Laser surface treatment technology beyond laser cleaning

For a long time, laser technology is widely used in welding, cutting and marking, and 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 attention and appears in people’s minds. The laser is processed in a non-contact way, with high flexibility, high speed, no noise, small non-destructive base material in the heat affected zone, no consumables and environmental protection and low carbon.

 

In addition to laser cleaning, there are actually many application categories, such as laser polishing, laser cladding, laser hardening, and so on. These methods are used to change the specific physical and chemical properties of the surface of the material, such as making the surface to have a hydrophobic function, or using laser pulses to create small depressions with a diameter of about 10 microns and a depth of only a few microns, in order to increase roughness, enhance surface adhesion and so on.

 

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 machining complex parts with high stress. It can increase the stress of parts with high wear, such as camshafts and bending tools, and extend the life of the parts.

 

Its principle is to rearrange the carbon atoms in the metal lattice by heating the skin of the carbon-containing workpiece to a slightly lower melting temperature (900-1400 °C, 40% of the irradiation power is absorbed), and then the laser beam stabilizes the heating surface along the feed direction, and the surrounding material cools rapidly as the laser beam moves. The metal lattice cannot be restored to its original form, resulting in martensite, which increases the hardness significantly.

 

The hardening depth of the outer layer of carbon steel achieved by laser hardening is usually 0.1-1.5mm, and 2.5mm or more can be achieved on some materials. Compared with the traditional quenching method, its advantages are:

1. The target heat input is limited to local areas, so there is little component warping during processing. Rework costs can be reduced or even completely eliminated;

2. It can also be hardened in complex geometric surfaces and precision parts, which can realize the accurate hardening of locally limited functional surfaces that cannot be quenched by traditional quenching methods;

3. No distortion. The traditional hardening process produces deformation due to higher energy input and quenching, but in the laser hardening process, the heat input can be precisely controlled thanks to laser technology and temperature control. Components remain almost in their original state;

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

 

02.

Laser hairification

Laser coating is one of the technological methods for surface modification of metal materials. During the structuring process, lasers create regular arranged geometries in layers or substrates in order to change technical properties and develop new functions in a targeted manner. The process of action is roughly the use of laser radiation (usually short pulses of laser light) to generate regularly arranged geometric shapes on a surface in a repeatable manner. The laser beam melts the material in a controlled manner and solidifies into a defined structure through proper process management.

 

For example, hydrophobic surface structures allow water to run off the surface. This property can be achieved by creating submicron structures on the surface with ultrashort pulsed lasers, and the structures to be created can be precisely controlled by changing the laser parameters. Opposite effects, such as hydrophilic surfaces, can also be achieved;

 

To paint the automobile panel, the surface of the sheet must be evenly distributed “micro pits” to enhance the adhesion of the paint, and the pulse laser beam focusing thousands to tens of thousands of times per second is incident on the roll surface, forming a tiny melt pool on the roll surface at the focal point, and blowing the tiny melt pool sideways, so that the melt in the melt pool is deposited to the edge of the melt pool as far as possible to form a circular arc convex according to the specified requirements. These small bumps and micro-pits can not only improve the roughness of the material surface, increase the adhesion of the paint, but also improve the surface hardness of the material and extend the service life.

 

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

 

Compared to traditional methods, surface laser structuring is more environmentally friendly and does not require additional sandblasting agents or chemicals. Repeatable and accurate, the laser achieves controlled structures down to the micron and is very easy to replicate; Low maintenance, laser is non-contact compared to fast wear mechanical tools, so absolutely no wear; No post-treatment is required, and no melt or other processing residues are left on laser-processed parts.

 

03.

Laser color surface treatment

Laser tempering is commonly used in laser dazzling surface treatment, also known as laser color marking. The principle of the process is that when the laser heats the material, the metal is locally heated to slightly below its melting point, and the structure of the gate will change under the appropriate process parameters. On the surface of the workpiece will form an oxide layer, this layer of film under the light irradiation, the incident light interference makes a variety of temper colors appear at this time, the surface generated by this layer of magic color marking layer, with the observation Angle changes, the marked pattern will also change a variety of different colors.

 

These colors maintain a stable temperature up to about 200 °C. At higher temperatures, the grid reverts to its initial state – marking disappears. The surface quality will remain intact. It has a high degree of security and traceability in the application of anti-counterfeiting. In recent years, it has matured in the field of medical technology, and in addition to the new black marking through ultra-short pulse lasers, it is also very suitable for product identification, thus achieving unique traceability according to the UDI Directive.

 

04.

Laser cladding

It is an additive manufacturing process suitable for metal and cermet mixed materials. This allows you to create or modify 3D geometries. Using this production method, the laser can also be repaired or coated. So in aerospace, additive manufacturing is being used to repair turbine blades.

 

In the field of tool and die making, cracked or worn edges and functional surfaces can be repaired, or even partially armored. To prevent wear and corrosion, in the field of energy technology or petrochemistry, coating bearing positions, rollers or hydraulic components. Additive manufacturing is also used in automotive manufacturing. A large number of components are modified here.

 

In conventional laser metal cladding, the laser beam first heats the workpiece locally and then forms a molten pool. The fine metal powder is then sprayed directly into the molten pool from the nozzle of the laser machining head. In the process of high-speed laser metal cladding, the powder particles are heated almost to melting temperature above the base surface. Thus, less time is required to melt the powder particles.

 

Effect: Significantly improve process speed. Due to the smaller thermal effect, it is also possible to coat materials that are very sensitive to heat, such as aluminum and cast iron alloys, with high-speed laser metal cladding. With the HS-LMD process, very high surface rates can be formed on rotationally symmetric surfaces, up to 1500 cm²/min. At the same time, feed speeds up to hundreds of meters per minute are achieved.

 

Quickly and easily repair expensive parts or molds with laser powder laser metal cladding. Injuries large and small can be repaired quickly and almost without trace. You can also change the design. This saves time, energy and materials. Especially for expensive metals like nickel or titanium, it’s worth it. Typical examples of applications are turbine blades, various pistons, valves, shafts or dies.

 

05.

Laser heat treatment

Thousands of micro-lasers (VCsels) are mounted on a single chip. Each transmitter is equipped with 56 such chips, and a module is made up of several transmitters. The rectangular radiant area can contain millions of micro-lasers and output several kilowatts of infrared laser power.

 

The VCSEL generates a near-infrared beam with a radiation intensity of 100 W/cm² through a large area of directional rectangular beam cross-section. In principle, this technology is suitable for all industrial processes where precise surface and temperature control is required.

 

The laser heat treatment module is particularly suitable for high-precision and flexible large-area heating applications. Compared to traditional heating methods, this new heating process offers greater flexibility, accuracy and cost savings.

 

The technology can be used to seal bagged batteries to prevent wrinkling of the aluminum foil, thereby extending the life of the battery. It can also be used for drying cell aluminum foil, photoinfiltrating solar panels, and precisely treating areas to be heated in specific materials such as steel and silicon wafers.

 

06.

Laser polishing

The mechanism of laser polishing technology is narrow melting of the surface and surface overmelting, relying on the remelting of the surface and the resolidification of the laser remelting layer. When the metal surface is irradiated by a sufficiently high energy laser, the surface undergoes a certain degree of remelting, redistribution, and through the action of surface tensile stress and gravity, a smooth surface is achieved before solidification.

 

The entire thickness of the melt layer is less than the height of the trough to the peak of the wave, so that the entire molten metal is filled into the nearby trough, the driving force of this filling is achieved by the capillary effect, and the thicker melt layer will cause the liquid metal to flow outward from the center of the melt pool, the driving force is the thermal capillary effect or the Marconi effect, so that it is redistributed.

 

Application cases such as light large telescope optical components (especially large size and complex shape mirrors) of the material silicon carbide ceramics. RB-SiC, as a typical high hardness and multiphase material, has a difficult surface precision polishing technology and low efficiency. By modifying the RB-SiC surface pre-coated with Si powder by femtosecond laser, the optical surface with a surface roughness Sq of 4.45 nm can be obtained after only 4.5 hours of polishing, and the polishing efficiency is more than 3 times higher than that of direct grinding polishing. Laser polishing is also widely used in the polishing of die, CAM and turbine blade.

 

07.

Laser shot blasting

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), and the surface metal (or absorption layer) instantly forms a plasma explosion under the action of the high power density laser, and the explosion shock wave is transmitted to the inside of the metal parts under the constraint layer, so that the surface grain produces compressive plastic deformation. Surface strengthening effects such as residual compressive stress and grain refinement are obtained in the thicker surface of parts. Compared with the traditional mechanical shot blasting has the following advantages

 

1. Strong orientation: the laser acts on the metal surface at a controllable Angle, and the energy conversion efficiency is high, while the mechanical projectile impact Angle is random;

2. Large force: the instantaneous pressure generated by laser shot blasting plasma is as high as several GPa; High power density: laser shock peak power density reaches several to dozens of GW/cm2;

3. Good surface integrity: laser shock has almost no sputtering effect on the surface, and after mechanical shot peening, the surface topography is damaged and stress concentration is generated.

 

The maximum compressive stress value after laser shock is better, the surface residual compressive stress is increased by about 40% to 50%, and the fatigue life of the workpiece, high temperature resistance and bending forming and other related indicators are significantly improved. At present, it has been used in aircraft surface treatment, aircraft engine surface treatment and other fields.