Laser polishing technology-ShangHai RunQia Electronics Technology Co.,Ltd.

Laser polishing technology

The importance of surface treatment technology is increasingly prominent in high-precision machining parts. Whether it is automobiles, aerospace, electronic products, or medical devices, high-precision and high smoothness surface treatment is the key to ensuring product quality and performance. Among various surface treatment technologies, laser polishing technology is gradually being applied in the industrial field due to its advantages of high efficiency, precision, and strong controllability.

What is laser polishing?

Laser polishing is an emerging surface polishing technology that has the advantages of no pollution, wide processing range, stable polishing quality, and easy automation. It has attracted high attention from researchers at home and abroad. The principle is that the laser causes the surface of the material to melt or evaporate, driving the flow of molten metal under the action of capillary force or thermal capillary force, achieving peak filling and valley filling, thereby obtaining a smooth surface.Compared to traditional polishing techniques, laser polishing has inherent advantages in industrial applications. It can polish various types of materials such as metals, glass, and ceramics, and is highly integrated with additive manufacturing, laser welding, and laser cleaning technologies to achieve efficient and intelligent manufacturing of products.

Laser polishing mechanism

According to the size of the thermal effect on the surface of laser polishing, laser polishing can be divided into hot polishing and cold polishing. The basic mechanism of hot polishing is the melting of surface materials and subsequent redistribution of materials. According to the depth of melting, it can also be divided into shallow surface melting mechanism and surface excessive melting mechanism. The thermal effect during cold polishing is relatively small, and rough surfaces are removed through ablation gasification or photochemical mechanisms, resulting in a smooth surface.

The specific mechanisms are as follows:

① Shallow surface melting mechanism

When the interaction between laser and metal materials is dominated by thermal effects, the laser polishing mechanism on the metal surface is a shallow melting mechanism at lower laser energy densities. After the surface peaks melt, the melt flows towards nearby valleys under capillary force, resulting in a smooth surface with a melting depth of only a few micrometers to tens of micrometers, hence also known as "laser micro polishing". The laser polishing surface under this mechanism is very smooth, with a surface roughness of several tens of nanometers, but the reduction rate of surface roughness (10%~60%) is relatively small.

② Excessive melting mechanism

When the interaction between laser and metal materials is dominated by thermal effects, at higher laser energy densities, the laser polishing mechanism on metal surfaces is often a surface over melting mechanism. Unlike the shallow surface melting mechanism, the surface melting depth under the excessive melting mechanism is greater than the vertical distance between the maximum peak and valley on the surface, and the melting depth can reach hundreds of micrometers. The melt duration is longer, and Marangoni convection occurs in the melt pool, causing the material surface to redistribute over a large area and obtain a smooth polished surface.
③ Ablation gasification mechanism

When the interaction between laser and material is ablative gasification, the surface material is instantly vaporized and removed, thereby achieving laser polishing. The lasers used include nanosecond, picosecond, and femtosecond lasers. Under the mechanism of ablative gasification, the substrate will also have a certain thermal melting effect, but this thermal melting effect is very small compared to the over melting mechanism. Part of the laser energy is consumed during material removal, resulting in a lower laser thermal effect on the substrate. In this case, the polishing process is the result of the combined action of ablation gasification mechanism and melting mechanism.
④ Photochemical mechanism

The photochemical mechanism also has the function of removing surface materials, but without thermal effects. The commonly used lasers are short wavelength ultra short pulse lasers and excimer lasers, such as ultraviolet/femtosecond lasers. The processing mechanism is that the photon energy absorbed by the surface of the material can directly break the chemical bonds in the material, promote the dissociation of the material, and thus achieve laser polishing. When photochemical reactions occur, due to the rate at which chemical bonds are broken being greater than their binding rate, the material in the processing area rapidly expands, local air pressure rapidly increases, and separates from the matrix material in the form of Coulomb explosion, taking away excess energy. The entire processing process generates very little heat, hence it is called "cold" processing.

Laser polishing process

Continuous laser polishing

Continuous laser has strong thermal effects, making it suitable for materials with good thermal and physical properties, the most common of which is metal materials, with remelting depths up to hundreds of micrometers. In recent years, this technology has gradually been applied to the field of additive manufacturing, especially for thin-walled and complex additive manufacturing parts.

Pulse laser polishing

Compared to continuous laser polishing, pulse laser polishing has a smaller thermal effect, so the processing objects of pulse laser polishing are diverse, such as sapphire, glass, metal materials, ceramics, and silicon. Researchers have used picosecond lasers to achieve efficient, large-area, non-destructive, and high-precision polishing of alumina ceramics. They found that the mechanism of picosecond laser polishing of alumina ceramics is not to directly remove surface materials, but to excite nanoparticles to melt and recrystallize, forming a dense microcrystalline structure on the ceramic surface, resulting in a decrease in surface roughness.

Dual beam laser asynchronous composite polishing

According to research, under the excessive melting mechanism dominated by continuous laser polishing, a significant reduction in surface roughness can be achieved, but the polished surface roughness is still relatively high; Under the shallow melting mechanism, although the reduction rate of surface roughness is small, a smaller surface roughness can be obtained. In recent years, the dual beam laser asynchronous composite polishing process combining continuous laser and pulsed laser has become one of the research hotspots in the field of laser polishing, in order to achieve both a large reduction rate of surface roughness and a small surface roughness. Tests have shown that the surface roughness obtained by the two-step composite laser polishing process is indeed smaller than that obtained by the one-step laser polishing process.

Techonology

Laser polishing technology