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Development of Laser Surface Strengthening Technology for Gear Mould

Apr 03, 2020

Laser surface strengthening technology of gear mold refers to the technology of using high energy density laser beam and coating or cladding material to treat the surface of gear or mold, change the structure or composition of the surface, and realize the surface transformation strengthening or enhanced repair under the numerical control environment.

Metal materials science of laser phase transformation strengthening

The so-called laser transformation strengthening is to use the laser beam to scan the workpiece so that the surface of the workpiece rapidly rises above the critical point of AC3. When the heating layer moves away from the spot, due to the heat conduction of the workpiece matrix, the temperature between Shun enters into the martensite area or bainite area, and the martensite transformation or bainite transformation occurs, completing the transformation strengthening process.

The phase transformation strengthening process has the advantages of good surface quality. According to different materials, workpiece heat capacity, and laser processing parameters, the hardness and strengthening layer depth can be controlled. In the traditional heat treatment process, the technical factors that affect the strengthening effect have changed a lot in the role of laser phase transformation strengthening.

1. Dispersion strengthening and distortion strengthening

When laser irradiation is stopped, martensitic transformation occurs on the metal surface. The austenite grains formed in this process environment, no matter in the surface layer or in the inner layer, have no chance to grow. The dispersed austenite grains form the dispersed martensite or bainite phase, which makes the structure have the effect of lattice strengthening and dispersion strengthening. Moreover, the martensitic lattice formed under the condition of quenching has a higher defect density than that of conventional quenching. At the same time, the retained austenite also has a very high dislocation density, which makes the metal materials have distortion strengthening effect and greatly improves the strength.

2. Non-oxidation decarburization quenching

In the traditional heat treatment, if there are no protection measures in the heating process, oxidation and decarburization will occur, which will reduce the hardness, wear resistance, service performance and service life of the workpiece.

The absorbing coating used for laser phase transformation strengthening has the property of protecting the surface of the workpiece from oxidation.

3. Anti-fatigue mechanism of laser strengthening

One of the reasons that affect the fatigue resistance of metal materials is the initiation time of fatigue cracks. Wear and fatigue promote each other in the process of material damage. Wear groove mark can be the initiation point of fatigue crack, and accelerate the initiation of a fatigue crack. After fatigue crack appears on the surface of the material, the surface roughness will be seriously deteriorated, and the wear will also be intensified.

The laser strengthened layer has a strong resistance to plastic deformation and adhesive wear.

4. Equal strength working layer

The cooling direction of conventional heat treatment is from the surface to the inside, the cooling speed of the surface is the fastest, and the cooling speed gradually decreases from the surface to the inside, so the gradient distribution of the hardness value decrease from the surface to the inside is obtained.

Although the heating direction of laser transformation strengthening is the same, the surface temperature is higher, and the healing time is relatively longer, up to 0.2-0.25s, while the inner layer austenitizing is completed between Shun and Shun, which makes the surface austenite have higher carbon concentration and stronger solid solution strengthening effect. The cooling direction of laser quenching is opposite to that of conventional heat treatment, which is from the inside to the outside. Although the temperature of the inner layer is low, the cooling speed is the fastest. Although the temperature of the outer layer is high, it has the advantage of solution strengthening, but the cooling speed is the slowest. Although the carbon concentration of the inner layer is slightly low, the distortion strengthening and dispersion strengthening are stronger. In this way, the hardness value distribution in the hardened layer is almost unchanged.

The strong working layer of laser strengthened parts can avoid the phenomenon that once the surface of conventional heat-treated parts is worn, the wear rate will accelerate.

The technology of laser phase transformation strengthening for gear

1. Material issues

The laser gear should be made of medium carbon steel instead of low carbon steel.

If low carbon steel is used, there will be no strength guarantee for the gear base and the bending fatigue strength will be reduced.

2. Original state

The best original state of laser gear is quenching and tempering state. The specific operation can be combined with stress relief heat treatment after the forging of gear blank. It is a low-cost way to obtain the desired quenching and tempering state of laser gear by normalizing forging stock and high-temperature tempering.

3. Scanning mode

The scanning mode of laser gear mainly includes circumferential continuous scanning and axial split scanning.

4. Pretreatment technology of gear laser strengthening

The proper pretreatment agent is one of the keys to ensure the laser strengthening treatment of gear, and it has always been a difficult problem in laser processing. The reasonable pretreatment agent and treatment process can prevent the quenching crack on the surface of the gear, reduce the surface burning sensitivity, ensure the tooth surface accuracy after laser treatment, and increase the thickness of the hardened layer.

5. No overlapping technology and defocusing difference

Due to the requirements of gear working conditions, the surface hardening layer of gear is required to be reasonably distributed along with the tooth profile, and the shape of gear is special. In addition, the circular surface of the gear pitch can not be overlapped by a quenching belt, so a special wide-band focusing system is required.

In addition, because the laser beam can not ensure the same defocusing amount in different parts of the tooth surface, the key to ensure the reasonable distribution of the hardness of the tooth surface is to choose the focal point.

6. Performance of laser gear

The performance of laser gear is mainly in three aspects: fatigue performance; if there is no broken tooth in laser gear and quenched and tempered gear, it is proved that it has high bending fatigue resistance; wear resistance; service performance.


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