Researchers at the Pacific Northwest National Laboratory (PNNL) have developed a process called Friction Stir Dovetailing that joins thick plates of aluminum to steel. The new method could play an important role in producing military vehicles that are lighter, more agile, and more fuel efficient. A full account of the development process can be found in the April 15 issue of Scripta Materialia.
The United States military spends billions of dollars annually on fuel consumption, which it feels could be reduced by losing some weight on ships, aircraft, ground vehicles, and cargo. To this end, the U.S. Army Tank Automotive Research Development and Engineering Center - or TARDEC - launched a campaign in 2014 seeking ways to make combat systems lighter.
One approach entailed the replacement of heavy steel components with thicker, yet lighter, aluminum. The problem is that aluminum and steel cannot be welded together because of their different melting points. Enter PNNL, which had previously developed unique material joining techniques for the automotive industry. 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, it didn’t work for the thicker plates needed in military applications. So, a special tool was designed to deform the aluminum into a steel dovetail groove, forming a mechanical interlock. Simultaneously, the device moves along the bottom of the dovetail to form a thin metallurgical bond that essentially glues the metals together.
Lab testing 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 capability allows the joint to give a bit, adding to its durability.
The research team now hopes to improve the technique and expand the process to include other metal combinations. These additional metals could include aluminum and copper, aluminum and magnesium, and magnesium and steel.