Absorption of laser irradiation in a porous powder layer

An analytical relationship was derived to describe the amount of energy absorbed within preplaced powder during the laser deposition process. The relationship, which reflects an exponential decay of Beer-Lambert, may be used to define internal absorption due to scattering within the powder layer regardless of the beam shape and energy distribution if the attenuation coefficient and bulk absorption are known. Experiments were conducted to estimate the attenuation coefficients for pure iron and pure copper powders, representing three powder size distributions, during CO₂ and Nd:yttrium-aluminum-gamet (YAG) laser irradiation. Qualitative observations and trends of the experimental data indicated that greater beam penetration, accompanied by a decrease in the estimated attenuation coefficients, was associated with the larger powder particles, the lower wavelength of the Nd: YAG laser, and the copper powder. Attenuation coefficients were determined for the original powder size distributions and a larger size distribution obtained by sieving. The estimated attenuation coefficients representing the original powder size distributions were found experimentally to be 0.0144 μm^-1 for the iron powder and 0.0113 μm^-1 for the copper powder. These values compared to those of the materials in the solid state indicate that the attenuation coefficients of the powders are four orders of magnitude less and represent significant penetration of irradiation into the powder layer.