Using neutrons at the Department of Energy's Oak Ridge National Laboratory, researchers from The Ohio State University are studying residual stress caused by friction stir welding (FSW) to reveal how to strengthen armor steel welds, like those used in military vehicles. Their findings were published in the Journal of Materials Processing Technology.
"We are trying to develop a new way to join armor steel that produces better ballistic and blast performance joints," said Antonio Ramirez, a professor of Materials Science and Engineering at OSU. "In the end, we want to be able to make structures that perform better."
The team's results will help fine-tune welding parameters to create a roadmap for engineering better armor systems.
They brought their research to ORNL's High Flux Isotope Reactor (HFIR) due to the facility's neutron production capabilities. Operating at 85 megawatts, HFIR's steady-state neutron beam is the strongest reactor-based neutron source in the United States. Neutrons can penetrate deeply and offer information other techniques cannot.
Identifying how to create stronger steel armor
Armor is commonly made from steel, which is a low-cost, high-performance material that has been around for many years and is no stranger to the welding community.
For their experiment, the team created samples using armor steel - specifically, rolled homogenous armor and high hardness armor. In armor steel, there is a property called hardness that determines how difficult it is to make an indentation.
Ramirez explained that the best way to join steel is by welding, but welding very high-hard steel softens the material and creates a weak point. Current solutions include adding more materials to the structure to cover up the weak spots, but Ramirez explained this method adds additional weight and can become too heavy to transport safely, not to mention that it's costly.
To avoid these potential setbacks, the team used FSW, which differs from conventional welding in that it doesn't melt the sample. Instead, it utilizes a rotating tool that uses friction to heat up material just enough to join the armor steel samples together.
Ramirez said, "So, our study began with the question, 'How can we optimize the parameters of FSW with other technologies to minimize the softening region and make better-performing armored steel joints?'"
Measuring residual stress with neutrons
When welding, material expands and contracts as it heats up and cools down. This process often creates stress around the area where the weld was created due to internal forces pushing both inward and outward at the same time, creating something called residual stress.
"Residual stress plays a very important role in a material's fatigue," said Ramirez. "Fatigue is failure that starts on the surface and grows like a little crack, ultimately breaking apart because the material literally gets tired over time."
The team wanted to measure the residual stress within their samples to see if FSW would be a plausible alternative route to conventional welding. Their research brought them to the High Intensity Diffractometer for Residual Stress Analysis (HIDRA), a neutron diffraction instrument at HFIR.
Completed in 1965, HFIR is a user facility that houses 12 neutron scattering instruments. Each year, hundreds of researchers like Ramirez and his team visit the facility, as well as the Spallation Neutron Source (SNS), to perform neutron scattering experiments.
Why neutrons?
To best measure residual stress found in their samples, the team needed neutrons.
"Neutrons can penetrate deep within the material and offer insights that might not be possible using other techniques," said Jeffrey Bunn, a neutron scattering scientist and lead instrument scientist at HIDRA.
Two big advantages HIDRA offers are that it specializes in spatial mapping and is optimized for determining residual stress in materials.
During these tests, the instruments measure specific, three-dimensional sections inside the material known as gauge volumes, or a region of the sample averaged over in the mapping.
"We are unique because our location at HFIR provides a high neutron flux, allowing HIDRA to measure much smaller gauge volumes than other lower-flux neutron instruments," said Bunn.
The team is working to develop models, and the data collected at HIDRA will then be used to validate those models.
"The next step is asking what we can do to not just predict but control these stresses," said Bunn.
What comes next?
The team will return to HFIR with samples created using conventional welding techniques to compare with the samples created using FSW.
"We're going to experiment with our collaborators to do actual ballistic testing, and with that we will know the effect of the residual stresses and the ballistic performance of the material," said Jhoan Guzman, a graduate research associate in Materials Science and Engineering at The Ohio State University.
SNS and HFIR are DOE Office of Science user facilities.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science - Kaeli Dickert
