LAUSR

Abstract Radiation thermometry methods used in powder bed fusion (PBF) additive manufacturing for in situ monitoring and control and quality assurance are increasing in importance. The most significant challenge associated with radiation thermometry methods arguably includes accurate determination or understanding of the emissivity of the surface (or entire emissive behavior of the region) being measured. This work describes a new approach for measuring the emissive behaviors of PBF materials during processing using a multi-wavelength (MW) or multi-spectral pyrometer operating in the spectral range from 1080-1640nm. The approach was implemented in an electron beam (EB) PBF machine, using the electron beam as a heat source, allowing for (1) measuring spectral emissive behavior of the surface in a fixed small region (~2.65 mm) throughout a variety of dynamic processing conditions including heating, melting, and cooling; (2) control of the scanning (heating) profile during processing without potential contamination of the measurements due to the wavelength of the fiber lasers (~1070 nm) commonly used in laser PBF; and (3) processing in an evacuated environment to assist with reduction of additional environmental effects that could impact the measurements. The experimental setup included a sight tube that prevented both metallization of the viewport and signal decay and enabled near-continuous measurements throughout processing. A type K thermocouple was placed in the vicinity of the measurement area and was used to compare with the temperatures measured by the MW pyrometer. Prior to the powder bed preheating experiment, the MW pyrometer was calibrated against a traceable blackbody source. The utility of the approach was demonstrated by acquiring measurements from the surface of a copper (d50~75μm) powder bed that was progressively heated up in a series of nine steps inside an Arcam A2 EB-PBF system through scanning with the electron beam. Following the preheat steps, seven consecutive melt steps were implemented enabling measurements of the emissive behavior for copper during its multiple solid-liquid-solid transitions. The unique capabilities of the MW pyrometer provided calculated values of target emissivity exhibiting temporal, spectral (1080-1640nm) and temperature dependence, hence indicating the non-graybody behavior for copper. Ongoing work will demonstrate the applicability of this technique across multiple powder metal alloy systems and PBF technologies.