Absorptivity measurement of solid and powder bed under IR laser beam

Abstract

The effective absorptivity of IR laser light for different powder beds were studied. The reflectivity of aluminum, titanium, stainless steel and copper alloys was measured using an appropriate Ulbricht sphere. Laser irradiation was reliably detected by a photodiode. Reflectivity was carefully measured as a function of illuminated area and powder bed density. Several powder size distributions and powder thicknesses were chosen to evaluate the impact on the laser absorption. Two spot diameters were tested to evaluate the variation of the reflectivity. The absorptivity of the powder bed was significantly higher than the absorptivity of a uniform surface for similar material due to multiple scattering. In addition, the substrate is responsible for a non-negligible variation in the powder bed absorption. The inhomogeneity of the powder bed strongly modified the laser absorption for a small spot size. The absorption fluctuated during the transition from the powder state to the molten pool state.