1. Principle of laser heat treatment
Laser heat treatment is a surface heat treatment technology that uses lasers to heat metal materials to obtain desired surface properties. The laser is directed at the metal surface and if the reflection is overcome, most of the laser energy penetrates the metal surface and is absorbed.
This causes the metal's electrons to become excited and collide with the lattice or other electrons, quickly transferring heat from the metal's surface to its interior. This results in a high cooling rate, which in turn leads to surface hardening.
Laser heating has high power density, which means that a large amount of power is concentrated in a small area of the workpiece. This high power density allows the part to quickly reach the austenitizing temperature in the area irradiated by the laser.
However, due to rapid heating, heat cannot be transferred from the part by conduction and heat dissipation in time. After laser heating is complete, the temperature in most of the part remains low.
This allows the heated area to be cooled quickly through heat conduction within the part itself, resulting in desired heat treatment effects such as floating fire.
Laser Extinguishing Schematic Diagram
Laser cladding technology
2. Advantages and disadvantages of laser heat treatment
Laser heat treatment is a method of modifying the surface properties of metals using high power density laser beams. It can induce hardening by phase transformation (such as surface floating fire), surface alloying and other surface modifications, resulting in changes in surface composition, structure and properties not achievable by other means.
Through laser heat treatment, the surface hardness of cast iron can reach more than 60 degrees of Rockwell hardness (HRC), and the surface hardness of medium and high carbon steel can exceed 70 degrees HRC, thereby improving its wear, fatigue, corrosion, oxidation resistance, and other properties, and extending its service life.
Compared with traditional heat treatment processes such as high-frequency quenching, carburizing and nitriding, laser heat treatment has several advantages:
- No additional materials are required, only the surface structure of the treated material is changed.
- The treated modified layer is thick enough and can be adjusted to a depth of 0.1-0.8mm as needed.
- Minimum deformation of the processed part.
The laser's high power density and short interaction time with the part (10^-2 to 10 seconds) result in minimal thermal deformation and overall part change, making it suitable for processing high-precision parts as a procedure final processing of materials and parts.
- Good processing flexibility and wide application.
The flexible light guide system can direct the laser to the processing part as desired, enabling the processing of deep holes, inner holes, blind holes and grooves. Selective local processing can also be carried out.
For large area scanning, multiple overlay techniques, large area spot technology, defocusing methods, broadband methods or rotating mirror methods must be used because the laser spot area is small.
When lapping multiple times, the microhardness fluctuates in the area between each adjacent scan band and a macroperiodic change in performance is observed in the lapped coating from a metallographic point of view.
When using large area spot technology, larger spot areas result in lower power densities when the output power is constant. Increasing the beam diameter may weaken the high energy density and rapid heating advantages of the laser.
3. Application of laser heat treatment
Laser heat treatment can be applied to almost all metal surface heat treatments. Currently, it is widely used in industries with high wear and tear, such as automobiles, metallurgy, petroleum, heavy machinery, agricultural machinery, and high-tech products such as aerospace and aviation.
1. Automobile industry
Laser heat treatment is widely used in the automobile industry and can be applied to many important parts of a vehicle, such as cylinder blocks, cylinder liners and crankshafts.
For example, General Motors in the United States uses more than a dozen kilowatt-level lasers for heat treatment in the automotive industry. In major automotive parts, CO2 lasers partially harden the inner wall of the switch housing, with a daily output of 30,000 sets, resulting in a fourfold improvement in work efficiency.
2. Manufacturing of large locomotives
Laser heat treatment has been adopted in the large locomotive manufacturing industry, improving the service life of locomotives. This includes laser heat treatment of large crankshafts and cylinder liners and main springs of diesel locomotive engines.
The mold manufacturing process in this industry is complex and requires high precision, with different shapes and wide applications. However, the short lifespan of molds often leads to increased costs and difficulty in repair.
To solve these problems, laser heat treatment of mold surfaces is gradually gaining recognition and adoption. This method can double the life of the mold without being limited by shape or size.
Laser surface treatment technology covers various techniques, including: laser phase transformation, laser cladding, laser alloying and laser composite surface treatment.
1. Laser surface quenching
(1) Laser surface quenching principle
In laser surface hardening, a laser is used to heat the surface of a metallic material to a temperature above its transformation point. As the material cools, it transforms from austenite to martensite, which hardens the surface. This results in a compressive stress layer, which improves the fatigue resistance of the surface. By using this process, the wear and fatigue resistance of materials can be greatly improved.
(2) Characteristics of laser surface quenching
Recent research has shown that applying laser surface quenching while the part is under pressure and removing external force after quenching can increase the residual compressive stress and improve the compressive and fatigue strength of the part.
Furthermore, the fast laser surface quenching speed results in less heat transfer to the material, which reduces thermal deformation by 1/3 to 1/10 compared to high-frequency quenching. This reduces the workload of subsequent operations and reduces manufacturing costs.
The process is self-cooling, making it a clean and hygienic heat treatment method. It is also convenient to carry out composite processing with the same laser processing system. This enables automatic production by directly integrating laser hardening supply and demand into the production line.
The non-contact nature of the process makes it ideal for surface hardening in narrow grooves and undersurfaces.
(3) Laser surface hardening application
Laser surface hardening is widely used due to its numerous advantages. It can increase the wear resistance of engine cylinder blocks by more than three times, double the service life of cutting edges in hot-rolled steel plate cutting machines, and is used for quenching guide rails of machine tools , gear tooth surfaces, engine crankshaft crank necks and cams, and various tool cutting edges.
