Engineers at Brigham Young University (BYU) in Utah and the University of Ulsan in South Korea have used friction bit joining (FBJ) to bond dissimilar combinations of aluminum alloy A356 and grey cast iron.
The process of friction bit joining uses a small, consumable bit to create a solid-state joint between metals. The method was invented by BYU manufacturing engineering technology professor Michael Miles and retired BYU professor Kent Kohkonen, in their collaboration with local Orem-based company MegaStir Technologies, a joint venture between Schlumberger and Advanced Metal Products. MegaStir provides both low- and high-temperature friction stir welding (FSW) systems.
(BYU has been involved in friction stir welding and processing (FSW&P) research since 1998.)
The automotive industry is pursuing an increased use of lightweight materials in auto body structures, for reduced weight and improved vehicle fuel efficiency. he use of different material combination presents a number of technical challenges, especially when light metals must be joined to steel or cast iron.
Joining of different steel alloys has been done using standard methods like resistance spot welding (RSW) or laser welding (LW). While some attempts have been made to use traditional fusion welding processes, like resistance spot welding (RSW) to join aluminum and steel, this approach is usually not suitable for welding of dissimilar metals, because of the metallurgical incompatibility that occurs during melting and solidification. On the other hand, a mechanical fastening method like self-pierce riveting (SPR) can be used, but this method is limited to the joining of relatively ductile metals like aluminum and mild steel. Friction stir spot welding )FSW)has also been employed for spot joining of dissimilar metal combinations, like aluminum/steel and magnesium steel. his method is viable in some cases, but is limited in the amount of join strength that can be produced.
Friction bit joining is another process with potential for joining of dissimilar alloys. This method has been used to join high strength steels and dissimilar combinations of aluminum and steel, and has been shown to achieve high levels of joint strength in these applications, with levels above produced by self-piercing riveting.—Miles et al.
The latest development in the FBJ process successfully bonds lightweight aluminum with cast iron by inserting a thin layer of steel between the two metals, which facilitates the bonding process. Welding development resulted in cycle times of approximately 5 seconds, and lap shear fracture loads of up to 6.8 kN. Findings of this research are published in the June issue of the International Journal of Precision Engineering and Manufacturing.
The motivation to make cars lighter was already there with a previous EPA mandate (34.5 mpg by 2016), but that motivation has now increased with the latest mandate. Our process is a technical success in the effort to spot join dissimilar metals together; now we need to go forward with our partners to make it commercially viable.—Michael Miles
Currently, the automotive industry uses resistance spot welding (RSW) to join steel stampings together into a completed body. RSW entails clamping the pieces to be joined between two electrodes. The clamping force holds the pieces while while current is passed from one electrode to the other. Resistance to the current flow generates heat, resulting in localized melting of the working pieces, which then allows the joining. While RSW could be used to join dissimilar materials such as aluminum and steel, the joints have defects and perform poorly.
In recent years, some aluminum parts have been introduced into the vehicle structure using a mechanical fastening method called self-piercing riveting. While this approach works to join lower strength steels with aluminum, it isn’t suitable for joining aluminum to ultra-high-strength steel.
Friction bit joining, by contrast, uses a consumable bit to drill through the materials to be joined, using sufficient rotational speed and force to degrade the bit, which then becomes friction-welded to the workpieces. FBJ has two basic phases:
An initial cutting phase, in which the top layer of sheet material is penetrated by the cutting action of the rotating joining bit under applied axial load.
The second phase, involving frictional joining of the bit to the bottom layer of material. The joining bit is consumable—it is left in the workpiece at the end of the process and forms an integral part of the weld. The bit has a cutting tip on one end and a Torx head on the other.
BYU’s friction bit joining method, which is being developed in collaboration with MegaStir and Oak Ridge National Lab, has received funding from the National Science Foundation, the Department of Energy, the state of Utah and some auto suppliers in South Korea.
Applications for the process include areas of the vehicle frame where ultra-high-strength steel needs to be joined to a light metal. For example, an automaker may want to use aluminum for the roof of a car while using ultra-high-strength steel for the A and B pillars of the frame that connect with the roof. Another example includes the incorporation of lighter weight metals on the interior parts of the car door.
Miles said the ability of friction bit joining to produce “incredible strength between two dissimilar metals” will eventually benefit both automakers and other industries, such as aerospace.
Michael Miles, Sung-Tae Hong, Coulter Woodward, Yong-Ha Jeong (2013) Spot welding of aluminum and cast iron by friction bit joining. International Journal of Precision Engineering and Manufacturing. Volume 14, Issue 6, pp 1003-1006 doi: 10.1007/s12541-013-0133-8
Siemssen, Brandon Raymond (2008) Technology Development and Characterization of Friction Bit Joining: A New Solid State Spot Joining Technology Applied to Dissimilar Al/Steel Joints (MS Thesis)