Speaker
Description
Temperature represents a key characteristic in additive manufacturing (AM) processes for metals. As a physical quantity, temperature provides a direct measure of the actual process state and can be an indicator for the process quality. Consequently, process evaluation based on temperature measurements, rather than solely on the monitoring of process radiation in gray values, is expected to offer increased robustness and explanatory power. However, dependable quantitative in-situ temperature measurements remain highly challenging due to extreme temperature gradients, emissivity changes and, in case of the widely used process laser powder bed fusion of metals (PBF-LB/M), due to the required high spatial and temporal resolution. To nevertheless obtain reliable temperature measurements, besides other factors, a robust thermal calibration of the measurement system is fundamental.
The de facto standard for the thermal calibration of thermal camera systems is the use of black-body radiators, as set out in several technical norms and standards (among others: ASTM E1933, IEC 62942, VDI 5585, VDI 3511). However, especially in the visible and near-infrared (VIS-NIR) range, this is not always feasible in practical applications. Apart from the high costs of calibrated blackbody radiators, their calibration is usually not valid in this wavelength range and significant deviation to black body radiation occurs. Furthermore, their physical size usually prohibits use within a PBF-LB/M build chamber, so it is not possible to calibrate the entire optical path with this type of calibration source. This limitation is exacerbated by machine- and system-specific optical interfaces (e.g., protective windows and viewports) that must be traversed in operation and can alter transmission and spectral response. In-situ solutions are therefore recommended, as these enable calibration of the complete on-machine optical path.
In this contribution, three different proof-of-concept approaches for the thermal calibration of monitoring systems for the PBF-LB/M process are presented and compared: 1) Calibration using blackbody-like radiation sources - For this purpose, the suitability of the pipetting hole of the graphite tube of a graphite furnace atomic absorption device and a graphite cylinder heated by the PBF-LB/M process laser with a suitable borehole are examined. 2) Calibration using a calibrated halogen light source and an isotopic (Ulbricht) sphere with a known color temperature – as special case for the VIS/NIR range and 3) the single-point calibration at the solidification plateau of molten metal samples – the melting also takes place within the PBF-LB/M process chamber using the process laser. While, as described, some of the approaches presented here are ex-situ, others can be carried out in situ in the PBF-LB/M process chamber, allowing the calibration of the complete optical path. As a ground-truth, additional measurements at a calibrated blackbody radiator were performed.
The calibration approaches are tested using a Multispectral Optical Tomography (MS-OT) sensor system. MS-OT operates in the VIS-NIR range and constitutes an approach for determining apparent maximum surface temperatures Tmax during the PBF-LB/M process.
