VIRTUAL

 

Thursday, December 16th    2021

3:45 – 4:45 pm (MT)

WEBEX

 

Speaker:  Prof. Anthony D. Rollett

Carnegie Mellon University 

 

“The Physics of 3D Printing with Laser Powder Bed Fusion”

 

 

Abstract: 

Superficially, 3D printing metal parts with the Laser Powder Bed Fusion (LPBF) method is as straightforward as arranging for a uniform layer powder to be spread in each cycle and a laser to scan over the surface wherever required by the design to melt the metal powder.  As such, the process is sometimes described as micro-welding, which, as a link to a long-established technology, implies that all the physics has long been worked out.  There are, however, a few issues that may intrigue a curious mind. LPBF readily prints parts to a resolution of ~100 microns, not much larger than the maximum powder size: this requires the ~500 W laser beam to be focused down to the same diameter spot.  At over 1 MW/cm2, the power density is enough to reach the boiling point and produce a vapor jet (then, plume) whose reaction force drills a hole in the liquid metal pool, aka a keyhole (in welding parlance). In the range used by current LPBF printers, the keyhole shape varies from long and deep at the low end of typical scan speeds (say, 0.3 m/s) to shallow at high speed (say, 1.0 m/s), all of which is readily reproduced by computational fluid dynamics.  Below a critical threshold (mainly in speed), however, the keyhole goes unstable and sheds bubbles from its base that can be trapped in the oncoming solidification front thereby introducing a substantial density of defects in the printed part that are deleterious to the mechanical properties.  This instability is crucial for production of good quality parts but lacks a quantitative description.  Moreover, as reported by Zhao et al. Science 2020), the observations of sudden movement of individual bubbles away from the base of a keyhole as it collapses from time to time suggests that an acoustic shock wave akin to cavitation contributes to bubble trapping. As a further example of challenges, increasing the productivity of LPBF systems would have us operate at high speed and high power but the physics of heat conduction mean the that melt pool may have the desired width of ~200 microns but extends to a millimeter or more behind the heat source. The large unidirectional extent of the liquid pool allows further instabilities and the occurrence of balling or humping, i.e., a tendency for the meltpool to break up into separate drops. Although this was thought of in terms of a Plateau-Rayleigh instability, it appears likely that the flow pattern in the melt pool influences this phenomenon.  Beyond this unsolved problem, there are questions about the formation of dislocations and when exactly this occurs and what controls the length scale of the cellular structures observed.  All these puzzles will be placed in the context of the practical value of 3D printing and how it is working its way into a wide variety of applications 

Bio:

Rollett has been a member of the faculty at Carnegie Mellon University since 1995, including five years as Department Head. He is the Co-Director of the NextManufacturing Center on additive manufacturing. Previously, he worked at the Los Alamos National Laboratory. There, he was Group Leader of Metallurgy 1991-1994 and Deputy Division Director of Materials Science & Technology for a year after that. He has been a Fellow of ASM since 1996, Fellow of the Institute of Physics (UK) since 2004 and Fellow of TMS since 2011. He received the Cyril Stanley Smith Award from TMS in 2014, was elected as Member of Honor by the French Metallurgical Society in 2015, and became the US Steel Professor of Metallurgical Engineering and Materials Science in 2017. He received Cyril Stanley Smith Award from the International Conference on Recrystallization and Grain Growth in 2019 and also the International Francqui Professor for 2020-2021, from the Francqui Foundation, Belgium. His research group is supported by industry, several Federal research agencies, and the Commonwealth of Pennsylvania. He is a member of the Basic Energy Science Advisory Committee and the Defense Programs Advisory Committee under the Dept. of Energy. His lecture notes on texture and anisotropy are widely known and used and he started a new course on Additive Manufacturing and Materials in 2016. He started a new Masters program in Additive Manufacturing in the Fall of 2018.

Rollett's research focuses on microstructural evolution and microstructure-property relationships in 3D, using both experiments and simulations. Interests include 3D printing of metals, materials for energy conversion systems, strength of materials, constitutive relations, microstructure, texture, anisotropy, grain growth, recrystallization, formability and stereology. Relevant techniques include high performance spectral methods in micro-mechanics, dynamic x-ray radiography (DXR) and high energy diffraction microscopy (HEDM). Important recent results include definition of process windows in 3D printing through characterization of porosity, 3D comparisons of experiment and simulation for plastic deformation in metals, the appearance of new grains during grain growth, and grain size stabilization. He has 250 peer-reviewed journal publications with an h-index of over 50.