Laser cladding with powder is a process that is widely used for repairing or improving surfaces and creating new components - also known as 3D printing or additive manufacturing. It can increase throughput and deliver high quality while keeping dilution levels very low, reducing the required application of heat, and minimizing distortion. It also results in extremely low porosity and better surface uniformity. All of these factors substantially reduce the need for subsequent processing, thus saving both time and costs. Laser cladding is also highly versatile. The available software-assisted manufacturing options permit the creation of a wide range of geometries. It is easy to apply a single layer, to sequentially apply multiple layers, or even to create a new object that is connected to an existing component.
Various process parameters—such as laser output, spot size, and speed—affect the efficiency and quality of cladding. One very important factor, although it is generally poorly understood and greatly underestimated, is energy coupling efficiency. This is the fraction of the laser’s output that actually reaches and is absorbed by the workpiece. Reflection of the laser beam by the part itself reduces the energy coupling efficient, and so does scattering and absorption by powder particles. Low transmission of the laser beam prevents adequate melting of the substrate’s surface. Another risk is that powder grains may absorb so much laser energy that they melt. The molten powder material then forms a bath on the workpiece surface that lacks sufficient coupling to the base material. The result is unusable cladding with poor adhesion. To prevent this from happening, the energy coupling efficiency of the process must be increased.
Absorption by Metal
Absorption of the laser beam by the part depends on the wavelength of the laser source. Diode-pumped lasers are most commonly used for laser cladding with powder. O.R. Laser offers diode- pumped fiber lasers with a wavelength of 1070µm for these applications. It is interesting to note that laser beam absorption by a metal surface is not completely constant for the same wavelength. It also depends on the surface temperature of the metal workpiece.
While the part absorbs energy from the laser beam and heats up, the absorption coefficient rises very gradually. When the metal’s melting point is reached, however, the absorption coefficient increases suddenly and sharply.
Not all of the laser energy reaches the part, however. Laser radiation is also absorbed by powder particles as they dissolve into the molten bath, or scattered and/or reflected by their surfaces.
The amount of laser energy absorbed by the powder can be determined by using Mie theory to calculate the scattering of incident radiation by small particles only a few microns in diameters.
With smaller powder particles, increasing the feed rate reduces transmission of the laser energy while also increasing absorption by the particles. The best solution is a particle size between 40 and 90 microns with relatively slow feed. Larger particles keep the transmission relatively constant even if the feed rate fluctuates, thus ensuring the stability of the overall process. O.R. Laser recommends a powder particle size of no more than 90 microns for its laser cladding systems. Even larger particles run the risk of blocking the coaxial powder nozzle, and smaller particles would reduce the efficiency and quality of the process.
The size of the powder particles used for laser cladding has a large influence on the overall quality and efficiency of the process. To achieve a stable cladding process, it is necessary for the laser beam to create a molten bath of the base material. The laser beam is disrupted by scattering or reflection by the surfaces of powder particles. Small particles and high feed rates block the laser radiation and reduce the efficiency and quality of the cladding process.back