By Andrea Svendsen, Managing Editor.
HRL Laboratories is a science, technology and engineering company that was first founded by Howard Hughes in 1948 under the name Hughes Research Laboratories. Renamed HRL Laboratories, LLC in 1997, the company is currently owned by Boeing and General Motors, with two facilities located in Malibu and Calabasas, CA. The company conducts pioneering research with the aim of realizing groundbreaking technologies and providing real-world solutions for the space, aircraft, automobile, and consumer products industries. Recently, HRL has turned its attention to the development of architected materials, with the introduction of a high strength aluminum alloy powder for additive manufacturing.
Additive manufacturing (3D-printing) is a unique technology that enables complete design freedom and performance optimization through increased functionality, lower weight, and reduced complexity and part counts in assemblies. “HRL’s mission is to constantly look forward and identify unique technical challenges and opportunities,” stated Dr. John “Hunter” Martin, a lead researcher at HRL. “An analysis of the additive industry found that significant advancements in machine configurations and component design were rapidly outpacing the capabilities of 3D-printable materials. We saw a critical need for broad industrial adoption of additive manufacturing.”
The company has addressed this need by developing a scalable, alloy-and-machine-agnostic system, which they are now working to commercialize. “Our alloy-design approach ultimately enables control of solidification during additive manufacturing, making previously unprintable alloys printable for the first time,” explained Martin.
Developing 3D-Printable Alloys
HRL began the development of its new high-strength, wrought-equivalent aluminum alloys for additive manufacturing in 2014. Over several years, a team of nearly a dozen scientists and engineers worked thousands of hours on research, production, simulation, testing, and analysis in order to achieve the desired results.
One of the keys to the team’s success was their ability to draw on the experience of its Architected Materials department, which works to develop novel materials. “Our Architected Materials team examines conventional and well-known materials and processes with the aim of redesigning and reengineering them in order to achieve enhanced performance,” said Zak Eckel, manager of Commercial Partnerships, HRL Laboratories. The department implements its unique micro-lattice process to architect lattice or repeating cellular structures in materials. “If you look at a large construction, such as the Eiffel Tower, it’s highly architected with lattice structures specifically designed to maximize stiffness, efficiency, and weight—which is how we approach our work. What Hunter and his team have done is apply that thinking to metallurgy and metal 3D-printing.”
Drawing on decades of existing alloying research, the company decided to focus on 7075 alloy, which is known for its high strength (ultimate tensile strength of around 570 MPa and yield strength of around 500 MPa) and is commonly used in the aerospace industry. The aim was to provide engineers and designers interested in utilizing additive manufacturing with access to an alloy they are already familiar working with. The challenge was that 7000 series alloys are generally considered to be unweldable. “If you try to directly weld 7075 with 7075, you will get a big crack down the center,” said Martin. “This has to do with the alloy composition, which provides the strengthening benefits that we want to keep, but also develops a microstructure that makes the material prone to cracking during solidification.”
One of the main methods for the additive manufacturing of aluminum involves using focused thermal energy in the form of a selective laser melting, electron beam melting, or direct energy deposition techniques to provide localized and selective fusion bonding of powders laid down in layers. In other words, additive manufacturing is analogous to welding and, therefore, has tended to implement lower strength alloys with higher weldability. In order to be able to achieve a 7075 additive manufacturing alloy, HRL had to find a way to maintain the alloy composition, while adjusting the microstructure to make it amenable to the 3D-printing process.
HRL looked into existing research on the subject and found that this susceptibility to cracking could be prevented by shifting the microstructure in the 7075 from columnar grain structures to small equiaxed grain structures. “Aluminum has 6-7% volumetric shrinkage during solidification and 7075 has one of the higher coefficients for thermal expansion,” explained Martin. “Because of this, if you have any rigid columnar grain structures in the material, then it’s easy for the material to pull itself apart as it’s solidifying. But if you’re able to achieve an equiaxed grain structure, then you can get more of a mushy granular solid that moves around as it’s solidifying and can accommodate the strain involved.”
There are a number of processing methods available to achieve these fine grain structures, such as controlling thermal gradients, solidification velocities, and other factors. Many researchers have been investigating applying these methods for additive aluminum alloys. However, rather than focusing on these processing routes, HRL determined that the most efficient method would be to inoculate the alloy.
The general approach was to control the microstructure during solidification using a nanoparticle grain refiner to promote nucleation of new grains in the melt.1 The refiners needed to be readily available, to remain in solution at very high temperatures, and to be extremely active as a nucleation site. Another requirement was that the compound needed to be lattice matched, so that it would mimic the atomic structure of the aluminum, making it energetically favorable for grain growth.
HRL applied a number of analytical approaches with large state analytics. Researchers studied thousands of actual intermetallic structures, examined their crystal structure, and then tried to computationally match those to the aluminum. “We identified the Al3Zr intermetallic as the ideal structure for this purpose,” said Martin. “We can add zirconium particulates at concentrations that enable the intermetallics to be readily available in the liquid during additive manufacturing.”
Through its detailed nanoparticle functionalization work, the company was able to create an aluminum alloy that was successfully tested using selective laser melting, showing no signs of cracking even at the first trial. “It was a great validation of over a year of physics, analytics, and other work that went into trying to understand this process,” said Martin.
HRL published the result of their study in Nature in 2017.1 According to the study, an analysis of the microstructure revealed “a substantial difference between components additively manufactured from stock powders and those produced with nanoparticle-functionalized powder.” Compared to the unmodified material, the grain refining nanoparticles were able to achieve “low-energy-barrier heterogeneous nucleation sites ahead of the solidification front [that] induces a fine equiaxed structure under the same processing conditions as for the unmodified powder. This results in crack-free microstructure with grain sizes of about 5 µm, 100 times smaller than the grains in the unmodified material.”
Following this publication, the company noted that they were only achieving 375 MPa, significantly lower than the strength of 7075. They discovered that this was due to the vaporization of the magnesium and zinc elements during the additive manufacturing process. “During 3D-printing, you’re hitting the aluminum with a high power laser, so the melt pool is very hot,” explained Martin. “We realized that we would probably not be able to avoid vaporizing these elements. Therefore, in order for the final product to achieve the correct concentration of magnesium and zinc, we needed to alter the initial powder composition to account for the vaporization.”
After further research into magnesium and zinc ratios, HRL was able to drastically increase the strength of the final 3D-printed aluminum alloy. The alloy was further refined through additional analysis, post-processing treatments, and the development of better processing parameters, enabling them to consistently achieve the same compositions and material properties in the final components.
“We basically figured out how to make 7000 series alloys 3D-printable, as such we have the strongest 3D-printable aluminum on the market,” noted Martin. “We’re pretty confident that we’re going to stay the strongest, since a lot more research would have to be done on aluminum in general in order to surpass the strength of the 7000 series alloys that we’re already taking advantage of.”
Moving Towards Commercialization
Following the successful development of the alloy, HRL moved toward commercialization, with Eckel leading the effort and Martin’s technical team providing support. One of the considerations was making sure that this was an industrialized and adoptable alloy and process, which required that the alloy be qualified through a third party. “Once we realized we would need some sort of qualification or certification to push the adoption of this alloy, we contacted the Aluminum Association and it turned out they were in the middle of their development of a new registration system,” said Martin.
The Aluminum Association announced its new Aluminum Alloy Designations and Chemical Composition Limits for Additive Manufacturing (AM) and Powder Metallurgy (PM) Feedstock and Products, otherwise known as the Purple Sheets, in April 2019.2 The Purple Sheets is a new registration system that details the important characteristics of powder and finished product alloys used in additive manufacturing. The system distinguishes between powder alloys and 3D-printed alloys, taking into account the concerns regarding vaporization previously described.
HRL was the first company to register an alloy in the Purple Sheets, including 7A77.50 for powder feedstock and 7A77.60L for the final 3D-printed products, as well as other alloys. “On a practical level, this registration will connect us to this particular alloy composition forever,” noted Martin. “These alloy registration numbers will always be traceable back to HRL, like a DNA signature. They also provide a level of confidence that the alloy will have a traceable and continuous composition far into the future, giving designers confidence that the alloy can be utilized for decades to come.”
In order to accelerate the commercialization of its new alloys, HRL launched a new commercial branch, HRL Additive. The company is working with end users to identify both existing and emerging applications for additively manufactured aluminum products. “With additive manufacturing there are so many new design possibilities, we really don’t know where it may end up,” said Eckel. “That’s the beauty of this breakthrough; the sky’s the limit.”
In addition to being able to achieve a high strength similar to 7075, the new 7A77 aluminum alloy powder can be used with any appropriate metal additive manufacturing system without modification. Furthermore, the technology is extendable to multiple alloy systems. “It has the potential to substantially reduce development time for alloys used in additive manufacturing,” said Eckel. “We have several new alloys in development that we expect to release after our first registered alloy system. We believe our technology can be adapted to make any unprintable metal printable, a true reinvention of metallurgy.”
HRL Additive already secured its first commercial sale of the 7A77 high strength alloy to NASA’s Marshall Space Flight Center in October 2019. “Certainly, the 7A77 feedstock powder could unlock the production of large-scale components produced via fusion-based additive manufacturing,” said Omar Rodriguez of NASA’s Marshall Space Flight Center. “Printed test articles will be subjected to a comprehensive characterization regime expanding several length-scales. The end-goal of this research effort is to expand MSFC’s range of fusion-based additive feedstock materials. If successful in our research endeavor, the feedstock powder could be part of the aerospace-related assets produced at the planned large-scale, advanced manufacturing facility.”
HRL has developed a system to produce high strength aluminum alloy powders capable of combining material performance, industrial acceptance, and cost benefits with the design freedom of additive manufacturing. The company is currently commercializing its 3D-printable aluminum alloys and has started operation of a production plant dedicated to producing the aluminum powders at their Malibu office.
“Additive manufacturing has immense opportunity to revolutionize manufacturing and engineering—enabling greater efficiency, performance, and safety for many industries including transportation, space travel, consumer products, and healthcare,” said Martin. “As with most industrial applications, success depends on proper materials, which is a glaring issue with additive manufacturing. We are proud to have developed a material that will enable engineers to combine the freedom of design inherent in additive manufacturing with the performance of high-strength aluminum alloys.”
In addition to working toward the development of new additive aluminum alloy powders, HRL is examining how their system can be applied to improve the weldability of high strength wrought aluminum alloys. “If you look at how an airplane is produced, you’ll notice that there’s not a single weld anywhere on the aluminum. Since welding the high strength aluminum is something that just isn’t done, you have to join the material using millions of rivets instead,” noted Martin. “We can envision a whole new level of manufacturability with welded components using high strength alloys. That’s something we’re actively investigating and we think there’s a large range of potential applications out there.”
- Martin, John H., et al., “3D printing of high-strength aluminium alloys,” Nature, Vol. 549, September 2017, pp. 365–369, www.nature.com/articles/nature23894.
- 2. Smith, Patricia, Jack Cowie, and John Weritz, “Registration System for Aluminum Alloys Used in Additive Manufacturing,” Light Metal Age, Vol. 77, No. 4, August 2019, pp. 72-75.