The main process parameters for laser cladding include laser power, spot diameter, cladding speed, defocusing amount, powder feeding speed, scanning speed, and preheating temperature. These parameters significantly affect the dilution rate, cracks, surface roughness, and density of the clad part. These parameters are also interdependent, making it a very complex process. Appropriate control methods must be employed to keep these parameters within the allowable range of the laser cladding process.
Laser cladding has three important process parameters:
Laser Power: The higher the laser power, the more cladding metal is melted, and the greater the probability of porosity. As laser power increases, the cladding layer depth increases, and the surrounding liquid metal fluctuates violently, dynamically solidifying and crystallizing, gradually reducing or even eliminating the number of pores, and also gradually reducing cracks. When the cladding layer depth reaches its limit, as the power increases, the substrate surface temperature rises, exacerbating deformation and cracking. If the laser power is too low, only the surface coating melts, and the substrate remains unmelted. In this case, localized balling and voids appear on the cladding layer surface, failing to achieve the surface cladding objective.
Spot Diameter: The laser beam is generally circular. The width of the cladding layer primarily depends on the laser beam spot diameter; as the spot diameter increases, the cladding layer widens. Different spot sizes cause variations in the energy distribution on the cladding layer surface, resulting in significant differences in the morphology and microstructure of the obtained cladding layer. Generally, smaller spot sizes result in better cladding layer quality, which decreases as the spot size increases. However, excessively small spot diameters are unfavorable for obtaining large-area cladding layers.
Cladding speed (V) has a similar effect to laser power (P). If the cladding speed is too high, the alloy powder cannot melt completely, failing to achieve a high-quality cladding effect; if the cladding speed is too low, the molten pool remains for too long, leading to powder over-burning, loss of alloying elements, and increased heat input to the matrix, resulting in increased deformation.
Laser cladding parameters do not independently affect the macroscopic and microscopic quality of the cladding layer; rather, they are interconnected. To illustrate the combined effect of laser power P, spot diameter D, and cladding speed V, the concept of specific energy Es is proposed:
Es = P/(DV) This represents the irradiation energy per unit area, which can be considered in conjunction with factors such as laser power density and cladding speed.
Lowering specific energy is beneficial for reducing the dilution rate and is also related to the cladding layer thickness. Under constant laser power, the cladding layer dilution rate decreases with increasing spot diameter. When both cladding speed and spot diameter are constant, the cladding layer dilution rate increases with increasing laser beam power. Furthermore, as the cladding speed increases, the melting depth of the substrate decreases, leading to a decrease in the dilution rate of the substrate material on the cladding layer.
In multi-pass laser cladding, the overlap rate is the main factor affecting the surface roughness of the cladding layer. Increasing the overlap rate reduces the surface roughness, but it is difficult to guarantee the uniformity of the overlap area. The depth of the overlap area between cladding passes differs from the depth at the center of the cladding pass, thus affecting the uniformity of the entire cladding layer. Furthermore, the residual tensile stress from multiple overlapping cladding layers will accumulate, increasing the local total stress value and thus increasing the susceptibility of the cladding layer to cracking. Preheating and tempering can reduce the cracking tendency of the cladding layer.