Nanotechnology enables engineers to weld previously un-weldable 7075 aluminum alloy

press release



Super-strong but lightweight, AA 7075 now could be more widely used in automobiles and other manufacturing thanks to UCLA research

An aluminum alloy developed in the 1940s has long held promise for use in automobile manufacturing, except for one key obstacle. Although it’s nearly as strong as steel and just one-third the weight, it is almost impossible to weld together using the technique commonly used to assemble body panels or engine parts.

That’s because when the alloy is heated during welding, its molecular structure creates an uneven flow of its constituent elements — aluminum, zinc, magnesium and copper — which results in cracks along the weld.

Now, engineers at the UCLA Samueli School of Engineering have developed a way to weld the alloy, known as AA 7075. The solution: infusing titanium carbide nanoparticles — particles so small that they’re measured in units equal to one billionth of a meter — into AA 7075 welding wires, which are used as the filler material between the pieces being joined. A paper describing the advance was published in Nature Communications.




Using the new approach, the researchers produced welded joints with a tensile strength up to 392 megapascals. (By comparison, an aluminum alloy known as AA 6061 that is widely used in aircraft and automobile parts, has a tensile strength of 186 megapascals in welded joints.) And according to the study, post-welding heat treatments, could further increase the strength of AA 7075 joints, up to 551 megapascals, which is comparable to steel.

Because it’s strong but light, AA 7075 can help increase a vehicle’s fuel and battery efficiency, so it’s already often used to form airplane fuselages and wings, where the material is generally joined by bolts or rivets rather than welded. The alloy also has been used for products that don’t require joining, such as smartphone frames and rock-climbing carabiners.

But the alloy’s resistance to welding, specifically, to the type of welding used in automobile manufacturing, has prevented it from being widely adopted.



“The new technique is just a simple twist, but it could allow widespread use of this high-strength aluminum alloy in mass-produced products like cars or bicycles, where parts are often assembled together,” said Xiaochun Li, UCLA’s Raytheon Professor of Manufacturing and the study’s principal investigator. “Companies could use the same processes and equipment they already have to incorporate this super-strong aluminum alloy into their manufacturing processes, and their products could be lighter and more energy efficient, while still retaining their strength.”



The researchers already are working with a bicycle manufacturer on prototype bike frames that would use the alloy; and the new study suggests that nanoparticle-infused filler wires could also make it easier to join other hard-to-weld metals and metal alloys.

The study’s lead author is UCLA graduate student Maximilian Sokoluk. The other authors are Chezheng Cao, who earned a doctoral degree from UCLA in December, and Shuaihang Pan, a current UCLA graduate student. Li holds faculty appointments in mechanical and aerospace engineering, and in materials science and engineering.






In Nature:

Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075
Abstract


Lightweight materials are of paramount importance to reduce energy consumption and emissions in today’s society. For materials to qualify for widespread use in lightweight structural assembly, they must be weldable or joinable, which has been a long-standing issue for high strength aluminum alloys, such as 7075 (AA7075) due to their hot crack susceptibility during fusion welding. Here, we show that AA7075 can be safely arc welded without hot cracks by introducing nanoparticle-enabled phase control during welding. Joints welded with an AA7075 filler rod containing TiC nanoparticles not only exhibit fine globular grains and a modified secondary phase, both which intrinsically eliminate the materials hot crack susceptibility, but moreover show exceptional tensile strength in both as-welded and post-weld heat-treated conditions. This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials.
https://www.nature.com/articles/s41467-018-07989-y