Lighter, Hotter, Stiffer, and Cheaper ... How Alloy R&D Is Working to Meet Casting Designers’ Requirements
Great advances in new alloy development are prevalent in the U.S. and around the world, including Germany, China, Japan, and Korea, but like all important product breakthroughs, alloys must be driven by customer need. Alloy researchers understand well that their work is tied tightly to meeting today’s widespread demands for improved light-weighting, performance in higher temperatures, as well as strength and stiffness.
“You can’t push a rope,” said Tom Prucha, president of MetalMorphasis and editor-in-chief of AFS’s International Journal of Metalcasting. “Alloy development needs to be customer-focused. With R&D, I like to use a small r and a big D––there needs to be innovation but driven by the customer. It takes a unique materials specialist to understand these needs, opportunities, and direct studies to tailor solutions for them.”
Toward that end, he said, the alloy R&D radar is definitely tracking with the demand for lighter weight castings as the transportation industry transitions away from legacy ICE-type components and moves toward integrating EVs. To the light-weighting driver, Prucha adds manufacturing’s need for cost reduction, as well as increased casting stiffness.
Not all casting buyers are seeking new alloys, of course. Many are focused on simply getting the best use from existing, listed alloys that are known in the industry. But for those needing improved characteristics for the demanding applications they’re working on, the heat is on.
“There are a couple areas that are hot right now, and that’ll sound like a pun after I explain—there’s more and more interest in aluminum alloys that perform better at elevated temperatures,” said David Weiss, president of Vision Materials who recently retired from Eck Industries as vice president of research and development. “That’s probably the most requested type of alloy development that I talk to potential customers about. Most aluminum alloys only maintain their strength up to about 155C, but above that temperature the properties rapidly deteriorate. Some alloys are better than others. But increasingly, customers are asking us to target properties for temperature ranges between 200C, and 350 C—they’re asking, ‘how can we get the best aluminum alloys to give us reasonable properties at those temperatures?’”
The big askers for high-temperature performing alloys are generally those from the aerospace industry, Weiss noted. Light-weighting is also another top priority because “the lighter you fly, the better the fuel economy, and that turns out to be a significant factor for the operating costs of an airplane,” he said. Currently, he added, for components that see greater than 200C, casting engineers must turn to an alloy like titanium, which is much more expensive and somewhat heavier than aluminum. Broadly speaking, the transportation market works castings hard and is therefore the dominant sector in need of alloys with high-temperature attributes.
Performance encompasses many different attributes of an alloy, Weiss added, such as strength (yield strength or fatigue strength), corrosion performance, electrical conductivity, or a controlled coefficient of thermal expansion. While customers would like to cherry-pick attributes––such as blending the elongation of a 206 alloy along with the castability of a 356 alloy––Weiss noted it’s not quite as easy as that.
“They’re interested in taking the best attributes of all the alloys and creating a new alloy with all of them, which, of course, is a challenging metallurgical proposition,” he said. “So we work to find compromise and see how close we can get them to the point where they will consider a new alloy that either we or somebody else has developed.”
Developing new alloys to meet these requirements is complicated by accessibility to certain metals, including rare earth elements, which involves a tangle of geo-political, supply chain, and environmental factors. Avoiding hard-to-acquire ingredients has become a leading challenge in the pursuit of new alloys.
Focus on Five
According to Diran Apelian, head of the Advanced Casting Research Center based at the University of California, Irvine, five areas of focus are colliding in the present day of alloy development, each making an indelible imprint on its progress and capabilities.
1) The power of ICME. First, he says, is the advent of Integrated Computational Materials Engineering (ICME), which has revolutionized alloy development with thermodynamic and kinetic software tools that inform researchers about what the microstructure may look like when various ingredients are combined––and thus informing and enabling alloy advancements in ways not possible even 10 or 15 years ago. As a result, the timeline for developing new materials is highly reduced, he said.
“As an example, one of the companies that I sit on the board of is QuesTek Innovations in Evanston, Illinois,” Apelian said. “We have developed alloys using ICME and got them to the market in record time––from infancy to commercialization in 18 months, when in the past it took years to accomplish the same.
2) Strengthening aluminum. An intense focus is in progress on how to make aluminum stronger and more ductile, both at room temperature and at high temperature (350C), according to Apelian.
For the last 50 years, he said, metalcasters have relied on a mechanism called precipitation hardening, which involves creating tiny precipitates in an alloy that become impediments to the flow of the material during deformation, and these impediments or resistances increase the strength.
“These precipitates, which are called GP zones, are not effective at temperatures above 250C, so there’s a limit of temperature for their effectiveness, Apelian said. “Now, we’re working on figuring out different mechanisms to strengthen the material. As examples, transition metals such as molybdenum, vanadium, titanium, and zirconium––these elements can provide different strengthening mechanisms in aluminum alloys in contrast to what has been done for the last 80 years with GP zones. This is a very exciting area in deploying new strengthening mechanisms in Al alloys.”
3) Good-bye heat treat. GP zones necessitate precipitation hardening, meaning they have to be heat treated, said Apelian. The alloy is first solutionized to ensure that all the phases go into one solution; subsequently, the alloy is quenched in water to capture the super-saturated solution at room temperature, and subsequently aging takes place to form the needed GP precipitates. During the quenching process, much distortion and residual stresses form, which would be good to eliminate, he said.
“Designing new alloys that are strengthened by mechanisms other than precipitation hardening is what we are working on at the ACRC consortium,” he added.
4) Neurotic alloys. One way researchers are eliminating GP zones is with high entropy alloys (HEA), which are comprised of equal parts of five different metal elements. It is unlike the traditional alloys, wherein there is a solvent and a solute, said Apelian. Rather, in HEA alloys, the atomic architecture is completely different from conventional alloys.
“Imagine you’ve got a 9’ x 12’ rug in your living room, and you’re trying to move it by 5 inches,” he said. “But if you got five people standing on the rug, it’s going to be hard to move it. The people standing on the rug are precipitates––they’re the impediments; they provide resistance to motion. If you have a lot of disorder, a crowd sitting on this rug, you’re going to have a hard time moving it.”
This disorder, he said, is created by adding equal parts atoms all mixed together—and by equal he means by number of atoms, not weight.
“I mix it all up together, and it’s chaotic. It’s a neurotic alloy,” he said. “This is another area of investigation, mostly being investigated for high-temperature applications, as well as cryogenic applications. There’s a lot of opportunities for aluminum-based alloys in this HEA space.”
5) Recovery and reuse. Apelian calls this the green alloy movement, and he is adamant that both industry and culture must keep the big picture at the forefront of any discussion about further alloy development.
“Materials are not renewable, but they are certainly recoverable and reusable,” said Apelian. “We are using most of the periodic table for all kinds of applications—energy, shelter, health care, mobility, aerospace—you name it. And it’s a zero-sum game. The big picture is not only alloy development, which we need—we need properties and we want things we don’t have today.
“The big question isn’t so much alloy development, but it should be how do we recover and reuse these alloys at the end of life? Technologies need to be developed, which we’re working on here at ACRC.”
Apelian said he’s currently working on a new metal recovery process that will potentially remove the need for sorting through technologies that will purify scrap metals prior to melting.
New Work at a Glance
Weiss said he, like Apelian, is involved in a number of development programs—some are funded by the federal government and are aimed toward anticipating future needs in energy and defense. One is focused on improving castability of 200 series alloys in order to effectively achieve more complicated geometries, he said.
“We have a program with the American Metal Casting Consortium where we’re looking at putting nano reinforcements in aluminum alloys––one of the attributes of doing that is it improves their castability with very small levels of reinforcements in the alloy. Usually the 200 series, which are really pretty good alloys, can be difficult in some geometries; we are working on how to make them more foundry friendly.
Another area to which Weiss has devoted considerable attention for the last eight years is with a range of high temperature alloys, specifically developing aluminum cerium alloys.
“Alloys containing cerium retain their strength better than alloys that contain silicon, like your 356 type of alloys,” Weiss said. “The personal focus of a lot of my work has been to continue to develop those alloys, which have a range of properties that customers are looking for. And that doesn’t mean just strength at elevated temperatures––it means fatigue strength, fracture toughness, coefficient of thermal expansion, and corrosion.
“Cerium is an element that behaves very much like silicon, but silicon doesn’t impart a lot of strength by itself, so it’s normally used in conjunction with magnesium. We’ve been looking through the periodic table and looking for those things that play well with cerium in order to both improve the castability and improve the strength of the alloys at both room temperature and elevated temperatures. Those can include magnesium, zinc, and nickel. There’s a class of cerium copper alloys that we have been working very hard on, as well.”
Another benefit of cerium in the alloy, he added, is that it can scavenge some elements that are usually not desirable in cast alloys like iron–– literally changing the morphology of the iron to reduce its negative impact on mechanical properties such as fracture toughness and conductivity.
“This has significant implications for the recycling of alloys,” Weiss said. “Over the last three or four years, customers have gotten serious about environmental and energy concerns. Our focus has been on how can we develop an alloy that is not as energy intensive, and how can we use more recycled product? The aluminum cerium effort is really a combination of trying to achieve properties as well as trying to achieve environmental improvement with the use of aluminum.
Stiffness—As Big a Deal as Temperature
For the aerospace community, Weiss has also worked on how to improve the ductility of the A 357 alloy, which contains beryllium, a carcinogen. He has worked on developing alloys that have less silicon, which increases elongation without losing appreciable strength in the alloy.
“I personally thought that was a great idea but it hasn’t gotten a lot of traction because it is outside of the alloys that are normally approved for aerospace applications,” Weiss said. “There’s a lot more work to be done before it can be commercialized.”
Weiss has also concentrated on a variety of approaches to increasing stiffness in aluminum alloys, one of its disadvantages compared to ferrous metals like iron and steel. Casting designs that are driven by the need for stiffness are often heavier than they need to be, he said. Irons and steels are chosen because they’re intrinsically three times as stiff as aluminum––to achieve stiffness with lighter weight, Weiss said he’s been doing a lot of work in metal matrix composites, where, for example, they’ve succeeded in affecting stiffness by adding certain kinds of particles into the melt.
“Recently, we’ve also been able to produce alloys that form purely metal reinforcements, which will also improve the stiffness of the alloy,” Weiss said. “That’s an area that has gotten a lot of attention recently, because you pretty much always would like to have improved stiffness for many applications. A common one in aluminum casting is gearboxes or transmission cases, where the designs are primarily stiffness driven. So, if you can improve the modulus of the aluminum alloy by forming some sort of composite either by adding ceramic particles or by forming a metallic phase that will improve the strength––that’ potentially as big a deal as designing alloys for elevated temperatures.
As active as alloy R&D is––with academia-industry partnerships making meaningful strides around the country––casting designers may wonder why they don’t see rapid, tangible evidence in the marketplace. Where are all the new properties manufacturing so earnestly desires?
First, commercializing a new alloy is very expensive–– easily $1 million, a cost that foundries may be hesitant to shoulder solo for a single customer. For this reason, many programs end up being government directed and funded to get them over the finish line, said Weiss.
Second, the metalcasting world is governed by numerous standards boards, to which part designers adhere when specifying alloys as well as processes––major industries won’t take the risk of deviating into new alloys until they’re adopted by a major specifications/standards board such as ASTM, SAE, ASME, ISO, FARS, and others. This is less of a problem for smaller, innovative customers, who often are the early adopters of new alloys, which was the case with Weiss’s aluminum cerium alloys for distributed hydropower. A company called Emrgy was designing from a clean sheet and was more interested in matching their exact requirements versus using standard alloys. Some of those alloys are also being sold into the medical equipment arena.
At the end of the day, collaboration will clear the path for responsive and responsible alloys that achieve the goals of humans while respecting the planet they live on. Just as researchers and universities around the world collaborate and share knowledge to advance the most successful ideas, so too will partnerships between casting designers and foundries yield the best results now and in the future.
“Collaborative efforts are so much better than just throwing something over the wall,” Prucha said. “We’re not enemies––we’re in the effort together.”