Laser cladding is a cutting-edge surface treatment technology that plays a vital role in enhancing the wear resistance of material surfaces. Laser cladding has become a key means to optimize the surface friction reduction and wear resistance of base materials. This technology clads high melting point, high hardness alloy or ceramic powder on the surface of the base material to form a cladding layer with excellent performance, thereby significantly improving the wear resistance, corrosion resistance and oxidation resistance of the material. This article outlines the basic characteristics of laser cladding and its current application status, discusses the impact of cladding materials on the performance of the cladding layer, and introduces related optimization processes.

Friction and wear are the main factors causing surface damage to parts and materials, which greatly restricts the reliability and safety of equipment operation. Friction and wear increase material and energy losses, reducing reliability and safety. Friction and wear of mechanical parts mainly occur on the material surface, and about 80% of parts failure is caused by surface wear. Laser cladding technology emerged in the 1980s and is now widely used to prepare harder and wear-resistant composite coatings. By selecting different parameters and coating materials, surface hardness and wear resistance can be effectively improved. It has the characteristics of resource saving, green cleaning, cost saving, etc. It has become a hot technology in the field of remanufacturing and has attracted much attention in industries such as aviation and automobiles. .

1 Laser cladding
1.1 Characteristics of laser cladding

Laser cladding uses a high-energy laser beam (10’4~10’6 W/cm2) to irradiate the surface of a metal substrate (see Figure 1), causing the thin layer on the surface of the metal substrate to interact with the cladding material on it. The process of melting and solidifying to form a coating layer with special physical and chemical properties such as high hardness, good wear resistance, and corrosion resistance. This is a new type of composite material technology that can supplement the excellent properties that the matrix does not have, give full play to the advantages of both, and overcome the shortcomings of each other, thereby significantly improving the wear resistance, corrosion resistance, and corrosion resistance of the base surface. It has physical and chemical properties such as heat resistance and oxidation resistance, thereby significantly improving the wear resistance, corrosion resistance, heat resistance, oxidation resistance and other properties of the workpiece, or achieving the effects of repairing the surface size of the workpiece, strengthening its performance, and extending its service life. Laser cladding technology is currently considered a sustainable remanufacturing technology and is used in the maintenance industry. Compared with other surface strengthening methods such as surfacing, thermal spraying, ion spraying, etc., the surface layer strengthened by laser cladding It has the advantages of good performance and low overall cost: 1) The molten pool cools quickly during the solidification process, and a small, dense and defect-free cladding layer can be obtained, which helps to improve the performance of the cladding layer; 2) The laser beam in the cladding process The energy density and energy utilization rate are high, which can reduce the thermal distortion and thermal damage of the matrix, and the matrix deformation is smaller and the performance is stable; 3) A variety of cladding materials can be selected, and the material composition can be adjusted or mixed with multiple materials according to needs. As a result, the cladding layer has excellent performance.

1.2 Classification of laser cladding technology

According to the supply method of cladding material, laser cladding is divided into preset laser cladding and synchronous laser cladding. Preset laser cladding requires pre-positioning of cladding material on the cladding surface of the base material, and then the laser beam is used to irradiate the cladding material. The melted cladding material can be placed in the form of powder, wire, or plate. Among them, powder The most commonly used form is the process flow: base material cladding surface pretreatment – preset cladding material – preheating – laser melting – post-heat treatment. Synchronous laser cladding requires the cladding material to be fed into the laser beam synchronously, so that feeding and cladding are completed at the same time. The cladding material is mainly fed in the form of powder. In a few cases, wires or plates can also be used for synchronous feeding. Process flow: substrate cladding surface pretreatment – feeding laser melting – post-heat treatment. Compared with preset laser cladding, synchronized laser cladding has the advantages of less consumables, good controllability, high laser energy utilization, uniform cladding layer, and less thermal influence on the substrate. Synchronized laser cladding has higher production efficiency in practical applications and is the main research direction of laser cladding technology.

2 Effect of cladding material on cladding layer

The quality of the laser cladding layer is closely related to the type of powder material selected. Different types of alloy powders have different properties and properties. 2.1 Self-fluxing powder Self-fluxing alloy is generally made into powder and used for thermal spraying. It has the characteristics of low melting point and good fluidity. During the melting process, the elements in the alloy can deoxidize and form slag by themselves, so that the alloy is protected. In addition, after the alloy solidifies, a high-hardness dispersion strengthening phase can be formed in the solid solution, which improves the strength and hardness of the alloy. Self-fluxing alloys currently used mainly include nickel-based alloys, cobalt-based alloys, iron-based alloys and copper-based alloys. The characteristics (advantages and disadvantages) of self-fluxing alloy powder are shown in Table 1.

2.1.1 Nickel-based alloy powder

Nickel-based alloy powder has excellent comprehensive properties, corrosion resistance, oxidation resistance, heat resistance, low-stress abrasive wear resistance, and good impact toughness; low melting point, wide solid-liquid temperature range, and is resistant to a variety of matrices and WC particles It has strong wetting ability and is easy to operate; it has excellent self-fluxing, wettability and spray weldability. The spray welding layer has the characteristics of high hardness, corrosion resistance, wear resistance and heat resistance. It is difficult to cut and wet grinding is suitable. . Through laser cladding nickel-based alloy powder experiments, Jing Peiyao and others proved that it has good self-fluxing properties, wettability, high hardness, high bonding strength with the matrix, low dilution rate, and few cracks and pores. In practical applications It can resist wear and damage, resist corrosion, and improve the strength of metal surfaces.

2.1.2 Cobalt-based alloy powder

Cobalt-based alloy is a hard alloy that can resist various types of wear, corrosion and high-temperature oxidation. It has high strength at temperatures above 980 ℃, good resistance to thermal fatigue, thermal corrosion and abrasion, and good weldability. Cobalt-based alloy powder is suitable for making guide blades and nozzle guide vanes of aviation jet engines, industrial gas turbines, ship gas turbines, and diesel engine nozzles, etc. Li Maochang et al. used Co-Cr-based alloy Stellite12 powder to perform multi-layer laser cladding in the substrate tooth groove. The microstructural characteristics of the Stellite12 cladding layer were studied by optical microscopes and other testers. The results showed that the cracks were brittle cracks caused by tensile stress and constraint stress of the coating. The average microhardness around the crack area was 450-540 HV, and the hardness was significantly improved. The wear surface of nickel-based cladding layer is relatively rough and the wear rate is large, the wear reduction effect is not good, and the hardness and strength are weak; although the wear resistance and corrosion resistance of cobalt-based coatings show certain advantages compared with iron-based coatings, the engineering cost of the former is lower and the overall effect is better.

2.1.3 Iron-based alloy powder

Among the laser cladding powders, iron-based alloy powders have the advantages of full range, good performance and low cost. Therefore, the laser cladding iron-based alloy cladding layer has good comprehensive mechanical properties and can be used as a surface protective layer of 45 steel to improve wear resistance, hardness and extend life. It is widely used in aviation, aerospace, automobile, electronics, petroleum, metallurgy and other fields. However, compared with other self-soluble alloy powders, it is very easy to oxidize, even if elements such as boron, nickel and chromium are added, it cannot make up for it. Therefore, its versatility as a corrosion-resistant cladding material is poor. Ding Ziyang et al. conducted experiments to study the organization and properties of laser cladding iron-based alloy powder cladding layers. The results showed that as the cladding speed increases, the hardness and corrosion resistance of the laser cladding layer will be affected.

2.2 Ceramic powder

Ceramic composite powders have the characteristics of high hardness, high melting point, and low toughness. At present, ceramic powders mainly include silicide ceramic powders and oxide ceramic powders, among which oxide ceramic powders (alumina and zirconia) are the main ones; zirconia has lower thermal conductivity and better thermal vibration resistance than alumina ceramic powders, so it is also often used to prepare thermal barrier coatings. Ceramic powders have excellent wear resistance, corrosion resistance, high temperature resistance and oxidation resistance, so they are often used to prepare high-temperature wear-resistant and corrosion-resistant coatings. Gu Jianqiang et al. deposited NiCr-Cr3C2 metal ceramic composite coating on 20Cr2Ni4A substrate by laser cladding and found that the Cr3C2 and Cr7C3 hard phases formed in the cladding coating had lower friction coefficient and wear rate in medium and high temperature working environment, and the wear resistance was significantly improved compared with the substrate.

2.3 Other metal powders

In addition to the above types of laser cladding powder material systems, the cladding material systems that have been developed and studied include copper-based, titanium-based, aluminum-based, magnesium-based, zirconium-based, chromium-based and intermetallic compound-based materials. Most of these materials use certain special properties of the alloy system to achieve one or more functions such as wear resistance, friction reduction, corrosion resistance, conductivity, high temperature resistance, and thermal oxidation resistance. Jiang Mingming et al. used laser cladding technology to prepare high entropy alloy coatings on the surface of the substrate, so that the coating and the substrate achieved good metallurgical bonding. The coating significantly improved the hardness, corrosion resistance and friction corrosion performance of the substrate, thereby reducing resistance and friction.

3 Surface laser cladding optimization process
3.1 Simulation technology

Numerical simulation technology can accurately understand the temperature field, stress field and flow field distribution during the cladding process, and effectively predict the cracks, pores, inclusions and interlayer bonding of the cladding layer. Li Yamin et al. analyzed the influence of process parameters on the molten pool based on the ANSYS simulation platform, and determined the optimal process parameters for laser cladding Inconel718 coating. These studies obtained the optimal process parameters for laser cladding under specific conditions, and the quality of the prepared cladding layer was significantly improved.

3.2 Ultrasonic assisted technology

Due to the characteristics of rapid heating and cooling of the laser cladding process, cracks are prone to occur in the cladding layer, which has become the main technical obstacle to its development. In order to improve the quality of the cladding layer, the combination of ultrasonic vibration, electricity and magnetism is the future development trend. Ultrasonic treatment is used to homogenize the composition of the cladding layer, refine the grains, and reduce residual stress, thereby solving the problems existing in the process. Ultrasonic vibration is used to achieve the cavitation effect, acoustic streaming effect, resonance effect and thermal effect of the alloy melt, increase the fluidity of the melt, promote the escape of bubbles in the melt, accelerate the diffusion of solute elements, break up coarse grains, and achieve grain refinement. After adding ultrasonic vibration, the forming quality of the cladding layer is significantly improved, the grains are significantly refined, and the average microhardness and surface roughness are improved.

3.3 Ultra-high-speed laser cladding technology

In 2017, the Aachen Institute of Laser Technology in Germany proposed ultra-high-speed laser cladding technology, which solves the shortcomings of traditional laser cladding and has attracted widespread attention from scholars. Compared with traditional laser cladding coatings, ultra-high-speed laser cladding coatings are thinner, have lower roughness and smaller grain size, significantly improve corrosion resistance, and have better wear resistance. In the coating preparation process, in addition to the cladding powder, the main factors affecting the coating performance are also the process parameters, which play a decisive role. Therefore, the study of process parameters is crucial. Ultra-high-speed laser cladding technology has made great improvements in terms of process compared to traditional laser cladding technology. The essence of ultra-high-speed laser cladding technology is to change the melting position of the powder, so that the powder intersects with the laser focus above the workpiece, melts under the action of laser energy, and then evenly coats the surface of the workpiece. The cladding speed is 100 to 250 times faster than traditional laser cladding. The laser energy acts on the substrate for a very short time, which minimizes the thermal effect of the laser on the substrate.

4 Summary and Outlook

In recent years, laser cladding technology has attracted much attention, especially in the field of friction-reducing and anti-resistance coatings, it provides excellent technical support. This technology can not only significantly extend the service life of the substrate, but also effectively reduce costs, which is of immeasurable value in promoting industrial upgrading and enhancing international competitiveness.

Looking to the future, when laser cladding technology gradually matures, its cladding materials will inevitably move towards standardization and serialization. This means that whether in aerospace, automobile manufacturing, or in medical equipment, electronic technology and other fields, laser cladding technology will play an increasingly important role with its unique advantages.