Approximation of absorptivity conditions for Inconel 625 from in situ radiation thermometry measurements in electron beam powder bed fusion

• This work presents a methodology for approximating the absorptivity conditions for Inconel 625 from in situ measurements done in PBF AM • The method uses a multi-wavelength or multi spectral sensor (pyrometer) that enabled capturing in process measurements from which thermal radiative behavior was approximated • Using energy balance and/or thermal equilibrium, the emissive and absorptance (or absorptivity) conditions are equated and the absorptance at shorter wavelengths approximated • Results show that only slight variations in the approximated absorptivity values (at 1064 and 1070nm) versus measured values (at 1500nm) are incurred. This is consistent with the observation of general decreasing trend of emissivity/absorptivity from short to long wavelengths • These results can help efforts in the AM community for both modeling and experimental works Highly dynamic conditions experienced during metals processing using powder bed fusion (PBF) additive manufacturing (AM) arise from the interaction of multiple process variables. Chiefly amongst them is the energy source-material interaction, which results in abrupt, temporally changing thermal conditions at the melt-pool. The absorption of the energy from the source by the powder bed is complicated complex because this interaction is highly dynamic and occluded by process emissions including the plasma plume and spatter. Typical approaches for measuring absorption conditions have relied on intricate ex situ setups requiring extensive sample preparation. This work involved approximating the absorptivity for a nickel-base material from in situ observations made using a multi-wavelength (MW) pyrometry sensor, while processing using electron-beam based powder bed fusion (PBF-EB/M). The prediction step involved fitting the spectral data captured by the MW sensor to then extrapolate values to shorter wavelengths. The approximation was done for wavelengths that are relevant for the PBF-LB/M process, where absorptivity of the laser energy is paramount. The results obtained provide a window into the behavior of this material as it transitions from powder to molten state, showing that the absorptivity substantially decreases (∼60% reduction) as the powder material changes phase induced by incipient melting. While this work focuses on a single material, the approach presented can help characterize the absorptivity of other materials employed in PBF. This work helps support both experimental and modeling efforts that are helpful to increase our understanding and practice of fusion-based AM processes, and other manufacturing techniques.