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Illustration of shrinkage porosity formation in solidifying metal. (a) Dendritic solidification with an adequate thermal gradient and cooling rate. (b) Flow channels between dendrites are blocked by secondary dendrites at an excessively low thermal gradient and/or an excessively high cooling rate. (c-d) Backscattered electron micrographs showing shrinkage pore networks in PBF-LB samples of Alloy 718, the laser scan direction is in and out of the frame, and build direction is the same for both micrographs.
© Acta Materialia (2023). DOI: 10.1016/j.actamat.2023.119632
Shrinkage Porosity

Carnegie Mellon Engineering Team Uncovers Overlooked Defect in Additive Manufacturing

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Autor: B S

Datum: 26. Feb. 2024

February 2024 | A study conducted by William Frieden Templeton, a Ph.D. candidate in Mechanical Engineering at Carnegie Mellon University, has led to the identification of shrinkage porosity in laser powder bed fusion (PBF-LB) additive manufacturing.

Shrinkage porosity, a defect well-known in metal castings, occurs during the metal’s transition from liquid to solid state, leading to volumetric contraction. This defect, typically challenging to detect, emerges when liquid metal cannot backfill the contracting volume due to blocked flow paths by the solidifying microstructure.

The results will particularly impact researchers and manufacturers working towards developing process parameters for printing at high temperatures closer to 500°C and printing complex geometries susceptible to local temperature build-up.

Carnegie Mellon Engineering Team Uncovers Overlooked Defect in Additive Manufacturing: Graphical abstract

Graphical abstract © Acta Materialia (2023). DOI: 10.1016/j.actamat.2023.119632

 

Importance of keeping a open mind

“These defects occur on the scale of the microstructure and are really hard to spot if you aren’t expecting them,” said Frieden Templeton. “Using a light optical microscope, they often look like small polishing scratches.”

“Beyond our research findings, I want to emphasize how important it is for researchers to keep an open mind when collecting and analyzing data. We started this project with no expectation to observe shrinkage porosity because it’s not often mentioned in existing literature.”

Frieden Templeton was taking a solidification processing course taught by co-author Chris Pistorius, Professor of Materials Science and Engineering; around the same time, he was characterizing the samples involved in this work, which enabled him to make the connections between his coursework and research quickly.

A previously overlooked defect

This discovery is significant as it highlights a manufacturing defect in PBF-LB additive manufacturing previously overlooked, especially concerning because these pores can be difficult to eliminate if they form deep enough that subsequent metal layers cannot remelt them. The research team, which included collaborations with the University of Pittsburgh’s Albert To, Shawn Hinnebusch, and Seth Strayer, was able to map shrinkage porosity as a function of processing conditions, offering a valuable resource for engineers and researchers to optimize process parameters.

The study was conducted under the guidance of Sneha Prabha Narra, Assistant Professor of Mechanical Engineering, and in collaboration with Chris Pistorius, Professor of Materials Science and Engineering. The findings, published in the journal Acta Materialia, underscore the importance of interdisciplinary collaboration and the application of academic coursework to research. They suggest that while current process parameters, typically set at lower printing temperatures, may not need adjustments, future research and development efforts, particularly those aiming for high-temperature printing and complex geometries, should consider the implications of shrinkage porosity.

The study, supported by NASA and leveraging additive manufacturing simulations from the University of Pittsburgh team, not only advances our understanding of defect formation in PBF-LB additive manufacturing but also proposes mitigation strategies to improve part quality. This research emphasizes the necessity of keeping an open mind during data collection and analysis, as it was initiated with no prior expectation to identify shrinkage porosity, a defect seldom mentioned in existing literature.

More information

The paper can be openly accessed at:

William Frieden Templeton et al, A mechanistic explanation of shrinkage porosity in laser powder bed fusion additive manufacturing, Acta Materialia (2023). DOI: 10.1016/j.actamat.2023.119632

 

(Provided by Carnegie Mellon University Mechanical Engineering/2024)