Monitoring of Laser Powder Bed Fusion with integrated Laser Heat Treatment based on in-process Eddy-Current Measurements

In additive manufacturing using powder bed fusion by laser beam melting of metal (PBF-LB/M), the formation of undesirable residual stresses during the manufacturing process still presents a challenge, particularly when processing heat-resistant materials such as highperformance steels. Within the framework of the BMBF project ADDAX, a process control is being developed aimed to generate advantageous residual stress distributions. This is achieved using integrated laser heat treatment to produce distortion-and crack-free components without the need for support structures, additional build chamber heating, or additional post-process heat treatment of the component. To control the laser heat treatment, in-process monitoring of local microstructural changes and changes in the residual stress state during the PBF-LB/M process is required using suitable sensors integrated into the build chamber. Eddy-current technology is generally suitable for this task in case of ferromagnetic materials, as microstructural transformations and residual stress changes in the material lead to changes in its magnetic permeability and electrical conductivity, which can be detected in the eddy-current signal. A particularly challenging aspect is that the eddycurrent sensor system has to be integrated into the build chamber of the PBF-LB/M machine in order to capture the local microstructural transformations and associated residual stresses during the layer build-up. This article presents initial results from experimental studies in which various eddy current measurement systems were used to monitor the local laser heat treatment. It was found that the residual austenite content in the heat-treated zone can be determined. Scanning eddy-current measurements were used to capture the spatial extent of the heat treated zone. Furthermore, the eddy current testing was statistically validated, revealing that certain measurement frequencies yield particularly good results.