The development of laser cladding technology is studied in depth. Laser cladding is analyzed from four aspects: technology, process, materials and the latest application in the field of mining machinery. The latest research results are summarized and its application direction is explored. In the field of mining machinery, laser cladding technology is widely used in the main components of three machines and one frame (excavator, coal mining machine, scraper conveyor and hydraulic support). The current problems and shortcomings of laser cladding are discussed, and its future development trend and prospects are prospected.
As a technology to change the surface properties of materials, laser cladding has the characteristics of dense cladding structure, high cladding interface bonding strength, easy to form metallurgical bonding coating, high degree of automation of cladding process, and can significantly improve the hardness/wear resistance and corrosion resistance of the substrate surface while maintaining the strength and toughness of the substrate. It is widely used in aerospace, automobile manufacturing, petrochemical and mining machinery.
As China’s largest energy source, more than 90% of coal mining operations are in coal mine underground environments, and the corrosion, wear and other failure phenomena of coal mine machinery are extremely serious. Therefore, the introduction of laser cladding technology to solve the wear, corrosion and repair and remanufacturing problems of mechanical equipment in the coal production process is of great significance to the efficient and safe production of coal mines.
This paper classifies laser cladding technology, discusses laser cladding process parameters and studies laser cladding material systems. Focus on the latest applications and research progress of laser cladding technology in the field of mining machinery, and look forward to future development trends.
1 Current status of laser cladding technology
Laser cladding technology is a method that uses a high-energy laser beam as a heat source to melt the surface of the substrate and the powder sent into the molten pool at the same time, and forms a metallurgical coating bonded to the substrate after solidification. According to the material addition method, laser cladding can be divided into the preset method and the synchronous powder feeding method. The preset method has the characteristics of simple process and flexible operation, but there are problems such as serious powder loss, the inability to accurately control the depth of the substrate, and the cladding layer is prone to pores and cracks. The synchronous powder feeding method has a high degree of automation, a fast cladding speed, and the quality of the cladding layer is also guaranteed. This technology is the main direction of future development of laser cladding.
According to the coupling form between the laser cladding material and the laser beam, the synchronous powder feeding method can be divided into paraxial powder feeding laser cladding, coaxial powder feeding laser cladding and high-speed laser cladding.
(1) Paraxial powder feeding laser cladding The nozzle is tilted at a certain angle, and the powder feeding and laser beam are carried out simultaneously. The process operation is simple, flexible and stable. However, during the paraxial powder feeding process, if the scanning direction is changed, the light-powder coupling will be anisotropic, resulting in uneven cladding layer.
(2) Coaxial powder feeding laser cladding The powder beam and the laser beam are coaxially coupled and output, and the powder is isotropic, which can ensure that the cladding layer is uniform under various paths. Coaxial powder feeding is divided into two methods: coaxial powder feeding outside the beam and coaxial powder feeding inside the beam according to the relative position of the laser beam and the powder beam. Coaxial powder feeding outside the beam adopts the powder-wrapped light mode, that is, multiple separate nozzles or a ring nozzle are set around the laser beam to feed powder, and the laser and powder are focused on the substrate surface to form a molten pool. In-beam coaxial powder feeding is also called light-coated powder mode, which is to let the nozzle feed powder vertically, set a ring cone laser generator around the nozzle, and let the laser wrap the powder to act on the substrate. In-beam powder feeding, since the powder is sprayed vertically on the processing surface, the powder spraying direction, the airflow direction and the axis direction of the ring cone beam are consistent and do not interfere with each other, which effectively improves the powder utilization rate and reduces the splashing phenomenon during the cladding process.
(3) High-speed laser cladding is a widely used laser cladding technology. Compared with traditional laser cladding technology, the laser beam focus of high-speed laser cladding is different. It is located 0.2~3mm above the substrate surface. The powder is fully melted before reaching the substrate and enters the molten pool in the form of droplets. The working principle of high-speed laser cladding is shown in Figure 1. Compared with traditional laser cladding technology, high-speed laser cladding has significant advantages:
① In the process of high-speed laser cladding preparation, the coating dilution rate and the substrate heat-affected zone are further improved. The laser directly acts on the powder, and only a small amount of energy will act on the substrate surface, forming a small molten pool. The heat-affected zone is only 5~10μm, and the dilution rate can be reduced to less than 1%, thereby effectively improving the coating quality;
② The preparation speed of the coating is significantly improved. In order to ensure that the powder is fully melted, traditional laser cladding needs to reduce the cladding speed, resulting in low cladding efficiency. However, since the powder is completely melted before reaching the substrate, high-speed laser cladding can be clad at a high line speed, which not only significantly reduces the coating thickness, but also the cladding efficiency can reach 500cm²/min, which is more than 10 times that of traditional laser cladding. Due to the high cladding line speed and small heat input, the high-speed laser cladding method is suitable for heat-sensitive materials such as Al-based, Ti-based materials and small-sized components, and can achieve low surface roughness and high powder utilization. The cladding layer prepared by high-speed laser cladding has a fine structure and a smooth surface. Its surface roughness is only 1/10 of that of traditional laser cladding. The requirements of fine processing can be achieved through simple grinding and polishing. The powder utilization rate of high-speed laser cladding exceeds 90%, which greatly improves the economic efficiency.
2 Laser cladding process parameters
Laser cladding process parameters directly affect the macroscopic mechanical properties and microstructure of the cladding layer, mainly including laser power, spot size, scanning speed, etc.
(1) Laser power The change of laser power will affect the maximum temperature of the molten pool. When the laser power is too low, there may be unmelted particles in the molten pool, resulting in poor melting effect on the surface of the base layer, which in turn causes problems such as uneven cladding layer structure and insufficient interface bonding strength. If the laser power is too high, the cladding material may be over-melted or even vaporized, which in turn aggravates the dilution effect between the cladding layer and the substrate.
(2) Spot size The role of the spot size is not limited to the width of the melting, but also directly affects the range and depth of the molten pool. If the spot is too small, the molten pool area is small, which causes the cladding material to heat up too quickly, resulting in a large temperature step, and the high dilution rate increases the formation of void cracks. If the spot is too large, the cladding structure will become rough because the cladding material is not completely melted, resulting in insufficient interface bonding strength.
(3) Scanning speed The scanning speed determines the heating time of the cladding material, which in turn affects the existence time of the molten pool. If the scanning speed is too slow, the cladding material is heated for a long time, and the grains grow fully, thus forming coarse crystals, affecting the overall performance. If the scanning speed is too fast, the heating is not sufficient, and the cladding material is not completely melted, resulting in poor interface bonding strength.
3 Laser cladding material system
Laser cladding material is the most important factor affecting the performance of the cladding layer, and directly determines the performance of the cladding layer. Currently, the common laser cladding materials mainly include self-fluxing alloy powder, ceramic powder and composite powder. The latter two are generally collectively referred to as hard particle reinforced alloy composite powder.
(1) Self-fluxing alloy powder Self-fluxing alloy powder refers to the addition of Si, B and other elements to Fe, Co, Ni-based non-ferrous metal alloy powder, which enables it to have self-deoxidation and slag formation and strong self-fluxing effect during the cladding process, and has good process formability.
① Fe-based self-fluxing alloy coating Fe-based self-fluxing powder is often used to prepare laser cladding coatings on the surface of low-carbon steel, cast iron and stainless steel substrates. It is widely used in the preparation and repair of easily worn parts such as oil drill pipes, mining machinery and engineering machinery due to its low cost, excellent wear resistance and good wettability with the substrate.
② Co-based self-fluxing alloy coating Co-based alloy powder is mainly composed of Co powder, containing a certain amount of Cr, Ni, W and a small amount of C and B elements. With its characteristics of high temperature resistance, corrosion resistance and wear resistance, it is mainly used in aerospace, metallurgical engineering, petrochemical engineering, and power engineering. Wear-resistant environment.
③ Ni-based self-fluxing alloy coating Ni-based alloy powder is currently the most fully studied and widely used cladding material. Compared with Fe-based alloys, Ni-based alloys have good ductility and toughness, and are widely popular due to their excellent wear resistance, good corrosion resistance and low price compared to Co-based alloys, especially in the remanufacturing and repair of parts such as turbine blades and hydraulic piston rods.
④ Other self-soluble alloy coatings In addition to the above three alloys, Cu-based, Ti-based, and Al-based alloy powders are also used in different laser cladding occasions due to their respective characteristics. For example, copper alloys are good conductors of electricity and heat, and the high potential of Cu brings good corrosion resistance, which can meet the corrosion resistance requirements in atmospheric and marine environments.
(2) Hard particle reinforced alloy composite powder
Hard particles have always been the preferred wear-reducing and wear-resistant reinforcement materials because of their good wear resistance, corrosion resistance and high temperature resistance. Therefore, hard particle reinforced alloy composite powder as a cladding material has attracted widespread attention in recent years. This metal-ceramic composite powder has both hard particles as a reinforcement phase and combines the good thermoplasticity and formability of the self-soluble alloy. It has good wear resistance, corrosion resistance, high temperature resistance and high temperature oxidation resistance, and is often used to prepare high temperature wear-resistant and corrosion-resistant coatings and thermal barrier layers.
4 The latest application of laser cladding technology in the field of mining machinery
Currently, laser cladding technology is widely used in the field of coal mining, mainly used to strengthen or repair key components of large equipment to improve their wear resistance and corrosion resistance.
(1) Excavators are key equipment in open-pit coal mines, and rotary reducers are key transmission assembly components of the equipment. One of the common faults of reducers is the wear of the oil seal lip of the input shaft. Literature
The wear of the rotary reducer input shaft is repaired by multi-pass and multi-layer laser cladding of Ni-based alloy powder. After the cladding is completed, the hardness of the cladding layer is detected to be 45HRC, and the size is restored to the original design tolerance range, and the repair effect is good.
(2) The guide shoe of the coal mining machine is a key component responsible for supporting and guiding the coal mining machine to move. Generally, the guide shoe is made of castings and is subjected to alternating loads for a long time in the working environment, so it is easy to cause wear or breakage. In order to improve the wear resistance of the coal mining machine shoe under oil-free conditions, the literature used laser cladding technology to clad FeNiMo and FeNiMoSi coatings on the surface of 45 steel respectively, and achieved significant results.
(3) The scraper conveyor is the primary transportation equipment for comprehensive mining operations, among which the middle trough is the largest number and the most frequently repaired and replaced component. Working for a long time under harsh conditions, the surface of the middle trough will produce deep grooves due to wear and rust, or cause serious wear and tear in the middle bottom plate and other positions. In order to solve the problem of easy wear and short life of the middle groove, the literature uses laser cladding technology to clad the wear-resistant layer of WC, Nb, Ti and other Fe-based alloys on the easily worn parts of the middle groove. The results show that the coating has good formability.
(4) Most of the failures of hydraulic supports are not caused by the design, manufacturing or quality problems of the equipment itself, but by the long-term use of contaminated emulsions, which causes the support column and jack cylinder to peel off and be damaged, and the support cylinder body to leak and flow, and cannot continue to be used. In order to repair the inner wall of the cylinder and improve the corrosion resistance, the literature clads Cr, Ni, Mo, and Si alloy powders on the inner wall of the hydraulic cylinder. Under an electron microscope, the cladding layer is observed to be metallurgically bonded, with good cladding quality. The average hardness of the cladding layer reaches 234HB, which is consistent with the original design hardness of the cylinder wall of 200~270HB. The cladding layer is first rough-machined on the cylinder with a deep hole boring machine and then fine-machined with a grinder. The surface processing accuracy of the completed surface meets the design requirements.
5 Conclusion
The application of laser cladding technology in the field of mining machinery has opened up a new direction for friction reduction and corrosion resistance, and also provided a new way for mining machinery to achieve green and high-quality development. At present, there are still some challenges and limitations in laser cladding technology, which hinder its promotion and application in a wider range of fields.
(1) The research and development of special materials for laser cladding is not sufficient. Although a lot of research on friction reduction and wear resistance has been carried out, there is no complete system of special cladding materials, so it is still difficult to meet the performance requirements under complex working conditions. New wear-resistant coatings are an important trend in future development. Whether it is a wear-resistant coating with self-lubricating properties or a high-entropy alloy wear-resistant coating, only by continuously developing and configuring a variety of composite materials, firmly promoting diversified innovation of materials, and comprehensively improving the functions of coatings to meet the requirements of different working conditions, can we further promote the development of laser cladding technology and create a broader prospect;
(2) The quality of laser cladding layers is still unstable. Although the quality of cladding layers can be improved by changing cladding methods, optimizing process parameters, and improving material ratios, defect control and process quantification standards have not yet been formulated. At this stage, defect control is still in the experimental exploration stage, and manufacturing experience has a great impact on the quality of cladding layers;
(3) In the research and development of laser cladding equipment, the main application directions include high-speed large-area laser cladding equipment and portable mobile in-situ repair equipment. The goal is to develop a complete set of laser cladding equipment with high speed, high power, intelligence and portability, as well as laser cladding nozzle technology with light-powder-gas coupling and all-round and all-position cladding technology and equipment.
