Despite recent advances in materials design, alternative metals still pose a challenge to manufacturers. Many are considered ‘un-weldable’ by traditional means, in part because high heat and re-solidification weaken them.
Glenn Daehn, professor of materials science and engineering at Ohio State, who helped develop the new technique said: "Materials have gotten stronger, but welds haven't. 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 of this method include generating high currents that consume a lot of energy, and that the melted portions of metal are never as strong afterward as they were before.
Over the last decade, Prof 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.
In VFA, a high-voltage capacitor bank creates a 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 speeds approaching thousands of miles per hour.
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 (pictured).
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 created strong bonds between commercial steel and aluminium alloys - a feat which is impossible normally. Also, 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 step in production.
Prof Daehn and his team now want to join with manufacturers to further develop the technology.
