Improving Microstructure Uniformity in Complex Inconel 718 and AlSi10Mg Geometries with PBF-LB/M Feedback Loop Control

In laser-based powder bed fusion of metals (PBF-LB/M), the part geometry influences the heat dissipation from the process zone and the cooling time between layers. Restricted heat flow and short layer times lead to local overheating and corresponding variations in microstructures. To achieve consistent microstructures in complex parts, the process parameters need to be adjusted dynamically based on the thermal conditions. In this study, near infrared (NIR) process emissions are monitored by an off-axis camera (EOS Exposure OT) and a feedback loop control system (Smart Fusion) is employed to correct deviations by adjusting laser power accordingly. Through a pixelwise comparison of the gray values measured in the last layer with a specified target, the controller computes a locally corrected laser power for the next layer. To investigate the effect of this closed loop control on microstructure formation, geometries prone to overheating are manufactured from Inconel 718 and AlSi10Mg. Differences in gray value profiles between the two materials and between the open loop and closed loop processes are analyzed. The controlled processes show a consistent thermal profile over the build height while overheating is found in the uncontrolled state. Porosity, microstructure and hardness in the open loop and closed loop conditions are compared. Microstructures are analyzed by means of optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). In the open loop condition, a significant hardness increase indicative of in-situ precipitation is measured in Inconel 718 samples. In contrast, hardness is reduced in the overheated AlSi10Mg owing to the partially dissolved Si-rich network. In both materials, the feedback loop control leads to more homogenous microstructures and hardness profiles.