Researchers at Pacific Northwest National Laboratory )PNNL) have developed and successfully tested a new solid-phase technique called Friction Stir Dovetailing (FSD) to join thick plates of aluminum to steel. The new process will be used to make lighter-weight military vehicles that are more agile and fuel efficient. An open-access paper on the work is published in the journal Scripta Materialia.
In FSD, mechanical interlocks are formed at the aluminum-steel interface and are reinforced by metallurgical bonds in which intermetallic growth has been uniquely suppressed. Lap shear testing shows superior strength and extension at failure compared to friction stir approaches where metallurgical bonding is the only joining mechanism.
|Illustration of FSD technique and tooling showing mechanical interlocking and metallurgical bonding in a dovetail groove. Reza-E-Rabby et al. Click to enlarge.|
According to the US Government Accountability Office, the military spends several billion dollars each year on fuel consumption, which could be reduced by lessening the weight of ships, aircraft, ground vehicles, and cargo.
To lower fuel costs and increase operational effectiveness while still maintaining the safety of military personnel, the US Army Tank Automotive Research Development and Engineering Center (TARDEC) launched a campaign in 2014 seeking ways to make combat systems—such as tanks, fighting vehicles, and personnel carriers—more lightweight.
One approach they are investigating is the replacement of heavy steel components with thicker, yet lighter, aluminum. However, aluminum and steel cannot be welded together due to vastly different melting points, so TARDEC sought a new process to join these materials.
TARDEC joined forces with PNNL—which had previously developed unique material joining techniques for the automotive industry (earlier post)—to develop Friction Stir Dovetailing. These previous techniques included Friction Stir Welding to join similar metals of differing thickness, and Friction Stir Scribe, which joins thin sheets of significantly different materials, like aluminum and steel.
While Friction Stir Scribe solved the challenge of joining thin sheets of aluminum with steel, that technique was found to not scale up to the thick plates of aluminum—measured in inches—needed for robust military vehicles.
In woodworking, dovetails and glue are used to securely join pieces of wood together. Friction Stir Dovetailing is a similar approach for metals. Using a specially designed tool, aluminum is deformed into a steel dovetail groove to form a mechanical interlock. At the same time, the tool rubs along the bottom of the dovetail to form a thin metallurgical bond—or intermetallic compound—which “glues” the metals together within the dovetail.
The combination of mechanical interlocking and metallurgical bonding formed during a single process is the innovation that produces joints of superior strength and ductility compared to joints created by the other friction stir methods.—PNNL engineer Scott Whalen, who leads the research
The research team discovered that using complex machine controls to precisely regulate temperature and pressure at the aluminum-steel interface inhibited growth of intermetallic compounds. These compounds grow thick and non-uniformly during the other friction stir techniques, causing joint brittleness and failure. However, growth of intermetallic compounds—iron aluminide or Fe3Al—during the Friction Stir Dovetailing technique is beneficial to the joint because they are so thin, one thousand times thinner than a human hair, which acts as “glue” without causing embrittlement.
Intermetallic compounds will form between aluminum and steel during all friction stir techniques as part of the heating process. We discovered that Friction Stir Dovetailing inhibits intermetallic compound overgrowth because temperature and pressure are much lower than other friction stir approaches.—Scott Whalen
Lab testing of joints made by Friction Stir Dovetailing showed that when combining metallurgical bonding with the dovetail configuration, the joint strength is not only superior, but the material can stretch to over a half centimeter before the joint breaks, illustrating five times more ductility than aluminum and steel joined with other friction stir techniques. This allows the joint to “give” or move farther before breakage—an attractive feature for military combat vehicles.
|Load per unit joint length vs. extension for joining trials A, B, C, and D. Reza-E-Rabby et al. Click to enlarge.|
The research team now plans to refine the technique and expand the process for other joint configurations. In addition to aluminum and steel, other material combinations such as aluminum to copper, aluminum to magnesium, and magnesium to steel can also be joined using Friction Stir Dovetailing.
While Friction Stir Dovetailing will help solve the fuel consumption challenge for TARDEC, it is also available for licensing for other potential applications as well as collaborative research opportunities via PNNL commercialization manager Sara Hunt.
Md. Reza-E-Rabby, Kenneth Ross, Nicole R. Overman, Matthew J. Olszta, Martin McDonnell, Scott A. Whalen (2018) “Joining thick section aluminum to steel with suppressed FeAl intermetallic formation via friction stir dovetailing,” Scripta Materialia, Volume 148, Pages 63-67, doi: 10.1016/j.scriptamat.2018.01.026.