The main problem of laser processing for metal materials is the absorption rate of laser energy.
In order to improve the radiation coupling efficiency of different metal materials in laser processing, the following methods are often used.
The laser wavelength of the critical wavelength of the irradiated metal material is selected.
For example, the critical wavelengths of Al, Au and Ti are about 1064nm, 630nm and 10000nm respectively.
When the laser beam wavelength is greater than the critical wavelength, the reflectivity of the metal surface to the laser beam increases sharply, the absorptivity decreases sharply, and more than 92% of the incident laser beam is reflected.
YAG solid-state laser is commonly used in laser micromachining, and its laser wavelength is 1064nm.
At this wavelength, the reflectivity of most metals such as Al, Cu, Ni, Ag, Pt, Zn and Pb is basically more than 80%.
CO2 gas laser is commonly used in laser processing with high power, and its laser wavelength is 10600nm, so the reflectivity of most metals is more than 90%.
The metal surface is covered with appropriate coating to improve the absorption of incident laser.
Generally, graphite or manganese phosphate is coated on the surface.
Due to the black absorption layer formed by graphite and manganese phosphate, the absorption rate can be increased by 60% ~ 80%.
However, the appropriate coating thickness is also a very important parameter.
If the coating is too late, considerable heat will evaporate the coating.
If the coating is too thin, it will evaporate completely before the end of laser processing.
The adverse phenomenon of metal reflection occurs during the laser radiation time.
When the coating thickness is, it will absorb light energy and transfer heat to the metal through heat conduction at a certain rate, and the coating will evaporate at a certain rate.
If the coating is basically evaporated at the end of laser processing, the coating thickness at this time should be the best value.
Its value can be determined by test method.
Optical pretreatment is an absorption enhancement technology without pollution to the material surface in recent years.
It mostly adopts the technology of simultaneous post-treatment of excimer laser with UV spectrum and CO2, which significantly increases the absorption rate of CO2 laser on the material surface.
The effect of optical pretreatment is generally related to the following three factors: laser energy, the number of laser pulses and the physical properties of the material itself.
Optical processing is a very complex technology.
At present, its application is mostly a summary of practice, and the theoretical research needs to be further improved.
Mechanical and chemical surface pretreatment technology
Mechanical and chemical surface pretreatment technology is adopted to improve the laser absorption rate of the material surface.
For example, grinding a smooth metal surface with a grinding wheel and corroding a thin layer of metal surface with acidic substances are the most direct surface treatment methods.
However, it is also the method with the greatest damage and pollution to sculptures.