A Natural Evolution
|Fig. 2—Made of Inconel 718 nickel, vent tubes printed for Lockheed Martin and used on the NASA Orion space capsule originally consisted of six parts welded together in a process that took weeks. Produced via AM in only 40 hr. as one part, including screens, four tubes fly on the Orion.
The printing of plastic patterns for investment casting, via stereolithography, marked the company’s introduction to AM 20 years ago, according to Don Larsen, Arconic’s vice president of R&D for advanced manufacturing and advanced powders. In 2015, two events signaled an all-in attitude with metal AM.
The mid-year acquisition by Alcoa (now Arconic) of RTI International Metals, Inc. included the RTI subsidiary, Directed Manufacturing, an Austin-based AM provider of metal and plastic components, production parts and prototypes. This signaled an entrance into laser-powder-bed printing. Simultaneously, it purchased a Sciaky electron-beam 3D metal-printing machine, a wire-fed HDR unit installed at its Technology Center.
“We make parts from materials such as aluminum, titanium and nickel, using all types of metalworking and casting processes, so it was natural that we would look at AM,” says Ed Colvin, Arconic’s vice president of technology in the Engineered Products and Solutions business. “We looked at the AM space for not just detailed, smaller laser-powder-bed parts, but also to see where AM might fit within our portfolio for manufacturing larger parts.”
On the laser-powder-bed side, Arconic produces parts from nickel, titanium and some stainless-steel alloys, while high-deposition-rate work mainly centers on titanium. As we’ll see, the company continues refining and qualifying its processes and developing AM-friendly materials while exploring new AM applications.
|Fig. 3—Arconic has developed its proprietary Ampliforge process, which combines AM with forging to produce large parts using fewer operations, using less material and taking less time. With Ampliforge, optimized 3D-printed preforms undergo final forging.|
While Arconic has a long history in metallurgy, material processing and manufacturing, AM demands a unique skillset and knowledge base, which the company has been working on.
“With traditional methods such as forging or casting, we deal with metal alloys cooling from liquid fairly slowly into a solid,” explains Larsen. “But with 3D printing, we need to understand the evolution of the microstructure as it cools very quickly from a molten to a solid state. If we understand that, we can achieve better and more consistent mechanical properties needed for aerospace applications.”
Arconic boasts an array of powder-bed machines—including EOS, Renishaw, SLM Solutions and ExOne—to produce metal parts for Airbus, Lockheed Martin and other aerospace heavy hitters. The company has expended much effort in researching and developing processes, and understanding microlayer behavior and characteristics, then getting machines to perform at their best via optimized parameter settings.
“You tune machines with printing parameters that get you from point A to point B,” Larsen says. “This is where many companies have issues. Either they buy machines without the needed parameter adjustability, or they don’t understand that parameter adjustment is so critical.
“We can examine the AM process layer by layer,” he continues. “In the future our goal is to identify potential problems during production of each layer, and fix them in-process. When we qualify a part for an aerospace customer, we must establish a fixed process with inspection steps to ensure that we produce defect-free parts. That all goes back to the metallurgy and the printing parameters. The best parameters end up producing a part with the lowest porosity. When post-processing, including hot isostatic pressing, if we only have a couple of pores, we likely can close those pores.”
Though today Arconic often employs AM to reproduce parts formerly manufactured using traditional processes, the company is working to design parts with 3D-printing capabilities in mind as customers become more comfortable with the process, offers Larsen. An example: airframe brackets with designs optimized for laser-powder-bed production.
“When we do that, we can produce, say, 50-percent-lighter designs—structures that look very organic but use 50-percent-less material—that perform the same functions,” he says. “Our goal: optimized designs that only place material where it is needed.”
Arconic, via the laser-powder-bed process, also has found success joining multiple parts into one structure printed as a single piece. A perfect example: vent tubes produced for Lockheed Martin and used on the NASA Orion space capsule. Made of Inconel 718, each tube is the size of a 1-L water bottle and originally consisted of six parts welded together in a process that took weeks. Produced via AM in only 40 hr. as one part, including screens, four tubes fly on Orion.
“Part of our job is to educate potential customers and designers, and unlock their design space to an area where they have never been,” says Larsen. “We want to open minds to a new level of design freedom.”
Freedom also arises from developing new alloy-powder blends and delivery, and Arconic has been busy in this area as well.
“The powder is our smart ink, and we can optimize how powder flows to print the best parts,” Larsen explains. “Rapidly solidifying layer by layer influences the microstructure and allows the development of new AM alloys that cannot be produced using the traditional ingot-production process.
“Resulting from such research,” he continues, “Arconic is developing a high-temperature aluminum alloy that performs in temperatures to 450 F, almost unheard of for aluminum alloys. For example, at 450 F, 6061 aluminum has a strength of about 350 MPa, while our new powder-bed AL alloy exhibits a higher tensile strength. This opens up new part possibilities for aluminum.”
At the Arconic Technical Center, the company atomizes nickel, aluminum and titanium powders, to develop new alloys and better understand the powder-making process. The payoff: metal powders optimized with the properties needed to print better-performing parts.
Looking to Larger Parts, Too
Beyond smaller, detailed parts, Arconic employs AM to produce larger parts as well.
“We make some very large titanium and aluminum forgings, nickel discs and similar types of parts,” says Colvin. “We bring material to a controlled temperature and give it a 3D shape. These fundamentals apply to our traditional processes and the same holds true for AM.”
By mid-2015, the company was operating the Sciaky HDR AM machine at its technology center, and learning how to produce AM-optimized larger parts that meet stringent aerospace standards.
“Arconic has a long history in developing new materials and processes, so we have a wealth of experience in undergoing testing, qualification and approval,” Colvin says. “Of course, the larger the part, the more places we have to check to meet all of these requirements. Our larger parts may have less detail than some of the smaller parts, but much more volume and more metal being deposited, and deposited much faster. As part size increases, there are more layers and they get longer, and every layer has an interface where we must ensure integrity. We have to ensure reliable deposition and, as we have very high performance criteria, the materials and parts must meet that criteria.”
With extensive experience in conventional processes, Arconic brings that knowledge and technology to bear in AM.
For example, Arconic retains an entire set of processes to produce low-residual-stress forgings.
“We call it Signature Stress Relief forging,” says Colvin. “Part of that involves understanding how the residual stress develops. Thermal processes and our working processes, the microstructure of alloys—all of those relate to residual stress. Residual stress also matters in AM. We can take models for traditional processes and apply them to the additive space for similarly sized parts, which puts us way ahead of the game. But we still have to verify and tweak—we have to prove the model and optimize it for the additive space. So we have a big head start but we are still working at it.”
Long a leader in forging, Arconic has developed its proprietary Ampliforge process, which combines AM with forging to produce large parts using fewer operations, less material and less time. With Ampliforge, Arconic creates an optimized 3D-printed preform that then undergoes final forging. Traditionally, forgers cast an ingot and that stock might be shaped into a cylinder or rectangle, then worked to create an asymmetric shape, or to introduce ribs or other features, with additional work needed to create the final forging. All of this requires multiple operations and a large amount of material shedded throughout. Preforms that closely approximate final part shape and contain required material characteristics ease the process considerably.
“We use our forging models to design the optimized preform with the shape and microstructure that we want,” Colvin explains. “Ampliforge results in fewer operations and lower material input into the finished form.”
As Arconic continues researching the economics of the AM process, the company is finding ways to employ AM as a low-cost way to produce larger parts, either through HDR or Ampliforge processes. A cooperative research agreement with Airbus is driving development on the HDR end, with the goal to produce aerospace parts to 1 m long.
Material-wise, near-term AM efforts for large parts at Arconic focus on titanium, and that is the material being researched with Airbus. Down the line, Colvin sees multi-material parts and other advances.
“Long-term,” he says, “we think that AM will open up material options, and Arconic is uniquely positioned to address that.” 3DMP
See also: Renishaw Inc., Sciaky Inc, EOS of North America, Inc., ExOne, SLM Solutions NA, Inc.
Technologies: Powder-Bed Systems