A Novel Methodology for the Thermographic Cooling Rate Measurement during Powder Bed Fusion of Metals Using a Laser Beam

Powder bed fusion of metals using a laser beam (PBF-LB/M) is a process that enables the fabrication of geometrically complex parts. In this process, a laser beam melts a metallic powder locally to build the desired geometry. The melt pool solidifies rapidly, which results in high cooling rates. These rates vary during the process in line with the geometric characteristics of the part, which leads to a non-uniform microstructure along with anisotropic mechanical properties. The unknown part characteristics prevent the process from being used in safety-critical applications. Thermographic in situ process monitoring provides information about the thermal field, enabling predictions of the resulting material properties. This study presents a novel methodology for the thermographic measurement of cooling rates during the PBF-LB/M process using a high-speed thermographic camera. The cooling rates occurring during the manufacturing of 316L tower-like specimens were measured. The cooling rate decreased with increasing build height, due to the heat accumulation in the parts. The microhardness profile of the parts was tested perpendicularly and parallel to the build direction. A significant decrease in hardness values was observed along the build height. The measured cooling rate was correlated to the microhardness profile of the specimens using a Hall–Petch type relationship. The results show a high level of reproducibility of the cooling rates between different specimens in the same build job as well as between subsequent build jobs. The presented methodology allows studying the effects of the geometry on the cooling rates and the resulting mechanical properties of 316L specimens.