Aerospace leads in additive manufacturing

  • 16-Jul-2012 03:40 EDT
Morris_combustor sector_dmls02.jpg

This combuster is an example of the intricate parts possible with additive metals manufacturing. (Morris Technologies)

Disruptive technology usually means something so uniquely different that it requires manufacturers to adapt to it in sometimes significant ways. The trade-off is the promise of substantial advantages in cost, time, or performance.

Ben Horine, Director of Advanced Manufacturing of GE Aviation, thinks of metal-based additive manufacturing as a disruptive technology—in particular, powder-bed technology for creating metal parts. While he sees issues, the potential advantages are worth it if they can solve certain problems.

Advantages are many

Powder-bed additive manufacturing builds up parts layer by layer, forming cross sections of the part in 20- to 80-micron thicknesses. Employing a concentrated heat source such as lasers or an electron-beam, it melts or welds metal powder into solid shapes. It produces highly complex parts, with internal structures and shapes simply not available to traditional machining methods.

It also consolidates components made in traditional ways to create complex, intricate mechanisms.

“Compared to traditional methods where you have to build parts with multiple steps or need multiple parts brazed together, additive manufacturing allows you to combine these into one process” to reduce cost and time, Horine explained to AE.

“Some of our biggest successes are making parts for customers where we combined multiple components into a single part,” agreed Tim Warden, VP of Sales and Marketing at Morris Technologies, a leader in the use of metal-based additive manufacturing.

The company started developing such parts in 2003 and now boasts 21 additive manufacturing machines running 13 various materials. They employ both direct metal laser sintering (DMLS) from EOS as well as electron beam melting (EBM) from Arcam.

One example of parts consolidation is a part once composed of 26 individual machined components that were braised or welded together to form a working unit. Using metals-based additive manufacturing, Morris created a single part in a single process. Using such a simple method for making complex parts also means lighter components optimized for strength in only critical areas.

Properties, process, and design 

Renishaw recently acquired technology for metals-based additive manufacturing, also using a powder bed of metals that are selectively melted using a laser. Common materials that they provide their customers include forms of Ti6Al4V, TiCP, 17-4 stainless, 316L stainless, and various Inconels.

“There are some challenges with this technology,” said Robin Weston, Global Product Manager for Renishaw. “If you put a standard material through our machine, you do not necessarily get the characteristics of that material as it would be in billet or wrought form.”

The differences in properties might be good for a particular application with, say, better hardness or toughness. “Or it might not be right because it must sacrifice ductility,” he said.

He believes that many designers are not now designing objects that could only be made through additive manufacturing. “Many are designing parts best made using CNC machining from a billet, then using additive manufacturing instead to create the part,” he said, missing the design freedoms accessible through additive manufacturing. “However, some designers have the ambition to capture the advantages of additive manufacturing.”

Others leading in this field recognize the challenge. “We are classifying the material properties that come out of [metals-based] additive manufacturing right now,” said Horine, also noting the different properties from additive manufacturing compared to wrought or billet. He shared that GE Aviation is using a number of materials suitable for the hot side of turbine engines, a particular focus for GE Aviation. “What it really comes down to is what material properties we can get out of this manufacturing technology that is critical to the parts,” said Horine.

Stresses are another concern. They build up in the part because they are essentially welded bit by bit. Internal sections of parts experience heat and thermal expansions that a CNC-machined part would never see. Thicker parts typically see higher stresses and potential distortions. “Every part is stress-relieved right out of the machine,” said Morris' Warden. “We solution heat treat and HIP, depending on some of the materials that we run,” referring to hot isostatic pressing techniques for stress-relieving parts.

Warden is enthusiastic about material advantages. “We are seeing exceptional material properties with the grade of materials we are using, with both EBM and DMLS,” said Warden. “In a couple of cases, we made parts with Ti6Al4V and Inconel 718 that surpassed wrought properties, after heat treat.” He notes this is a vast improvement in just the last few years.

While other industries are certainly using metals-based additive manufacturing, aerospace in particular is embracing the technology, according to Scott Killian, Key Account Manager of Aerospace for EOS of North America.

“There is a company today that is manufacturing parts that are flying,” he said. “Some people are looking at airframe parts, but engine manufacturers are the ones that have found the economies of scale attractive in bringing their complicated parts to production.” Simply put, airframe parts are not particularly challenging geometrically, though some companies are looking at specialized hinge parts and other complicated components. “Parts where they want to avoid wasting a lot of material that is typical in subtractive machining processes,” he said. His company provides primarily cobalt chrome, Inconel 718, and Inconel 625 to engine manufacturers, with Ti6Al4V going more to airframe and structure parts.

Low production rates are also a factor in its popularity in aerospace. “Remember, in aerospace, 50 to 100 parts a year is production, unlike automotive where production means 100,000's of parts,” said Warden.

Improvements for the future

GE Aviation's Horine expects much improvement from the existing and relatively immature state the metals-based additive manufacturing industry is in today.

“The equipment is currently experiencing an exponential scale of improvement, similar to advancements in the computer industry,” he said.

He also points to software and supporting technology upgrades as similarly improving rapidly as well. Improving speed of manufacturing individual parts is important. “Any improvement in speed is welcome, but we would like to see at least a 4 to 8 times speed improvement,” said Horine.

The other improvement many would like are bigger parts. Today, size is in some ways limited.

For example, the Renishaw AM250 builds parts only within a 250 x 250 x 300 mm volume, extendable to 360 mm height as an option. However, in the case of larger volumes, faster laydown may also be a key improvement.

“Under a funded research project, we built one machine with a larger build envelope,” said Renishaw's Weston. “It was 500 x 500 x 500 mm, installed at the University of Liverpool to understand the feasibility of such a size.”

They found that, while it was feasible, they were dealing with unfamiliar problems such as heavier parts, the machine pumping out more waste products, and the machine itself having a bigger footprint.

“What became obvious is that, until you can scan more quickly and process more material, you do not really gain any process efficiencies by scaling up the machine. You can build a bigger object, but it takes a very long time,” he explained.

EOS also reports developing faster laydown methods. But there is more to making this a mainstream production method.

“What our customer base is looking for is an end-to-end solution,” said Andy Snow, EOS Regional Director. “Although EOS offers one aspect [of additive manufacturing], there is also design and data generation. The core technology is really systems and software along with parameters associated with the build. We talk about different alloys and materials, but training, intelligent interfaces, or software that helps you design weight reduction geometries especially for aerospace is also needed. Quality assurance methods as well.”

He notes that when an organization develops a process, they connect specific hardware such as their M280 with a particular material specification, process parameters, and quality assurance methods to ensure the part meets its intended use.

Thus, ensuring and improving the ecosystem these machines reside in is vital. The surface finish of a part is similar to a cast part, for example.

“One of the attractions to our customers is that we are a lot more than an additive manufacturer,” said Morris' Warden. “We are a one-stop shop with CNC machining and our sister company dedicated to production, RQM, is adding a HIPing furnace. Ninety percent of the parts that we make from our additive machines we do some type of secondary machining operations using our installed CNC equipment.”

Weston from Renishaw also pointed out that international standards are another important factor needed for widespread acceptance in production.

“There are no ISO standards for metals-based additive manufacturing currently,” he said. He noted that the ASTM F42 committee is meeting regularly and drafting standards for additive manufacturing.

“Once those standards are in place, it will help a great deal with acceptance of this in production,” he said.

Another improvement Renishaw is looking at is developing machines guided by in-process feedback, somewhat like in-process touch probes used on CNC machines today. Currently, the process is linear, driven by up-front data fed to the machine in the form of .stl files. He points to new file formats under development that will unlock more potential to change values and intervene as the process runs.

The future includes more actual production. “We are looking at using additive manufacturing for our future engines in the combustor side of the engine,” said Horine. “We are looking at parts in multiple areas—both internal and external parts of the engine as well.

“We should be increasing the number of parts made from additive manufacturing later in this decade in some of our newer engines.”

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