UCLA team devises method to weld previously unweldable aluminum alloy AA 7075; potential for wider use in automobiles
Engineers at the UCLA Samueli School of Engineering have developed a way to weld the super-strong but lightweight aluminum alloy known as AA 7075. Developed in the 1940s, AA 7075 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.
The problem arises when the alloy is heated during welding, as its molecular structure creates an uneven flow of its constituent elements—aluminum, zinc, magnesium and copper—which results in cracks along the weld.
To circumvent this, the UCLA engineers infused titanium carbide nanoparticles into AA 7075 welding wires, which are used as the filler material between the pieces being joined. An open-access paper describing the advance was published in Nature Communications.
Gas tungsten arc welding (GTAW) of AA7075. a Two 152.4 × 76.2 × 3.175 mm AA7075 sheets were arc welded using three different types of filler materials for the weld bead (purple). b and c Macroscopic solidification cracks in the bead’s melting zones in the welds performed with conventional filler materials AA7075 and ER5356, respectively. d Using AA7075 + 1.7 vol% TiC as filler material, the weld yields an even weld bead without signs of cracking. Scale bars, 10 mm. Sokoluk et al.
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.) 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. 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.—Xiaochun Li, UCLA’s Raytheon Professor of Manufacturing and the study’s principal investigator
The researchers already are working with a bicycle manufacturer on prototype bike frames that would use the alloy; the new study also 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.
Maximilian Sokoluk, Chezheng Cao, Shuaihang Pan & Xiaochun Li (2019) “Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075” Nature Communications volume 10, Article number: 98 doi: 10.1038/s41467-018-07989-y