Laser Surface Strengthening Technology for Precision Gear Molds-ShangHai RunQia Electronics Technology Co.,Ltd.

Laser Surface Strengthening Technology for Precision Gear Molds

Laser Surface Strengthening Technology for Precision Gear Molds refers to a technology that processes the surface of gears or molds with high-energy-density laser beams and coatings or cladding materials in a numerical control environment. It modifies the microstructure or composition of the surface layer to realize surface phase transformation strengthening or restorative strengthening with enhanced performance.I. Metallic Materials Science of Laser Phase Transformation StrengtheningLaser phase transformation strengthening involves scanning the workpiece with a laser beam to rapidly heat the workpiece surface layer above the Ac3 critical point. When the laser spot moves away, the temperature of the heated layer instantly drops to the martensite or bainite region due to thermal conduction of the workpiece substrate, triggering martensitic or bainitic phase transformation and completing the phase transformation strengthening process.

The phase transformation strengthening process features excellent surface quality. The hardness and depth of the strengthened layer can be controlled according to different material properties, workpiece heat capacity, and laser processing parameters. The technical factors that affect the strengthening effect in conventional heat treatment exert significantly different influences in laser phase transformation strengthening.1. Dispersion Strengthening and Distortion StrengtheningAustenite forms during laser phase transformation strengthening, and martensitic transformation occurs on the metal surface once laser irradiation stops. The austenite grains formed under this process condition, whether in the surface layer or inner layer, have no chance to nucleate and grow.

Dispersed austenite grains transform into dispersed martensite or bainite phases, endowing the microstructure with both lattice strengthening and dispersion strengthening effects. Moreover, the martensite lattice formed under rapid cooling conditions has a higher defect density than that obtained via conventional quenching. Meanwhile, retained austenite achieves an extremely high dislocation density, bringing about a distortion strengthening effect and substantially improving the strength of metal materials.2. Non-oxidizing and Non-decarburizing QuenchingIn conventional heat treatment, workpieces are prone to oxidation and decarburization without protective measures during heating, which reduces the hardness, wear resistance, service performance and service life of workpieces.

The light-absorbing coating used in laser phase transformation strengthening can protect the workpiece surface from oxidation.3. Anti-fatigue Mechanism of Laser StrengtheningOne of the factors affecting the fatigue resistance of metal materials is the initiation time of fatigue cracks. Wear and fatigue interact and accelerate material damage: wear grooves can serve as initiation sites for fatigue cracks and accelerate their generation. Once fatigue cracks appear on the material surface, the surface roughness deteriorates severely, which in turn intensifies wear.

The laser-strengthened layer possesses excellent resistance to plastic deformation and adhesive wear.4. Equal-Strength Working LayerIn conventional heat treatment, cooling proceeds from the surface to the interior with the fastest cooling rate at the surface and a gradual decrease inward, resulting in a gradient hardness distribution that declines from the surface to the core.

Although the heating direction of laser phase transformation strengthening is the same, the surface has a higher temperature and a relatively longer heating duration (up to 0.2–0.25 seconds), while austenitization of the inner layer completes instantaneously. This leads to a higher carbon concentration in the surface austenite and a stronger solid solution strengthening effect.

Conversely, the cooling direction of laser quenching is from the interior to the surface. The inner layer has a lower temperature but the fastest cooling rate; the outer layer has a higher temperature with the advantage of solid solution strengthening but the slowest cooling rate. Despite the slightly lower carbon concentration in the inner layer, distortion strengthening and dispersion strengthening are more pronounced. Consequently, an almost uniform hardness distribution is formed within the hardened layer.

The equal-strength working layer of laser-strengthened parts avoids the accelerated wear phenomenon of conventional heat-treated parts once surface wear occurs.II. Laser Phase Transformation Strengthening Process Technology for Precision Gears1. Material SelectionMedium carbon steel is suitable for gear manufacturing, while low carbon steel is not recommended. The use of low carbon steel fails to guarantee the strength of the gear matrix and reduces the bending fatigue strength.2. Initial StateThe optimal initial state of gears is quenched and tempered state, which can be combined with stress relief heat treatment after forging gear blanks. Normalizing followed by high-temperature tempering of forged blanks to obtain the desired quenched and tempered state of gears is a cost-effective approach.3. Scanning ModeThe main scanning modes for laser strengthening of gears include circumferential continuous scanning and axial tooth-separated scanning.4. Pre-treatment Technology for Gear Laser StrengtheningAppropriate pre-treatment is one of the keys to ensuring the laser strengthening effect of gears and has long been a difficult problem in laser processing. A reasonable and applicable pre-treatment process can prevent quenching cracks on the gear surface, reduce sensitivity to surface burning, guarantee the tooth surface precision after laser treatment, and increase the thickness of the hardened layer.5. Non-overlapping Technology and Defocus DifferenceDue to operational requirements, the hardened layer on the gear surface needs to be reasonably distributed along the tooth profile. Given the special shape of gears and the prohibition of quenching zone overlapping on the pitch circle surface, a special broadband focusing system is required.

In addition, the laser beam cannot ensure a consistent defocus amount at different positions of the tooth surface. Selecting an optimal focal irradiation position is the key to ensuring reasonable hardness distribution on the tooth surface.6. Performance of Laser-Strengthened GearsThe performance of laser-strengthened gears is mainly reflected in three aspects:(1) Fatigue performance. If neither laser-strengthened gears nor quenched and tempered gears experience tooth breakage, it indicates excellent bending fatigue resistance;(2) Wear resistance;(3) Service performance.

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