Graduate Student Seminar

Keyhole dynamics in laser powder bed fusion additive manufacturing 

Laser powder bed fusion (LPBF) is the most extensively used metal additive manufacturing technology due to its unique capabilities in building parts with high geometric complexity and fine features. During the LPBF process, sparks (i.e., spattered particles) can be observed following the laser scanning path, indicating the presence of high-velocity vapor arising from the melt pool. Indeed, strong metal vaporization occurs in LPBF, resulting in recoil pressure that creates a depression in the melt pool, often referred to as a keyhole. The keyhole is an important dynamic structural feature in LPBF as it influences energy coupling, metal melting mode, and defect generation. Without a keyhole, the laser is absorbed by the metal surface only once, with significant reflection. However, with the presence of a keyhole, multiple laser absorption events occur, significantly increasing laser absorption efficiency. The laser melting mode can shift from conduction to transition, stable keyhole, and then unstable keyhole as the energy input increases. When the conduction mode is applied, lack-of-fusion voids may be generated during the build. An unstable keyhole condition also leads to porosity.

In this seminar, I will present the direct observation and quantitative measurement of keyhole dynamics using the high-speed, high-resolution synchrotron x-ray imaging technique. The characteristics of stable and unstable keyholes will be elucidated through x-ray visualization and multiphysics simulation. Lastly, I will introduce a novel approach based on thermal imaging and machine learning for localized, real-time detection of keyhole pore generation during the LPBF process. 

Tao Sun, Northwestern University

Dr. Tao Sun is an Associate Professor of Mechanical Engineering at Northwestern University. He obtained his B.S. and M.S. degrees in Materials Science and Engineering (MSE) from Tsinghua University and completed his Ph.D. degree in MSE at Northwestern University. Following his graduation, Dr. Sun embarked on postdoctoral research at Argonne National Laboratory, where he was later promoted to the positions of Assistant Physicist and then Physicist. In 2019, he commenced his academic career at the University of Virginia and has recently relocated to Northwestern. Dr. Sun possesses extensive expertise in additive manufacturing (AM), sophisticated instrumentation, and x-ray science. His research team is dedicated to comprehending the underlying physics involved in energy-matter interactions, heat and mass transfer, multiphase flow, and non-equilibrium material evolution within AM processes. His research underscores the importance of fundamental studies in manufacturing science and emphasizes the significant role of state-of-the-art in situ characterization and monitoring techniques.