The dynamic solidification behavior of metal additive manufacturing has a direct impact on the microstructure as built, defects and mechanical properties. In metal additive manufacture, the effects of temperature variation on these features (e.g. solidification front velocity and thermal gradient magnitude) have been extensively studied. Synchrotron imaging has become a crucial tool to monitor this process.
Cornell researchers have used a new method to study the microstructure of a 3D printed metal alloy. They irradiated it with X rays while printing.
The researchers can build specialized materials that integrate such performance-enhancing properties by monitoring how thermomechanical deformation generates localized microscale phenomena like bending, fragmentation, and oscillation in real time.
Atieh Moridi, assistant professor in the Sibley School of Mechanical and Aerospace Engineering at Cornell Engineering and the paper’s senior author, said, “We always look at these microstructures after processing, but there’s a lot of information that you’re missing by conducting only postmortem characterizations. Now we have tools to watch these microstructural evolutions as they are happening.”
“We want to understand how these tiny patterns or microstructures are formed because they dictate everything about the performance of printed parts.”
The group focused on 3D-printing, where a powder is deposition via a nozzle, melted using a high power laser beam, and then cooled and solidified. The powder in this case was a nickel-based alloy called IN625. It is widely used for additive manufacturing and aerospace.
Researchers built a portable version of their 3D-printing system. They brought it to Wilson Laboratory’s Center for High Energy X-ray Sciences, at Cornell High Energy Synchrotron Source CHEXS@CHESS because high-energy X-rays are not accessible in the laboratory.
The CHESS team developed safety protocols to operate a high-power Laser and flammable Powders.
During this experiment, a focused X ray beam was used to travel through IN625 during its heating, melting, and cooling. The diffraction patterns were recorded by a detector located on the opposite side of the printer.
Moridi said, “How these diffraction patterns form gives us much information about the material’s structure. The microstructural patterns are like fingerprints, capturing the history of a material’s processing. Depending on the interaction and what caused it, we get different patterns, and from those patterns, we can back-calculate the material’s structure.”
Researchers usually try to combine diffraction data to analyze it. In this study, the researchers took on a harder task and analyzed the raw images of detectors. This method required more time and work, but revealed a more complete picture of the IN625’s development, including unique features.
The team observed important microstructural properties such as torsion, bending, fragmentation, assimilation, oscillation, and interdendritic development that were produced by the process’ heat and mechanical effects.
The scientists think their method could be used to 3D print other metals such as stainless steel, titanium, high entropy alloys and any material that has a crystalline structure.
The technique can be used to create stronger materials. For instance, pulsing a laser beam would promote crystal fragmentation and decrease grain size, increasing the material’s strength.
Dass said “The final goal is to have the best material system for that particular alloy for a particular application. If you know what is happening during processing, you can choose how to process your materials, so you get those specific features.”
- Dass, A., Tian, C., Pagan, D.C. et al. By operando-x-ray diffraction, dendritic deformations in additive manufacture are revealed. Commun Mater 4, 76 (2023). DOI: 10.1038/s43246-023-00404-0