Using neutrons at the Department of Energy's Oak Ridge National Laboratory (ORNL), 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 and create a roadmap for engineering better armor systems.

The team brought their research to ORNL's High Flux Isotope Reactor (HFIR) because of 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 that other techniques cannot.

Armor is commonly made from steel — a low-cost, high-performance material long familiar to the welding community. For their experiment, the team created samples using rolled homogenous armor and high hardness armor. In armor steel, hardness determines how difficult it is to make an indentation.

Ramirez explained that welding very high-hardness steel softens the material and creates a weak point. Current solutions include adding more material to cover the weak spots, but that adds weight, becomes costly and can be too heavy to transport safely.

To avoid those setbacks, the team used FSW, which differs from conventional welding in that it does not melt the sample. Instead, a rotating tool uses friction to heat the material just enough to join the armor steel samples together.

"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?'" Ramirez said.

When welding, material expands and contracts as it heats and cools, often creating stress around the weld due to internal forces pushing both inward and outward at the same time — 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 residual stress within their samples to see whether FSW would be a plausible alternative 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 visit HFIR and the Spallation Neutron Source (SNS).

"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 advantages HIDRA offers are that it specializes in spatial mapping and is optimized for determining residual stress in materials. During tests, the instruments measure specific three-dimensional sections inside the material known as gauge volumes.

"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," Bunn said. Data collected at HIDRA will be used to validate the team's models. "The next step is asking what we can do to not just predict but control these stresses," Bunn said.

The team will return to HFIR with samples created using conventional welding techniques to compare with the FSW samples. "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 Ohio State.

SNS and HFIR are DOE Office of Science user facilities. UT-Battelle manages ORNL for the DOE Office of Science, the single largest supporter of basic research in the physical sciences in the United States.

— Kaeli Dickert, ORNL