As an alternative technology to electroplating and thermal spraying, ultra-high-speed laser cladding technology has attracted much attention since its inception. By comparing with conventional laser cladding technology, the technical principles of ultra-high-speed laser cladding are clarified; by comparing with electroplating, thermal spraying, conventional laser cladding and other technologies, its technical characteristics and advantages are analyzed. This review summarizes the research status of ultra-high-speed laser cladding technology from the aspects of equipment, materials, processes, organization and applications, analyzes the shortcomings of this technology, and makes suggestions for its research and application development.

Most of the failures of metal parts originate from microscopic damage caused by surface wear and corrosion. Preparing a special metal coating on the surface of parts can greatly improve the wear resistance and durability of parts without significantly increasing costs. Corrosion, fatigue and oxidation resistance, thereby extending the service life of components. Common metal coating preparation techniques include electroplating, hot-dip galvanizing, thermal spraying and laser cladding. Due to environmental, health and other issues, the use of electroplating and hot-dip galvanizing is gradually banned; the thermal spray coating and the substrate are generally mechanically bonded, with low bonding force and high porosity, which limits its application in extreme environments; laser There is a metallurgical bond between the cladding coating and the substrate, and the bonding strength is high. However, conventional laser cladding has a low scanning rate and low production efficiency, which limits its large-scale application. Ultra-high-speed laser cladding (EHLA) is an emerging, efficient, green coating manufacturing technology that can effectively solve the above-mentioned manufacturing problems. This technology was first jointly developed by Germany’s Fraunhofer Institute for Laser Technology and RWTH Aachen University. Its original intention was to replace electroplating and thermal spraying. As soon as the technology came out, the German company Bosch applied it to manufacturing cast iron brake discs. China Institute of Mechanical Engineering and the German Fraunhofer Institute of Laser Technology have carried out technical cooperation and jointly applied to undertake the national key research and development program “Research and Demonstration Application of Key Technologies for Ultra-high-speed Laser Cladding of Large Metal Components and Its Powder Preparation” Project; Xi’an Institute of Optics, Chinese Academy of Sciences and Xi’an Jiaotong University have also developed ultra-high-speed laser cladding equipment and technology with independent intellectual property rights.

This article introduces the technical principles, technical characteristics and application advantages of ultra-high-speed laser cladding, reviews the current research status of ultra-high-speed laser cladding technology from the aspects of equipment, materials, processes, organization, applications, etc., analyzes the shortcomings of this technology, and Provide suggestions for its research and application development.

1 Basic principles and technical features of ultra-high-speed laser cladding

The basic principle of conventional laser cladding (CLA) is: a high-energy laser beam is irradiated onto a metal substrate, the substrate melts to form a molten pool; metal powder enters the molten pool in a solid form, melts under heat, fuses with the substrate metal and undergoes a metallurgical reaction; the high-energy laser beam moves away from the molten pool, and the molten pool cools and solidifies to form a coating, as shown in Figure 1(a). Unlike conventional laser cladding, ultra-high-speed laser cladding changes the laser optical path and powder feeding path. The high-energy laser beam is focused at the appropriate position above the metal substrate. The laser interacts with the metal substrate and powder at the same time. The powder is melted before entering the substrate molten pool and enters the molten pool in liquid form, as shown in Figure 1(b). Table 1 lists the optical path, powder path, and light-powder interaction of conventional laser cladding and ultra-high-speed laser cladding.

Ultra-high-speed laser cladding technology has the following characteristics: 1) The powder is melted before entering the molten pool, which greatly shortens the molten pool formation time and increases the scanning rate, which is 10 to 50 times the efficiency of conventional laser cladding. 2) The laser beam energy acts more on the powder, greatly reducing the thermal impact on the substrate, which is particularly suitable for coating preparation on substrates with high crack tendency; the molten pool can solidify quickly, which is particularly suitable for amorphous coating preparation. 3) The prepared coating is thin, which greatly reduces the material cost. 4) Although the heat input is small, the coating forms a metallurgical bond with the substrate and has high bonding strength. 5) The dilution rate of the coating is extremely low, and the substrate material has little interference with the chemical composition of the coating. 6) Under the same laser energy density, the powder melts more fully, with less pores and slag inclusions. 7) The cooling rate is fast, the supercooling degree is large during solidification, the nucleation rate is high, and the coating grains are fine and evenly distributed. 8) The coating is thin, the overlap is uniform, the surface quality is high, and the subsequent machining workload is small. 9) No sandblasting pretreatment is required, and no fuel gas such as hydrogen or propane is used, so no waste gas or wastewater is generated. Considering the coating quality, manufacturing cost, production efficiency, environmental factors, etc., compared with conventional laser cladding technology, thermal spraying technology, and electroplating technology, ultra-high-speed laser cladding shows obvious competitive advantages, as summarized in Table 2.

2 Research status of ultra-high-speed laser cladding technology

2.1 Ultra-high-speed laser cladding equipment
The existing ultra-high-speed laser cladding equipment is designed for rotating body components, and ultra-high-speed laser cladding is achieved through high-speed rotation of the rotating body. The entire cladding system is mainly composed of a laser, a powder feeder, a cladding head, a water cooler, and a machine tool. In order to achieve intelligent control, online monitoring devices such as high-speed cameras can be selected, as shown in Figure 2. Lasers, powder feeders, and water coolers generally use mature commercial brands, and machine tools are mostly modified from existing ordinary lathes. In order to realize the integrated operation of ultra-high-speed laser cladding and turning, Wang Wei and others invented a device that combines ultra-high-speed laser cladding and turning processing, which solves the problem of axis deviation caused by the need to turn shaft parts before ultra-high-speed laser cladding, and multiple disassembly and clamping of parts. The cladding head is a key component for realizing ultra-high-speed laser cladding and is also a research hotspot. The research and development of the cladding head focuses on ensuring that the powder material is fully melted before entering the molten pool through the control of the optical path and powder path, and most of the relevant documents are patents. Duan Kailiang and others invented a multifunctional coaxial powder feeding ultra-high-speed laser cladding head, which can realize the coaxial and directional movement of the sprayed powder with the laser at ultra-high speed. Wang Yuyue and others invented a coaxial powder feeding head for ultra-high-speed laser cladding process, which realizes ultra-high-speed laser cladding by optimizing the angle between the powder feeding cavity and the beam cavity. The ultra-high-speed laser cladding head invented by Liu Changyong and others adopts a three-powder feeding channel design, and the powder can be accurately gathered at the laser focus position. Huang Zhaoming and others invented a ring-shaped coaxial powder feeding device for ultra-high-speed laser cladding. The laser beam passes through the center of the powder feeding device and is coaxial with the ring-shaped powder curtain. It acts on the ring-shaped powder curtain in the air to melt it and spray it onto the surface of the workpiece at high speed.

2.2 Ultra-high-speed laser cladding powder materials
The chemical composition, particle size, fluidity, sphericity, etc. of the powder material have an important influence on the effect of ultra-high-speed laser cladding, and will directly affect the structure and performance of the coating. Swedish metal powder manufacturer Hegnac has long begun to develop ultra-high-speed laser cladding special powder materials to replace electroplating. At present, the research and development of ultra-high-speed laser cladding powder materials pays more attention to the design of powder composition. By adjusting the composition, the melting and solidification conditions of the powder are changed to achieve the regulation of the coating structure and performance. Chu Qiaoling and others designed an iron-based powder suitable for ultra-high-speed laser cladding. Through a reasonable composition ratio, the coating forms a high-toughness and high-hardness iron-based coating under the conditions of rapid heating and cooling of ultra-high-speed laser cladding. He Nan and others invented a nickel-based alloy powder for high-speed and ultra-high-speed laser cladding. An appropriate amount of Zr was added to the powder, which played a role in refining the grains in the alloy coating, which could significantly reduce the grain size, reduce the tendency of cracks in the alloy coating after cladding, and obtain good mechanical properties.
Another focus of the research and development of ultra-high-speed laser cladding powder materials is the design of powder morphology. By improving the sphericity, controlling the particle size range, and increasing the powder fluidity, the powder melting state can be controlled, thereby improving the coating structure and performance. Wang Miaohui and others used atomization to prepare iron-based, nickel-based and cobalt-based powders suitable for ultra-high-speed laser cladding. The sphericity was ≥90%, the particle size range was 15-65 μm, the average particle size was 25-40 μm, and the fluidity was 16-20 s/50 g. By controlling the powder particle size distribution, ultra-high-speed laser cladding preparation of fully equiaxed crystal coatings was achieved.

2.3 Ultra-high-speed laser cladding process
Similar to conventional laser cladding, the main process parameters of ultra-high-speed laser cladding are laser power density, scanning rate, powder feeding rate, overlap rate, shielding gas flow rate, etc. In addition, the spot defocus and powder spot defocus are also important process parameters of ultra-high-speed laser cladding. Early research on ultra-high-speed laser cladding process mainly focused on the influence of process parameters on the morphology and size of the coating, especially the coating thickness. The Fraunhofer Institute for Laser Technology used ultra-high-speed laser cladding technology to prepare nickel-based alloy coatings on chromium-nickel alloy steel substrates. It was found that the coating thickness decreased with the increase of scanning rate and increased with the increase of powder feeding rate. When the scanning rate reached 200 m/min, the coating thickness could be reduced to 25 μm. Xi’an Jiaotong University used the third-generation ultra-high-speed laser cladding system (SHE-LSC3000) developed by itself to conduct experiments and found that under the condition of constant laser power and powder feeding rate, the coating thickness gradually decreased with the increase of scanning rate and the dilution rate gradually decreased.
Traditional laser cladding does not need to consider the interaction between laser and powder, but in the process of ultra-high-speed laser cladding, the powder material has been melted before entering the molten pool, so the melting state of the powder in the air will have an important impact on the structure and performance of the coating. KORUBA et al. successfully collected the morphology and temperature distribution of the powder ejected by ultra-high-speed laser cladding using a high-speed camera and an infrared thermal imager, as shown in Figure 3, which laid a technical foundation for the in-depth study of the ultra-high-speed laser cladding process. KELBASSA et al. and SCHOPPHOVEN et al. measured the energy attenuation of the laser after passing through the powder, thereby obtaining the absorption rate of the powder material to the laser. The test results show that the remaining energy after the laser passes through the powder decreases with the increase of the powder feeding rate and decreases with the decrease of the powder particle diameter. Numerical simulation can effectively describe the physical changes such as the movement trajectory of the powder, temperature change, phase transformation, and droplet deformation during the laser cladding process, which is helpful for in-depth research on the ultra-high-speed laser cladding process. SCHOPPHOVEN et al. established a mathematical model to describe the density distribution of powder particles. This model is an important part of the ultra-high-speed laser cladding process model. It can reflect the powder gas flow, especially the running trajectory of powder particles of different particle sizes, the powder mass, the relationship between the carrier gas and the shielding gas, as shown in Figure 4. Based on this model, the interaction between the laser high-energy beam and the powder particles can be studied in depth.

2.4 Coating structure of ultra-high-speed laser cladding
The microstructure of the coating, the microscopic defects inside the coating and the interface state between the coating and the substrate determine the basic properties of the coating. Harbin Institute of Technology used ultra-high-speed laser cladding and conventional laser cladding to prepare a dense, defect-free, and well-metallurgically bonded 431 stainless steel corrosion-resistant coating on a 27SiMn substrate. Compared with conventional laser cladding, the ultra-high-speed laser cladding coating has a lower dilution rate, and the coating structure is mainly fine dendrites, as shown in Figure 5. The Cr in the coating is more evenly distributed inside and between dendrites, and the uniformity of the structure and composition improves the corrosion resistance of the coating. Beijing Jike Guochuang Lightweight Science Research Institute Co., Ltd. [38] conducted a similar comparative experiment and reached similar conclusions. Xi’an Jiaotong University compared ultra-high-speed laser cladding 431 stainless steel corrosion-resistant coating and electroplated hard chrome and found that the 431 coating prepared by ultra-high-speed laser cladding had good bonding with the substrate, the coating density was almost 100%, there were no defects such as holes and cracks, and it showed higher corrosion resistance. Beijing Jiaotong University used ultra-high-speed laser cladding to prepare AISI4340 low-alloy coating and found that the coating had no cracks, the porosity was between 0.15% and 3.45%, and the metallurgical bonding between the coating and the substrate was good. Xi’an Jiaotong University [40] used ultra-high-speed laser cladding technology to prepare Ti-Cu-NiCoCrAlTaY heat-resistant coating on TC4 alloy substrate. The coating had a dense structure and no pores or cracks. Compared with conventional laser cladding, the ultra-high-speed laser cladding coating has a more uniform chemical composition, finer grains, higher density, fewer microscopic defects, and can achieve good metallurgical bonding with the substrate, showing better performance.

2.5 Application of ultra-high-speed laser cladding technology
At present, the “high speed” of ultra-high-speed laser cladding technology is mainly achieved through the high-speed rotation of the rotating body. Therefore, this technology is mainly used for the preparation and repair of corrosion-resistant and wear-resistant coatings on rotating body parts such as shafts and discs. IHC, a Dutch offshore platform manufacturer, was the first to use ultra-high-speed laser cladding instead of electroplating for the preparation of corrosion-resistant coatings on the surface of hydraulic cylinders for offshore platforms, which improved the corrosion resistance of the product. Bosch, Germany, has developed corresponding ultra-high-speed laser cladding processes and coating products for automotive cast iron brake discs, as shown in Figure 6, which extends the service life of the brake discs. Shandong Energy Heavy Equipment Group Dazu Remanufacturing Co., Ltd. conducted an ultra-high-speed laser cladding process verification experiment on the pillars of mining hydraulic supports. The results showed that the macroscopic morphology of the ultra-high-speed laser cladding coating was smoother and more uniform, and the hardness, corrosion resistance and bonding strength all reached or even exceeded the effects of traditional laser cladding processes, and the bonding strength far exceeded that of electroplating coatings. Ningbo Rayspeed Laser Technology Co., Ltd. successfully applied ultra-high-speed laser cladding technology to the preparation of surface coatings of hydraulic pillars, as shown in Figure 7, which improved the wear resistance and corrosion resistance of the products and extended the service life of the products. Beijing Jike Guochuang Lightweight Science Research Institute Co., Ltd. successfully applied ultra-high-speed laser cladding technology to repair nuclear power seawater pump shafts and oil equipment sucker rods, further expanding the application scope of this technology.

3 Development direction of ultra-high-speed laser cladding technology

It has been less than 10 years since the conception, experimental equipment development and commercial promotion of ultra-high-speed laser cladding technology. The development of equipment and materials, process structure performance research and engineering application promotion related to it are all in the exploratory stage. The basic research is not comprehensive and has not formed a system. There is still great potential for research and development.
1) Integration and intelligence of cladding equipment The difficulty of ultra-high-speed laser cladding is to achieve “all-position” high-speed movement in space and ensure that the movement accuracy reaches the micron level. To address this problem, the Fraunhofer Institute for Laser Technology and PONTICON use a fixed laser cladding head to achieve “all-position” high-speed movement by introducing an ultra-high-speed mobile platform, but this design lacks flexibility. Therefore, how to flexibly achieve “all-position” high-speed cladding is still an important direction for future equipment research and development. In addition, although the surface accuracy of ultra-high-speed laser cladding is very high, it still has a certain gap with electroplating. Therefore, the development of “ultra-high-speed laser cladding + grinding and polishing” integrated equipment will help promote the application of this technology. Ultra-high-speed laser cladding has many process parameters. If any process parameter is not well controlled, it will cause defects in the product, which will affect the product performance. Real-time monitoring of the ultra-high-speed laser cladding manufacturing process, online detection of cladding defects and even intelligent repair can greatly improve the quality of ultra-high-speed laser cladding products and promote their engineering applications.
2) Systematization and specialization of cladding materials At present, the powder materials used in ultra-high-speed laser cladding include stainless steel, nickel-based alloys, iron-based alloys, etc., and the powder material system is not complete. Ultra-high-speed laser cladding is more suitable for the preparation of amorphous or high-entropy alloy coatings, but there are few reports on special powder materials for this purpose. Moreover, the chemical composition design of existing powder materials and the evaluation standards of powder quality (particle size range, fluidity, sphericity, etc.) are based on the needs of ordinary laser cladding technology or thermal spraying technology. Therefore, it is crucial to study special powder materials suitable for ultra-high-speed laser cladding and establish special evaluation standards for powder materials suitable for ultra-high-speed laser cladding.
3) Diversification and coordination of cladding processes Affected by conventional laser cladding technology, existing process research still uses laser power density, scanning rate, powder feeding rate, overlap rate, shielding gas flow rate, and carrier gas flow rate as the main process parameters. In fact, parameters such as spot diameter, powder spot diameter, optical path focus position, and powder path focus position have a greater impact on ultra-high-speed laser cladding. In addition, the research on ultra-high-speed laser cladding process generally adopts the “single variable method”. In fact, the combined effect of various parameters has a greater impact on the microstructure and performance of the coating. Therefore, the process research of ultra-high-speed laser cladding should focus on the diversification and coordination of parameters. The Fraunhofer Institute for Laser Technology has developed a new cladding head suitable for ultra-high-speed laser cladding and its corresponding measurement and control system. The system can accurately measure and control the amount and flow rate of deposited powder, the position and diameter of the convergent powder spot, and through data integration, it can obtain the appropriate powder-laser action time to improve the geometric accuracy and product quality of the ultra-high-speed laser cladding coating. 4) Comprehensive and in-depth research on microstructure The coating prepared by ultra-high-speed laser cladding has uniform chemical composition, fine grains, and dense microstructure. However, ultra-high-speed laser cladding is a typical metal rapid solidification forming process. Rapid solidification will inevitably lead to a sudden increase in internal stress in the coating and an increased risk of coating cracking. When the author used ultra-high-speed laser cladding to prepare iron-based amorphous coatings, a large number of through cracks were found, as shown in Figure 8. In addition, the dilution rate of ultra-high-speed laser cladding is extremely low, and its bonding state with the substrate is closely related to the coating performance, but there are few literature reports on this aspect. Therefore, the rapid solidification, crack formation, defect control mechanism of ultra-high-speed laser cladding coatings, and the interface morphology between the coating and the substrate still need to be comprehensively and deeply studied.

5) Diversification and compounding of technology applications
Limited by the “high-speed” implementation method, existing ultra-high-speed laser cladding is mostly used for surface coating preparation of rotary structures such as shafts and disks. If the “all-position” high-speed operation device is successfully developed, ultra
High-speed laser cladding can be applied to surface coating preparation of flat and curved structures, and even in the field of additive manufacturing, achieving diversification of technical applications. In addition, ultra-high-speed laser cladding only increases the surface scanning rate and does not significantly increase the deposition efficiency. Therefore, this technology is more suitable for use in combination with other additive manufacturing technologies to improve the surface accuracy of additively manufactured parts.

4 Conclusion and outlook
The technological process development and structural performance research of ultra-high-speed laser cladding are all carried out with “high speed” as the core. However, these studies are still in the exploratory stage, the basic research is not comprehensive, and a complete scientific research system has not been formed. In the future, the development of this technology will still be oriented towards achieving “high-speed” cladding, specifically in the following four aspects:
1) Guided by high-speed and precise cladding in all positions, realize the systematization, high precision, intelligence and integration of ultra-high-speed laser cladding equipment, and establish evaluation standards for equipment stability and compatibility.
2) Oriented to achieve rapid melting and rapid solidification, develop special powder materials and preparation processes suitable for ultra-high-speed laser cladding, and establish special quality evaluation indicators.
3) Using numerical simulation technology as a means, focus on studying the in-flight melting mode of powder materials, clarify the key process parameters of ultra-high-speed laser cladding, and carry out “multi-variable” process parameter optimization.
4) Oriented by improving the overall performance of the coating, focusing on key issues such as the formation mechanism and prevention measures of tissue defects, as well as the bonding interface between the extremely low dilution rate coating and the substrate.