Our Sites

Nanotechnology meets metal fabrication

Nanoparticle-infused metal makes unweldable material weldable

Xiaochun Li in the lab

Professor Xiaochun Li and his team spent years uncovering ways to uniformly self-disperse nanoparticles in molten metal. Photo courtesy of Oszie Tarula, UCLA.

In late 2018 Travis Widick finished welding a prototype bike frame that looked like any other bike frame, only it wasn’t. The tubular components were aluminum, but not just any grade. They were aluminum 7075.

The aluminum 7075 alloy, widely used in the aerospace industry, is about twice as strong as its widely used cousin, aluminum 6061, but it’s nearly impossible to weld by conventional means without cracking. Until now, that is. Widick used conventional gas metal arc welding with no special shielding gas, welding gun, or power source. In fact, he welded the material slowly, actually intending to increase thermal stress and shrinkage.

Why? He was testing a unique welding wire. Widick works at the UCLA Samueli School of Engineering with professor Xiaochun Li, who in 2016 launched a company called MetaLi to manufacture and market nanoparticle-infused materials. The bike frame base material Widick welded was conventional 7075, but his 7075 welding wire was anything but. It was infused with titanium-based ceramic nanoparticles, dispersed in just the right way so that they became, as Li described them, “the ultimate peacemakers.” Those nano-sized peacemakers made the unweldable weldable.

Nanotechnology’s Potential for Fabrication

If you walked the FABTECH® show floor in Chicago last November, you would have witnessed hundreds of different technologies designed to make sheet metal and plate fabrication not only faster, but also more consistent.

Welding has specialized processes like friction stir welding designed for materials that are difficult or impossible to weld by conventional methods, at least to the desired quality levels. And aluminum 7075 is a prime example. Elements in its microstructure cool at different rates, creating a thermal war of sorts, inducing pressure until something has to give. When something gives, cracks emerge. To overcome the problem, most either avoid the material or choose a different welding process like FSW or another cold joining method. The metal can’t cool without cracking? Fine. We’ll weld without a significant amount of heat, so it doesn’t have to cool in the first place.

Precision forming is another example. Every year FABTECH attendees witness numerous examples of technology designed to deal with forming challenges. Angle measurement devices on press brakes account for variation of grain direction and other material characteristics. Then you have finite element analysis (FEA). In sheet metal stamping, tube bending, and metal forming in general, an entire software industry has sprung up around metal manufacturing’s need to form challenging materials.

The material can’t form easily without cracking? Fine. I’ll use software and tweak my part or tool design to ensure we avoid cracking and the process is repeatable and reliable. In this sense, the “peacemakers” come in the form of tools, machines, and software. They adapt technology so that it can cut, weld, and form finicky, unwieldy material.

MetaLi takes a fundamentally different approach. The material can’t form or weld without cracking? Fine. Let’s change how the material behaves by infusing nanoparticles into the metal when it’s in the molten state, as it’s being made.

Li’s nanoparticle “peacemakers” create a homogeneous microstructure designed to equalize and distribute forces. This prevents that thermal war during postweld cooling, and it could prevent the “stress war” during metal forming, be it on a plate roll, roll former, press brake, folder, panel bender, or stamping press.

7075 aluminum welded

Two pieces of aluminum 7075 are welded together with a nanoparticle-infused filler wire. Photo courtesy of Oszie Tarula, UCLA.

Li has researched nano-infused particles in various areas of manufacturing. But at this writing, his company is tackling challenges in aluminum fabrication. Specifically, it offers aluminum powders (2000, 6000, and 7000 series) for metal additive manufacturing; nano-treated high-strength aluminum alloys, including barstock, tube, and sheet; and, especially, nanoparticle-infused aluminum wires and rods for welding and additive manufacturing to weld the unweldable or print the unprintable.

Li is certainly aware of the broader implications of nanoparticle-infused metal. He’s even been in talks with major metals producers regarding the adoption of and scaling up the technology (though it’s too early to disclose specifics). Smart use of nanotechnology might one day make metal become the best of all worlds: strong, formable, and weldable, all thanks to nano-sized peacemakers maintaining metallurgical calm and order.

Calm in the Molten State

A metal’s crystalline grains—their size, shape, and orientation—all directly affect a material’s strength and manufacturability. “A grain is kind of like a little family of atoms that are ordered in a lattice structure, “ Li said. “Metals in their solid form are made up of these many families. Each grain has an atomic lattice oriented in one direction, while its neighbors will be ordered in other directions.”

During aluminum smelting or steelmaking, a kind of metallurgical battle takes place when the metal cools from a molten state. As the material cools, grains transform, grow, deform, and segregate, establishing grain boundaries and other elements in a nonuniform metallurgical soup. This creates inconsistencies in the metallurgy that in turn lead to variations in material properties, which again lead to variations in manufacturing processes like forming and welding.

Oversimplified, that metallurgical inconsistency springs from the atomic lattice and grains, the microscopic scaffold that holds the bulk metal together. A dense grain is stronger than a looser one. But lattice orientations in different grains vary, creating the heterogenous grain structure at the heart of material property variation.

“Think of metal in three dimensions,” explained Li. “Metal does not fail in all directions. It fails because one direction is strong and another is weak.”

In the case of welding aluminum 7075, segregation (grouping of atoms for primary and secondary grains) in the microstructure causes certain areas of the material to cool faster than others. The temperature and grain differences within the material create so much stress that it rips the lattice apart, resulting in cracking.

“But nano-treated materials are much more homogeneous,” Li explained. “Under uniform stress loading, they fail in all directions, or they do not fail at all.”

In various facets of metalworking, thanks to grain inhomogeneity, metal tends to fail in one direction. Postweld solidification cracking in aluminum 7075 comes from the straw that broke the camel’s back, not a complete breakdown.

UCLA Samueli School of Engineering lab

UCLA Samueli School of Engineering lab technician Travis Widick holds a bike frame welded using aluminum alloy 7075 with graduate student Maximilian Sokoluk. Photo courtesy of Oszie Tarula, UCLA.

Years of Work

Infusing nano-sized particles into metal required researchers to overcome myriad hurdles. First, they had to determine the optimal size. Li said the actual size of these particles is a trade secret, but all are less than 100 nanometers, more than a thousand times smaller than the width of a human hair. Particles that are too large or small tend to have problems serving their function as peacemaker—effecting a homogeneous metal microstructure with small, consistently shaped and spaced grains.

Researchers also needed to ensure the tiny particle didn’t react with existing chemical elements in the molten material. “If certain micro-particles in molten metal spur some limited chemical reactions, that could be fine,” Li explained. “But if nanoparticles chemically react even a little bit, they’re so small that they effectively just disappear.”

At the same time these particles need to have good wetting characteristics, able to incorporate themselves between metal atoms without oxidation. Put nonscientifically, the nanoparticles need to “play nice” with existing elements of the metal microstructure.

The last characteristic is the most critical: The nanoparticles need to disperse in the right way. That is, they need to enter the molten metal and disperse in an orderly fashion within the microstructure. Properly dispersed, the nanoparticles can prohibit excessive grain growth and help control the grain’s shape: small, globular, not stretched or extended in any distinct direction; and consistently spaced, at least relative to conventional materials. For aluminum 7075, a titanium carbide ceramic fits the bill.

But Li added that identifying the material type and size of the nanoparticle is only half the battle. The other half is the method of infusion. How does one infuse nanoparticles into molten metal, considering the harsh environment? Even more challenging, how does one infuse these particles in a scalable way? Sure, a researcher could develop a contraption, using vibratory techniques and other methods, that infuses nanoparticles uniformly into material. But the processing costs would probably be high and scaling it up into a large operation would be a pipe dream.

But Li and his research team found a way to infuse the nanoparticles in such a way that they self-disperse—or “self-assemble,” in nanotech-speak—in molten metal, an extreme environment. The finding was significant enough in nanotech circles that the journal Nature published two papers on the topic, one in 2015 and another in 2019.

“Self-dispersion is absolutely key to this technology,” Li said. “Without self-dispersion, you cannot do mass production. It really was a milestone.”

Part of self-assembly has to do with the choice of material, which can vary depending on the application. Titanium-based ceramic works well for aluminum 7075, but according to Li, steel will require another grade of nanoparticle.

Even more critical is the exact method of infusion and nanoparticle dispersion, a challenge that for Li consumed years of lab and industrial work. “The first challenge was to find out how to disperse nanoparticles inside molten metal,” Li explained. “It took us 12 years to discover a solution for that problem. Then we had another problem: How do we disperse these nanoparticles uniformly to achieve grain refinement and modification in mass production? That took us another 10 years. Then, finally, in 2015 we made a breakthrough.”

nanoparticle morphology in sheet metal

Images from a scanning electron microscope show a material’s postprocessing morphology without (left) and with nanoparticles. Photo courtesy of Oszie Tarula, UCLA.

What was that breakthrough, exactly? The fundamental science behind the process has been published in academic journals, and it involves the battle between the physical world’s entropic tendencies—that is, things tend to evolve from order into disorder. “The principles behind entropy determine how these nanoparticles self-disperse,” Li said.

Although the science behind it is published, the exact mechanism Li and his team use to infuse these nanoparticles on an industrial scale is, alas, a trade secret—and yet understandably so. After all, that secret led to MetaLi’s founding in 2016. According to Li, recent breakthroughs have made nanoparticle-infused material practical and cost-effective. Because such a small amount of ceramic materials is required—these are only nanoparticles, after all—and the fact that these particles self-disperse, additional material costs are negligible.

“The nanoparticle-infused filler wire we now have basically modifies the metallurgical solidification process to solve the cracking problems,” Li said. “That modified solidification essentially makes the unweldable weldable.

“We’ve found that we can change the temperature by more than 450 C, and the grain size doesn’t change,” he added. “The grains remain uniform and exceptional.”

For years fabricators have worked to engineer sheet metal fabrication solutions for customers to suit the application. Thanks to nanotechnology, the industry might soon be able to engineer the sheet metal grains themselves, making every downstream process—from laser cutting and punching to forming and welding—more stable and reliable than ever.

About the Author
The Fabricator

Tim Heston

Senior Editor

2135 Point Blvd

Elgin, IL 60123

815-381-1314

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.