Researchers at theUniversity of Texasat El Paso, working withSandia National LaboratoriesandFAR Associates, have developed a method to approximate the absorptivity of Inconel 625 during powder bed fusion by analyzing in situ radiation thermometry data collected during electron beam powder bed fusion (PBF-EB/M). Reported inAdditive Manufacturing Letters, the study shows that absorptivity decreases sharply as the material transitions from heated powder to a fully molten state, falling by approximately 60% at elevated temperatures.

Absorptivity strongly affects powder bed fusion by controlling how efficiently energy from the heat source is absorbed by the material. Yet process models usually treat it as a constant, mainly because direct in situ measurements are hard to make due to plasma, spatter, and other emissions. The authors suggest that multi-wavelength pyrometry, paired with an energy balance and spectral extrapolation, offers a practical way to estimate absorptivity under real processing conditions.

Approximating absorptivity from in situ spectral measurements

The experiments used a commercialGE AdditiveArcam A2X electron beam powder bed fusion system to process Inconel 625 powder. A cylindrical part was monitored for 18 consecutive layers with a multi-wavelength pyrometer, which recorded temperature and spectral emission data during preheating, melt scanning, and cooldown.

Unlike standard two-color pyrometry, the multi-wavelength method resolves temperature without relying on emissivity, while also measuring spectral signal strength across a calibrated 1080–1637 nm range. Using these measurements, the researchers fit polynomial models to the spectra and extended the emissivity estimates to shorter wavelengths (1064–1070 nm) matching the near-infrared laser wavelengths used in laser powder bed fusion (PBF-LB/M).

Using Kirchhoff’s law of thermal radiation and assuming local thermal equilibrium, the extrapolated emissivity values were taken to be equal to absorptivity at the same wavelength and temperature.

Absorptivity drops as Inconel 625 melts

Analysis of the aggregated data shows a clear and repeatable trend. During melt scanning, as temperatures rose from preheated conditions into the early melting range and then into the fully molten state, the measured signal strength declined steadily.

Following the onset of melt scanning, absorptivity values were in the ~0.5–0.6 range. As the material transitioned into the molten state, absorptivity fell below 0.2 at both 1064 nm and 1070 nm. This pattern was consistent across all monitored layers.

The authors discuss how absorptivity can change as powder transitions into a liquid state, and compare their findings with prior studies that report higher absorptivity values under different conditions, including keyhole-mode laser processing where absorption can increase. They also note that measurements in PBF-LB/M are needed to account for process-specific effects.

Source: 3D Printing Industry