The main challenge in laser processing of metallic materials is the laser energy absorption rate. To increase the coupling efficiency of laser radiation in the processing of different metallic materials, the following techniques are commonly employed.
The laser wavelength used in irradiating metallic materials is selected to be the critical wavelength. For example, Al, Au, and Ti have critical wavelengths of approximately 1,064 nm, 630 nm, and 10,000 nm, respectively.
When the wavelength of the laser beam is greater than the critical wavelength, the reflectivity of the metal surface to the laser beam increases dramatically and its absorptivity decreases drastically, resulting in the reflection of more than 92% of the incident laser beam.
The YAG solid-state laser, commonly used in laser micromachining, has a laser wavelength of 1064 nm. At this wavelength, the reflectivity of most metals such as Al, Cu, Ni, Ag, Pt, Zn and Pb is greater than 80%.
The CO 2 gas laser, which is often used in high-power laser processing, has a laser wavelength of 10,600 nm. As a result, the reflectivity of most metals with this laser is greater than 90%.
T The metal surface is covered with appropriate coating to improve the absorption of the incident laser.
Typically, graphite or manganese phosphate is applied as a coating on the surface. This coating results in a black absorption layer, which can increase the absorption rate by 60% to 80%.
However, appropriate coating thickness is crucial. If the coating is too thick, it may evaporate due to excessive heat. If it is too thin, it may completely evaporate before the end of laser processing, leading to reflection from the metal during laser radiation.
The thickness of the coating affects the absorption and transfer of light energy to the metal through heat conduction. If the coating has evaporated at the end of laser processing, this is considered the ideal coating thickness. This value can be determined through testing.
o optical pretreatment
Optical pretreatment is a recent, non-polluting technology that improves absorption on material surfaces. It mainly employs the combination of excimer laser with UV spectrum and CO 2 for simultaneous post-treatment, resulting in a significant increase in CO 2 laser absorption on the surface of the material.
The effectiveness of optical pretreatment is largely influenced by three factors: laser energy, number of laser pulses and physical properties of the material.
Optical processing is a complex technology and currently depends mainly on practical experience, requiring more theoretical research.
M mechanical and chemical surface pretreatment technology
Mechanical and chemical surface pretreatment technologies are used to increase the laser absorption rate on material surfaces.
For example, grinding a smooth metal surface using a grinding wheel and corroding a thin layer of the metal surface with acidic substances are the simplest surface treatment methods. However, these methods are also those with the greatest potential for damage and pollution to sculptures.
