Laser surface treatment technology beyond laser cleaning

For a long time, laser technology has been widely known for its applications in welding, cutting, and marking. However, in recent years, with the gradual popularization of laser cleaning, the concept of laser surface treatment has become an increasing focus, entering the minds of many. Laser processing is non-contact, highly flexible, high-speed, noiseless, with a small heat-affected zone, and it causes no damage to the substrate, without consumables, making it environmentally friendly and low-carbon.

In addition to laser cleaning, there are many other applications of laser surface treatment, such as laser polishing, laser cladding, and laser hardening. These methods are used to alter specific physical and chemical properties of material surfaces, such as making surfaces hydrophobic or using laser pulses to create small indentations (about 10 microns in diameter and just a few microns deep) to increase roughness, enhance surface adhesion, and more.

Beyond laser cleaning, here are some other laser surface treatments you may be aware of:

01. Laser Hardening

Laser hardening is one of the solutions for processing high-stress, complex components, allowing parts such as camshafts and bending tools to withstand higher stress and extend their lifespan.

Its principle involves heating the carbon-containing surface of a workpiece to just below its melting temperature (900-1400°C, with 40% of the irradiated power being absorbed), causing the carbon atoms in the metal lattice to rearrange (austenitization). The laser beam then continuously heats the surface along the feed direction, and as the laser moves, the surrounding material cools rapidly. The metal lattice cannot return to its original form, resulting in martensite formation, significantly increasing hardness.

The carbon steel surface hardness achieved by laser hardening typically reaches a depth of 0.1-1.5 mm, and can even reach 2.5 mm or more for certain materials. The advantages over traditional hardening methods include:

1. The targeted heat input is limited to localized areas, so there is virtually no component warping during processing. Rework costs are reduced or even eliminated.
2. Hardening can be applied to complex geometries and precision parts, enabling localized, functional surface hardening that traditional methods cannot achieve.
3. No distortion. Traditional hardening methods cause deformation due to higher energy input and quenching, but laser hardening allows for precise control of heat input, keeping components nearly unchanged.
4. Rapid, on-the-spot modification of the hardness geometry of components without needing to change optical devices or the entire system.

02. Laser Texturing

Laser texturing is one of the surface modification processes for metal materials. During the structuring process, laser creates geometrically arranged shapes in layers or substrates to change specific technical properties and develop new functions. The process involves using laser radiation (typically short-pulse lasers) to generate geometrically arranged shapes on the surface in a repeatable manner. The laser beam melts the material in a controlled manner, and the material solidifies into a defined structure through appropriate process management.

For example, hydrophobic surface structures can allow water to flow off the surface. Using ultrashort pulse lasers to create sub-micron structures on the surface achieves this feature, with precise control over the structure by varying laser parameters. Conversely, hydrophilic surfaces can also be achieved.

For automotive panels to be painted, the sheet surface must be evenly distributed with “micropits” to enhance paint adhesion. Focused laser pulses of several thousand to tens of thousands per second are directed onto the surface of the rollers. At the focal point, tiny molten pools form on the roller surface, and with side-blown air, the molten material from the pool accumulates at the edge of the pool, forming an arc-shaped bump. These small bumps and micropits both increase the surface roughness, improving paint adhesion, and enhance the material’s surface hardness, extending its service life.

Certain properties, such as friction characteristics or conductivity, are generated by the laser structure. Additionally, laser structuring increases the bonding strength and lifespan of the workpiece.

Compared to traditional methods, laser surface structuring is more environmentally friendly, as it doesn’t require additional abrasive agents or chemicals; it is repeatable and precise, achieving micron-level controlled structures that are easy to replicate; it requires low maintenance since laser is non-contact and does not wear out like mechanical tools; and no post-processing is needed, as there are no melt residues or other processing remnants left on the parts.

03. Laser Color Surface Treatment

Laser tempering is commonly used in laser color surface treatment, also known as laser color marking. The principle involves laser heating of the material, locally heating the metal to just below its melting point. At suitable processing parameters, the structure of the grating changes, forming an oxide layer on the workpiece surface. This thin film causes interference with incident light, producing various tempering colors that change with the viewing angle, resulting in a dynamic, iridescent marking layer.

These colors remain stable at temperatures up to approximately 200°C. At higher temperatures, the grating returns to its initial state, causing the marking to disappear while preserving the surface quality. It has high security and traceability for anti-counterfeiting applications. In recent years, it has been successfully used in the medical technology field, not only for new black marking using ultrashort pulse lasers but also for product identification, enabling unique traceability per UDI regulations.

04. Laser Cladding

Laser cladding is an additive manufacturing process suitable for metal and metal-ceramic mixed materials, allowing the creation or modification of 3D geometries. This method can also be used for repair or coating, and in aerospace, it has been applied to repair turbine blades.

In the tool and mold manufacturing sector, it can repair cracked or worn edges and functional surface shapes or even add armor in localized areas. To prevent wear and corrosion in energy technology or petrochemical fields, coatings can be applied to bearing locations, rollers, or hydraulic components. In automotive manufacturing, it is used to modify a large number of components.

In conventional laser metal cladding, the laser beam first locally heats the workpiece, then forms a melt pool. Fine metal powder is directly injected into the melt pool from the nozzle of the laser processing head. During high-speed laser metal cladding, powder particles are almost heated to melting temperature above the substrate surface. As a result, the melting powder requires less time to melt.

Effect: It significantly increases process speed. With lower thermal effects, high-speed laser metal cladding can also coat heat-sensitive materials such as aluminum alloys and cast iron alloys. The HS-LMD process can achieve high surface speeds on rotating symmetrical surfaces, up to 1500 cm²/min, while achieving feed speeds of hundreds of meters per minute.

Laser powder laser metal cladding enables the fast and easy repair of expensive parts or molds. Large and small damages can be repaired quickly and almost seamlessly, and designs can also be modified. This saves time, energy, and materials, especially for expensive metals such as nickel or titanium. Typical applications include turbine blades, various pistons, valves, shafts, and molds.

05. Laser Heat Treatment

Thousands of miniature lasers (VCSELs) are installed on a single chip. Each emitter contains 56 such chips, and a module is composed of several emitters. The rectangular radiation area can contain millions of micro-lasers and output thousands of watts of infrared laser power.

VCSELs generate a radiation intensity of 100 W/cm² in the near-infrared beam through a large-area, directional rectangular beam cross-section. In principle, this technology is suitable for all industrial processes that require extremely precise control of surface and temperature.

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

This technology can be used for sealing bag-type battery cells to prevent aluminum foil wrinkles, thus extending the battery’s lifespan. It can also be used for drying battery aluminum foil, light-soaking solar cells, and precision heating of specific materials (such as steel and silicon wafers), among other applications.

06. Laser Polishing

Laser polishing technology works on the mechanism of narrow remelting and over-melting of the surface, relying on surface remelting and the re-solidification of the laser remelted layer. When the metal surface is irradiated with high-energy laser, it undergoes a certain degree of remelting. Through surface tensile stress and gravity, the metal smooths before solidifying.

The entire thickness of the melt layer is less than the height from trough to peak, causing the molten metal to fill the nearby troughs. This filling is driven by capillary effect, and thicker melt layers cause molten metal to flow from the center of the melt pool outward, driven by thermal capillary effects or Marconi effects, leading to redistribution.

Example applications include lightweight large telescopic optical components (especially large and complex shaped mirrors) made of silicon carbide ceramics. As a typical high-hardness, multi-phase material, RB-SiC is difficult to polish precisely with low efficiency. By modifying the RB-SiC surface coated with Si powder using femtosecond lasers, a polished optical surface with a roughness of Sq 4.45 nm is achieved after just 4.5 hours, improving polishing efficiency by more than three times compared to direct grinding and polishing. Laser polishing is also widely applied to polishing molds, cams, and turbine blades.

07. Laser Peening

Laser shock peening, also known as laser shot peening, involves irradiating metal parts with high energy density, high focus, short-pulse laser (λ=1053nm). Under the high power density of the laser, the metal (or absorbing layer) instantly forms a plasma explosion. The shockwave generated by the explosion transfers to the metal part’s interior under the confinement of the constrained layer, causing compressive plastic deformation of the surface grains, generating residual compressive stress, and grain refinement over a thick range on the part’s surface, resulting in surface strengthening effects. Compared to traditional mechanical shot peening, it has the following advantages:

1. Strong Directionality: Laser acts on the metal surface at a controllable angle, with high energy conversion efficiency, while mechanical shot impacts are random.
2. High Force: The instantaneous pressure generated by plasma explosion during laser peening can reach several GPa, with a peak power density of several to tens of GW/cm².
3. Better Surface Integrity: Laser shock impacts have almost no splashing effect on the surface, while mechanical shot peening causes surface morphology damage and stress concentration.

Laser peening results in better maximum compressive stress