The importance of additive manufacturing (AM) isn’t lost on the aerospace industry, with industry leaders like Boeing and Airbus long having adopted the technology to create parts. But it’s not just the giants that recognise the advantages: smaller, innovative companies like Aurora Flight Sciences also see the benefits.

Aurora developed and flew the first 3D-printed jet-powered unmanned aerial vehicle (UAV), capitalising on the strength of fused deposition modelling (FDM) ASA thermoplastic for the main wing and fuselage structures. The aircraft’s main purpose was to demonstrate the speed at which a design can go from concept to a flying aircraft. It also illustrates the validity of AM for flight-capable parts, beyond the traditional role of prototyping.

While AM methods and applications may differ among these companies, the reason they use it is common: it provides multiple benefits that collectively improve their bottom line. That might come in the form of meeting delivery schedules, improving performance, reducing waste, optimising the supply chain, or a combination of the above.

From rapid prototyping to flight parts

Since its inception, a common use case for AM has been rapid prototyping, allowing aerospace companies to validate fit, form and function in addition to reducing product development time. The next step is the production of flightworthy parts for use on certified aircraft.

The reasons for this are obvious: AM helps aerospace companies attain important goals of reduced weight and lower buy-to-fly ratios (the ratio of procured material weight to the final part weight). For example, 3D printing allows the creation of organic shapes that aren’t otherwise possible with conventional manufacturing methods. This lets engineers design optimal strength-to-weight geometries, reducing weight by minimising the amount of material needed to carry the load. Aurora Flight Sciences used this approach on their UAV to achieve a stiff but lightweight structure, using material only where it was necessary.

This capability to apply material only where it’s needed also results in little to no scrap, unlike subtractive processes. AM thereby offers a much more favourable buy-to-fly ratio, using only what’s necessary to create the part. Buy-to­fly ratios for machined aircraft components can be in the range of 15-20, compared to close to one for AM parts, making material waste an important cost consideration.

Other benefits of AM include part count reduction by 3D printing multiple components as a single part. This results in fewer individual parts, less manufacturing and inventory, and reduced assembly labour. United Launch Alliance reduced the part count on an environmental control system duct for its Atlas V flight vehicle from more than 140 pieces to just 16 with ULTEM 9085 resin.

AM also enables creation of complex designs and intricate geometries without the time and cost penalty of traditional manufacturing methods. In some cases, it allows creation of parts that otherwise wouldn’t be possible. The fuel nozzle for the GE LEAP engine is a good example. The configuration engineers developed to meet restrictive performance requirements included intricate internal passages and geometries. The final design was ultimately not possible to manufacture with machine tools and could only be achieved with AM.

From a supply chain perspective, the ability to economically produce parts on-demand gives manufacturers much greater flexibility to make and locate parts when and where they’re needed. This alleviates the expensive process of producing and stocking sufficient spares to support demand scenarios that are difficult to predict. It also gives manufacturers the flexibility to overcome hiccups in the supply chain. Airbus used this strategy in producing the A350 XWB aircraft, 3D printing parts to maintain the aircraft delivery schedule.

Certification headwinds

These are just a few examples of the merits of AM for end-use aircraft parts. But the challenge faced by aerospace companies in achieving these benefits lies with airworthiness certification. Parts installed on aircraft must be certified flightworthy as part of the overall certification of the aircraft. For engineers wanting to design additively manufactured flight parts, that’s easier said than done, since unlike traditional materials and processes, there are no industry-standard “design allowables” to characterise the properties of AM materials.

Without this information, aerospace companies face either avoiding the use of AM flight parts altogether or developing the design allowable themselves. Depending on the application, the latter scenario probably requires expensive and lengthy testing. In the highly competitive aerospace industry, neither option is optimal.

Nonetheless, some companies have done what it takes to qualify non-metallic materials, such as polymer-based composites, for use on aircraft. However, manufacturers that take this approach typically view the data as proprietary due to the cost and effort involved, and they don’t share it within the aerospace community. This creates an “everyone­for-themselves” environment, resulting in a lack of industry-wide standards. While large companies may be able to justify the time and cost for this effort, it can be prohibitive for smaller companies without publicly funded support programs. At a practical level, requiring each manufacturer to duplicate the process for a material that another manufacturer has already evaluated is simply counterproductive, driving up industry costs and inhibiting innovation.

An industry solution

A solution to this was indirectly borne out of an effort in the mid-1990s to rejuvenate the general aviation market. The Advanced General Aviation Technology Experiments (AGATE) initiative involved the participation of NASA, the US Federal Aviation Administration (FAA), the aerospace industry and academia in the development of improved technologies and the standards and certification methods governing them. Part of this project involved the development of standards to qualify new materials and their production while abbreviating the certification timeline.

AGATE eventually evolved into a new process, named after the organisation that administers it: the National Centre for Advanced Materials Performance (NCAMP). The NCAMP process is now the established method for qualifying new material systems and developing a shared database of design allowables. The benefit for aerospace companies looking to certify their designs, beyond access to resultant material data, is that the certification authorities, the FAA and the EU Aviation Safety Agency (EASA) accept dataset and material process specifications for key components of the qualification process.

The initial focus of AGATE and the NCAMP process was qualification of polymer-based carbon-fibre composites, as composite technology offered benefits in the form of strong, lightweight structures. As a result, a number of different composite material systems have been qualified through the NCAMP process, providing a shared database of corresponding design allowables.

This has been a benefit to companies that want to use those materials on certified aircraft, while avoiding the long, expensive process of qualifying the material on their own. Instead, they must simply demonstrate “equivalency” – proving they can duplicate the material characteristics of the base qualification dataset, but on a smaller and less costly scale than a full qualification program.

Unfortunately, the same cannot be said for manufacturers looking to use AM parts on certified aircraft programs. Although aerospace companies have leveraged AM for more traditional benefits like faster prototyping and agile tooling, the barrier to certification endures due to the lack of a qualified AM material that is tested and validated using the NCAMP process. This prevents the aerospace community from fully benefitting from AM production parts, with real performance, supply and cost efficiencies.

The problem was taken up by America Makes, also known as the National Additive Manufacturing Innovation Institute, which chartered the initiative to qualify the first AM material for certification purposes using the NCAMP process. This qualification process is what ultimately results in the establishment of design allowables that companies can use to design aircraft parts, a major piece of the certification puzzle. ULTEM 9085 resin was chosen by an industry steering group as the first AM material for NCAMP qualification. The material and the FDM process were selected because of their wide acceptance and use within the aerospace community.

Material testing and qualification using the NCAMP process is performed by the National Institute for Aviation Research (NIAR) at Wichita State University. The process begins with the establishment of a material specification to control the material manufacturing and processing quality. Next, a process specification is developed to control the build process using that material. This is necessary to remove any process variability and establish a controlled means of production.

These specifications exist because an important part of the certification process is the assurance that each manufacturer is making parts to the same standard. As Paul Jonas, Technology Development Director at NIAR, described it: “The first part you make has to be equivalent to the hundredth part, to the thousandth part, to the part you make ten years from now, to be good enough to be certified for the FAA.”

Once the production method is established, any decision to create parts using another system, process or material requires a separate qualification process for that method or material. The specifications also include controls for ongoing process validation, to ensure standards are maintained.

After the process specification was established, the ULTEM 9085 resin material qualification plan began. Around 6,500 test coupons were evaluated for specific mechanical properties and exposed to fluids typically found in an aircraft operating environment like engine oil, hydraulic fluid and jet fuel. Test results are then analysed to determine the material characteristics and corresponding design allowable dataset.

This data is currently published on the NCAMP website and findings will be incorporated into CMH-17, the composite materials handbook that provides standardised engineering data on composite and other non-metallic materials.

The Stratasys solution

The final step in the AM certification solution path involves the demonstration of equivalency to the NCAMP dataset. Companies that want to leverage the NCAMP process need to show that their AM process with ULTEM 9085 CG resin results in material properties statistically equivalent to the original data. But this is achieved with a much smaller sample size, drastically reducing the time and cost for qualification, with a ten-fold estimated cost savings. Once equivalency is achieved, manufacturers can leverage the NCAMP process for key components of the airworthiness qualification process, having established that their AM process is equivalent to the allowable database.

The equivalency process is reliant on two factors: use of a properly configured Fortus 900mc Production System, in conjunction with certified ULTEM 9085 CG material. Stratasys developed specific capabilities for the Fortus 900mc that include enhanced material deposition, ensuring consistent, repeatable build results to produce equivalency test coupons. An AIS Machine Readiness package is available to validate the proper set-up and operation of the AM system and demonstrate a means of compliance with the NCAMP process specification. ULTEM 9085 CG resin undergoes more testing than standard ULTEM 9085 material and is accompanied by documentation that gives manufacturers full traceability back to the raw material. The certified ULTEM 9085 CG resin, the configured Fortus 900mc, and the AIS Machine Readiness package are available as a comprehensive solution from Stratasys.

AM is no stranger to the aerospace industry. The next level of efficiency involves the manufacture of interior (or other non­flight-critical) components using the NCAMP-certified material and process – a major step in defining the roadmap toward using AM in aerospace production. The NCAMP process sets the precedent for the qualification of other AM materials, clearing the path for faster, broader use of AM in aerospace.