Towards the determination of real process temperatures in the LMD process by multispectral thermography

Additive manufacturing of metals offers the opportunity to build parts with a high degree of complexity without additional costs, opening a new space for design optimization. However, the processes are highly complex and due to the rapid thermal cycles involved, high internal stresses and peculiar microstructures arise, which influence the parts mechanical properties. To systematically examine the formation of internal stresses and the microstructure, in-process spatially resolved measurements of the part temperature are needed. Usually, thermography is used to measure temporally resolved thermal fields. The thermal cameras are calibrated at black body reference radiators (unity emissivity) for the conversion of the measured thermal radiation intensity to temperatures. If the emissivity of the inspected part is known, its thermodynamic temperature can be reconstructed by a suited radiometric model. However, in additive manufacturing of metals, the emissivity of the part surface is strongly inhomogeneous and rapidly changing due to variations of, e.g., the degree of oxidation, the material state and temperature. However, measuring the process thermal radiation at different wavelengths simultaneously enables one to separate temperature and emissivity spatially resolved to obtain further insight into the process. Here, we present results of a study using multispectral thermography to obtain real temperatures and emissivities in the laser metal deposition (LMD) process. For a better understanding of the basic processes, the measurements have been performed first without powder supply and by recording images at different wavelength in subsequent runs.