The laser-treated surface makes the workpiece more load-resistant. Laser quenching, melting and coating make the workpiece more load-resistant: it improves the hardness and toughness, changes the surface structure, and generates pressure tension or protective coating on the surface. Laser marking and laser micromachining can also change the surface of the workpiece.
【Laser Hardening】
The principle of laser hardening: the laser beam heats the surface layer of the metal, and the rapid cooling increases its hardness. The advantage of laser hardening technology is that it requires very little follow-up processing and can process irregular three-dimensional workpieces. Due to the small heat input, the deformation of the workpiece is very small, reducing or even eliminating the need for subsequent processing.
Laser quenching belongs to the surface layer hardening process. It can only be used for iron-based materials that can be hardened. That is, steel and cast iron with a carbon content of more than 0.2%.
In order to harden the workpiece, the laser beam heats the metal surface layer to near the melting point in most cases, that is, 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 surface of the workpiece along the new direction. Under the action of high temperature, the carbon atoms in the metal lattice change their position (austenitization). Once the laser beam leaves a location, the material around that location quickly cools down the hot surface layer. This phenomenon is called "self-quenching". Due to the rapid cooling, the metal lattice does not return to its original shape, but martensite is produced. Martensite is a metal structure with extremely high hardness. Conversion to martensite can increase the hardness of the material.
The laser beam heats the surface layer of the workpiece. The typical surface hardening depth is 0.1 to 1.5 mm, with some materials reaching 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 dissipated, so that the hardened zone can be cooled quickly enough. The laser hardening process requires a relatively small power density. At the same time, the workpiece should be processed on the same plane. Therefore, it is necessary to make the laser beam irradiate as large a plane as possible. At present, the square irradiation surface is commonly used. Similarly, the scanning mirror group is also used in the laser quenching process to make the laser beam of the circular spot move back and forth very quickly. A line with substantially uniform power density is formed on the surface of the workpiece. It is possible to generate hardened tracks with a width of up to 60 mm. As shown in the figure above, the bearing part near the shaft of the turbocharger has been laser hardened.
【Laser Cladding】
In order to improve the wear resistance of materials or to modify the surface, people use laser surfacing technology. The laser cladding system can be used to coat metal coatings on the surface of existing workpieces with the same quality as casting. There is no quality loss, sealing, no pores and cracks.
The laser cladding system makes the laser surfacing process very simple: on the prepared surface, a laser is used to create a molten pool. The powdered material is sprayed onto the surface through the nozzle, and when the new material is solidified, the welding of the next layer or subsequent processing is started.
Generally, the laser cladding system is composed of three main functional units: powder conveyor, powder conveying line and processing mirror group with powder nozzle. The powder conveyor is a movable unit next to the laser processing machine. The powder gas mixture from several containers is mixed into a powder flow in the powder conveyor, and is introduced into the powder nozzle at a precisely set flow rate. The integrated sensor system ensures the high quality of material coating at all times.
