High-speed laser cladding repair processing parameters

High-speed laser cladding is a rapid laser surface treatment technology, primarily involving technical parameters divided into two aspects: one is the equipment adjustment and setting parameters during the laser cladding process, referred to as processing parameters; the other is the evaluation and measurement parameters for the quality of the cladding effect after completion, known as inspection parameters.

I. Processing parameters mainly include laser power, spot shape, spot size, processing distance, overlap rate, cladding speed, powder feeding method, and shielding gas pressure—a total of 8 key parameters.

1. Laser power: the energy output by the laser per unit time.

2. Spot shape: common spot shapes are circular and rectangular, selected by users based on the characteristics of the processing object.

3. Spot size: spot size mainly affects the optical power density, i.e., the amount of optical energy per unit area. Under the same power conditions, a smaller spot size results in higher optical power density. High-power density spots are suitable for cladding high-melting-point metal powders.

4. Processing distance: refers to the distance between the laser output port and the substrate surface. If the processing distance is too far, metal powder tends to disperse, resulting in low powder utilization; if the distance is too close, the surface temperature of the laser cladding head due to laser radiation becomes excessively high, potentially causing powder blockage.

5. Overlap rate: the overlap rate is a major factor affecting the surface roughness of the cladding layer. Increasing the overlap rate reduces the surface roughness of the cladding layer. However, the uniformity of the overlapping sections is difficult to ensure. The depth of the overlapping areas between each cladding layer differs from the depth at the center of each layer, thereby affecting the entire cladding layer. The overlap rate in high-speed cladding can be as high as 70%-80% (compared to 30%-50% in conventional cladding).

6. Cladding speed: both cladding line speed and cladding area rate can represent the cladding speed.

7. Powder feeding method: the primary powder feeding method for high-speed laser cladding is annular powder feeding.

8. Shielding gas pressure: the shielding gas pressure is adjustable during processing. Shielding gases, typically nitrogen or argon, are mainly used for powder feeding and forming a protective zone around the laser cladding molten pool to reduce oxidation.

II. Inspection parameters are measurement parameters for evaluating the quality of the cladding layer after high-speed cladding is completed, mainly including porosity, hardness, bonding strength, dilution rate, thermal fatigue resistance, and surface roughness.

1. Porosity: pores are inevitable during high-speed laser cladding. The degree of porosity is related to the temperature and speed of the metal powder, as well as the angle of powder movement. Generally, slower powder movement speed results in higher porosity in the cladding layer.

2. Hardness: due to rapid cooling and high-speed impact during the formation of the high-speed laser cladding layer, the grains are refined, and lattice distortion occurs, strengthening the coating. Therefore, the hardness of the laser cladding layer is higher than that of conventional materials.

3. Bonding strength: the high-speed laser cladding layer and the substrate form a metallurgical bond, meaning atoms at the interface between the cladding layer and the substrate diffuse mutually to create bonding. This bond is formed under high temperatures generated by the laser acting on the substrate and metal powder, along with the high-speed movement of the powder. The bonding strength between the high-speed laser cladding layer and the substrate can reach up to 360 MPa.

4. Dilution rate: refers to the degree to which the cladding metal is diluted, expressed as the percentage of the substrate material in the cladding layer. The dilution rate significantly affects the performance of the cladding layer. In high-speed cladding processes, the dilution level can be controlled by adjusting metal powder flow rate, optical power density, and cladding rate.

5. Thermal fatigue resistance: refers to the cladding layer's resistance to thermal fatigue or thermal shock. Poor thermal shock resistance can lead to cracking during use. The quality of thermal shock resistance mainly depends on the difference in thermal expansion coefficients between the metal powder and the substrate, as well as the bonding strength between the cladding layer and the substrate.

6. Surface roughness: the flatness of the cladding layer surface. In process testing, laser energy density, powder feed rate, and carrier gas pressure all affect surface roughness. Each of these parameters has an optimal range; values set too high or too low will reduce surface flatness. In practical high-speed laser cladding processing of substrates, appropriate processing parameters must be set based on the characteristics of the powder and substrate to ensure that all inspection parameters meet standards and application requirements.