For a long time, laser technology has been known for its extensive use in welding, cutting and marking, and it is only in these two years, with the gradual popularization of laser cleaning, that the concept of laser surface treatment has become more and more the focus of attention and appeared in people's minds. Laser processing in a non-contact manner, high flexibility, high speed, no noise, small heat-affected zone without damage to the substrate, no consumables and environmental low-carbon.

Laser surface treatment actually has a very large number of application categories in addition to laser cleaning, such as laser polishing, laser cladding, laser quenching and so on. These methods are used to change the specific physicochemical properties of the material surface, for example, to make the surface processed into a hydrophobic function, or laser pulses to produce a diameter of about 10 microns, the depth of only a few microns of small depressions, as a way to increase the roughness, enhance the surface adhesion, and so on.

In addition to laser cleaning, do you know the following types of laser surface treatment?

01. Laser hardening

Laser hardening is one of the solutions for machining highly stressed and complex parts. It enables parts with high wear, such as camshafts and bending tools, to be subjected to higher stresses and to have a longer lifespan.

It works by heating the skin of a carbon-containing workpiece to a temperature slightly below the melting temperature (900 - 1400°C, 40% of the irradiated power is absorbed), causing the carbon atoms in the metal lattice to rearrange themselves (austenitization), and then the laser beam steadily heats the surface in the direction of the feed, and the material around the laser beam cools so quickly as the laser beam moves that the metal lattice is unable to return to its original form, resulting in martensite, which gives rise to a This results in martensite and a significant increase in hardness.

The depth of hardening of the outer layers of carbon steel achieved by laser hardening is typically 0.1-1.5mm, and can be 2.5mm or greater in some materials. The advantages over conventional hardening methods are:

1. Targeted heat input is limited to a localized area, resulting in virtually no component warpage during machining. Rework costs are reduced or even eliminated altogether;

2. hardening even on complex geometries and precision components, allowing precise hardening of locally restricted functional surfaces that cannot be hardened by conventional hardening methods;

3. without distortion. Conventional hardening processes produce distortion due to higher energy input and quenching, but during laser hardening the heat input can be precisely controlled due to laser technology and temperature control. The component remains virtually pristine;

4. The hardness geometry of the component can be changed quickly and “on the fly”. This means that there is no need to convert the optics/entire system.

02. Laser Haircutting

Laser hairpinning is one of the processes for surface modification of metallic materials. In the structuring process, the laser creates regularly arranged geometries in layers or substrates in order to target changes in technical properties and develop new functions. The process generally involves the use of laser radiation (usually short pulses of laser light) to generate regularly arranged geometries on a surface in a reproducible manner. The laser beam melts the material in a controlled manner and solidifies it into a defined structure by appropriate process management.

Hydrophobic surface structures, for example, allow water to flow off the surface. Creating sub-micron structures on surfaces with ultrashort pulsed lasers allows this property to be realized and the structure to be created can be precisely controlled by varying the laser parameters. The opposite effect, such as a hydrophilic surface, can also be achieved;

Automotive panels to be painted, it is necessary to make the surface of the thin plate uniform distribution of “micro-pits” to enhance the adhesion of paint, with thousands to tens of thousands of times per second pulsed laser beam focusing on the surface of the roll, in the focusing point at the surface of the roll to form a micro-soluble pool, at the same time on the side of the micro-soluble pool of blowing, so that the soluble pool of the soluble pool of melt according to the specified requirements as much as possible to the accumulation of the pool of melt! The edge of the formation of arc-shaped tabs, these small tabs and micro-pits can not only enhance the roughness of the material surface to increase the adhesion of paint, but also improve the surface hardness of the material to extend the service life.

Certain properties are generated by laser structuring, such as the frictional properties or the electrical and thermal conductivity of some metallic materials. In addition, laser structuring increases the bonding strength and service life of the workpiece.

Compared to traditional methods, surface laser structuring is more environmentally friendly, requiring no additional abrasive blasting agents or chemicals; repeatable and precise, lasers achieve controlled structures that are accurate down to the micron and are very easy to replicate; low-maintenance, lasers are non-contact and therefore absolutely wear-free compared to fast-wearing mechanical tools; and there is no need for post-processing, with no melts or other machining residues left on the laser-processed part.

03. Laser Flare Surface Treatment

Laser tempering is commonly used in laser dazzle surface treatment, also known as laser color marking. The principle of the process is that when the laser heats the material, the metal will be locally heated to slightly below its melting point, in the appropriate process parameters, at this time, the structure of the gate will change; in the surface of the workpiece will form an oxide layer, the film in the light irradiation, the incident light interference so that a variety of tempering color at this time, the surface of the generation of a layer of colorful marking layer, along with no need to change the angle of observation, the marking pattern will be changed out of a The marking pattern will also change into different colors as the viewing angle changes.

These colors remain temperature stable up to about 200 °C. At higher temperatures, the gate is stabilized. At higher temperatures, the gate returns to its original state - the marking disappears. The surface quality remains intact. A high degree of security and traceability is achieved in anti-counterfeiting applications. In addition to the new black marking with ultrashort pulsed lasers, it is also ideally suited for product marking and thus for unique traceability according to the UDI directive.

04. Laser cladding

is an additive manufacturing process suitable for metal and metal-ceramic hybrid materials. With this, 3D geometries can be created or modified. With this production method, the laser can also be used for repair or coating. In the aerospace sector, additive manufacturing is therefore used to repair turbine blades.

In tool and mold making, cracked or worn edges and shaped functional surfaces can be repaired, or even locally armored. To prevent wear and corrosion, bearing locations, rollers or hydraulic components are coated in energy technology or petrochemistry. And additive manufacturing is also used in automotive manufacturing. Numerous components are modified here.

In conventional laser metal melting, the laser beam first locally heats the workpiece 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. During high-speed laser metal melting, the powder particles are already heated almost to melting temperature above the substrate surface. As a result, less time is required to melt the powder particles.

The effect: a significant increase in process speed. Due to the lower thermal effect, high-speed laser metal melting also makes it possible to coat materials that are very sensitive to heat, such as aluminum alloys and cast iron alloys. With the HS-LMD process, high surface rates of up to 1500 cm²/min can be achieved on rotationally symmetric surfaces, while feed rates of up to several hundred meters per minute can be realized.

Expensive parts or molds can be repaired quickly and easily with laser powder laser metal cladding. Damage, large or small, can be repaired quickly and almost without marks. Design changes are also possible. This saves time, energy and material. Especially for expensive metals such as nickel or titanium, it is quite worthwhile. Typical examples of applications are turbine blades, various pistons, valves, shafts or molds.

05. Laser Heat Treatment

Thousands of miniature lasers (VCSELs) are mounted on a single chip. Each emitter is equipped with 56 such chips, while a module consists of several emitters. The rectangular radiation area 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 by means of a large, directional rectangular beam cross-section. In principle, this technology is suitable for all industrial processes that require extremely precise surface and temperature control.

Laser heat treatment modules are particularly suitable for large-area heating applications where precision and flexibility are required. Compared to conventional heating methods, this new heating process offers a higher degree of flexibility, precision and cost savings.

The technology can be used to seal pouches of battery cells, preventing the aluminum foil from wrinkling and thus extending the service life of the battery. It can also be used in applications such as drying cell foils, photo-impregnation of solar panels, and precise treatment of the area to be heated for specific materials such as steel and silicon wafers.

06. Laser Polishing

The mechanism of laser polishing technology is surface narrow fusion and surface overfusion, which relies on remelting of the surface and reconsolidation of the laser remelted layer. When a metal surface is irradiated by a laser with a sufficiently high energy, the surface undergoes a certain degree of remelting, redistribution and smooth surfaces are achieved by surface tensile and gravitational stresses prior to solidification.

The entire thickness of the melting layer is less than the trough-to-peak height, thus allowing the entire molten metal to fill the nearby troughs, this filling being driven by the capillary effect, while a thicker melting layer induces the liquid metal to flow outward from the center of the molten pool, the driving force of which is the thermo-capillary effect or the Marconi effect, which allows for its redistribution.

Application examples include silicon carbide ceramics, the material for lightweight large telescope optics (especially large and complex shaped mirrors.) RB-SiC, as a typical high-hardness, complex-phase material, is technically difficult to surface-precision-polish with low efficiency. By modifying the surface of RB-SiC pre-coated with Si powder by femtosecond laser, an optical surface with surface roughness Sq of 4.45 nm can be obtained after only 4.5 hours of polishing, which improves the polishing efficiency by more than three times compared with direct grinding and polishing. Laser polishing is also widely used in the polishing of molds, cams and turbine blades.

07. Laser Shot Peening

Laser impact peening, also known as laser shot peening, is a high energy density, high focus, short pulse laser (λ = 1053nm) irradiation of the surface of the metal parts, the surface metal (or absorption layer) in the high power density of the laser under the action of the instantaneous formation of the plasma explosion, the explosion of the shock wave in the constraints on the bounding layer of the bounding layer of the metal parts inside the transmission, so that the surface layer of the grains to produce compressive plastic deformation in the surface layer of the parts in a thick range of Obtain residual compressive stress, grain refinement and other surface strengthening effects. Compared with the traditional mechanical shot blasting has the following advantages

1. Strong directionality: the laser acts on the metal surface at a controlled angle, high energy conversion efficiency, while the mechanical projectile impact angle is random;

2. Large force: laser blasting plasma burst generated by the instantaneous pressure up to several GPa; power density: the peak power density of the laser impact reaches several dozens of GW/cm2;

3. Good surface integrity: laser impact on the surface is almost no sputtering effect, while the mechanical shot peening, the surface morphology is damaged to produce stress concentration.

Laser impact after the maximum compressive stress value is better, the surface residual compressive stress increased by about 40% to 50%, the fatigue life of the workpiece, resistance to high temperature and bending molding and other related indicators of numerical value have been significantly improved. Currently it has been applied in the field of aircraft surface treatment, aero-engine surface treatment and so on.