Scientists at Oak Ridge National Laboratory (ORNL), located in Oak Ridge, Tennessee, USA, have demonstrated their ability to measure strain in a material as it changes over time and observe the movement of atoms in response to stress through OpeN-AM platform. The findings were published in Nature Communications.
OpeN-AM can measure evolving residual stress during manufacturing using the VULCAN beamline at ORNL’s Spallation Neutron Source (SNS), which is a US Department of Energy (DOE) Office of Science user facility. By combining this system with infrared imaging and computer modelling, researchers have gained insight into material behaviour during manufacturing. In this case, researchers utilised low-temperature transformation (LTT) steel and physically measured the movement of atoms in response to stress, such as temperature or load.
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Residual stresses are the stresses that remain in a material even after the load or the cause of the stress is removed. These stresses can deform the material and, in some cases, lead to premature failure. Dealing with residual stresses is a significant challenge when it comes to fabricating components that have accurate dimensions and desired properties and performance.
The scientists conceived and, over a two-year period, conducted an experiment that can measure strain in the material as it evolves. This measurement determines how stresses will be distributed.
“Manufacturers will be able to tailor residual stress in their components, increasing their strength, making them lighter and in more complex shapes. The technology can be applied to anything you want to manufacture,” stated Alex Plotkowski, materials scientist in ORNL’s Materials Science and Technology Division and the lead scientist of the experiment.
“We have successfully shown that there is a way to do that,” he added. “We are demonstrating we understand connections in one case to anticipate other cases.”
The scientists utilised a custom Wire Arc Additive Manufacturing (WAAM) platform to conduct operando neutron diffraction of the LTT metal at SNS. They processed the steel using SNS’s VULCAN beamline and collected data at different stages throughout the manufacturing process and after the material cooled to room temperature. To validate the findings, they combined the diffraction data with infrared imaging. The Manufacturing Demonstration Facility (MDF), a user consortium of the DOE Advanced Materials and Manufacturing Technologies Office, designed and constructed the system. A replicate platform was also created at MDF to plan and test experiments before conducting them at the beamline.
SNS operates a linear particle accelerator that generates beams of neutrons for studying and analysing materials at the atomic scale. The research tool they have developed enables scientists to observe the internal structure of a material as it is being produced.
The LTT steel was melted and deposited in layers. As the metal solidified and cooled, its structure underwent a phase transformation, a process in which atoms rearrange and occupy different positions, resulting in a change in the material’s behaviour.
Usually, it is difficult to comprehend transformations that occur at high temperatures by simply examining a material after it has been processed. However, through their experiment on LTT steel, the scientists have demonstrated that they can comprehend and control the phase transformation by observing it during processing, thus understanding what stresses occur, how they occurred and how to control them.
“These results provide a new pathway to design desirable residual stress states and property distributions within additive manufacturing components by using process controls to improve nonuniform spatial and temporal variations of thermal gradients around key phase transformation temperatures,” the authors wrote.
The scientists recently earned a 2023 R&D 100 Award for this technology. R&D World magazine announced the winners in August. Plotkowski and other winners will be recognised at the organisation’s award ceremony on November 16 in San Diego.
This research was funded by ORNL’s Laboratory Directed Research and Development programme. This programme supports high-risk research and development in areas that have the potential for high value to national programs.
Co-authors include ORNL’s Chris Fancher, James Haley, Ke An, Rangasayee Kannan, Thomas Feldhausen, Yousub Lee, Dunji Yu and Joshua Vaughan; University of Tennessee-ORNL Governor’s Chair Suresh Babu; and former ORNL researchers Kyle Saleeby, Guru Madireddy and C Leach.
The full article is available here.