Detailed interaction between laser and material

Guangzhou Delan Laser Technology Co., Ltd.: professional manufacturing and production of laser printers, carbon dioxide laser marking machines, semiconductor laser marking machines, fiber laser marking machines, laser coding machines, if you want to understand the laser force on the material process, It is necessary to understand the basic principle of the laser. The laser beam as an energy source can be focused into a small spot, which can interact with the material without any direct contact with the material. Due to the increasing performance of lasers, lasers on the market today come in a variety of wavelengths, powers and pulse widths. Different combinations of these parameters are suitable for a variety of different processing needs. To better understand the potential of lasers, engineers must be familiar with the technology and the nuances. Before deciding which laser to use, engineers should understand how lasers work, laser-material interactions, laser parameters, and when lasers can be used for material processing. Only after length understanding of this knowledge can an engineer make the right choice for the application of the laser.

Nowadays, the laser marking machine industry is mainly divided into CO2 laser marking machine, semiconductor laser marking machine, fiber laser marking machine and YAG laser marking machine. At present, laser marking machine is mainly used for some finer precision and precision. Higher occasions. Used in electronic components, integrated circuits (IC), electrical appliances, mobile communications, hardware products, tool accessories, precision equipment, glasses and watches, jewelry, auto parts, plastic buttons, building materials "target=_blank> building materials, PVC pipe.

The laser marking machine industry parameters are in the infrared band, CO2 laser is 10.6um, YAG laser is 1.06um, and the temperature increase caused by the absorption of laser energy from the material can be divided into the following parts:

When the laser beam is projected onto the surface of the material, part of the energy is reflected, partially absorbed, and partially transmitted, depending on the type of material and the wavelength of the laser. Of the light energy reaching the surface of the material, the portion of the energy absorbed by the material is useful for material processing. Light energy is absorbed by the vibrational excitation of electrons and atoms, and is converted into heat energy and diffused to adjacent atoms. As more and more photons are absorbed, the temperature of the material increases, thereby increasing the proportion of light energy absorption. This process can initiate a chain reaction that causes the temperature to rise sharply in a very short time (typically within one millisecond of soldering). The rate at which the temperature rises depends on the ratio between energy absorption and energy dissipation in the material.

The laser absorption length refers to the distance that the beam travels when the photon energy is absorbed and the beam intensity is reduced to 1/e (37%). The thermal energy diffusion distance of the material absorbed energy conversion within the distance is

L = [4Dt] 1/2,

Where L is the diffusion distance, D is the thermal diffusivity, and t is the pulse width of the laser.

If the thermal diffusion distance is much larger than the absorption length, the temperature rise at the laser spot will be limited. Conversely, if the diffusion distance is less than the absorption depth, the temperature will rise sharply, causing the material to melt and even vaporize. To achieve the desired results, whether it is heating, soldering, welding, drilling, marking, cutting or micromachining, engineers must choose the right laser wavelength and pulse width. When the light absorption depth is equal to the thermal diffusion distance, a critical value can be reached, and the pulse width of the laser of a specific frequency can be selected according to the value.

Table II above lists the calculation results of the pulse width required to limit thermal diffusion when using a 248 nm wavelength laser. Since the absorption depths of various metals are close, the difference in pulse width mainly depends on the difference between the diffusion distances. For example, stainless steel "target=_blank> stainless steel has a lower thermal conductivity than nickel, so a longer pulse width can be used for micromachining; on the other hand, silicon has better thermal conductivity than nickel, so when ablated Need a shorter pulse

Some people think that when using femtosecond pulse, the interaction between laser and material occurs in the multiphoton nonlinear process due to high power density and short time frame. This process is extremely rapid, so it can be assumed that the beam actually removes atoms from the surface in an instant without affecting adjacent atoms. Since femtosecond lasers do not leave a disturbing layer on the exposed surface, they are suitable for micromachining.

For ablation, the pulse width used must be less than the critical value calculated in Table II, but this is not enough. It must also be ensured that the pulses have sufficient energy so that each pulse can heat a sufficient volume of processing material. For a certain pulse energy, as the pulse time is shortened, the heat is more and more confined near the laser spot, and gradually produces heating, melting, ablation, and finally vaporization. Once the appropriate wavelength is selected, the combination of pulse energy and pulse width is determined to determine the type of material processing. The pulse width and energy density values ​​commonly used in different processing applications are listed in Table III.

Although the interaction between laser and material is basically similar, different materials such as metal, ceramic, glass and plastic have different characteristics. Figure 1 shows the absorption length-wavelength curves for metals, plastics, ceramics, and glass. The curves in the figure are schematic and are for discussion only and are valid only at room temperature. The absorption characteristics of various materials are listed in "Industrial Applications of Lasers" and "LIA Handbook of Laser Materials Processing" 1,2.

figure 1

The laser cannot pass through the metal material and some of the energy is absorbed and reflected off. Metals have a weaker ability to absorb carbon dioxide lasers. The smaller the laser wavelength, the higher the absorption rate and the higher the energy transfer efficiency. Although metal absorbs less carbon dioxide lasers, carbon dioxide lasers are still effective for metal welding and cutting as long as the energy density is high. In contrast to metals, ceramics and glass absorb well for lasers of all wavelengths. However, due to the poor thermal shock resistance of the ceramic and the high melting point, the processing is more difficult than metal. Glass can only absorb a small portion of the YAG laser incident energy, but because of the poor thermal conductivity of the glass, it is easier to melt.

Plastics can better absorb laser energy, especially ultraviolet lasers and carbon dioxide lasers. Some lasers with wavelengths in the ultraviolet range can break specific chemical bonds in plastic molecules (see Table IV), which adds some new uses to the laser. By the laser light of these wavelengths, the surface properties of the material can be selectively changed. In addition, if the plastic is sufficiently transparent, engineers can also change the material properties under its surface.

Table IV. Some lasers with wavelengths in the UV range can break chemical bonds in polymer materials during processing.

For more information on the laser industry, please pay attention to Guangzhou Delan Laser - the laser industry expert around you

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Guangzhou Delan Laser Technology Co., Ltd.: professional manufacturing and production of laser inkjet printer, carbon dioxide laser marking machine, semiconductor laser marking machine, fiber laser marking machine, laser coding machine, contact Zhao Sheng, 020-66230280 UV laser Marking machine, Shantou laser marking machine, Guangzhou laser marking machine, Guangzhou CO2 laser marking machine, Guangdong semiconductor laser marking machine, Changsha laser marking machine, metal cutting machine, non-metal cutting machine

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