Welders will tell you that joining two dissimilar metals is fraught with difficulty because they literally do not mix well with others, especially when the party gets hot.
Steel and aluminum, for example, are unsuited to resistance spot welding, which is the low-cost method long used in the auto industry to bond two pieces of sheet steel together. The mismatched metals react in the molten pool to create brittle intermetallic and carbide compounds that weaken the joint after it cools. Steel similarly has little affinity for welding to other low-density metals such as magnesium and titanium.
Increasingly, however, it's looking as if the designers of next-generation lightweight cars may want to use both high-strength steel and aluminum alloys to produce strong, safe structures that are light enough to enable automakers to meet the tough 54.5-mpg average CAFE fuel-economy standards for 2025. Such structural hybrids may benefit from the ability to marry aluminum roofs to high-strength-steel B-pillars and other high-strength and ultrahigh-strength steel frames or to incorporate aluminum parts into steel car-door structures to shave weight.
Honda has reportedly had success in joining the steel yin to the aluminum yang using friction stir spot welding, which it first used in building the 2013 Accord. The friction stir technique, which was developed at The Welding Institute in England in 1991, uses a tool to mechanically mix the two metals without heating them to their melting points; instead, they are softened just enough to allow applied pressure to produce a solid-state joint. Steel melts at 1425 to 1535°C (2600 to 2800°F); aluminum at 650°C (1220°F.)
Friction stir welding works well for joining straight sections but is less amenable to compound curves. It also often suffers from high tool wear, which raises production costs, said Michael Miles, a professor of manufacturing engineering technology at Brigham Young University in Utah. Other techniques such as self-piercing riveting have also found use in car factories, he said, but the method does not work with advanced steel grades because the joints lack durability since the metal does not readily form around the rivets as needed.
“Those were some of the issues that we were considering about in early 2006 when we started batting around the concepts that led to the new method,” Miles recalled. Together with Kent Kohkonen, a retired BYU professor, and Scott Packer, an engineer with local Orem-based company MegaStir Technologies, which is a joint venture between Schlumberger and Advanced Metal Products, Miles came up with an assembly technique that they now call friction bit joining (FBJ). The process creates extremely strong joints between aluminum and high-strength steel sheets.
In 2008, the team began working with Zhili Feng’s group in the materials joining group at Oak Ridge National Laboratory, which had purchased a MegaStir machine for study and also had worked with Honda. Besides this expertise, the ORNL group provided microstructural analysis of the joints. Currently, BYU, MegaStir, and ORNL are focusing on optimizing the technology for low-cost mass production with funding from the National Science Foundation, the U.S. Department of Energy, the state of Utah, and a few automotive suppliers in South Korea.
FBJ uses a small, driven bit to create a solid-state joint between metals both by cutting and by friction. “It’s sort of like friction rivet,” Miles said. The steel joining bit features a cutting tip and a Torx head for driving it. With moderate pressure applied, the bit first cuts through the top aluminum sheet at 500 to 600 rpm. Then with the spin rate raised to, say, 2000 rpm, the bit causes heating that soon friction-welds the bit tip to the bottom sheet.
“The frictional heating is just over the melting point of steel, so it forms a strong metallurgical (diffusion) bond—a solid-state joint with little or no brittle components inside,” said Miles.
The initial application of FBJ was attaching cast iron to aluminum, he noted, in a project conducted with researchers at the University of Ulsan in South Korea, who collaborate with Korean-based suppliers of Hyundai. As reported in the June issue of the International Journal of Precision Engineering and Manufacturing, the process successfully bonded a lightweight aluminum hub with a cast iron brake rotor. This was accomplished by inserting a thin layer of steel between the two metals; this facilitates the bonding process, which is normally hindered “by stealthy lubricating action of the graphite flakes in the iron,” said Miles.
FBJ is good for bonding anything very hard to something very soft, he said. It works well with joining titanium to aluminum, for instance.
“Our process is a technical success in joining dissimilar metals together in the lab,” said Miles, “but converting a process that works in the lab into a production application is very difficult because automotive manufacturing processes are so high-volume and so cost-conscious. Now we need to go forward with our partners to make it commercially viable.”
Miles predicted that FBJ eventually will benefit both automakers and other industries such as aerospace.