Additive Manufacturing

Implications for accurate surface temperature monitoring in powder bed fusion: Using multi-wavelength pyrometry to characterize spectral emissivity during processing

Alfonso Fernandez a,b, Ralph Felice c, César A. Terrazas-Nájera a,b, Ryan Wicker a,b

a W. M. Keck Center for 3D Innovation, The University of Texas at El Paso, El Paso, TX 79968, USA
b Department of Mechanical Engineering, The University of Texas at El Paso, El Paso, TX 79968, USA
c FAR Associates, Macedonia, OH 44056, USA

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. Arguably, the most significant challenge associated with radiation thermometry methods is the limited understanding of the emissivity, that is the emissive behavior of the entire 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 Spectropyrometer operating in the spectral range from 1000 to 1650 nm. 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) controlling the scanning (heating) profile during processing while rejecting radiative interference in the measurements due to heating 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 resultant signal decay, which enabled near-continuous measurements throughout processing. Measurements from the MW pyrometer were compared against those of a type K thermocouple that was placed in the vicinity of the measurement area. Prior to the powder bed preheating experiment, the MW pyrometer was calibrated against a NIST traceable blackbody source. The utility of the approach was demonstrated by acquiring measurements from the surface of a copper (d50~75 μm) powder bed that was progressively heated 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 measured values of emissivity of copper that exhibited temporal, spectral (1080–1640 nm) and thermal dependence, verifying the non-graybody behavior for copper. Ongoing work will demonstrate the applicability of this technique across multiple powder metal alloy systems and PBF technologies.

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Keywords: multi-wavelength pyrometry, accurate surface temperatures, electron beam melting, radiation thermometry, solid and liquid metal emissivity