Researchers at the Lawrence Livermore National Laboratory (LLNL) have taken a major step toward answering a question plaguing a common metal 3D-printing technique: What interactions can lead to the porosity found in parts produced by laser powder-bed fusion processes?

LLNL researcher Ibo Matthews and his team discovered that gas flow, due to evaporation when the laser irradiates the metal powder, is the driving force that clears away powder near the laser’s path during a build. This “denudation” phenomenon reduces the amount of powder available when the laser makes its next pass, causing tiny gaps and defects in the finished part.

(L-r): LLNL researchers Ibo Matthews, Gabe Guss and Phil Depond examine a 3D-printed set of metal test cubes. Image credit: LLNL.(L-r): LLNL researchers Ibo Matthews, Gabe Guss and Phil Depond examine a 3D-printed set of metal test cubes. Image credit: LLNL. “During this process, you get to temperatures that are near or at the boiling point of the metal, so you have a strong vapor flux emitted from the melt pool,” Matthews explains. “Prior to this study, there wasn’t an understanding of what effect this flux of metal vapor had on the powder bed.”

Using a custom-built microscope setup, a vacuum chamber and an ultra-high-speed camera, Matthews’ team observed the ejection of metal powder away from the laser during the melting process and, through computer simulation and fluid dynamics principles, built models to help explain the particles’ movement.

“[Matthews] has discovered a phenomenon that we didn’t know was present in metal powder-bed additive manufacturing, and this is an effect that has important implications for part quality and build speed,” says Chris Spadaccini, director of additive manufacturing initiatives at LLNL. “It is also something we now know we will have to capture with our models, so new physics is being added to the simulation codes.”

“It’s one of those things that nobody really had a clue about why it happened,” says Wayne King, director of the Accelerated Certification of Additively Manufactured Metals project at LLNL. “What this [research] does is bring us closer to an understanding of the process, which will eventually lead to a reduction in defects of parts. The [updated] models should help us optimize the process and give us the best chance at getting the best part.”

Matthews says the next steps are to investigate how porosity develops in real time and, using the new information, explore both advanced diagnostics and modifications to the process for improving build quality.

“Now, having the physics better understood, we can simulate the process more accurately and make enhancements to our manufacturing efforts,” Matthews says. “In the end, we want to be able to use simulation to build the confidence that we’re making parts with little or no defects.”

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