Multi-Wavelength Pyrometry for Emissivity Mapping and Accurate Surface Temperature Measurement in Powder Bed Fusion Systems
Accurate surface temperature measurement within powder bed fusion (PBF) systems during fabrication remains elusive for many reasons, despite the importance of knowing temperature for improving part quality, process control, repeatability and reproducibility, simulation capabilities, and more. Multi-wavelength (MW) pyrometry has been used previously to measure off-axis temperatures of a small region (~2.6mm in diameter) within an electron beam PBF (EBPBF) system. While this small region measurement makes it difficult to get a full field temperature map of the powder bed, it also allows for inline (on-axis) with laser temperature measurements for laser PBF (LPBF) systems. The MW pyrometry technique determines temperature, which allows the calculation of emissivities at various sensor wavelengths (ranging from ~1080nm to ~1650nm) using measured intensities. The emissivity of a surface is affected by different parameters such as- temperature, surface morphology, surface chemistry, instrument wavelengths. In PBF processes, the powder bed undergoes different temperatures (preheating, melting, cooling), and surface morphology (surface with metal powder, surface with molten metal, solidified printed surface). By calculating spectral emissivities over a range of temperatures for a material, emissivity maps can be obtained. These emissivity maps can work as a quantitative tool for understanding the change in surface characteristics because of the affecting parameters. Besides, the emissivity values of different materials are also required as an input for emissivity dependent monitoring devices such as infrared (IR) cameras. In PBF, different surface morphologies can be found during a printing process, including high-temperature printed surface to preheated powder samples. In this work, a method has been developed, including an experimental setup, for determining emissivity maps as a function of three different surface morphologies of metal (pure metals and alloys) printed samples, polished samples, and powders. These emissivity maps will be discussed based on changing temperature and sensor wavelength. The development of the experimental setup includes; designing sample holders for accommodating different types of samples; resistive coil heaters; calibrating the pyrometer with the same optical setup with a blackbody at 1000°C; AC and DC power supply with data logging system; validating the setup with view-factor analysis, and thermal modeling of the radiated intensity of different surfaces at high temperatures. Part of the project was to understand the working principle of the MW pyrometer and to perform a successful demonstration of temperature measurements from raw data files of the pyrometer using an in-house developed MATLAB script with ±1.5°C accuracy over 550°C and a variation of 0.27% with the pyrometer’s reported temperature. The emissivity maps at different temperatures and sensor’s wavelength have been studied for Copper and Inconel 718 samples. The experimental results are discussed with surface images of the printed and polished samples before and after the experiment. The transition temperature where the surface starts acting like a greybody has been addressed for all the morphologies, and the intensity pattern from low to high temperature is also discussed. For both copper and Inconel 718 samples, the highest emissivity values were found for powder samples, and the lowest emissivity values were from the printed samples. This difference in emissivity values is supported by previous literature, where the higher porosity and surface roughness resulted in higher emissivity values. From the results of the polished copper samples, an emissivity value of 0.033 at 552°C was found, which increased to 0.052 at 700°C. For printed copper samples, the emissivity values increased with increasing temperature, from 0.08 at 523°C to 0.106 at 664°C. For copper powder, an emissivity value of 0.307 was found at 490°C. For both solid samples of Inconel 718- as printed and polished, the emissivity values had an increasing trend with increasing temperature up to 700°C. The emissivity values of powder samples showed a decreasing pattern with increasing temperature. Both the solid samples showed a transition from greybody to non-greybody behavior at higher temperatures compared to the powder samples. The transition from greybody to non-greybody behavior occurred at 797.1±25.23°C for polished samples, and 798.51±17.63°C for printed samples, and 588.29±5.69°C for powder samples. For both the polished and printed samples, surface oxidation became apparent, as observed in a distinctive bluish color from the surface of the specimen above 700°C. However, finding reasoning behind the level of oxidation on surface chemistry and how it affects the emissivity behavior needs further investigation and remains as an opportunity for future work.
Shuvo, Md Moinuddin, "Multi-Wavelength Pyrometry for Emissivity Mapping and Accurate Surface Temperature Measurement in Powder Bed Fusion Systems" (2020). ETD Collection for University of Texas, El Paso. AAI28087295.