RICHLAND, WA—Engineers at the Pacific Northwest National Laboratory (PNNL) here have developed a new process for joining aluminum and steel. Friction stir dovetailing was inspired by assembly techniques used in woodworking.
“In woodworking, dovetails and glue are used to securely join pieces of wood together,” says Scott Whalen, a PNNL engineer who is leading the R&D project. “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 that “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 welding methods,” explains Whalen.
Whalen and his colleagues discovered that complex machine controls that precisely regulate temperature and pressure at the aluminum-steel interface inhibite growth of intermetallic compounds.
“These compounds grow thick and non-uniformly during the other friction stir techniques, causing joint brittleness and failure,” Whalen points out. “However, growth of intermetallic compounds during the friction stir dovetailing technique is beneficial to the joint because they are so thin—1,000 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,” says Whalen. “We discovered that friction stir dovetailing inhibits intermetallic compound overgrowth, because temperature and pressure are much lower than other friction stir welding approaches.”
Lab testing of joints made by friction stir dovetailing show that the joint strength is not only superior, but the material can stretch more than 0.5 centimeter before the joint breaks. According to Whalen, that’s 5 times more ductility than aluminum and steel joined with other friction stir welding techniques.
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.
The new application for the new process will be to produce lightweight military vehicles that are agile and fuel efficient.
To lower fuel costs and increase operational effectiveness while still maintaining the safety of military personnel, the U.S. Army Tank Automotive Research Development and Engineering Center (TARDEC) launched a campaign in 2014 seeking ways to make tanks, fighting vehicles and personnel carriers more lightweight. It is investigating new ways to replace heavy steel components with thicker, yet lighter, aluminum.
“We are constantly looking for innovative multi-material joining methods to help us select the right material for the right purpose in the right location,” says Jason Middleton, associate director for product lifecycle engineering at TARDEC. “Friction stir dovetailing offers us precisely such a state-of-the-art method.”