What is Laser Heat Treatment: Principles, Advantages and Applications

1. Principle of laser heat treatment

Laser heat treatment is a surface heat treatment technology.

Namely, the laser is used to heat the surface of metal materials to achieve surface heat treatment.

When a strong laser irradiates a metal surface, if the high reflection is overcome, most of the penetration into the metal surface will be absorbed.

The excited electrons collide with the lattice or other electrons, rapidly transferring the local heat of the metal surface to the interior, forming a very high cooling rate, and hardening the surface.

What is Laser Heat Treatment: Principles, Advantages and Applications 1

Laser heating has a very high power density, that is, high power is concentrated on the unit area of the laser irradiation area.

Due to the extremely high power density, the heat can not be transferred from the workpiece by conduction and heat dissipation in time, which makes the workpiece rapidly rise to the austenitizing temperature in the laser irradiated area to realize rapid heating.

When the laser heating is over, because the temperature in the basic large volume of the workpiece is still low during rapid heating, the heated area can be rapidly cooled through the heat conduction of the workpiece itself, so as to achieve heat treatment effects such as floating fire.

What is Laser Heat Treatment: Principles, Advantages and Applications 2

Schematic diagram of laser quenching

What is Laser Heat Treatment: Principles, Advantages and Applications 3

Laser cladding technology

2. Advantages and disadvantages of laser heat treatment

Laser heat treatment is a method of surface treatment of metals by using high power density laser beams.

It can achieve phase transformation hardening (or surface floating fire), surface alloying and other surface modification treatments on metals, resulting in changes in surface composition, structure and properties that cannot be achieved with its large surface luster.

The surface hardness of cast iron can reach more than HRC (Rockwell hardness) 60 degrees, and the surface hardness of medium carbon and high carbon steel can reach more than HRC 70 degrees, thus improving the wear resistance, fatigue resistance, corrosion resistance, oxidation resistance and other properties, and extending its service life.

Compared with other traditional heat treatment processes, such as high-frequency quenching, carburizing and nitriding, laser heat treatment technology has the following characteristics:

1. There is no need to use additional materials, but only change the structure of the surface of the treated material.

The treated modified layer has sufficient thickness, and the depth can be adjusted as required, generally up to 0.1-0.8mm.

2. The deformation of the processed part is very small.

Because of the high laser power density and the short interaction time with the part (10-2-10 seconds), the thermal deformation area and overall change of the part are very small, so it is suitable for high-precision part processing as the final processing procedure of materials and parts.

3. Good processing flexibility and wide application.

The flexible light guide system can guide the laser to the processing part at will, so that the deep hole, inner hole, blind hole and groove can be conveniently processed, and selective local processing can be carried out.

For large area scanning, because the laser spot area is small, multiple overlapping technology or large area spot technology, defocusing method, broadband method and rotating mirror method must be used.

When lapping for several times, there is an area at the joint of each adjacent scanning band.

Therefore, its microhardness is fluctuating.

From the metallographic point of view, the lapped coating shows a macro periodic performance change.

For large area spot technology, when the output power is constant, the larger the spot area is, the lower the power density is.

Increasing the beam diameter may weaken the high energy density and super fast heating advantages of the laser.

3. Application of laser heat treatment

Almost all metal surface heat treatment can be applied.

At present, there are many machine industries with serious 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 automobile industry, and it can be almost used in many key parts of automobile, such as cylinder block, cylinder liner, crankshaft, etc.

For example, General Motors of the United States uses more than a dozen kilowatt level lasers for heat treatment in the automotive industry.

On many key automotive parts, CO2 lasers partially harden the inner wall of the commutator shell, with a daily output of 30000 sets, improving work efficiency by four times.

2. Large locomotive manufacturing

Laser heat treatment has been adopted in large locomotive manufacturing industry, which has greatly improved the service life of locomotive, mainly including the laser heat treatment of large crankshaft of locomotive and the laser heat treatment of cylinder liner and main spring of locomotive diesel engine.

Their mold manufacturing process is complex, with high precision requirements, different shapes, and wide applications.

However, because of the short life of the mold, the cost is often increased, and it is difficult to repair.

Laser heat treatment of mold surface has been gradually recognized and adopted, which can double the life of the mold without being limited by the shape and size.

What is Laser Heat Treatment: Principles, Advantages and Applications 4

Laser surface treatment technology includes: laser phase transformation technology, laser cladding technology, laser alloying technology, and laser surface composite treatment technology

What is Laser Heat Treatment: Principles, Advantages and Applications 5

1. Laser surface quenching

(1) Principle of laser surface quenching

Laser is used to heat the surface of metal material above the transformation point.

As the material cools, austenite transforms into martensite, which hardens the surface of the material.

At the same time, there is considerable compressive stress in the hardened layer, which increases the fatigue strength of the surface.

Using this feature to implement laser quenching on the surface of parts can greatly improve the wear resistance and fatigue resistance of materials.

(2) Characteristics of laser surface fire

The latest research results show that if laser surface quenching is carried out when the workpiece is under pressure, and the external force is removed after quenching, the residual compressive stress can be further increased, and the compressive and fatigue strength of the workpiece can be greatly improved.

As the laser surface quenching speed is fast, there is less heat entering the workpiece material, which leads to less thermal deformation (the deformation amount is 1/3~1/10 of high-frequency quenching).

Therefore, the workload of the next working procedure (correction or grinding) can be reduced, and the manufacturing cost of the workpiece can be reduced.

In addition, the process is self cooling without quenching liquid, which is a clean and sanitary heat treatment method;

And it is convenient to realize compound processing with the same laser processing system.

Therefore, the supply and demand of laser quenching can be directly arranged on the production line to achieve automatic production.

Because the process is non-contact, it can be used for surface quenching of narrow grooves and bottom surfaces.

(3) Application of Laser Surface Quenching

Laser quenching is widely used because of the above advantages.

The surface hardening of the engine cylinder block can increase the wear resistance of the cylinder block by more than 3 times;

The service life of the cutting edge of the hot rolled steel plate shearing machine is about twice as long as that of the same untreated cutting edge;

Moreover, laser surface quenching is also applied to the quenching of machine tool guide rail, gear tooth surface, local quenching of engine crankshaft’s crank neck and cam, and laser quenching of various tool cutting edges.

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