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LLNL researchers discover cause of porous defects in laser powder-bed fusion

Researchers at Lawrence Livermore National Laboratory (LLNL) have identified the cause of porous defects in parts produced by laser powder-bed fusion 3D printing processes.

The research team, led by LLNL researcher Ibo Matthews, has discovered that evaporation caused by the laser irradiating the metal powder produces a gas flow that acts as a driving force, clearing away powder near the laser’s path during a build. This reduces the amount of powder available when the laser makes its next pass, causing tiny gaps and defects in the finished part. The team’s findings were published in the online journal Acta Materialia on 20 May.

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.

‘During this process you get to temperatures that are near or at the boiling point of the metal, so you have a strong vapour flux emitted from the melt pool,’ Matthews explained. ‘Prior to this study, there wasn’t an understanding of what effect this flux of metal vapour had on the powder bed.’

Chris Spadaccini, director of additive manufacturing initiatives for the lab, commented on the team’s findings: ‘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. 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.’

Matthews said 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 said. ‘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.’

The Laboratory Directed Research and Development programme funded the research.

Further Information
www.llnl.gov

Acta Materialia Paper

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