High temperature reflectometer for spatially resolved spectral directional emissivity of laser powder bed fusion processes

Additive manufacturing involving layer-wise selective laser melting of a powder material, or laser powder bed fusion (LPBF), is a fast-growing industry. At the Additive Manufacturing Metrology Testbed (AMMT) at the United States National Institute of Standards and Technology (NIST) an integrating hemispherical reflectometer has recently been developed to facilitate measurements of spatially resolved reflectance of the laser-melting heat affected zone (HAZ) during the LPBF process. Reflectance is then used to determine spatially resolved emissivity. The design features of the hemispherical-directional reflectometer are discussed. Then, the reflectometer performance and measurement uncertainties are detailed. A two-dimensional map of emissivity and emissivity uncertainty of the HAZ around a meltpool of high-purity nickel are presented. It is found that emissivity measurements are in good agreement with literature values at the melting point of high-purity nickel with acceptable uncertainty.