Additive Manufacturing
On the thermal emissive behavior of four common alloys processed via powder bed fusion additive manufacturing
César A. Terrazas-Nájera a,b, Alfonso Fernández a,b, Ralph Felice c, 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 Aerospace and Mechanical Engineering, The University of Texas at El Paso, El Paso, TX 79968, USA
c FAR Associates, Macedonia, OH 44056, USA
Additive Manufacturing 82 (2024)
Abstract. Current thermal monitoring methods used in metal powder bed fusion (PBF) additive manufacturing (AM) rely on a priori knowledge of the emissivity, which is usually assumed temperature, wavelength, and time invariant. Given the true dynamic nature of emissivity for a given material, these assumptions result in the calculation of inaccurate process temperatures, or in the reporting of radiant intensity or radiance temperature as a proxy for the absolute temperature. In this work, we detail the use of a multi-wavelength (MW) pyrometry technique, operating in the spectral range from approximately 900–1700 nm, to capture radiant intensity emissions during processing of various materials using the electron-beam powder bed fusion (PBF-EB) process. The technique measured spectral intensity and analyzed the results of these data after processing with Planck’s distribution law in its two-color or ratio pyrometry form to establish the spectral and temporal emissive behavior in the active range from 1080–1637 nm. Four commonly used alloys (Ti6Al4V, TiAl, 316 L, and IN625) were examined here as each of these alloys underwent transitions (from powder to liquid to structured solid) induced during processing. For each material studied, analysis of the aggregated data for ten layers, superimposed in time, identified specific and repeatable trends in the emissivity, with plotted measurements forming clusters or regions in each of the processing stages (preheating, melt scanning, liquid, and cooldown). Also, the data recorded by the MW pyrometer was used to analyze the spectral variability of measurements as well as temporal changes in emissive behavior (from gray to non-gray) at similar temperatures and points in time in each processing stage but during different layers or scans. The results show that the measured emissive behaviors for the four materials were highly variable during processing, with typical differences during transitions ranging from 20% to 75%, and as high as 300% for the case of Ti6Al4V, indicating emissive behaviors that are highly dynamic rather than temporally and spectrally invariant. This dynamic emissive behavior is associated with changes in temperature, morphology, phase, and chemistry of the processed metal that happen during the highly transient and non- equilibrium conditions of PBF AM, and that can only be accounted for by performing in situ measurements during processing using techniques that do not rely on prior knowledge of the emissivity. These results are intended to (1) better inform the additive manufacturing community on the physical nature of emissivity of metallic materials during processing, and (2) provide foundational emissivity data that can be used to improve numerical modeling and the application of radiation thermometry techniques in PBF AM. Further, these results indicate that the emissive behavior during processing can result in significant variations temporally and spectrally, and although the results can be used as foundational emissive behavior measurements, the authors recommend the use of in situ techniques that operate without prior knowledge of emissivity to reduce uncertainties in measurements during PBF AM.
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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
Additive Manufacturing 46 (2021)
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.
Keywords: multi-wavelength pyrometry, accurate surface temperatures, electron beam melting, radiation thermometry, solid and liquid metal emissivity