Laser cladding is a surface alloy layer that is completely different from the composition and properties of the matrix material by cladding the alloy material on the collective surface with high energy density laser beam. Both of these are the use of laser technology to change the surname of the surface of the material, but the difference is that the laser spraying is the use of laser beam to heat the coating material to the melting state, and then the compressed gas is accelerated by the spray deposition to the surface of the substrate to form a coating, and the laser cladding is the rapid solidification of the coating material and the surface of the substrate. During the process, the cladding layer is completely metallurgic combined with the substrate surface layer. The common feature of laser spraying and cladding is that most coating materials can be used from low melting point to ultra high melting point; it can be sprayed under atmosphere, inert gas or vacuum state; the porosity of the coating is low, and the coating structure is basically the same as that of the original powder. At present, laser cladding is more applied than laser spraying.
The important parameters of laser cladding process are: laser power density, optical power in unit spot. Laser power density is mainly in the range of 104-106W/cm2 according to the type of laser treatment and the characteristics of coated materials. Light spot, laser cladding requires flat topped spot. The shape of the spot is mainly 3 kinds of rectangle, line and circular spot. The shape of the spot should be suitable for the requirements of the workpiece being machined. The scanning speed and the velocity of the workpiece surface relative to the laser spot. When the scanning speed is controlled by laser power density and powder feeding rate, the higher the scanning speed is, the lower the dilution rate of cladding layer is. The amount of powder supplied and the amount of powder supplied per unit time are closely related to the spot power density.
The characteristics of laser cladding process are as follows: the laser cladding layer is metallurgical bonded to the substrate, and the bonding strength is not less than 95% of the original matrix material. The thermal influence on the substrate is small, and the deformation caused by it is also small. The material has a wide range of materials, such as nickel base, cobalt base, iron base alloy, carbide composite, etc., which can meet the requirements of different uses of the workpiece and take into account the properties of the heart and the surface. The cladding layer and its interface have compact structure, fine grain, no holes, no inclusion cracks and so on. It can repair parts and moulds of large equipment with local wear or damage and prolong the service life. The cladding process has good controllability and is easy to realize automatic control. For damaged parts, it can achieve high quality and quick repair, reduce downtime due to failures, and reduce equipment maintenance costs.
Laser cladding repair technology is a material surface modification technology. It is the use of astronomy laser pointer high power density beam, which is controlled by the laser processing system under the control of numerical control, and forms a very thin layer of micro melting layer on the specified part of the surface of the matrix. The self melting alloy powder, such as nickel base, cobalt base and iron base alloy, is added by preposition or synchronization, such as nickel base, cobalt base and iron base alloy, so that it is spread evenly in the parts in the melting state. The surface layer has reached the predetermined thickness and formed a good metallurgical bond with the matrix metal material of the micro melt, and there is only a small dilution between each other. In the subsequent solidification process, the surface of the part is completely different from the matrix, and has a functional cladding material with predetermined special properties, so that the surface of the material can be completely changed. The performance will ultimately make the surface of the cheap material obtain extremely high wear resistance, corrosion resistance and high temperature resistance.
According to the different ways of material addition, laser cladding is divided into preset method and synchronous powder feeding method. Prefabricated materials are pre placed on the surface of the pretreated substrate by spraying or bonding, and then remelting through laser beam radiation to make appropriate heat treatment. Synchronous powder delivery is the surface of the pre treated cladding substrate, and the powder is directly sprayed on the laser radiation. On the mobile pool, the coating is formed in one time. Synchronous powder feeding is the development trend of laser cladding technology. It can make full use of laser energy, control process parameters, improve production efficiency and quality of coating. However, synchronous powder feeding is also required for particle size and fluidity of powder, and depends on specific circumstances.
The repair and remanufacturing process of laser cladding, one is strengthening function, can enhance the performance of substrate through cladding layer; two is repair function, mainly embody the holes and cracks on the surface of material, restore the geometric size and performance of the worn parts, and has great application value for almost the whole machine manufacturing industry.
In the machinery and equipment industries such as stone, chemical, metallurgical, electric, cement, etc., the shaft neck and blade of the turbine, the neck of the roll shaft and the archway of the steel plant are aged and damaged with the time of use. Because of the long-term gas high temperature and high pressure and corrosion medium, and the mechanical stress caused by the volume load, most of the damage occurs on the surface or surface, and the failure mode is mainly the fragmentation and cracking, wear or corrosion of the internal metal parts to the local exfoliation. Therefore, the application of laser cladding technology to enhance the surface performance of parts can effectively prolong the service life, and in the process of periodic maintenance, the damaged parts can be remedied by surface remanufacturing technology.
In a word, the traditional processing methods include surface hardening, surface carburization or nitriding, thermal spraying, and surfacing, whether it is surface hardening before service or failure to repair after service. With the continuous upgrading and improvement of processing technology, laser mobile Remanufacturing Technology (laser cladding) has been widely applied. The laser remanufacturing technology can be used not only for the repair of damaged parts, but also for laser surface quenching. Compared with the traditional heat treatment, laser quenching is a fast hot and fast cold processing technology, and the fine hardened layer with fine grain can be obtained on the surface. In addition, combined with high end multi axis machine tool or 6+2 type manipulator, laser can also repair the damaged 3D complex components, which fully embodies the flexibility and advanced of laser remanufacturing technology.