Identification of Shrinkage Porosity in Laser Powder Bed Melting Additive Manufacturing

When William Frieden Dempton, a PhD candidate in mechanical engineering, planned to carefully study how laser powder bed melting process parameters affect microstructure, he never imagined discovering manufacturing defects that had previously been overlooked in PBF-LB additive manufacturing.

Shrinkage porosity is a common defect in metal castings, formed as the metal transitions from liquid to solid. Metal undergoes volume shrinkage during cooling and solidification. If the flow path of liquid metal is blocked by the solidified microstructure, this shrinkage cannot be filled by the remaining liquid metal, and we will obtain a shrinkage porosity.

"These defects occur at the scale of the microstructure, and if you don't anticipate them, it's really difficult to detect," Frieden Templeton said. Using an optical microscope, they usually look like small polishing scratches.

Due to the layer by layer PBF-LB printing process, remelting can remove these shrinkage holes, or if they are closer to the surface of the part, they can be removed during post-printing processing, making these defects no problem. However, problems arise when these pores are formed deep enough that the next metal layer cannot remove them during the remelting process.

"This is the time for anyone to explain the occurrence of shrinkage porosity based on solidification and L-PBF processing principles," said Sneha Narra Prabha Narra, Assistant Professor of Mechanical Engineering.

In addition, we can also map it as a function of processing conditions and present this information in a form that is easy for researchers and engineers to explain during the process of developing process parameters. This is possible because this project has interdisciplinary and collaborative properties.

Frieden Dempton is studying a solidification process course taught by co author and materials science and engineering professor Chris Pistorius; Around the same time, he was describing the samples involved in this work, which allowed him to quickly establish a connection between his course assignments and research.

"This is a suitable example of what happens when students are willing to apply their graduate courses to their ongoing research," said Narra. This situation often occurs in CMU.

Understanding the mechanism of defect formation enables us to propose mitigation strategies“
In recent years, significant progress has been made in the development of process parameters for PBF-LB, enabling the process to achieve consistent high part quality. Although the results of this work may not change the previously established process parameters, as most are created at low printing temperatures. In future work, manufacturers will need to pay attention to this defect in additive manufacturing.

Frieden Templeton said, "This will have a special impact on researchers and manufacturers who are committed to developing process parameters for printing at temperatures close to 500 ° C and printing complex geometric shapes that are susceptible to local temperature accumulation.".

In addition to our research findings, I would also like to emphasize how important it is for researchers to maintain an open mindset when collecting and analyzing data. We did not expect to observe shrinkage porosity when we started this project, as it is not often mentioned in existing literature.

This study was conducted in collaboration with researchers from the University of Pittsburgh; Albert To, Shawn Hinnebuch, and Seth Strayer used their additive manufacturing simulations to design these experiments and manufactured these samples as part of a continuous project supported by NASA.

The work was published in Acta Materia magazine.

Source: Laser Net