Application of Laser Surface Processing

The laser treated surface makes the workpiece more resistant to load. Laser quenching, melting and coating make the workpiece more load-resistant: improve hardness and toughness, change the surface structure, generate pressure tension or protective coating on the surface. Laser marking and laser micromachining can also change the surface of the workpiece. Principle of laser quenching: Laser beam heating metal surface layer, rapid cooling to increase its hardness. The advantage of laser quenching technology is that it requires very little follow-up processing and can process irregular three-dimensional workpieces. Because the heat input is very small, the deformation of the workpiece is very small, reducing or even needing no further processing at all.

Laser quenching belongs to surface hardening process. It can only be used for hardening iron-based materials. That is to say, steel and cast iron with more than 0.2% carbon content. In order to harden the workpiece, laser beams in most cases heat the metal surface to near the melting point, i.e. about 900 to 1400 C. When the surface reaches the required temperature, the laser beam leaves this position and continues to move forward, continuously heating the workpiece surface along the new direction. Carbon atoms in metallic lattices change their positions (austenitizing) at high temperatures. Once the laser beam leaves a position, the material around that position cools the hot surface layer down quickly. This phenomenon is called "self-quenching". As a result of rapid cooling, the metallic lattice does not return to its original shape, but produces martensite. Martensite is a kind of metal structure with very high hardness. The transformation into martensite can improve the hardness of the material.

Laser cladding has several advantages over arc welding and thermal spraying methods. Specifically, precise and limited application of heat can control the thermal deformation of components to a minimum or even no thermal deformation, thus eliminating the need for reprocessing in subsequent processing. At the same time, laser cladding also results in little mixing (dilution) of the deposited material and the matrix material, resulting in a truly strong metallurgical bond between the cladding layer and the substrate.

However, several researchers have noticed that sometimes the rapid cooling of materials that occur during laser cladding produces bond defects and creates some voids in the cladding that result in grain or other heterogeneous microstructures. form. The special properties of these structures are highly dependent on the precise laser process parameters and the cladding materials used. They also observed the presence of cracks, pores and various columnar and ribbon-like grain structures. Each such structure affects the life and effectiveness of the cladding. For example, cladding cracks can provide a hotbed for corrosion and may even penetrate the cladding to the substrate. Grains or other microstructures can affect the mechanical properties of the cladding layer and have been shown to reduce the tensile strength of the cladding layer in some cases.

The effects of various process parameters, such as laser power, laser beam scanning speed, feeding speed and precise formulation of cladding materials, were studied. By properly controlling these factors, the formation of poor microstructure of cladding can be minimized or even these defects can be avoided. Specifically, a high performance cladding system can be built by following methods, including accurately simulating the cladding process, optimizing the cladding material, and carefully controlling the cladding process to reproduce the calculation results.

Laser beam heating the surface layer of workpiece. Typical surface hardening depths are 0.1 to 1.5 mm, and some materials reach 2.5 mm or higher. If the surface hardening depth is to be greater, the surrounding volume must be larger, so that heat can be quickly derived, so that the hardening zone can be cooled sufficiently quickly. Laser quenching requires relatively small power density. At the same time, the workpiece should be processed on the same plane. Therefore, the laser beam can be irradiated on the plane as large as possible. At present, the square irradiation surface is commonly used. Similarly, the scanning mirror group is also used in laser quenching process to make the laser beam of circular spot move back and forth very quickly. A line with uniform power density is formed on the surface of the workpiece. The hardening trajectory with a maximum width of 60 mm can be generated. As shown above, the bearing part near the turbocharger shaft has been laser quenched.

In order to improve the wear resistance of the material or to modify the surface, a laser surfacing process is employed. With the laser cladding system, the surface of the existing workpiece can be coated with a metal coating, the quality is the same as casting. No mass loss, seals, no porosity and cracks. The laser cladding system makes the laser surfacing process very simple: using a laser to create a molten pool on the surface to be treated. The powdered material is sprayed onto the surface through the nozzle, and when the new material solidifies, the next layer of welding is started, or subsequent processing is performed.

A typical laser cladding system consists of three main functional units: a powder conveyor, a powder conveyor line, and a processing mirror set with a powder nozzle. The powder conveyor is a movable unit that sits next to the laser processing machine. The powder gas mixture from several vessels is mixed in a powder conveyor into a powder stream that is introduced into the powder nozzle at a precisely set flow rate. The integrated sensor system ensures high quality coatings at all times.