What's actually happening with L-PBF absorptivity
Surface absorptivity, powder amplification, and the non-constant factor that most calculations quietly assume away.
Absorptivity is one of the most important numbers in L-PBF. A flat surface of 316L at 1064 nm absorbs roughly 36% of the laser energy. Use that same material as a powder and it can jump to over 70%.
The amplification comes from multiple reflections between particles, and it shifts with various factors like temperature, laser angle of incidence, wavelength, powder size distribution, and packing fraction. Most of the time it gets treated as a fixed number pulled from a single paper.
I’ve built a tool to calculate laser absorptivity and show what’s happening. Fresnel equations give the single-surface absorptivity, a Hagen–Rubens correction scales it with temperature, and the Gusarov two-flux radiative transfer model turns that into an effective powder bed absorptivity based on the actual powder geometry. A 2D ray trace illustrates the multiple-reflection mechanism visually, if incompletely. It doesn’t account for everything — like oxide layers or power-dependent melt pool effects — but it’s fast enough to build intuition before committing to a simulation or a build.
An often overlooked part when doing calculations is that the amplification factor is non-constant. It depends on a number of factors: the ratio of layer thickness to particle size, the packing fraction, the angle of laser incidence. A material with low flat-surface absorptivity (like copper at 1064 nm) gets a larger relative boost from powder geometry than one that already absorbs well.
I’d be interested to hear how other people approach absorptivity in their calculations when starting to design a new parameter set or process.