Meaning
Physical reduction in the power of a laser beam occurs as it passes through the vapor and plasma generated during metal processing. During powder bed fusion or welding, plume attenuation can cause inconsistent melt pool depth and poor fusion between layers. The plume consists of metal vapor, ions, ejected nanoparticles, and droplets that scatter or absorb the incoming radiation.
Managing this effect is necessary for maintaining a stable energy density at the workpiece.
Energy Absorption
Thermal energy from the laser is partially diverted into the vapor cloud instead of the solid metal. This loss of intensity reduces the effective temperature of the melt pool and may lead to lack of fusion defects. The wavelength of the laser determines the degree of interaction with the specific species in the plume.
Shorter wavelengths typically experience different plume attenuation characteristics than longer wavelengths used in industrial CO2 lasers.
Particle Scattering
Solid and liquid particles ejected from the melt pool act as physical barriers to the light. These small fragments deviate the beam from its intended path and widen the focal spot on the powder bed. High speed photography reveals that the density of these particles varies throughout the duration of the scan.
Mitigation Strategy
Cross-flow systems are employed to blow the vapor and particulates away from the path of the laser. By maintaining a constant flow of inert gas, the process minimizes the buildup of material that causes plume attenuation. Sensing equipment can monitor the light intensity in real time to adjust the power of the laser dynamically.
Consistent removal of the plume ensures that every layer of the part receives the same amount of energy.
