The U.S. military annually spends several billion dollars on fuel, an expense that could be reduced by lowering the weight of ships, aircraft, ground vehicles, and cargo. A recent discovery made by researchers at Pacific Northwest National Laboratory may offer a way to cut some pounds from military gear. They have developed and tested a process, friction stir dovetailing, that joins thick plates of aluminum to steel. This new process will be used to make lighter-weight military vehicles that are more agile and more fuel efficient.
PNNL has already developed friction stir welding, which joins similar metals of differing thickness, and friction stir scribe, which joins thin sheets of significantly different materials (such as aluminum and steel). Although friction stir scribe solved the challenge of joining thin sheets of aluminum with steel, it could not be scaled up to handle the thick plates of aluminum (those measured in inches) needed for military vehicles.
The new method uses a form of dovetailing. In woodworking, dovetails and glue securely join pieces of wood together. Friction stir dovetailing uses a specially designed tool with a spinning head that generates enough friction to heat and form the aluminum into dovetail that fits into a mechanically cut dovetail groove in a piece of steel. A mechanical interlock is created between the two metals. At the same time, the new tool heats 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 produces joints of superior strength and ductility compared to joints created by the other friction stir methods,” says 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 new dovetailing technique is beneficial because the growths are so thin—one thousand times thinner than a human hair—they act as “glue” without causing embrittlement.
“Intermetallic compounds form between aluminum and steel during all friction stir techniques as part of the heating process,” says Whalen. “We discovered that friction stir dovetailing inhibits intermetallic compound overgrowth because temperature and pressure are much lower than other friction stir approaches.”
Lab testing of joints made by friction stir dovetailing showed that combining metallurgical bonding with the dovetail configuration creates joints that are stronger, but the joined materials can stretch to over a half-centimeter before the joint breaks.
Lab testing of friction stir dovetailed joints showed that when combining metallurgical bonding with the dovetailing, the joint strength is not only superior, but the material can stretch to over a half centimeter before the joint break. This indicates the joints have five times the ductility of aluminum and steel joined with other friction stir techniques. This lets the joint “give” or move farther before breaking, an attractive feature for military combat vehicles.
The researchers are refining the technique and expanding 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. And although friction stir dovetailing will partially help solve the fuel consumption challenge for the military, it is also available for licensing for other potential applications as well as collaborative research opportunities.