Approximation of absolute surface temperature measurements of powder bed fusion additive manufacturing technology using in situ infrared thermography

Additive manufacturing (AM) has several possible advantages over traditional manufacturing including increased design freedom, reduced material usage, and shorter lead-times. A noteworthy capability of AM is the ability to monitor the process during material deposition and interrupt the process during fabrication if necessary. Recently, such monitoring, feedback, and control have been made possible by implementing in situ infrared (IR) thermography in powder bed fusion AM technologies. The purpose of the current research was to investigate the acquisition of absolute surface temperatures using in situ IR imaging of the melted or solid surfaces layer-by-layer during fabrication within an electron beam melting (EBM) system. The thermal camera was synchronized with the system's signal voltages of three synchronized events (pre-heating, melting, and raking) to automatically capture images. To acquire absolute temperature values from the IR images, a calibration procedure was established to determine the solid material's emissivity and reflected temperature or mean radiant temperature of the build chamber, which are necessary input parameters for the IR camera. A blackbody radiator was fabricated via EBM and was used as a tool to determine the emissivity of Ti-6Al-4V (determined to be 0.26 in the temperature range of the current study). Furthermore, a mathematical model was developed to determine the view factors associated with the system's interior (e.g. heat shielding) that were used in calculating the mean radiant temperature of the manufacturing environment (∼342 °C). Experimental validation of the model was performed using a thermocouple embedded during fabrication that showed a 3.77% difference in temperature. A temperature difference of ∼366 °C (1038 °C vs. 672 °C) was observed when comparing uncorrected IR temperature data with corrected temperature data. Upon validation of the IR parameters for a melted area, experimentation was conducted to also determine powder emissivity (found to be 0.50). The thermal model presented here can be modified and implemented in other AM technologies for consideration of radiation energy to acquire absolute temperatures of layered surfaces, leading to improved thermal monitoring and control of the fabrication process.