Revealing melt pool dynamics and mechanisms of powder spattering in laser powder bed fusion of nickel-based superalloy by in-situ high-speed synchrotron and optical imaging

Spatter formation during laser powder bed fusion (LPBF) process of nickel-based superalloys detrimentally affects the processing reliability and consequently impacts part quality through defects and powder bed contamination. However, due to high-Z characteristics of superalloys, directly observing melt pool dynamics and related mechanisms of powder spattering and evolution remains challenging and less understood. This study presents a comprehensive in-situ investigation into powder spattering down to melt pool scale during LPBF of nickel-based GH4169 superalloy under various processing parameters by utilizing high-energy high-speed synchrotron X-ray imaging and high-speed optical imaging. Four distinct stages of droplet formation, void formation, molten liquid fluctuation and stabilization are revealed during melt pool formation process and periodic forming and wetting behavior of large droplets are characterized in subsequent stable stage. Five types of spattering formation associated with laser-irradiated dominant forces are established: gas-flow entrainment from distal powder bed region, gas-flow entrainment from proximal powder bed region, droplet separation from melt pool, direct acceleration by vapor plume, and initial gas-flow entrainment followed by vapor-plume acceleration. The spattering characteristics are profoundly dependent on keyhole fluctuation by tailoring processing parameters of laser power and scan speed. The periodic vortices in front of and behind the laser beam are experimentally captured and quantitatively characterized, arising from the entrainment effect of the ejecting vapor plume. These findings establish critical relationships between processing parameters and spattering dynamics, provide in-depth insights into the mechanisms of spattering formation and evolution, and offer effective guidance for spatter suppression and process optimization related to melt pool dynamics during the LPBF process of superalloys.