New welding process opens up uses for formerly un-weldable lightweight alloy
Developed in the 1940s, AA7075 is an aluminum alloy that's almost as strong as steel, yet it weighs just one third as much. Unfortunately its use has been limited, due to the fact that pieces of it couldn't be securely welded together. That's recently changed, however, thanks to the use of titanium carbide nanoparticles.
The problem with welding 7075 lies in the fact that when the metal is heated, the aluminum, zinc, magnesium and copper of which it's composed flow unevenly. The phenomenon is known as phase segregation, and as a result of it, cracks form along the length of the weld.
Led by Prof. Xiaochun Li, a team at the UCLA Samueli School of Engineering set about addressing that problem. In the course of doing so, they fabricated thin rods made of 7075 infused with minuscule particles of titanium carbide. These rods were then placed between larger pieces of the aluminum alloy which were about to be joined, acting as a filler. When the rods and adjacent material were subsequently melted in an arc welding process, the nanoparticles eliminated the uneven flow issue.
"Nanoparticles make the elements in the liquid metal solidify together more uniformly, thus preventing phase segregation," Li tells us. "Phase segregation normally will block the liquid metal flow during cooling, thus inducing shrinkage and cracks without liquid filling. Since the metal solidifies more uniformly after adding nanoparticles, the liquid metal flows better during solidification, thus no cracking due to segregation."
The resulting welded joints have a tensile strength of up to 392 megapascals – by contrast, the commonly-used 6061 aluminum alloy has a weld strength of 186 megapascals. What's more, it is believed that post-welding heat treatments could boost the strength of the 7075 welds up to 551 megapascals, which is on par with the weld strength of steel.
Although the alloy is already used in items such as airplane wings and fuselages, sheets of it typically have to be riveted together. The scientists now hope that the ability to weld it will increase its applications.
"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," says Li. "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."
Li and his team are in fact already working with a bicycle manufacturer, which is interested in utilizing the technology to make 7075 frames.
A paper on the research was recently published in the journal Nature Communications.