Some things just seem out of the realm of possibilities, like the four minute mile, putting a man on the moon or air travel – and then it happens. That’s the case with the technology behind MetaLi, a company founded in 2016 and devoted to designing and manufacturing a new generation of super metals using nanotechnology, or as the company’s founder, Xiaochun Li, calls the process, “nanotech metallurgy.”
MetaLi produces aluminum 7075 welding wire that has been infused with nanoparticles such as titanium carbide, which is used to join aluminum 7075 via an arc weld. That might not mean much to anyone who hasn’t followed nanotechnology as it relates to metals, but infusing molten metal with dispersed nanoparticles was thought to be impossible until roughly five years ago.
Nanotechnology involves engineering conducted at the nanoscale. For reference, a nanometer is one billionth of a meter. Not that it makes it any easier to grasp, but there are more than 25 million nanometers in an inch. The National Nanotechnology Initiative, a U.S. government research and development organization, says that on a comparative scale, “if a marble were a nanometer, then one meter would be the size of the earth.”
Nanotechnology is already being utilized in some fairly common household items, such as films on eyeglasses, windows, camera lenses and computer displays. Li’s company is working to introduce nanotech metallurgy to metals manufacturing, especially in industries with a heavy focus on lightweight materials.
It is difficult to discuss the state of the automobile industry today without talking about dropping weight. Reducing gas consumption has a lot to do with it, not to mention controlling emissions. For example, reducing the weight of a vehicle by 10 percent can reduce fuel consumption by 6 to 8 percent. And it’s not just automobiles – shipbuilding, aviation and construction can also benefit through reducing weight.
The problem is that one of the strongest and affordable lightweight materials, aluminum 7075, which is roughly a third the weight of steel and nearly as strong, cracks when welded. There has been some success using friction stir welding to join aluminum 7075, but it has limits in practical application due to the tooling involved, which can’t always reach the joints. Furthermore, friction stir welding just doesn’t perform well on complicated weld shapes.
Thanks to Li’s breakthrough, aluminum 7075 is no longer “unjoinable” through arc welding or any fusion welding. In fact, his technology completely eliminates hot cracking defects and improves weld quality.
A new alloy
Li earned his undergraduate degree in engineering from the “MIT of China,” Tsinghua University in Beijing. He completed his master’s degree through Ohio State University’s welding and engineering program (which has gotten a lot of attention in recent years for its breakthroughs in vaporizing foil actuator welding) and earned his doctorate in mechanical engineering at Stanford University. He was a professor at the University of Wisconsin-Madison for 12 years and has taught at the University of California-Los Angeles for seven years.
“I’ve been studying on how to integrate nanotechnology and mass manufacturing for a long time,” Li says, adding that his nanotechnology work in metals began in earnest in 2001 when he was a professor in Wisconsin. “A lot of people had doubts – it’s incredibly difficult. Even today, a lot of nanotechnology has not yet reached the industry-mature stage.”
Li says he spent 12 years trying to solve one big problem, and that was getting nanoparticles to “behave” the way the “theoretical guys” wanted them to behave in metals.
“How do you control the size of nanoparticles?” Li asks. “How do you get the nanoparticles into the metal, and second, how do you disperse them uniformly into molten metal? In the early 2000s, people thought this was not real, that it was impossible to disperse nanoparticles into molten metal. Nanoparticles don’t work well with metal. To get the nanoparticle in, you have no wiggle room to react.”
For the first three years, Li used mechanical mixing to try to introduce nanoparticles into metal in an attempt to create a stronger alloy. But because nanoparticles are attracted to each other so quickly, and because shear stress cannot pull apart nanoparticles, it was impossible for mechanical mixing to disperse the nanoparticles into the alloy. Li and his team then found some promise with ultrasonic processing, which involves applying sound to generate cavitation for localized shock waves to separate nanoparticles, but this process only worked in small-volume processing of materials.
“We got a lot of research through that,” Li says of the ultrasonic processing, “but we couldn’t penetrate industry by the end, especially the metal industry, because they tend to look for processes that are inexpensive and offer massive production. We couldn’t compete at that time.”
The “ah ha” moment
When looking at his attempts with ultrasonic processing through a microscope, Li often saw what appeared to be clusters of nanoparticles, which would indicate that the nanoparticles hadn’t dispersed as they were supposed to. But one day, when looking at some silicon carbide nanoparticle clusters in a magnesium sample through a more powerful transmission electron microscope, he saw something quite strange.
“We were supposed to be seeing clusters with nanoparticles connected or fused together, but they actually were separated by metal at the nanoscale,” Li says. “I was surprised by that, and it inspired us to understand the physics behind this ‘strange’ phenomenon of nanoparticle separation. After intensive fundamental study and within a few years, we finally uncovered a thermally activated self-dispersion mechanism for nanoparticles in molten metals, which establishes a new platform for nanoparticle dispersion and their use for mass metals manufacturing.”
Li and his team of researchers first focused on infusing magnesium with silicon carbide to demonstrate the physics and their effect, which is well-documented in a milestone paper published in December 2015 in Nature. It should be noted that the thermally activated self-dispersion mechanism can be applied to all types of metals, such as steel, superalloys, aluminum and copper alloys, but only if the nanoparticles are well-selected.
Once the mechanism of nanoparticle dispersion was discovered, Li’s team was faced with another tough issue: how to incorporate nanoparticles into the molten metals without oxidation and even burning at elevated temperatures. Li and his team used a process called flux-assisted liquid-state incorporation. By adding flux to the nanoparticles while feeding it into molten material, they saw a drastic improvement in the rate of nanoparticle incorporation.
The self-dispersion mechanism does its job in distributing and dispersing nanoparticles to create a stronger material or materials with significantly improved manufacturability. Li’s group is also working on in-situ nanoparticle synthesis and production in molten metals to generate nanoparticles for self-dispersion.
With this new discovery of nanoparticle self-dispersion and its significance for new metallurgy, Li coined a new term for this emerging field as “nanotech metallurgy,” which adds weaponry of the four traditional metallurgy methods, namely alloying, plastic deformation, grain refinement and heat treatment. Nanotech studies how nanophases (both ex situ and in situ) can be engineered and applied to significantly improve the processing/manufacturing, micro/nano-structures, and behaviors of metals and alloys.
To showcase the promises of nanotech metallurgy for practical applications, Li’s team produced titanium carbide-infused aluminum 7075 welding wire. When welders use nanoparticle-enhanced AA 7075 filler rod, they can achieve welds on aluminum without cracks, but the added benefit is they achieve a high megapascal rating of 392 MPa as welded or 551 MPa with a post-weld heat treatment.
Early adopters of MetaLi’s products manufactured from nanotech metallurgy were bicycle manufacturers, yet another industry focused on lightweighting products. High-end bicycles are mostly made of carbon fiber, which offers a comfortable and extremely light ride, but makes them expensive. Aluminum is not as lightweight as carbon fibers, but it’s a more affordable option for cyclists. With the ability to utilize a lighter weight aluminum and join it, Li saw a window of opportunity in the cycling industry.
“With nanoparticles, we can gear up aluminum to significantly higher strengths,” he says. “It has strength and stiffness and reduces vibration. Carbon bikes are better and have reduced vibration, but with nanotechnology, we can do a lot of cool metal stuff there. Nano bikes are much cheaper than carbon fiber. It’s a little more expensive than (non-nano) aluminum, but cheaper than carbon.”
The aerospace industry certainly took notice of this breakthrough, too, as light weighting is paramount in all air and space transportation. MetaLi has around 30 regular customers worldwide in the bike, tooling and aerospace industries.
Cost continues to be somewhat of a barrier to jumping into other mass markets, such as manufacturing automobiles. Another barrier is that MetaLi’s technology isn’t widely known. However, the nanotechnology-improved metal, which Li said was at one time around 50 times the cost of the materials it can replace, has dropped considerably in the last few years and is now probably only 10 to 20 percent more costly. It would seem that being fully embraced by the auto industry isn’t far away.
“The welding wire will continue to improve, and we’ll find other markets,” he said, adding that they have already begun to process nanoparticle material for 3-D printing. “We’re in a new age of metals – it looks like sunrise again for metallurgy as nanotech metallurgy is creating a new and exciting processing/manufacturing space while pushing the performance envelope of metals to meet energy and sustainability challenges in human society.”