Tiny bubbles in the melt: Study could lead to more energy efficient alloys
In the college’s foundry, the research team uses an ultrasonic transducer (upper left) to produce cavitation and disperse ceramic nanoparticles in molten magnesium for casting. The group includes (front row, from left) graduate students Zhichao Duan, Yong Yang and Michael Miller, and (back row, from left) Professor Xiaochun Li, graduate student Mike DeCicco, and Professors Rod Lakes and Sindo Kou
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hen a liquid is blasted with ultrasound, bubbles form, grow and then collapse so violently they can reach temperatures hotter than the surface of the Sun. Now, a team of engineers led by Assistant Professor Xiaochun Li plans to harness some of that energy to cast stronger magnesium alloys for the automotive and aerospace industries.
Funded by a four-year, $1.1 million grant from the National Science Foundation, the team will test whether ultrasonic cavitation—a process in which countless tiny bubbles grow within a liquid and then burst forcefully inward—could help mix nano-sized bits of ceramic within molten magnesium for casting. Joining Li on the project are Materials Science and Engineering Professor Sindo Kou and Wisconsin Distinguished Professor of Engineering Physics Rod Lakes.
Although magnesium is one-third lighter than aluminum, its tendency to degrade at high temperatures has hampered its widespread use. If magnesium could be strengthened by adding nanoparticles—specks of material in the size range of atoms and molecules—this lighter alloy could be used to increase the performance and energy efficiency of all sorts of vehicles. For this reason, the project has attracted four collaborators from industry: Ford Motor Company, INTERMET Corporation, Eck Industries, Inc. and SPX Contech.
The effort to marry novel nanotechnology to conventional casting techniques has also garnered support from the American Foundry Society, which hopes to see the casting industry revitalized.“Ultrasonic cavitation is a new materials processing method that could be applicable to other alloys, such as titanium and even steel,” says Li. “In this research, we’re applying emerging technology to traditional manufacturing.”
The department has a strong history of casting research and collaboration with the casting industry, which Li hopes his new project will help to revitalize. In this photo from the 1970s, students perform a casting operation in the foundry.
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Magnesium under load begins to break down at the relatively cool temperature of 300 degrees Fahrenheit, making it unsuitable for components such as engine blocks and cylinder heads. Researchers have tried reinforcing it with expensive rare earth elements; however, these additives tend to cause the alloy to crack during casting. Others have successfully strengthened magnesium by adding micron-sized particles, but this can reduce ductility by 90 percent, says Li.
Still other engineers have created magnesium alloys containing nanoparticles of ceramic, as Li wants to produce. These so-called metal matrix nano-composites exhibit increased strength while maintaining their ductility. But the specialized techniques for making them consume large amounts of energy and limit the size and complexity of the parts that can be manufactured. Li and his team now want to produce metal matrix nano-composites through the more flexible and cost effective process of casting.
“With casting, we can form any shape we want—that’s the beauty of the process,” Li says. “And if you can make any shape you want, then industry will be interested.”
But casting metals that include such tiny particles is no easy task, because the particles tend to clump together in the melt. “When they cluster,” Li says, “we lose the advantage of adding nanoparticles.”
Conventional stirring technologies fail to break the particles apart and distribute them more evenly. In fact, separating two nanoparticles would require at least 1,000 atmospheres of pressure, Li estimated with a simple theoretical model. That’s 1,000 times the air pressure experienced by people walking around at sea level.
The needed force seems impossibly high, but ultrasonic cavitation can easily achieve it, Li says. Cavitation occurs when alternating low- and high-pressure ultrasonic waves travel through a liquid. Falling pressure during the low-pressure phase lets dissolved gases pop out of solution, forming bubbles, or cavities.
When the pressure jumps back up, the bubbles collapse violently, generating a micro-scale shock wave approaching the speed of sound.
Churning submarine propellers can cause cavitation and often sustain damage as a result. And at the microscale, scientists have shown that the force generated by cavitation in water can weld two micron-sized metal particles together.
The UW-Madison team will now use the phenomenon to drive nanoparticles apart in molten metal—a feat that hasn’t been attempted before. In preliminary experiments, Li has already cast a half-pound of a magnesium metal matrix nano-composite using the technique. He will next attempt to cast three and then five pounds of material. The collaborators will also study the material’s micro- and nano-scale structure, its functional properties, and how its structure and properties change under different processing methods.
Eventually the technique might be extended to other lightweight, structural metals, such as aluminum, leading to more energy-efficient engines, airplanes, or even tanks.
“If we can do this,” says Li, “it will solve a big problem for industry.”