials (mainly metals and non-metallic materials) to complete micro-processing operations such as welding, cutting, through-holes, and surface cladding. It is an advanced material forming technology that integrates multiple disciplines such as computer science, numerical control, and mechanics. As a typical representative of laser processing technology, laser cladding is an emerging material processing method and an important application direction in the field of laser processing. Specifically, laser cladding processing is a rapid additive processing technology that irradiates a high-energy-density laser beam to the surface of the base material to melt the metal powder or coating, and after rapid cooling, a cladding layer is formed and can produce a good metallurgical bond with the base material.

The process flow of laser cladding processing can be briefly described as follows: 1) Based on According to the appearance size of the parts, the processing area and coordinate range are determined through computer-aided design modeling and CNC system; 2) the cladding process parameters (such as laser power, powder feeding amount, powder material, etc.) are formulated to start stacking materials line by line and layer by layer, directly pre-generating or repairing metal parts or large castings and forgings; 3) the repaired large castings and forgings and parts products are inspected and tested, and finally formed after machining. Its working principle and process steps are shown in Figure 1. The large castings and forgings finally processed or repaired by laser cladding technology have good comprehensive material properties. On the basis of meeting the original performance, it can even further improve the physical, chemical or mechanical properties of the base material surface such as corrosion resistance, wear resistance, conductivity and tensile strength. At present, the laser cladding system is mainly composed of a computer (including software operating system), a powder feeding system (including multi-axis robot, nozzle and protective gas equipment), a laser head and a CNC workbench (including environmental With the improvement of technology in my country’s laser additive equipment manufacturing industry, it has now developed towards an integrated, customizable intelligent mobile laser additive system. As shown in Figure 2, this is a mobile laser cladding equipment produced by Huirui Group (Chengdu Qingshi Laser Technology Co., Ltd.).

Laser cladding can be used for surface modification. Compared with traditional surface modification technologies (such as chemical plating, thermal spraying, surfacing, vapor deposition, etc.), the advantages of laser cladding technology are as follows:
(1) The material has a wide range of applications. High-energy laser beams can perform laser surface cladding on common metal powders and ceramic materials on the market.
(2) The cooling speed is fast, and the cladding structure is a typical rapid solidification feature. The coating material composition can be adjusted according to actual needs to obtain excellent material properties.
(3) Thanks to the rapid heating characteristics of the laser, the substrate material The input energy and thermal deformation are small, the dilution rate of the cladding layer is low, and it has a good metallurgical bond with the base material, which can improve the powder utilization rate and the workpiece forming rate.
(4) The cladding layer can be precisely controlled, with a high degree of automation, and can be used for local printing or repair of waste workpieces.

In the late 1980s, since laser cladding technology was listed as a national key technical research project, it has received a lot of attention and research from all walks of life and scientific researchers. It has successfully provided practical verification and solutions for the surface engineering processing and repair of large castings and forgings in my country’s aerospace, water conservancy and hydropower industries, and has achieved considerable technical and economic benefits. As shown in Figure 3, the preparation and repair of the babbitt alloy bearing bushing of the sliding bearing. The melting point of babbitt alloy is only 240 ~ 370℃. Adding a babbitt alloy layer on the surface of the iron-based bushing with a melting point above 1100℃ requires process, The innovative design of the equipment. This is because the traditional casting method has inherent defects (including loose pores, coarse grains and even crack damage), that is, it is impossible to produce a Babbitt alloy layer with high production bonding strength, no element damage, no component segregation and a well-formed surface; in addition, the casting process is cumbersome and there are phenomena such as insufficient or insufficient pouring of molten iron, resulting in a high proportion of processing costs. However, the laser cladding technology is used to add Babbitt alloy to the surface. The laser is used as a heat source to form a molten pool on the substrate, and then the Babbitt alloy powder is sent into the molten pool in a certain way for rapid melting and rapid solidification. The microstructure of the obtained tin-based Babbitt alloy is uniform and fine, and it forms a metallurgical bond with the matrix, and the bonding strength is better than that of the traditional casting process.

Table 1 shows the comparison of the mechanical properties of several alloy parts formed by laser cladding and forging at room temperature, which can be seen from this It can be seen that laser cladding forming has the advantage of fast solidification speed, which makes the internal structure of the material fine, refines the grains, and improves the comprehensive mechanical properties of the material. In addition, the laser cladding technology has no pollution emissions, is green and clean during the forming process, and does not produce excess scraps. Moreover, the unmelted metal powder during the process can be partially recycled and reused. Compared with traditional cutting processing, it has high material utilization rate, short R&D cycle, and low manufacturing cost.

Thanks to the layer-by-layer overlapping forming processing method and precise powder feeding control system, laser cladding can manufacture thin-walled metal parts with complex geometric shapes. By adjusting the working state and powder feeding rate of different powder barrels, metal powders with different properties and compositions can be clad at any position of the parts, thereby realizing the rapid preparation of gradient functional materials, making the physical and chemical properties of the materials “anisotropic”, that is, different positions and directions have different antioxidant, high temperature resistance, impact resistance or water erosion resistance, etc. This is based on This feature is incomparable to the traditional large casting and forging preparation methods. By using this feature, old parts can be “repaired and reshaped”. Figure 4 shows the turbine blades and gradient functional parts with complex shapes repaired and formed by laser cladding technology.

1.2 Overview of laser cladding material system and cladding layer performance research

Since laser cladding technology uses a laser with high energy density, the power of the current fiber laser can even reach 4000 ~ 6000 W, so it can easily complete the cladding of metal powders, alloy coatings, etc., and even achieve the cladding forming of some high melting point ceramic materials. At present, most of the materials used in laser cladding are still based on powder materials used for spraying, which can be roughly divided into metal powders, alloy powders and ceramic powders. Commonly used pure metal powders are W, Cu, Ni, Al, Fe, Ti Alloy powders generally use a composition similar to that of the base material. Common alloy powders include Co-Cr-W, Ni-Ti, Fe-based, Ni-based alloys, etc. Ceramic powders include oxide ceramic powders such as Al2O3, ZrO3, TiO2, carbide ceramic powders such as WC and TiC, and metal ceramic powders represented by WC-Co. With the increasing service requirements of large castings and forgings, the popularity of composite materials has continued to rise in recent years. Laser cladding materials have gradually changed from single powder spraying such as pure metal powder and alloy powder to composite materials. Typical examples are pure metal powder + alloy powder or pure metal powder + ceramic powder. For example, Kunt et al. [7] used Hastelloy X NiCrAlY high temperature resistant coating was laser clad on 300mm (nickel-based high temperature alloy), and the oxidation behavior of the coating was evaluated under the conditions of 1100℃ and 450h. The results of high temperature oxidation experiments showed that the coating successfully achieved a double-layer oxide film (the inner film was continuous aluminum oxide, and the outer film was mixed spinel oxide), which was beneficial to prevent the diffusion of oxygen and achieved long-term continuous high temperature resistance. Zhang Jie et al. determined the optimal process conditions for laser cladding repair of alloy coating on the surface of aged IN718 alloy based on orthogonal experimental design, and studied the interface structure characteristics and mechanical properties of IN718 cladding layer under this condition. The results showed that the cladding layer structure under the process parameters of laser power 900W, scanning speed 6mm/s, and powder feeding amount 10g/min was better than that under the conditions of 1000℃ and 1000℃, the cladding layer structure … It presents a typical dendrite morphology, and the fusion zone and the substrate show good metallurgical bonding. The morphology is different from the cladding layer and the substrate, and no planar crystals appear.

As a surface modification technology with controllable organizational composition and performance, the laser cladding technology can not only greatly improve the mechanical properties of the workpiece such as fatigue resistance, hardness, wear resistance, and tensile strength, but also greatly extend the service life of parts, especially in the maintenance of industrial-level large castings and forgings. It has great application prospects and research significance. At present, it has been widely used in many fields such as nuclear power, hydropower, petrochemicals, aerospace, and medical equipment. Chao Mingju et al. used laser cladding doped (Ta2O5 + C) Ni60 alloy powder mixture to synthesize TaC particle-reinforced nickel-based composite coating on the surface of low-carbon steel in situ. The results show that the coating is metallurgically bonded to the substrate, and the γ(Ni) solid solution and Nearly cubic TaC particles and needle-shaped chromium carbides are distributed in the dual-phase matrix of Cr3C2, Fe2B and γ(Ni) eutectic; compared with Ni60 coating, the hardness of TaC/Ni60 composite coating is increased by 1.38 times, and the block-ring wear rate of hardened steel is reduced by 5 times, which is mainly attributed to the existence of in-situ synthesized TaC particles and their good distribution in the coating. Guo Baoguang et al. reported a TiN-reinforced Ti3Al intermetallic compound-based composite coating (TiN/Ti3Al IMC) synthesized in-situ on a pure titanium substrate. The composite coating was made of titanium and aluminum mixed metal powder as the cladding material and laser cladding was performed in a nitrogen atmosphere. It is prepared by cladding method and laser nitrogen fixation method; the study found that the nitrogen flow rate affects the morphology of TiN reinforcement phase in the composite coating, that is, granular and well-developed dendrites are presented at high flow rate, and granular, flaky and poorly developed dendrites are presented at low flow rate. In addition, relevant experimental results show that the hardness and wear resistance of TiN/Ti3Al IMC are higher than those of Ti3Al coating.

It can be seen that in the field of engineering materials, laser cladding processing and repair technology have been continuously explored and developed by researchers, providing a large number of experimental cases for improving the wear resistance, oxidation resistance, high temperature resistance, wear resistance and other properties of traditional protective coatings. It is worth noting that in addition to the above, the strong adaptability of laser cladding can also be used to obtain some special functional protective coatings, such as bio-ceramic coatings, functional gradient coatings (materials), etc., which have also been used in recent years. Gradually become a research hotspot in the field of surface processing.

2 Practical application of laser cladding repair technology

Since the beginning of the 21st century, laser cladding technology has become increasingly closely related to the forming of large castings and forgings with metal materials as the main body in the heavy industry field due to its unique advantages in material surface modification and a large amount of previous practical foundation. It has successfully taken a solid step. The following briefly introduces and summarizes some actual cases of using laser cladding technology to repair workpieces in important industrial fields.

2.1 Laser cladding repair of large civil aircraft landing gear

The development of long-life large ultra-high strength steel landing gear has important research value for the low-cost and high-safety development of single-aisle large civil aircraft. Taking the main strut of the ultra-high strength 300M steel (large die forging) landing gear of a civil aircraft as an example, fatigue tests (more than 100,000 cycles of stress cycles) showed obvious about 30 mm in its main force transmission part. Cracks, see Figure 5.

Through subsequent failure analysis, it was determined that the reason was that after the roughness around the small hole decreased, the high stress alternating load borne by the drainage hole was greater than the fatigue strength of 300M steel. Before repair, the crack was pre-treated by grinding through machining to form a long straight groove. The process repair plan was determined as: laser powder feeding cladding repair (including mechanical arm) + A100 steel powder raw material. The repair process is shown in Figure 6. By analyzing the mechanical properties of the material after laser cladding repair, it can be seen that: the tensile strength of the repaired area is significantly higher than that of the substrate area (the average values ​​of multiple tests are 2015 MPa and 1969 MPa respectively); all samples did not break after fatigue testing, meeting the fatigue performance requirements, indicating that the performance of the heat affected zone meets the requirements of fatigue testing; the surface residual stress is not greater than 200 MPa, which also meets the requirements. Finally, the repaired 300M The steel landing gear main bracket was subjected to subsequent take-off and landing fatigue tests, and the remaining 135,000 stress cycle fatigue tests were successfully completed, indicating that laser cladding technology has reliable technical guarantees in the repair of large metal parts in the aviation industry.

2.2 Laser cladding repair of nuclear reactor charging pump rotor

The charging pump is the most important power equipment for the chemical and control system in the nuclear reactor. The pump core consists of an inner pump shell and a rotor. As a core component, its importance to the operation of the nuclear reactor is self-evident. However, the repair of the damaged part of the charging pump rotor under long-term working conditions is a thorny problem generally faced by the industry. Not only is the rotor itself highly radioactive, but the rotor is also expensive. During the overhaul process, the rest of the rotor is in good working condition, and the procurement cost of direct replacement is too high and wasteful. Conventional repair methods such as electroplating and thermal spraying will cause damage during the processing process. This leads to irreversible thermal deformation and poor bonding strength of the components, which cannot meet the repair requirements. Taking the double-shell 11-stage centrifugal pump rotor of a nuclear power plant as an example, laser cladding technology was selected to solve the problem of the repair project after comprehensive consideration of various factors. The bonding strength reached more than 90% of the original material, the surface hardness could reach 100% of the original material, and the forming was excellent. The cladding material selected was Ni-based alloy powder with the same composition as the matrix (CA6NM martensitic stainless steel). The repair process is shown in Figure 7. Some process parameters are as follows: laser power 1000 W, spot diameter 3.5 mm, laser scanning speed 10 ~ 12 mm/min, powder feeding rate 1.4 g/min, cladding layer thickness of about 0.7 mm.

Finally, the charging pump rotor repaired by laser cladding was put back into use. According to the current situation, it has been running smoothly for nearly one and a half years, and the parameters such as flow rate, outlet pressure and bearing temperature are normal, and the working condition is good. This shows that using laser cladding technology to repair the charging pump rotor of a nuclear power plant is a good strategy that not only meets the cost-effectiveness requirements, but also meets the performance requirements, and saves time and effort.

2.3 Laser cladding repair of turbine blades

The turbine is the core functional equipment of a hydropower station. According to the operating environment of the turbine, the damage forms of its blades can be divided into cavitation (clean water environment) and erosion (mud and sand environment). For the cavitation and erosion problems of turbine blades, the current traditional method is to select a matrix material with high cavitation resistance (usually high-strength stainless steel such as 0Cr13Ni4Mo). This type of stainless steel It is considered to be one of the most widely used and effective parent materials in the current field of water conservancy machinery manufacturing; the second is to use surface strengthening methods to give the parent material surface a special protective coating to ensure long-term normal operation in different water environments. The first method has certain limitations due to the geographical restrictions on the operation of turbines, so people pay more attention to the second method—the research and development of blade surface strengthening (or repair) technology. However, traditional repair and strengthening methods are mainly based on plasma spraying, surfacing, etc. Although they can solve some existing problems (such as good wear resistance and erosion resistance of spray coatings, but poor cavitation resistance), there is still poor bonding strength between the coating and the substrate, and it is impossible to solve the wear resistance and impact resistance problems at the same time. Therefore, it is necessary to develop new turbine blade surface strengthening and repair technologies, among which laser surface cladding technology is one of them. Take the damage repair of turbine blades running in a clear water environment as an example At present, laser cladding is mainly used to enhance its cavitation resistance. The sample materials and methods are as follows: the blade base material is 0Cr13Ni4Mo stainless steel, the cladding material is Co-based alloy powder (element mass fraction: Co 48.8%, Cr 26%, Mo 1.2%, W 15%, Ni 2.5%, and the remaining non-metallic content such as Si, B and C is the remainder), the laser power is 1.6 kW, and the laser scanning rate is 10 mm/s. The comparison of the sample before and after cladding is shown in Figure 8.

The cavitation resistance of the coating is analyzed by the ultrasonic cavitation mass loss method and the surface morphology of the sample. The results show that the Co-based alloy powder has a good metallurgical bond with the base material, and the coating is near the surface. The surface grains are fine (mainly equiaxed crystals) and there is a certain amount of W-C reinforcement phase, so a higher surface hardness can be obtained, even 1.5 times that of the base material. In addition, the mass loss of the coating under the same working conditions is less than that of the base material, which proves its excellent anti-erosion performance in a clean water environment. The laser cladding technology may provide a new strategy for the surface strengthening and repair of high anti-erosion blades in the future. In response to the cavitation and erosion problems of the Gezhouba runner blades, Nanjing Pfizer Group used laser cladding to clad a layer of Steelite 6 on the 0Cr13Ni5Mo plate after process debugging. The coating is flat and the surface finish is not high. After laser remelting, the surface finish of the cladding layer is greatly improved. This shows that the use of laser cladding + The laser remelting process is feasible for preparing Steelite6 strengthening coating on a large area on the substrate, and the coating has good forming quality and no metallurgical defects. The “nine-square grid” method is feasible for cladding on the forming path, and has a certain effect on improving the deformation of the substrate. After subsequent grinding and polishing, the cladding layer has a smooth and bright surface, and the roughness can reach Ra0. 8 μm, which can meet the technical requirements. The process effect is shown in Figure 9.

3 Conclusion

As an advanced material forming technology with multidisciplinary cross-cutting and huge economic benefits, laser cladding technology provides a new research direction and solution for the forming and repair of large castings and forgings in traditional manufacturing. Thanks to the superiority of high-energy laser processing, the application scope of laser additive repair technology in the large casting and forging industry is becoming wider and wider. This paper analyzes the principle and characteristics of laser cladding. This paper briefly introduces the cladding material system and cladding layer performance, and summarizes and reviews the practical application of laser cladding repair technology in large castings and forgings. Although this technology has made many progresses, we should still face up to some of the current shortcomings. There are still many key technical problems that have not been overcome, such as basic theoretical research on cladding, research and development of domestic laser additive equipment and accessories, cladding powder system and key process development. This is not only conducive to the innovation of laser cladding technology, but also can broaden its future application areas and promote the development of the large casting and forging industry.