Laser cladding has been developed in China for nearly ten years and is a relatively advanced metal surface modification technology. Laser cladding technology is a method that uses high-energy-density laser beams to fuse the cladding material (metal powder) with a thin layer on the surface of the substrate to form a metallurgically bonded cladding layer on the surface of the substrate. The laser cladding layer has low dilution but strong bonding force and is metallurgically bonded to the substrate, which can significantly improve the wear resistance, corrosion resistance, heat resistance, oxidation resistance or electrical properties of the surface of the substrate material.
Laser cladding can be divided into side-axis powder feeding, coaxial circumferential powder feeding, and center powder feeding according to the mode of action of laser and cladding powder (i.e. feeding method). These three powder feeding modes determine the relationship between laser and metal powder. differences in the interaction process. Furthermore, in terms of technological iteration and advancement, laser cladding can be divided into traditional laser cladding and high-speed laser cladding. High-speed laser cladding is developed on the basis of traditional laser cladding, which solves the problems of low processing efficiency, large heat input and uneven cladding layer, and large post-processing volume of traditional laser cladding. The traditional powder-feeding methods of laser cladding are side-axis powder feeding and coaxial powder feeding. High-speed laser cladding technology has developed a new powder feeding method based on the original coaxial powder feeding technology, namely center powder feeding. Compared with the first two methods, the powder method has higher efficiency and higher powder utilization rate, which solves the technical shortcomings of traditional side-axis and coaxial powder feeding methods. The following mainly focuses on side-axis powder-feeding laser cladding technology and coaxial powder-feeding laser cladding technology. Let’s talk about the differences, advantages, and disadvantages of these two technologies.
1. Side-axis powder feeding laser cladding technology
Side-axis powder-feeding technology is also called lateral powder-feeding laser cladding technology. It refers to a powder feeding method in which the powder conveying device and the laser beam are separated and independent of each other. Generally, the external powder feeding tube is used. The powder feeding tube is located in front of the laser processing direction. The metal powder is accumulated on the surface of the substrate in advance under the action of gravity, and then the rear laser beam scans the pre-deposited powder to complete the laser cladding process.
In actual production, the powder nozzle of the powder feeder and the laser head have relatively fixed positions and angles. At the same time, side-axis laser cladding completely relies on the action of gravity, and protective gas cannot be applied to prevent the cladding powder preset on the substrate from being blown away, reducing powder utilization and cladding efficiency. The range-axis powder feeding laser generally uses a semiconductor direct output laser or a semiconductor fiber output laser and a gravity powder feeder. The cladding head adopts a rectangular light spot and a range-axis broadband powder feeding solution.
<1>Disadvantages of side-shaft powder feeding technology
1. The choice of metal powder is limited
Due to the lack of protective gas, the laser-molten pool can only rely on the slag when the cladding powder is melted to protect itself. Therefore, in current industrial production, side-shaft powder feeding systems mostly use self-fluxing alloy powder. The cladding powder relies on the slagging effect of elements such as B and Si to produce self-protection on the surface of the molten pool.
2. The surface of the cladding layer has large fluctuations
The side-axis powder feeding adopts the preset powder feeding method. After cladding, the melt channels on the surface of the cladding layer are very obvious and the gullies are large and uneven. After the cladding is completed, corresponding turning, polishing, and grinding are required, and the processing cost is high.
3. The processing surface is restricted
Since the side-axis powder feeding is gravity feeding, it is not suitable for use on inclined workpieces, nor is it suitable for inner hole cladding, and its application range is limited.
<2> Advantages of side-shaft powder feeding technology
1. High utilization rate of metal powder
Compared with coaxial powder feeding, the material utilization rate of side-axis powder feeding laser cladding technology can reach more than 95%. The powder of coaxial powder feeding laser cladding technology is blown into the laser molten pool through inert gas. During this process, due to the collision between powders, the splash of the molten pool and the accuracy of the powder feeding channel, a considerable proportion of the metal powder cannot be The cladding layer is formed and wasted, causing its material utilization rate to be only about 50%-80% (the smaller the spot, the lower the material utilization rate). The side-axis powder-feeding laser cladding can achieve a very high material utilization rate and save a lot of material costs by pre-positioning the powder on the surface of the workpiece and then scanning and irradiating the laser beam to melt it.
2. The cladding efficiency is relatively high
Since the side-axis powder-feeding laser cladding technology adopts a rectangular spot scheme, while ensuring that the energy density of the spot in the cladding direction remains unchanged, the laser power and spot width can be increased, which greatly improves the cladding efficiency. At present, in actual production, the width of a single pass of cladding can reach more than 30mm, and the cladding line speed can reach 1.0m/h.
3. No inert gas consumption and low cost
On the one hand, side-axis powder feeding laser cladding technology generally uses gravity powder feeding, which does not require the consumption of inert gas; on the other hand, due to the use of preset powder feeding, the airflow will affect the preset and accumulation of the powder, so the cladding head does not Specialized inert gas protection function. Therefore, the side-axis powder-feeding laser cladding technology does not need to consume other gases except compressed air. From a cost perspective, it saves more inert gas costs; from a technical perspective, this technology has certain requirements for the oxidation resistance of powder materials, which limits its application fields.
4. Scope of application
Due to its high efficiency and low cost, side-axis powder-feeding laser cladding technology is generally used in surface cladding and additive remanufacturing of parts with large areas and simple shapes such as hydraulic cylinders and rollers.
2. Coaxial powder feeding laser cladding technology
Coaxial powder feeding technology means that the laser (circular light spot) is output from the center of the cladding head, and the metal powder is distributed in a ring or multiple beams around the laser. The cladding head is equipped with special protective gas channels, metal powder channels, and cooling water channels. During cladding work, multiple beams of metal powder and laser intersect at a point outside the cladding head. For traditional laser cladding, the intersection point is generally set above the laser molten pool, that is, the laser and metal powder interact on the surface of the substrate, and the metal powder and substrate melt under the action of the laser at the same time, forming a cladding layer on the surface of the workpiece.
When this technology is applied to high-speed laser cladding, the intersection point of the metal powder and the laser is required to be located in the space above the surface of the workpiece. That is, the metal powder falls into the molten pool on the surface of the workpiece after fully absorbing the laser energy and becoming a molten or semi-molten state. A dense and smooth cladding layer is formed on the surface of the substrate. Coaxial powder-feeding lasers generally use semiconductor fiber output lasers or fiber lasers, and the powder feeders use disk-type airborne powder feeders.
<1> Disadvantages of coaxial powder feeding technology
1. Low utilization rate of metal powder
Coaxial powder feeding laser cladding technology uses pneumatic powder feeding. The inert gas assists in the output of metal powder and is blown into the laser molten pool. Collisions between metal powders under the action of the gas, splashing in the molten pool, and a considerable proportion of Metal powder cannot absorb laser melting and is wasted. Therefore, in practical applications, the powder utilization rate is only about 50%-70%, and the faster the powder output speed, the lower the powder utilization rate.
2. Easy to block powder and high maintenance cost
In the ring powder feeding or multi-beam powder scheme, the powder feeding channel is narrow, and the powder needs to be divided evenly. Due to gravity or the influence of airflow fluctuations, uneven powder distribution will occur, and the powder outlet channel will be easily blocked, affecting the continuity of production operations. In severe cases, the nozzle needs to be replaced, and the maintenance cost is high.
3. Poor safety and stability
The coaxial powder feeding cladding head uses a central output laser. The surrounding powder path, gas path, and water path structure are complex. The cooling effect of the cladding head is poor. The temperature of the cladding head will be too high when working for a long time, which will cause the adhesion of the splashed powder.
<2> Advantages of coaxial powder feeding technology
1. The cladding surface is relatively smooth
Compared with the side-shaft powder feeding, the coaxial powder feeding surface is relatively flat, the post-processing process is simple, and the processing volume is small.
2. Integrated design of powder feeding and laser, high degree of freedom and easy automation integration
Coaxial powder feeding is pneumatic powder feeding, which can obtain cladding layers with the same quality at different angles and in any direction. Since it can move to any plan during cladding, it can obtain cladding layers with the same appearance and the same quality, so its cladding There are no restrictions on the coating direction. With industrial robots or multi-axis motion machine tools, surface cladding of parts of any shape or shape can be carried out. It can also be used for laser coaxial powder feeding 3D printing.
3. The inert gas protection effect of the molten pool is good
The coaxial powder feeding adopts the airborne powder feeding method, and a special inert gas flow channel is provided on the cladding head. During the cladding process, the molten pool is always in a good inert gas atmosphere, which greatly reduces the oxidation of the molten pool and cladding layer. , there are fewer oxide inclusions in the cladding layer, the molten pool is small, the powder is heated evenly, and the cladding layer has good crack resistance.
4. The molten pool is small, the powder is heated evenly, and the cladding layer has good crack resistance.
The spot size of coaxial powder-feeding laser cladding is generally Φ1-Φ5mm. The powder and beam are in uniform contact, and the heat transfer during the cladding process is more uniform, so the cladding layer has good crack resistance. Especially for the cladding of materials containing ceramic particles such as tungsten carbide, it is suitable for preparing a coating with no cracks and evenly distributed tungsten carbide.
5. Small penetration and low heat input
Due to the small spot diameter and high cladding linear speed of coaxial powder feeding laser cladding, compared with wide spot side powder feeding laser cladding and arc welding, the cladding heat input is low, with moderate dilution rate and smaller thermal impact. zone depth, excellent cladding performance, and little impact on the properties of the base material.
6. Scope of application
Due to the above characteristics of coaxial powder-feeding laser cladding technology, it is usually used in surface cladding modification and additive remanufacturing of high-precision parts and complex-shaped parts such as spindles, gears, and boxes. At the same time, metal 3D printing based on coaxial powder-feeding laser cladding technology is mainly used for net near-forming of large parts and the preparation of gradient materials.
