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Aluminum

Perfect powder

By Lynn Stanley

Above: HRL produced a demonstration pair of thrusters with its new AI 7A77 high-strength aluminum 3D-printing powder.

Research lab invents first aluminum additive feedstock registered by the Aluminum Association and signs NASA as its first customer

December 2019 - Historians call the 1940s the decade that changed America. In the wake of World War II, Chuck Yeager broke the sound barrier, the first atomic bomb was tested and the first atomic clock was built for the National Bureau of Standards. The Polaroid camera made its debut along with Elmer’s Glue-All and freeze-dried coffee.

The “Flying Forties” also saw Hollywood film producer and aeronautical engineer Howard Hughes emerge as an architect of the airline industry. A millionaire at the age of 18, he set four world records for straight-line speed, cross country and around-the-world flights. He developed and flew three ingenious new airplanes, and built and piloted the world’s largest wooden aircraft—the Spruce Goose.

In 1948 he established Hughes Research Laboratories (HRL) in Culver City, California. It was relocated to Malibu in 1960. In 1997, the physical science and engineering research centers were restructured as HRL Laboratories, a limited liability company performing research and development for The Boeing Co. and General Motors Co. as well as government and commercial clients.

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Theodore Maiman, left, demonstrated the first ruby laser in 1960 at HRL’s Malibu facility, right.

With inventions such as the world’s first laser; first ion-implanted, self-aligned gate metal-oxide semiconductor; and the first ion-propulsion engine under its belt, the company has helped to shape America’s technological trajectory for more than 70 years.

Housed in 250,000 sq. ft. of lab space and a 10,000-sq.-ft. Class 10 clean room, HRL’s scientists and engineers hold more than 1,100 patents.

Milestone

In September 2017, the organization hit another technology milestone when it introduced Al 7A77 high-strength aluminum 3D-printing powder. In February 2019, Al 7A77 became the first additive feedstock to be registered by the Aluminum Association. The first commercial sale of Al 7A77—to NASA’s George C. Marshall Space Flight Center—was announced in September 2019.

“We saw a critical need for broad industrial adoption of additive manufacturing (AM) but found that advancements in machine configurations and component design were quickly outpacing the capabilities of 3D-printable materials,” says HRL Chief Metallurgist John “Hunter” Martin.  “Out of the thousands of commercially available metal alloys systems, only 10 alloys can be 3D printed.”

The researchers chose aluminum for several additional reasons. The aerospace and automotive industries covet the alloy because it reduces weight without the loss of strength associated with other metals and it’s corrosion resistant. But previous attempts to print with aluminum resulted in cracks that formed during melting and solidification.

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Engineer Rod Smith developed the rifle-like Colidar Mark II laser range-finder, able to emit a light beam at 186,000 miles per second to measure target distances.

“Additive manufacturing is essentially equivalent to welding because you are taking layer by layer and welding each of those layers on top of the other,” Martin explains. “So the metal you use has to be weldable. We took an unweldable metal and made it weldable so that it could be used in conventional additive manufacturing.”

And in true Howard Hughes fashion, Martin adds, “It seemed like the hardest problem to solve so we picked it. Hot cracking in alloys has been a problem for a century—with aluminum proving particularly difficult.”

The team began looking for answers to the conundrum in 2014. “It required throwing away the rule book,” Martin says. “We didn’t invent new physics, we just leveraged physics in individual spots and integrated it in a way that hasn’t been done before.”

HRL used low-cost alloying elements such as zinc, magnesium and copper to strengthen the aluminum with targeted grain refiners that control solidification. The fine, uniform grain structure controls metal hardening for a stronger additively manufactured part.

Next steps

“The development of Al 7A77 is a real breakthrough,” says Zak Eckel, corporate partnerships manager for HRL, “because it’s the first time the inherent performance, cost and scale advantages of industrially relevant, wrought aluminum alloys are available with the design freedom of additive manufacturing. 3D-printable versions of high-strength aluminum alloys in the 2000 and 7000 series are very desirable for the aerospace, automotive and consumer product industries.”

With commercialization in mind, HRL contacted the Aluminum Association in December 2018 to identify a method for codifying the alloy.

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HRL Corporate Partnerships Manager Zak Eckel and Chief Metallurgist John “Hunter” Martin.

“We had a project to create such a system under development for some time but there was no need for it until HRL approached us,” says Jack Cowie, director of standards and technology for the Aluminum Association. “This alloy is very, very unique because it’s the first successful attempt to make a 7000 series aluminum alloy for printing.

“The ability to place tiny particles of zirconium on the outside of the powder enhances grain nucleation,” Cowie notes. “Without zirconium, you get a giant grain structure which is notorious for poor ductility and strength. You could only get the lowest strength alloys to respond this way, which is why conventional aluminum would not work.”

Formed by an Act of Congress in 1933, the Aluminum Association began developing aluminum alloy standard designations in the 1950s. One driving force was the aerospace industry, which recognized that standards were needed to ensure consistent material quality. Since then, the organization has registered more than 500 alloys.

“It’s important to know what you are buying,” Cowie says. “When you specify a nomenclature, you know exactly what it is. For 7A77, we’ll have another registration system for the cast and wrought tempers [heat treatment that enhances and defines mechanical properties] of this alloy. When you get into military and aerospace standards like AMS, ASME and PPD, the ability to have our standard listed among those is critical for commercialization.”

For example, he continues, Boeing can ask for 7A77.60L-ST6, the most common high-strength temper, register it with a set of properties and know exactly what they are going to get. “I think we will displace a lot of heavy nickel parts because this new alloy is high strength and will be able to compete. It’s changing the product mix.”

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Hunter Martin, left, examines an intricate truss structure made with A1 7A77, right.

It’s official

The Aluminum Association assigned registration number 7A77.50 for HRL’s aluminum powder that will be used to additively manufacture the alloy and 7A77.60L for the printed alloy. The organization’s “purple sheets” are the latest addition to its long-running “rainbow sheet” series, which provides alloy designations and chemical composition limits for various types of aluminum.

“It’s exciting,” says Matt Meenan, senior director of public affairs at the Aluminum Association, “because it’s the first new material registration record we’ve created in 20 years.”

The alloy numbers will be trackable back to HRL like a DNA signature. But the registration also lends immediate credibility to the product.

“We are the architecting materials group for HRL,” says Eckel. “We look at how we can leverage a material’s properties and capabilities in a new way; how we can recombine and reformulate within the known world of metallurgy.”

Familiarity is important for prospective customers. “When we can provide a registration, it raises people’s comfort level and tends to reduce the skepticism that can occur when you say you’ve created something brand new.”

The launch and registration of 7A77 was timely for Omar Rodriguez, a mechanical engineer on the materials science team at the George C. Marshall Space Flight Center’s Metals and Processes Laboratory. Located in Huntsville, Alabama, the center has been solving complex technical problems and developing science instruments and complex space systems for nearly six decades.

“We cover the materials science paradigm by developing processing parameters and characterizing the microstructural and overall performance of a myriad of materials of interest to us and the aerospace sector in general,” says Rodriguez. “We read about 7A77 in Nature.”

As HRL’s first customer, the lab procured 7A77 in two particle sizes—50µm for the powder bed fusion (PBF) method of AM and 80µm for the direct energy deposition method. Characterization of the printed test articles will be split between Marshall’s materials and processes laboratory and the Professor Kavan Hazeli laboratory at the University of Alabama’s Mechanical and Aerospace Department.

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The U.S. Cygnus space freighter is seen while the Canadarm2 robotic arm, guided by NASA astronaut Jessica Meir and fellow Flight Engineer Christina Koch as her backup, reaches out to grapple the 12th resupply ship from Northrop Grumman.

Potential applications

“Broadly speaking, [using] 7A77 could mean we will be able to produce additively manufactured aluminum structures for critical and/or high-performance applications,” says Rodriguez. “If the comprehensive characterization of the test articles validates 7A77, NASA could potentially benefit on two fronts: ground-based and orbital manufacturing activities.”

Rodriguez defines ground-based AM efforts as the production of components with reduced lead times and manufacturing-associated costs while meeting stringent performance requirements.

“As an example, we are exploring the use of additive manufacturing techniques for the production of next-generation rocket engines with the goal of cutting their cost by a considerable fraction,” he says. “One of our goals is to develop a sustainable exploration program.”

Orbital manufacturing involves the transformation of raw materials into usable components in a reduced gravity environment. “This area remains at a concept development stage; at Marshall Space Flight Center, we are working to mature it alongside other NASA centers,” Rodriguez says. “Ultimately, we don’t want our space assets to be constrained by fairing capacity or the harsh mechanical conditions experienced by payload at the launch event.”

As a first step to realize this vision, the researchers are concentrating their efforts on technology aspects such as the identification of  material systems and processing techniques candidates. “Once fully matured and infused, orbital manufacturing technologies could signify better asset management and decreased technology refresh times, giving us sustainable operations that will pave the way for large-scale orbiting structures and construction on lunar and Martian surfaces.” Rodriguez explains.

HRL believes Marshall Space Flight Center is taking the right approach. “To realize the potential of 3D printing, you have to design for additive manufacturing,” Martin says. “Next-generation machines also need to be built that can add more lasers and change how a powder coats, spreads  and builds to gain higher production efficiencies. And you need the right materials.”

Eckel agrees. “It will take time for the additive manufacturing infrastructure to build up before we start seeing common metal parts being 3D printed. We see the headlines and want the technology now, but additive manufacturing is not a new iPhone app,” he says. Materials and manufacturing require capital and time to mature stable and reliable new technologies. “It has to be done right the first time, and that takes patience and optimism.” FFJ

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