Carnegie Mellon University and Argonne National Laboratory Seek to Understand Gas-Driven Micro Defects in Metal AM Parts

Better understanding of "keyhole" formations resulting from boiling metal will lead to better control over laser powder-bed fusion.

Laser powder-bed fusion is a process that plays out at the microscopic level. The microscopic interaction of laser and metal determines the strength and material structure of the part built this way. In the trial-and-error search for a repeatable process for a given part made this way, manufacturers tweak process parameters at the macro level without necessarily knowing the effect at the micro scale.

One organization with tools to look into this is Argonne National Laboratory. Researchers from Carnegie Mellon University along with Argonne researchers have been using high-energy X-rays at Argonne’s Advanced Photon Source to study how microscopic gas pockets form in metal additive parts. These gas pockets—“vapor depression” is a related term—produce small imperfections in metal integrity that might seed significant defects.

“Most people think you shine a laser light on the surface of a metal powder, the light is absorbed by the material, and it melts the metal into a melt pool,” says Anthony Rollet, co-director of the NextManufacturing Center at Carnegie Mellon, in an article by the University’s Karen Mellen. “In actuality, you’re really drilling a hole into the metal.” The effects of the phase change include pressure on the surrounding stock. One outcome of particular interest is the “keyhole” shape resulting from the melt pool departing from its ideal semicircular form. Keyhole melting can lead to microscopic voids.

Mellen writes, “The research shows that keyholes form when a certain laser power density is reached that is sufficient to boil the metal. This, in turn, reveals the critical importance of the laser focus in the additive manufacturing process, an element that has received scant attention so far, according to the research team.”

Understanding this influence is, well, key to controlling keyhole formation. The promise is not just that AM users will understand this effect, the research team says, but also that AM machine makers might offer better control over process variables directly relevant to this phenomenon playing out at the microscopic level of the build.

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