Laser coating technology involves placing selected coating materials on the surface of the coated substrate using different filling methods. After laser irradiation, they simultaneously melt with a thin layer on the substrate surface and quickly solidify to form a layer with extremely low dilution. a surface coating that is metallurgically bonded to the base material, thereby significantly improving the wear resistance, corrosion resistance, heat resistance, oxidation resistance and electrical properties of the surface of the material basic.
Laser cladding technology is a new technology with high economic advantages. It can prepare high-performance alloy surfaces on inexpensive metal substrates without affecting the properties of the substrate, thereby reducing costs and saving valuable and rare metal materials.
Lasers used in laser cladding mainly include CO2 lasers and solid-state lasers, including disk lasers, fiber lasers and diode lasers.
Process characteristics of laser cladding technology
Laser cladding can be divided into two categories according to different powder feeding processes: powder presetting method and synchronous powder feeding method. Both methods have similar effects. The synchronous powder feeding method has the advantages of easy automatic control, high laser energy absorption rate and no internal pores. In particular, cermet coating can significantly improve the crack resistance of the coating layer, thereby enabling curing. ceramic phase uniformly distributed in the coating layer.
1. Laser cladding has the following characteristics:
(1) The cooling rate is fast (up to 106 K/s), which corresponds to a rapid solidification process. It is easy to obtain fine-grained structures or produce new phases that cannot be obtained at equilibrium, such as unstable phases or amorphous states. , etc.
(2) The dilution ratio of the coating is low (generally less than 5%) and it has a strong metallurgical bond or interface diffusion bond with the substrate. By adjusting the laser process parameters, a good coating with a low dilution rate can be obtained. and the coating composition conforms to The dilution is controllable.
(3) Heat input and distortion are small, especially when using high power density and fast coating, the distortion can be reduced to the assembly tolerance of the part.
(4) There are almost no restrictions on powder selection, especially for deposition of high melting point alloys on the surface of low melting point metals.
(5) The thickness range of the coating layer is wide, and the thickness of a coating with single-pass powder feeding is 0.2-2.0mm.
(6) It can perform selective deposition, consumes less material and has an excellent performance-price ratio.
(7) Beam aiming can weld inaccessible areas.
(8) The process is easy to automate. It is very suitable for the wear and repair of common wearing parts in the petroleum field.
2. Similarities and differences between laser cladding and laser alloy
Both laser cladding and laser alloying are rapid melting processes produced by high energy density laser beams, forming alloy coatings with completely different compositions and properties on the substrate surface which are fused with the matrix. The processes of both are similar, but they are essentially different. The main differences are:
(1) During the laser cladding process, the coating material is completely melted, and the melted layer of the base material is extremely thin, so it has minimal impact on the composition of the coating layer. Laser alloy, on the other hand, adds. alloy to the surface fusion coating layer of the elements of the base material, the aim is to form a new alloy layer based on the base material.
(2) Laser cladding basically does not use the molten metal on the substrate surface as a solvent, but melts the configured alloy powder separately to make the subject alloy of the cladding layer at the same time, a fine coating layer. The substrate alloy is also melted to form a metallurgical bond. The preparation of new materials by laser cladding technology provides an important basis for the repair and refurbishment of defective parts under extreme conditions and for the direct manufacturing of metal parts. It has received great attention from the scientific community and businesses around the world.
Evaluation of laser cladding effect
To evaluate the quality of laser cladding layer, we mainly consider two aspects. The first is to macroscopically examine the shape of the coating channel, surface roughness, cracks, pores and dilution rate, etc. ; the second, under the microscope, is to examine whether a good structure is formed and whether it can provide the required performance. In addition, the types and distribution of chemical elements in the surface coating layer should also be measured, and care should be taken to analyze whether the transition layer is a metallurgical bond. If necessary, life tests should be carried out.
The research work focuses on the research and development of coating equipment, molten bath dynamics, alloy composition design, crack formation, expansion and control methods, and strength connection between the coating layer and the substrate.
The main problems facing the further application of laser cladding technology are:
(1) The main reason why laser cladding technology has not yet been fully industrialized in China is the instability of the quality of the cladding layer. During the laser cladding process, the heating and cooling rates are extremely fast, with the highest speed reaching 1012°C/s. Due to the difference in temperature gradient and thermal expansion coefficient between the coating layer and the base material, various defects may occur. in the coating layer mainly include pores, cracks, deformation and surface roughness.
(2) Automatic detection and control of the optical deposition process.
(3) The cracking susceptibility of laser cladding layer remains a difficult problem that worries domestic and foreign researchers, and also poses an obstacle to technical application and industrialization. Although crack formation and propagation have been studied, control methods are. not yet ripe.
Application of laser cladding technology
The scope and application areas of laser cladding processing technology are very wide, covering almost the entire machinery manufacturing industry.
Currently, laser coating of self-melting cobalt, nickel and iron alloy powders and ceramic phases on the surfaces of stainless steel, mold steel, malleable cast iron, of gray cast iron, copper alloy, titanium alloy, aluminum alloys and special alloys has been successfully carried out.
Iron-based alloy powder laser coating is suitable for parts that require local wear resistance and are easily deformed. Nickel-based alloy powder is suitable for components requiring local wear resistance, hot corrosion resistance and thermal fatigue resistance. Cobalt-based alloy powder is suitable for parts requiring resistance to wear, corrosion and thermal fatigue. Ceramic coatings have high high temperature resistance, good thermal stability and high chemical stability. They are suitable for parts requiring resistance to wear, corrosion, high temperature and oxidation.
Here are some typical applications of laser cladding technology:
1. Manufacturing and refurbishment of mining equipment and parts
Coal mining machinery equipment is heavily used and wears out quickly. Due to their harsh working environment, parts and components are damaged relatively quickly. Laser manufactured and remanufactured coal machine equipment parts include: three machines and one machine.
(1) Coal shear: main frame, rocker arms, gears, gear shafts, various bushings, hinged frames, oil cylinders, oil cylinder seats, guide shoes, sprockets, rail axle wheels, driving wheels, picks, etc.
(2) Tunnel boring machine: cylinder, support, shaft, various rings, picks, etc.
(3) Scraper conveyor: central chute, transition chute, gearbox, gears, gear shafts, spiral bevel gears, shaft parts, etc.
(4) Hydraulic support: hinge holes of oil cylinders, bases and supports, various bushings, etc.
2. Manufacturing and refurbishment of electrical equipment and parts
Electrical equipment is distributed in large quantities and operates continuously, and its parts have a high probability of being damaged. Steam turbines are the basic equipment for thermal energy production. Due to the special working conditions of high temperature and heat, damaged parts such as main shaft diameter, moving blades, etc. may be damaged. must be repaired regularly every year. Gas turbines often suffer damage due to operating at high temperatures of up to 1,300°C. Laser refurbishment technology is used to repair any defects and restore its performance at a cost that is only 1/10 of the price of a new unit.
Laser coating of motor rotor shaft
3. Manufacturing and refurbishment of petrochemical equipment and parts
The modern petrochemical industry essentially adopts a continuous mass production model. During the production process, machines operate for a long time in harsh environments, resulting in damage, corrosion and wear of equipment components, which often cause problems, including valves. pumps, impellers, journals, discs, bushings, bearings, etc. large rotors, and these components are very expensive, and many types of parts are involved, and most of them are complex in shape, which makes them difficult to repair, But thanks to the emergence of rotor technology laser coating, these problems are no longer a problem.
Laser coating of hard ceramic coatings on oil drilling rods, drilling tools, etc.
4. Manufacturing and refurbishment of railway equipment and parts
Rail transport is developing rapidly with the growth of the social economy. The demand for new railway vehicles is very high, and the quantity and performance requirements for major components are also increasing. As a new resource reuse technology, remanufacturing technology can be applied to the refurbishment of vehicle parts prone to wear. Laser surface strengthening is the core technology and process method of remanufacturing, among which laser surface coating technology can be applied to repair and strengthen the surface of remanufactured parts.
5. Refurbishment of key components of equipment in other machinery industries
Remanufacturing of key parts in other machine manufacturing industries involves sectors such as metallurgy, petrochemicals, mining, chemicals, aviation, automotive, shipping and machine tools, considering the wear and tear of precision equipment, large equipment and valuable parts. These fields, erosion and corrosion areas are repaired and performance optimized using laser cladding processing technology.
Laser coating of highly wear-resistant steel continuous casting rollers
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