Above: Deposition rates typically range from 7 to 20 lbs./hr., depending on part geometry and the material selected, Sciaky states.
For aerospace applications, additive manufacturing has evolved from impracticality to essential tool
September 2015 - When thinking about the factory of the future, Bill Flite, senior manager for Lockheed Martin’s Corporate Engineering, Technology and Operations office, considers how advanced manufacturing will become a hybrid of conventional and additive techniques. “It really is another tool in an ever-growing toolkit,” Flite says. Aircraft manufacturers like Boeing represent the vanguard of technological innovation on such projects as the 702MP satellite, constructed using 3-D printed metal parts. Boeing has 3-D printed parts on 10 different military and commercial aircraft production programs. To date, these parts are used on non-flight critical systems only so if a part fails, it poses no risk to the safe operation of the aircraft.
Boeing is working to accelerate use of 3-D printing technology in its products in ways that make sense for the company without increasing risk for its customers, according to Nathan Hulings, communications specialist, Boeing research & technology.
“We are conducting research into 3-D printing applications for both metals and polymers. Metals research is focused on applications that offer challenges in the supply chain,” Hulings says. “These would include difficult-to-cast or machined parts that are demanded in a limited quantity.” Boeing evaluates all categories of additive metals, including large, wire-fed systems; direct metal deposition systems; and powder bed systems. Powdered metal allows for smaller, finer features while wire allows for larger parts but at the expense of feature resolution.
GE Aviation recently created a cobalt chrome fuel nozzle component using additive manufacturing, a critical part able to withstand harsh environments within gas turbines and jets. Design cycles in aerospace typically required five to ten years, with a product’s life being 30 years.
“Additive manufacturing is exponentially speeding up the design cycle of components and systems,” says Steve Rengers, principal engineer at GE Aviation, Cincinnati. This gives GE “the ability to design, test and release a complicated product or very complex product like a gas turbine, for example,” in a tighter timeframe. “This is a dramatic difference from the way it used to be.”
Advances in rocket engine technology often lead to tomorrow’s aircraft manufacturing processes. 3-D printing systems are increasingly capable of creating larger parts with a high degree of geometric tolerance and yield properties capable of surviving the harsh and stressful environment of rocket engines at Sacramento, California’s Aerojet Rocketdyne.
“Five or six years ago the largest powder bed system was roughly a 9 in. cube,” says Jay Littles, director of advanced launch vehicle propulsion at Aerojet Rocketdyne, which primarily uses powder bed processes such as selective laser melting (SLM) and electron beam melting (EBM) to form parts. These systems are getting larger, allowing for larger components, which makes complex assemblies possible, says Littles. Some companies expect to realize a greater return on investment by shifting to additive versus traditional manufacturing processes.
Building large parts quickly is economically attractive for aerospace applications. At NASA, Karen Taminger, materials engineer, studies how using additive manufacturing technology can advance future subsonic transport aircraft, commercial, regional and big jumbo jets. Electron Beam Freeform Fabrication (EBF3) is a large-scale 3-D printing process that uses an electron beam to melt metal wire. The head is programmed to follow the shape of the part, depositing the melted wire along the path.
NASA is also using additive manufacturing to tailor stiffness along the length of the wings to alleviate gust loads generated by instabilities in the air that can cause the wing of a jet to vibrate or flutter, Taminger says. The trick is to combine structural design with specialized material properties, reducing weight without sacrificing stability.
Traditional forging is a high-cost process, with tremendously long lead and delivery times, says John O’Hara, global sales manager at Sciaky Inc., a Chicago company pioneering electron beam additive manufacturing (EBAM) and electron beam welding technology. “We commonly hear metal forging taking between 12 and 18 months to complete,” O’Hara says. “With EBAM, you can produce a near-net shape part, also known as a preform, in a matter of hours. From there, the preform undergoes post-process machining” and, depending on the part size and complexity, it can take days to weeks instead of months.
Sciaky has been developing its 3-D printing technology for 15 years. It first offered an EBAM system for purchase last fall and has since sold three. Previously, EBAM was used in defense research and development projects, as well as selective contract service engagements.
EBAM starts with a 3-D model from a CAD program. Sciaky’s fully articulated, moving electron beam gun deposits metal via wire feedstock, layer by layer, until the part is built and ready for minor finish machining, according to the company.
While the cost of this new technology remains high at the moment, as research continues, acquiring the technology will become more affordable, says GE Aviation’s Rengers. “It’s going to take time and the technology we’re using isn’t something you can buy online and have in your garage,” he adds. “Additive manufacturing is a very exciting tool [that] opens up so much design space and so much capability we’ve never had before. We’ll always need conventional processes but this gives us more flexibility.”
“The real power of additive manufacturing comes out when you have a designer that knows how to take advantage of the process,” says Timothy W. Simpson, Ph.D., professor of mechanical and industrial engineering at Pennsylvania State University and co-director of the Center for Innovative Materials Processing through Direct Digital Deposition (CIMP-3D). “Now designers can design and produce an organic shape or lattice structure, which wasn’t possible before additive manufacturing.”
Here the manufacturing process is reversed. Instead of treating metal with heat first like with casting or forging, the shape is created first, then heat is applied, explains Simpson, who also has used a Sciaky EBAM system at the university.
By looking at combining design, fuselage and wing structure, Taminger says, NASA aeronautical engineers are asking, “Can we change aircraft design instead of using straight ribs and spars as the frames? What we are looking at is considering a new structural design. Right now when you look at a plane, everything is laid out on a rectangular grid,” the easiest format under which “to assemble and build,” she explains. “But as we continue to look at new ways to reduce weight, we are revisiting the structural design.”
NASA wants to increase yields and lower energy consumption. For example, when refining blocks of material to be machined into components, “Often times we start with 100 lbs. [blocks] of aluminum and the final part is only 10 lbs.,” Taminger says. “Additive offers a lower-cost energy solution if you can build the structure up rather than take a block and machine it down.”
At Penn State, Optomec’s directed energy deposition systems feature multiple powder feeders. The Albuquerque, New Mexico-based company’s production grade 3-D printers make printing full functional end-use devices possible. “This [technology] allows us to print different materials at different places on a single part—better hardness here, better corrosion resistance here, better wear resistance here, etc., but our engineering analysis tools aren’t capable of designing these parts yet. We’re only scratching the surface in terms of that work,” says Simpson.
Lockheed Martin is also exploring how 3-D printing best lends itself to tailoring alloy composition, says Slade Gardner, fellow at the company’s Space Systems division. “We’re very eager to address physical properties—such as thermal expansion, radiation tolerances, low densities of materials—in our design thinking,” Gardner says.
With a single structure there is no longer the weight of fasteners to join it together. Instead, the properties of the structure can be locally tailored, Taminger notes.
Changing material composition creates a new generation of alloys and functionally graded materials (FGM) for potential performance advantages that are otherwise unobtainable today, Sciaky’s O’Hara says. EBAM systems can be equipped with multiple wirefeed nozzles on a single EB gun, allowing it to simultaneously feed, with independent program control, two or more different metal alloys into a single molten pool. “It is very beneficial for creating graded or super alloy parts or ingots,” O’Hara says.
Mass production is the next step in additive manufacturing for aerospace components, says Spartacus3D CEO Charles de Forges. Based in La Clayette, France and part of Paris-based Farinia Group, the company uses SLM and EBM processes to build parts. Titanium and Inconel 718 are priority materials, according to de Forges. Static parts are on the docket, with rotating parts further down the road.
The qualification process to inspect parts made using 3-D printing is the next development front. “Everything that I’ve seen so far has been inspired by casting, including x-ray inspection and fluid penetrant testing, similar to what’s used in forging and casting,” says de Forges.
Aerojet Rocketdyne relies on a combination of select traditional nondestructive inspection processes combined with closely controlled additive processes. “There’s still a lot of work to do to develop the ability to perform in situ process monitoring it for qualification,” Littles says.
Process monitoring, inspection, the digitization of the production environment, and using that in a meaningful way to control the process, is what “it’s all about,” adds Gardner.
General Electric designers and engineers believe the latest advances in the last two to three years have not been around the core technology but rather more around using process monitoring for general quality assurance, says Rengers. “Nondestructive means a combination of CT scans and x-rays—it won’t be one or the other. Moving forward, there will be a migration to more quality assurance of the process versus post-process inspection.”
The learning curve stretches onward. “There are so many knobs and dials that we can turn—we don’t have them all figured out yet—not to say that we won’t,” Penn State’s Simpson says. “Companies are spending money to figure this out, but they won’t share that information because of the competitive advantage it provides. We’re trying to get the knowledge out there by collaborating with many companies and sponsors. It’s part of our center’s mission as DARPA’s Manufacturing Demonstration Facility for Additive Manufacturing and our partnership with America Makes.
“Two and a half years ago when I gave tours [of the CIMP-3D facility], people were asking, ‘Can you really 3-D print metal parts?’ and then we’d show them sample parts,” Simpson continues, adding that expectations have since risen significantly. “Now they’re asking: ‘When can we start making multi-material parts?’” FFJ