In ‘Pore elimination mechanisms during 3D printing of metals,’ researchers from around the world explore more about refining laser powder bed fusion (LPBF) technology through the use of imaging and modeling techniques meant to prevent porosity.
While 3D printing with metal has become extremely popular, and especially within the industrial realm, there are still numerous challenges to overcome before many users are able to completely embrace such processes. In this research, the authors use in-situ high-speed high-resolution synchrotron x-ray imaging and multi-physics modeling to understand more about how to control porosity issues as they so often occur in the melt pool and are then found in parts.
It is often difficult for users to take measures to reduce porosity through post-processing:
“For example, the hot isostatic pressing (HIP) cannot close the surface pores; and the gas pores closed by HIP can reopen and grow during subsequent heat treatment,” state the researchers, who began looking for the most logical route to fix the problem—during the printing process.
Pores exhibiting high velocity and small size make it hard for researchers to examine their motion while the printing process ensues. X-rays have been somewhat successful previously, but with inferior resolution. The research team discovered that pores move not only according to temperature and thermocapillary force, but also drag force caused by the melt flow. Further, they found that the high thermocapillary force could overcome the drag force, thus halting pores from forming during printing.
In situ X-ray imaging consisted of the following:
- Powder bed system – 100 µm layer of powder on a substrate sandwiched between two glassy carbon plates
- Selective laser melting system (for scanning the powder bed and forming a melt pool)
- High-speed X-ray imaging system – showing the LPBF process
“To study the effect of thermocapillary force on pore dynamics in different locations of the entire melt pool, we developed a force map based on the ratio of thermocapillary force to drag force (Ft/Fd) for a 10 μm-dimeter pore, using the local temperature gradient and the average velocity of the melt flow (1.1 ± 0.5 m s−1). The buoyant force is neglected because it is orders of magnitude smaller than the thermocapillary force and the drag force for the pore size range studied here,” stated the researchers.
Experiments were also performed to see how this process would affect 3D printing not driven by powder. The results showed ‘similar pore motion behaviors.’
Overall, the researchers found that they were able to use thermocapillary force to eliminate pores while LPBF was in progress. They demonstrated this during printing, as well as showing that pores in previous layers were eliminated by ‘thermocapillary force by laser rescanning with proper laser scan parameters.’
The researchers expect this study to have positive implications for applications like:
- Laser polishing
- Laser cladding
- Melt spinning
- Nuclear reactors
- Chemical reactors
“We achieved pore elimination by thermocapillary force in both alloys, which indicates that the thermocapillary force driven pore elimination mechanism is not limited to a specific alloy system,” stated the researchers.
Porosity is an ongoing topic of research 3D printing users continue to refine manufacturing processes, and especially in metal—but also extrusion-based printing, FDM 3D printing, and in bioprinting too. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.[Source / Images: ‘Pore elimination mechanisms during 3D printing of metals’]
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