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New welding technique welds 'un-weldable' metals

30 October 2015

US researchers have developed a novel technique involving electrical pulses, vaporised foils and impact bonding to seamlessly weld dissimilar metals.

Microscopic view of copper (top) welded to titanium (bottom) using a new technique developed at Ohio State University (image: Glenn Daehn, courtesy of Ohio State University)

Engineers at Ohio State University have developed a new welding technique that they claim consumes 80 percent less energy than a common welding technique, yet creates bonds that are 50 percent stronger. The new technique could have implications for the automotive industry, which is seeking to combine traditional heavy steel parts with lighter, alternative metals to reduce vehicle weight.

Despite recent advances in materials design, alternative metals still pose a challenge to manufacturers in practice. Many are considered 'un-weldable' by traditional means, in part because high heat and re-solidification weaken them.

"Materials have gotten stronger, but welds haven't," says Ohio State's Professor Glenn Daehn who helped develop the new technique. "We can design metals with intricate microstructures, but we destroy the microstructure when we weld. With our method, materials are shaped and bonded together at the same time, and they actually get stronger."

In a common technique called resistance spot welding, manufacturers pass a high electrical current through pieces of metal, so that the metals' natural electrical resistance generates heat that partially melts them together and forms a weld. The drawbacks: generating high currents consumes a lot of energy, and the melted portions of metal are never as strong afterwards as they were before.

Over the last decade, Daehn and his team have been trying to find ways around those problems. They've amassed more than half a dozen patents on a system called vaporised foil actuator (VFA) welding.

Diagram showing vaporised foil actuator welding, a technique developed at Ohio State University (image: Glenn Daehn, courtesy of Ohio State University)

In VFA, a high-voltage capacitor bank creates a very short electrical pulse inside a thin piece of aluminium foil. Within microseconds, the foil vaporises, and a burst of hot gas pushes the two pieces of metal together at great speed. The pieces don't melt, so there's no seam of weakened metal between them. Instead, the impact directly bonds the atoms of one metal to atoms of the other.

Seen under a high-powered microscope, the bond often features delicate curlicues in spots where veins of both materials extend outward and wrap around each other (see top image).

The technique uses less energy because the electrical pulse is so short, and because the energy required to vaporise the foil is less than what would be required to melt the metal parts.

So far, the engineers have successfully bonded different combinations of copper, aluminium, magnesium, iron, nickel and titanium. They have also created strong bonds between commercial steel and aluminium alloys. High-strength steel and aluminium join together with weld regions that are stronger than the base metals.

The technique is powerful enough to shape metal parts at the same time it welds them together, saving manufacturers a production step.

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