Experimental Determination of the Emissivity of Powder Layers and Bulk Material in Laser Powder Bed Fusion Using Infrared Thermography and Thermocouples

Abstract

Recording the temperature distribution of the layer under construction during laser powder bed fusion (L-PBF) is of utmost interest for a deep process understanding as well as for quality assurance and in situ monitoring means. While having a notable number of thermal monitoring approaches in additive manufacturing (AM), attempts at temperature calibration and emissivity determination are relatively rare. This study aims for the experimental temperature adjustment of an off-axis infrared (IR) thermography setup used for in situ thermal data acquisition in L-PBF processes. The temperature adjustment was conducted by means of the so-called contact method using thermocouples at two different surface conditions and two different materials: AISI 316L L-PBF bulk surface, AISI 316L powder surface, and IN718 powder surface. The apparent emissivity values for the particular setup were determined. For the first time, also corrected, closer to real emissivity values of the bulk or powder surface condition are published. In the temperature region from approximately 150 °C to 580 °C, the corrected emissivity was determined in a range from 0.2 to 0.25 for a 316L L-PBF bulk surface, in a range from 0.37 to 0.45 for 316L powder layer, and in a range from 0.37 to 0.4 for IN718 powder layer.

Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts

“…The camera was calibrated by its vendor for black body radiation. A temperature adjustment was conducted by a determination of emissivity values of 316L powder layers and 316L L-PBF surfaces for the same set-up in previous work [35]. The camera was sensitive in the spectral range from 2 µm to 5.7 µm.…”

“…The following black body calibration ranges were used at a converter resolution of 14 bit: 60 • C-200 • C at an integration time of 89 µs, 125 • C-300 • C at an integration time of 27 µs, 200 • C-400 • C at an integration time of 193 µs, and 300 • C-600 • C at an integration time of 45 µs. They are referred to the following abbreviation scheme: IR-CB 60-200 for the calibration range of 60 • C-200 • C. The conversion of the received IR signal values (apparent temperatures) into temperatures was conducted using a MATLAB (The Mathworks Inc., Natick, MA, USA) routine considering the experimentally determined apparent emissivity values of 316L powder from previous work [35]. For simplification, a constant apparent emissivity value was used over each respective calibration range, i.e., ε = 0.33 for IR-CB 60-200 and for IR-CB 125-300, ε = 0.43 for IR-CB 200-400 and for IR-CB 300-600.…”

“…For simplification, a constant apparent emissivity value was used over each respective calibration range, i.e., ε = 0.33 for IR-CB 60-200 and for IR-CB 125-300, ε = 0.43 for IR-CB 200-400 and for IR-CB 300-600. Additional information about the temperature adjustment and emissivity determination using this set-up as well as some theoretical background on that matter can be found in [35].…”

“…This value is then defined as current preheating temperature of the respective specimen. The measurement uncertainty of the emissivity determination has to be considered [35].…”

“…The alloy composition of the produced specimens as well as a potential oxygen intake is measured by different methods of chemical analysis. By applying recently conducted temperature adjustments of the infrared (IR) monitoring set-up [35], real temperatures of the powder surfaces could be quantified. The layer-wise increase of the preheating temperatures of the specimens was measured in-situ over the entire build process of the specimens.…”