Joining Lightweight Metals to Nonmetals via Friction Stir Interlocking

The automotive, aerospace, and construction industries need parts and materials that are strong and durable, but of lighter weight to save on energy, production, and transportation costs. Steel has strength. Aluminum and magnesium are lighter weight. And carbon fiber reinforced composites are lightweight, strong, and corrosion resistant. When you put all these materials together, they can form the strong, lightweight parts these industries need.

But, until now, joining these materials has been a problem because of their vastly dissimilar physical properties. PNNL’s Friction Stir Interlocking method overcomes the problems in joining these dissimilar materials and can even increase the strength of the joint in the process.

These dissimilar materials joined by conventional means—whether mechanical fasteners or adhesive bonds—produce weak joints that are subject to fatigue. Conventional joining can also damage the structure of carbon fiber and creates an opportunity for corrosion. The standard processing speeds for joining dissimilar metals are also very slow, and these slow processing speeds create additional opportunities for corrosion.

An exciting new alternative, the patented Friction Stir Interlock joins dissimilar materials and easily overcomes another hurdle of standard joining methods—connecting thick and thin sections of metal or thermoplastic materials. The joint is completely encapsulated in the Friction Stir Interlock, thereby further reducing corrosion potential.

Pieces of different materials are interlocked and strengthened to the point that in some cases the joint itself becomes stronger than the base materials themselves. That beats industry standards, which require that a weld or a joint needs to be at least 80 percent of the strength of the starting materials.

Friction Stir Interlocking involves pins and holes embedded in two different metals or materials to start the joining process. But that physical joint is then strengthened by a customized Friction Stir Welding tool. The device moves across the inserted pins, using friction and the low heat it creates to plasticize the material at the interface so that the pins and holes are indistinguishable from the base material. This solid-phase technique locks the two dissimilar materials together without high heat or the need for pre-melting or pretreating the base materials and without external fasteners.

The processing takes just a few seconds and the low process temperatures--as low as 250 degrees Celsius for magnesium--make the Friction Stir Interlocking technique ideal for joining magnesium and aluminum to carbon fiber composites without degrading its properties. Specifically, the Friction Stir Interlocking process leaves the carbon fibers, which provide its strength and stiffness, intact.

Numerous interlocks or joint areas can be created quickly and uniformly, in a single pass, offering reduced cost and improved process efficiency compared to conventional fasteners, adhesives, and coatings used to join metal to nonmetal materials.

New, Improved Way to Join Different Metals and Nonmetals

Friction Stir Interlocking offers a new and improved way to join different metals and nonmetals, such as carbon fiber composites, without creating an opportunity for galvanic corrosion. It strengthens the joint compared to conventional joining methods and enables far greater use of diverse lightweight and strong materials.

The ability to join different materials seamlessly, strongly, and quickly is not only key to making strong, lightweight automobile and aerospace parts, it will allow carbon fiber to be used more widely in different commercial applications. PNNL envisions that Friction Stir Interlocking can also be employed at offshore drill rigs, in building construction, and in bridge building--and maybe even in elevator cars. We invite you to think outside the box with us.