Laser cladding is a new type of coating technology. It is a high-tech technology involving multiple disciplines such as optics, machinery, electricity, materials, detection and control. It is the most important supporting technology for laser advanced manufacturing technology. It can solve the problems that traditional manufacturing methods cannot solve. It is a high-tech technology that the country focuses on supporting and promoting. At present, laser cladding technology has become one of the important means for the preparation of new materials, rapid and direct manufacturing of metal parts, and green remanufacturing of failed metal parts. It has been widely used in aviation, petroleum, automobile, machinery manufacturing, shipbuilding, mold manufacturing and other industries.

In order to promote the industrialization of laser cladding technology, researchers from all over the world have conducted systematic research on the key technologies involved in laser cladding and have made significant progress. There are a large number of research and conference papers and patents at home and abroad that introduce laser cladding technology and its latest applications: including laser cladding equipment, materials, processes, monitoring and control, quality inspection, process simulation and emulation and other research contents. But so far, laser cladding technology has not been applied industrially on a large scale. Analyzing the reasons, there are factors such as government guidance, the limitation of the maturity of laser melting technology itself, and the degree of recognition of laser cladding technology by all sectors of society. Therefore, in order to achieve comprehensive industrial application of laser cladding technology, it is necessary to increase publicity efforts, be guided by market demand, focus on breaking through the key factors that restrict development, and solve the key technologies involved in engineering applications. It is believed that in the near future, the application field and intensity of laser cladding technology will continue to expand. The following introduces several development trends of laser cladding technology for readers.

Advantages of laser cladding

The focusing power density of the laser beam can reach 10^10 to 10^12 W/cm², and the heating rate can reach up to 1012K/s when acting on the material. This comprehensive characteristic not only provides a strong foundation for the growth of new disciplines in materials science, but also provides an unprecedented tool for the realization of new materials or new functional surfaces. The rapid cooling conditions of the melt created by laser cladding far away from the equilibrium state under high temperature gradients, which form a large number of supersaturated solid solutions, metastable phases and even new phases in the solidified structure, have been confirmed by a large number of studies. It provides new thermodynamic and kinetic conditions for manufacturing functional gradient in-situ self-generated particle reinforced composite layers. At the same time, the preparation of new materials by laser cladding technology is an important basis for the repair and remanufacturing of failed parts under extreme conditions and the direct manufacturing of metal parts. It has been highly valued and studied in many aspects by the scientific community and enterprises in various countries around the world.

At present, laser cladding technology can be used to prepare metal-based composite materials such as iron-based, nickel-based, cobalt-based, aluminum-based, titanium-based, and magnesium-based. From the functional classification: single- or multi-functional coatings such as wear resistance, corrosion resistance, high temperature resistance, and special functional coatings can be prepared. From the perspective of the material system that constitutes the coating, it has developed from a binary alloy system to a multi-element system. The alloy composition design and multifunctionality of the multi-element system are important development directions for the preparation of new materials by laser cladding in the future.

The latest research shows that steel-based metal materials dominate in my country’s engineering applications. At the same time, the failure of metal materials (such as corrosion, wear, fatigue, etc.) mostly occurs on the working surface of the components, and the surface needs to be strengthened. The use of large in-situ self-generated particles to reinforce steel-based composite materials to meet the service conditions of the workpiece not only wastes materials, but also has extremely high costs. On the other hand, from the perspective of bionics, natural biomaterials are composed of dense outside and sparse inside, and their performance is hard outside and tough inside. The density-sparseness, hardness-toughness change gradually from the outside to the inside. The special structure of natural biomaterials makes them have excellent performance. According to the special service conditions and performance requirements of materials in engineering, new surface metal matrix composite materials with strong and tough technology and gradient performance are urgently needed. Therefore, the use of laser cladding to prepare gradient functional in-situ self-generated particle reinforced metal matrix composite materials that are metallurgically bonded to the substrate is not only an urgent need in engineering practice, but also an inevitable trend in the development of laser surface modification technology. The preparation of in-situ self-generated particle reinforced metal matrix composite materials and functional gradient materials by laser cladding technology has been reported, but most of them remain in the stage of organization, performance analysis, and process parameter control. The size, spacing and volume ratio of the reinforcement phase cannot reach a controllable level. The gradient function is formed by multi-layer coating, and there is inevitably a problem of weak interface bonding between layers, which is still a long way from practical application. The use of laser melting technology to prepare metal-based surface composite materials with controllable particle size, quantity, and distribution, appropriately matched strength and toughness, and integrated gradient functions and in-situ self-generated particle reinforcement is an important development direction in the future. The research content involves:

• Technology, means and principles for the design of composition, organization, and performance of cladding materials and control technology for process implementation.
• Establishment of thermodynamic and kinetic models for the precipitation, growth and strengthening of particle reinforcement phases of functional gradient in-situ self-generated particle-reinforced metal-based composites prepared by laser cladding.
• Bionic design of particle reinforcement phase morphology, structure, function and composite, and control technology of size, quantity and distribution.
• Research on the principles, key factors and process methods of coating composition, organization and performance gradient control.
• Observation, analysis, control and characterization of macro and micro interfaces; analysis and detection of conventional properties of functional gradient in-situ self-generated particle-reinforced metal-based composites, as well as wear behavior and failure mechanism under different working conditions.

The breakthroughs in these research contents may solve the problem of mismatch between coating and substrate and easy cracking, and promote the expansion of the application field of laser cladding technology.

Laser composite cladding technology

Laser cladding uses laser as a heat source to coat a layer of alloy with extremely good performance on the substrate. Its performance will depend on the specific requirements of the parts being processed. The advantages of the laser cladding method are fine coating structure, excellent performance, small thermal stress, small deformation and no pollution. Its disadvantages are also obvious: it requires a very high-power laser, single-channel overlap scanning is not suitable for large-area processing, and it is difficult to achieve industrialization. To solve these problems, the use of laser composite cladding technology is one of the effective ways and an important direction for future development. Laser composite cladding is to use ordinary heating methods and add laser composite heating to complete the cladding treatment work. Ordinary heating methods can be electric heating, various types of induction heating, etc. according to needs. In summary, laser composite cladding technology has the following characteristics:

• “Conventional (such as induction) + laser” composite heating cladding combines the advantages of two heating processes, while overcoming the shortcomings of each single method, fully reflecting the characteristics of complementary advantages.
• Conventional methods are used to assist laser heating, so that large-area cladding that was originally difficult to complete with very high power can be achieved with a relatively low-power laser, which is difficult to achieve with a single method.
• Laser composite cladding technology has expanded the new and wider application of conventional technology, and the adoption of conventional technology has further promoted the application and industrialization of laser cladding technology.
• Laser composite cladding technology is particularly suitable for slender rods, shafts within a certain range of sizes, such as oil pump plungers, certain types of rollers, and shafts for special purposes.

Cladding technology of new laser sources

At present, laser cladding mainly uses CO2 gas lasers for laser cladding of large parts, and a small number of YAG lasers. YAG laser cladding often uses pulsed laser cladding. Recent engineering applications have shown that YAG laser cladding has more advantages in small parts.

Another important development trend is the use of high-power semiconductor lasers, using red light or near-infrared lasers with a wavelength range of 808-965μm. Compared with CO2 lasers, metals are easily absorbed, pre-treatment can be omitted, and it is convenient and easy to operate. High-power semiconductor laser cladding technology has significant advantages over other cladding methods. At the same time, semiconductor lasers can realize integrated control with coaxial powder feeding and use optical fiber transmission and beam expansion technology for light guiding and focusing, realize fully enclosed transmission or optical fiber transmission, and realize highly integrated control of light, machine, electricity, powder, and control; combined with robot arms (people), miniaturization, mobile online services can be realized to meet the needs of different levels. It can be foreseen that in addition to traditional CO2 and YAG laser cladding technologies, new high-power semiconductor laser cladding equipment and processes will gradually develop and meet the needs of high-quality surface engineering, becoming an important part of laser surface treatment.

Laser cladding technology under extreme conditions

With the maturity and development of laser cladding technology, it has been successfully applied to laser cladding of corrugated rollers, direct laser cladding manufacturing of cylinder flame rings, and engine component repair. It has achieved the geometric loss of various metal parts with laser as the main processing method, and restored the geometric dimensions and improved the performance according to the original manufacturing standards. With the development and needs of science and technology and engineering technology, the working conditions of metal parts are becoming more and more harsh, and they often work under extreme conditions such as high alternating stress, high temperature, high speed and high corrosion. Therefore, the materials used to manufacture metal parts need to have multiple properties at the same time to meet the special service conditions of the parts. Moreover, the manufacturing cost and manufacturing cycle of these parts are long, and once they fail, huge economic losses and safety accidents will occur. For example, in turbine equipment, various important parts such as blades, rotor journals, valve stem impellers, valves, etc.; aircraft engines, internal combustion engine parts, etc. These engineering technical difficulties have posed new challenges to laser melting technology. Therefore, how to solve the problem of repairing failed parts under extreme conditions is very urgent and complex. It is necessary to analyze the failure form of parts under extreme conditions, evaluate the remaining life, and select appropriate materials and process methods. Therefore, taking the strengthening and repair of key components under extreme conditions as the starting point, the laser cladding strengthening and remanufacturing technology is systematically studied. Through the joint research of several key technologies, the overall technology suitable for strengthening and repairing various components under extreme conditions is obtained. The key directions to be tackled are:

• Life assessment technology before and after repair (strengthening) of failed components under extreme conditions;
• Research on damage-free repair technology of failed components under extreme conditions;
• Research on special alloy materials for laser repair of failed components under extreme conditions;
• Research on entity measurement, three-dimensional entity accumulation modeling repair control system, repair process temperature, geometric dimensions and quality intelligent monitoring system;
• Research on special repair auxiliary equipment;
• Research on repair layer performance testing technology and processing technology.

Prospects of laser cladding technology

Laser cladding technology integrates material preparation and surface configuration, is one of the important supporting technologies of green remanufacturing technology, and is in line with the national sustainable development strategy. High-tech. Chinese scientists are at the international advanced level in basic theoretical research and have made great contributions to the development of laser cladding technology. But on the other hand, the application level and scale of laser cladding technology cannot meet the needs of the market. It is necessary to solve the key technologies in engineering applications, research and develop special alloy powder systems, develop special powder conveying devices and technologies, systematically study the process methods of damage-free repair, establish a quality assurance and evaluation system, increase efforts, and cultivate talents in engineering applications. I believe that in today’s increasingly competitive manufacturing market, laser cladding technology has great potential.