For a long time, laser technology has been known for its widespread use in welding, cutting and marking. In the past two years, with the gradual popularization of laser cleaning, the concept of laser surface treatment has become more and more the focus of people's attention and appeared in people's minds. Laser processing is non-contact, highly flexible, high-speed and noise-free, with a small heat-affected zone and no damage to the substrate, no consumables, and environmentally friendly and low-carbon.
In addition to laser cleaning, laser surface treatment actually has many application categories, such as laser polishing, laser cladding, laser quenching, etc. These methods are used to change specific physical and chemical properties of the material surface, such as making the surface process hydrophobic, or using laser pulses to generate small depressions with a diameter of about 10 microns and a depth of only a few microns, so as to increase roughness and enhance surface adhesion.
In addition to laser cleaning, do you know the following laser surface treatment methods?
01. Laser quenching
Laser quenching is one of the solutions for processing high-stress complex parts. It can make parts with high wear such as camshafts and bending tools withstand higher stress and extend life.
Its principle is to rearrange the carbon atoms in the metal lattice (austenization) by heating the surface of the carbon-containing workpiece to slightly below the melting temperature (900-1400°C40% of the irradiation power is absorbed), and then the laser beam stably heats the surface along the feed direction. As the laser beam moves, the surrounding material cools rapidly, and the metal lattice cannot return to its original form, thereby producing martensite, which significantly increases the hardness.
The hardening depth of the outer layer of carbon steel achieved by laser hardening is usually 0.1-1.5mm, and can be 2.5mm or more on some materials. Compared with traditional quenching methods, its advantages are:
1. The target heat input is limited to the same area, so there is almost no component warping during processing. Rework costs are reduced or even completely eliminated:
2. It can also harden on complex geometric surfaces and precision parts, and can achieve precise hardening of locally restricted functional surfaces that cannot be quenched by traditional quenching methods:
3. No distortion. Traditional hardening processes produce deformations due to higher energy input and quenching, but in laser hardening processes, heat input can be precisely controlled due to laser technology and temperature control. The component remains almost in its original state:
4. The hardness geometry of the component can be changed "instantly". This means that there is no need to convert optics/the entire system.
02. Laser texturing
Laser texturing is one of the process means for surface modification of metal materials. During the structuring process, the laser creates regularly arranged geometries in the layer or substrate in order to change the technical properties in a targeted manner and develop new functions. The process is roughly to use laser radiation (usually short-pulse lasers) to generate regularly arranged geometries on the surface in a reproducible manner. The laser beam melts the material in a controlled manner and solidifies into a defined structure through appropriate process management.
For example, a hydrophobic surface structure allows water to flow off the surface. This feature can be achieved by creating submicron structures on the surface with ultrashort pulse lasers, and the structure to be created can be precisely controlled by changing the laser parameters. The opposite effect, such as hydrophilic surface, can also be achieved:
For painting of automobile panels, "micro-pits" must be evenly distributed on the surface of the thin plate to enhance the adhesion of the paint. A pulsed laser beam with a frequency of thousands to tens of thousands of times per second is focused and incident on the roller surface. A tiny melt pool is formed on the rolling surface at the focus point. At the same time, the tiny melt pool is blown sideways to allow the melt in the melt pool to accumulate as much as possible to the edge of the melt pool according to the specified requirements to form an arc-shaped boss. These small bosses and micro-pits can not only increase the roughness of the material surface and increase the adhesion of the paint, but also increase the surface hardness of the material and extend the service life.
Some characteristics are generated by laser structuring, such as the friction characteristics or electrical and thermal conductivity of some metal materials. In addition, laser structuring also increases the bonding strength and service life of the workpiece.
Shuishang Boguang
Compared with traditional methods, surface laser structuring is more environmentally friendly and does not require additional sandblasting agents or chemicals: Repeatable and precise, the laser achieves a controlled structure accurate to microns and is very easy to replicate: Low maintenance, compared to mechanical tools that wear quickly, the laser is non-contact and therefore absolutely wear-free: No post-processing is required, and no melt or other processing residues are left on the laser-processed parts.
03. Laser colorful surface treatment
Laser tempering is often used in laser colorful surface treatment, also known as laser color marking. The process principle is that when the laser heats the material, the metal is heated to slightly below its melting point. Under appropriate process parameters, the structure of the gate will change: an oxide layer will be formed on the surface of the workpiece. When this film is exposed to light, the incident light interferes to make various tempering colors appear at this time. The colorful marking layer generated on the surface changes with different viewing angles. The pattern of the mark will also change into various different colors. These colors remain stable at temperatures up to about 200 "C. At higher temperatures, the gate will return to its initial state-the marking disappears. The surface quality will be completely preserved. It has a high degree of security and traceability in anti-counterfeiting applications. In recent years, it has been maturely used in the field of medical technology. In addition to the new black marking by ultrashort pulse lasers, it is also very suitable for product identification, thereby achieving unique traceability according to the UDI directive.
04. Laser cladding
It is an additive manufacturing process suitable for metal and metal-ceramic hybrid materials. This can be used to create or modify 3D geometries. The laser can also be used to repair or coat them using this production method. In the aerospace sector, additive manufacturing is therefore used to repair turbine blades.
In tool and die manufacturing, broken or worn edges and shaped functional surfaces can be repaired or even armored. In energy technology or petrochemicals, bearings, rollers or hydraulic components are coated to protect against wear and corrosion. Additive manufacturing is also used in automotive construction. A large number of components are modified here.
In conventional laser metal deposition, the laser beam first heats the workpiece locally and then forms a molten pool. Fine metal powder is then sprayed directly into the molten pool from the nozzle of the laser processing head. In high-speed laser metal deposition, the powder particles are already heated to almost the melting temperature above the substrate surface. Therefore, less time is required to melt the powder particles.
The effect: significantly increased process speed. Due to the reduced thermal effects, very heat-sensitive materials such as aluminum alloys and cast iron alloys can also be coated using high-speed laser metal deposition. High surface velocities of up to 1500 rpm can be achieved on rotationally symmetrical surfaces using the HS-LMD process. cm/min. At the same time, feed speeds of up to several hundred meters per minute are achieved.
Repair expensive components or molds quickly and easily with laser powder deposition. Damage of all sizes can be repaired quickly and almost without leaving a mark. Design changes are also possible. This saves time, energy and material. This is especially worthwhile for expensive metals such as nickel or titanium. Typical application examples are turbine blades, various pistons, valves, shafts or molds.
05. Laser heat treatment
Thousands of micro lasers (VCSELs) are mounted on a single chip. Each emitter is equipped with 56 such chips, and a module consists of several emitters. The rectangular radiation field can contain millions of micro lasers and can output several kilowatts of infrared laser power.
VCSELs generate near-infrared beams with a radiation intensity of 100 W/cm² with a large, directional rectangular beam cross section. In principle, this technology is suitable for all industrial processes that require extremely high precision in surface and temperature control.
Laser heat treatment modules are particularly suitable for large-area heating applications with demanding and flexible requirements. Compared with traditional heating methods, this new heating process has higher flexibility, precision and cost savings.
This technology can be used to seal bag-type cells to prevent aluminum foil from wrinkling, thereby extending the service life of the battery. It can also be used to dry battery aluminum foil, light-soak solar panels, and precisely process the heating area of specific materials (such as steel and silicon wafers).
06. Laser polishing
The mechanism of laser polishing technology is surface narrow melting and surface over-melting, relying on surface remelting and re-solidification of the laser remelted layer. When the metal surface is irradiated by a sufficiently high-energy laser, its surface undergoes a certain degree of remelting and redistribution, and through the action of surface tensile stress and gravity, a smooth surface is achieved before solidification.
The entire thickness of the molten layer is less than the height from the trough to the peak, so that the entire molten metal is filled into the nearby trough. The driving force for this filling is achieved through the capillary effect, while the thicker layer will cause the liquid metal to flow outward from the center of the molten pool. The driving force is the thermal capillary effect or the Marconi effect, so that it can be redistributed.
Shuici Bieguang
Application cases include silicon carbide ceramics, which are used as optical components of light and large telescopes (especially large-size and complex-shaped reflectors). RB-SiC is a typical high-hardness, complex-phase material, and its surface precision polishing technology is difficult and inefficient. The surface of RB-SiC pre-coated with Si powder is modified by femtosecond laser. After only 4.5 hours of polishing, an optical surface with a surface roughness Sq of 4.45 nm can be obtained. Compared with direct grinding and polishing, the polishing efficiency is increased by more than 3 times. Laser polishing is also widely used in the polishing of molds, cams and turbine blades.
07. Laser shot peening
Laser shock strengthening, also known as laser shot peening, is to irradiate the surface of metal parts with high-energy density, high-focus, short-pulse laser (λ = 1053nm). The surface metal (or absorption layer) instantly forms a plasma explosion under the action of high-power density laser. The explosion shock wave is transmitted to the inside of the metal part under the constraint of the constraint layer, causing the surface grains to produce compressive plastic deformation, and obtaining residual compressive stress, grain refinement and other surface strengthening effects in the thicker range of the surface of the part. Compared with traditional mechanical shot peening, it has the following advantages:
1. Strong directionality: The laser acts on the metal surface at a controllable angle, with high energy conversion efficiency, while the mechanical projectile impact angle is random:
2. Large force: The instantaneous pressure generated by the laser shot peening plasma blasting is as high as several GPa: High power density: The peak power density of the laser impact reaches several to tens of GW//cm2:
3. Good surface integrity: The laser impact has almost no sputtering effect on the surface, while after mechanical shot peening, the surface morphology is damaged and stress concentration occurs.
The maximum compressive stress value after laser impact is better, and the surface residual compressive stress is increased by about 40%~50%, which significantly improves the values of related indicators such as fatigue life, high temperature resistance and bending forming of the workpiece. It has been applied in the fields of aircraft surface treatment and aero-engine surface treatment.
