Additive Manufacturing for Aerospace: Standards as a Path Forward

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Amidst the backdrop of last week’s FAA/EASA workshop in Wichita, Bill Bihlman, Consultant and Volunteer to SAE, analyzes the state of AM aerospace standards.

Commercial aviation is notoriously risk averse. Military and space systems are somewhat less so. During the technology revolution epoch from 1940 to 1970, aerospace led the development of various novel material and manufacturing systems. Examples include the commercialization of titanium, superalloy, single crystal investment castings, isothermal die forgings, vacuum arc remelting, and even carbon fiber composites. Each required extensive empirical substantiation. These material qualification tests are costly and painstakingly slow – development of a novel aerospace alloy can cost between $2M to $10M and take up to 10 years. Consequently, the commercialization of additive manufacturing (AM) for aerospace has lagged, according to some, but for good reason. This article discusses the vital role of public consensus standards to facilitate this process.

Since the 1980s, Western governments have played a diminished role in commercial research and development (R&D). Government-sponsored industrial laboratories have contracted, and regulatory agencies seem to pivot more towards consumer protection than R&D. Part of this research shortfall and intellectual property (IP) transfer has been offset by university engagement, and the efforts of public consensus standards development organizations (SDOs). Along with regulatory support, various United States and European-based SDOs have played a fundamental role in industrializing additive manufacturing globally.

SDOs help to ensure product quality and safety. Moreover, they often facilitate production efficiencies, promote global supply chain integrity, and lower overall program costs. These consensus documents ultimately help mitigate the risk of adopting novel material systems in risk-adverse industries such as aerospace. Additive is no exception.

Navigating Intellectual Property

Additive manufacturing – particularly powder-bed fusion (PBF) – is inherently complex. Successful adoption for critical applications requires collaboration between public and private stakeholders. Along with publicly available design handbooks, consensus standards are vital to this process. In theory, standards should afford a sufficiently flexible, yet robust framework to ensure consistent quality and process efficiency at a reduced cost. The process to develop high-quality standards is non-trivial, however.

EOS and i3DMFG make a deal at RAPID + TCT 2024.

Public consensus standards require a certain amount of IP to be shared between the original equipment manufacturer (OEM) – which often controls the design – and its suppliers. The burgeoning AM “service bureaus” offer a critical perspective as well. Many of these small enterprises possess unique institutional knowledge in print parameter development, build layout maximization, and part design-for-manufacturing (DfM) optimization.

The consensus standards development process strives to balance the resources and priorities of these disparate stakeholders. Large OEMs and their major supply partners (i.e. Tier 1s and Super Tier 2s) are well endowed with resources – qualified personnel, equipment, and procedures – although smaller organizations (i.e. service bureaus and Tier 3 machine shops) typically lack scale and certification-related expertise. This can be a significant challenge when managing complex work packages. An analogy is the traditional “build-to-spec” versus “build-to-print” tenders – typically lacking engineering depth, smaller firms often prefer the latter, more prescriptive contracts.

Publicly Available References

The aviation industry has at least two public mechanisms to facilitate the adoption of new material systems – design handbooks and consensus standards. Both routinely support federal regulations.

In the late 1930s, the US military issued handbooks that eventually became known as Metallic Materials Properties Development and Standardization (MMPDS) for metals and Composite Materials Handbook (CMH-17) for polymers/composites. The goal was for these guides to be the single source of acceptable properties for aircraft materials. Both have become indispensable references for military and civil aircraft and engines. These tomes are regularly updated and provide material “allowables” (i.e. properties) that – in conjunction with influence factors, such as corrosion and temperature effects – are used to calculate a structure’s margin-of-safety. This is a critical process in structural design.

Consensus-based standards are complementary to data offered in these handbooks. For additive manufacturing, standards address critical aspects of the part-production process, ranging from initial material handling and material- and process-control parameters to part non-destructive evaluation (NDE) and part qualification/certification.

Historically, SDOs are engineering societies that have responded to a persistent, widespread need to publish technical information on a pre-competitive basis. There are several relevant global SDOs involved in additive manufacturing. One example is SAE International. Since its AM Materials Committee inception in 2015, it has released 38 documents, with an additional 50 under development – with an increased focus on qualification – to support the adoption of additive manufacturing in aerospace worldwide.

Current AM Part Use Cases

To date, arguably, the GE/CFM LEAP fuel nozzle tip is still the most prominent example of a successful application of additive manufacturing in aviation. This predated most AM standards and guidance documents. Thus, the development and substantiation process started at least a decade before the planned entry-into-service (EIS) date for the nozzle. As the technology matured, both time and cost were significantly reduced.

The 3D printed LEAP fuel nozzle. Image courtesy of GE.

Along with the growth of AM-related technical documents, there have been several other success stories. Examples include low-pressure turbine blades (GE9X), engine casings (GE turboprop engine), actuator flight controls (A380 by Liebherr), and thrust reverser cascade arrays (Collins Aerospace).

The business case for new parts using additive manufacturing is heavily influenced by a new product launch. This design freedom is necessary to fully leverage the technology’s potential. New material systems, nonetheless, need to be extensively vetted to guarantee failure-proof performance throughout the design life (and operating aircraft envelope) of the component under development at the time the design is frozen. This necessarily adds programmatic risk.

The 3D printed flex shaft, consolidated from seven conventional manufactured components into a single 3D printed part. Image courtesy of Liehberr-Aerospace.

Maintenance, repair, and overhaul (MRO) parts follow a different business case. These are largely driven by replacement parts for the military, especially for the US Department of Defense (DOD) that struggles to maintain their fourth-generation fighters, among other aircraft. Indeed, an aircraft-on-ground (AOG) can be prohibitively costly for both military and commercial aircraft. Parts obsolescence is a persistent threat.

Nevertheless, in either business case, changing material systems (and manufacturing) is an arduous and a risky endeavor.

Future AM Part Production Challenges

Three fundamental challenges remain in this journey for additive manufacturing in aviation – part cost, consistent part quality, and part production speed (i.e. manufacturing throughput).

The dedicated supply base is still nascent. In the United States, for instance, there are less than 30 metal PBF service bureaus actively involved in aerospace. And only a dozen would be sufficiently qualified for serialized production for aviation, which requires a robust quality management system (QMS) (e.g. first-article inspection and part-depositing process) and the requisite facility’s credentials (e.g. Nadcap, AS9100). Regardless of enterprise type, at the heart of additive manufacturing adoption is diligent data management.

The technical data packages (TDP) for additive manufacturing need to be well defined and meticulously curated to qualify for a certified part. In addition to the myriad SDOs’ taxonomies, the National Institute for Standards and Testing (NIST) has published important guidelines via their FAIR (Findable, Accessible, Interoperable, Reusable) common data dictionary. This helps direct the public dialog.

Furthermore, in the United States, government-funded America Makes serves as a public-private nexus for AM R&D. This complements activities at various US National Laboratories like Sandia, Lawrence Livermore, and Oak Ridge. In Europe, several government-supported research institutes are engaged to further AM R&D, including Germany’s Fraunhofer, France’s CETIM Institute, and the United Kingdom’s MTC. Within the European Union’s governance are programs such as Clean Sky that support broader technology development. However, the effort for additive manufacturing is less orchestrated than in the United States.

Additional Barriers, yet a Path Forward

There are still several factors that hamper the industrialization of additive manufacturing, especially in a post-Cold War market economy. Most pronounced are: a) the data glut and academic-like need to constantly analyze and optimize data sets; b) the central role of capital markets to fund R&D and its impatience for a healthy return-on-investment (ROI); and, c) the erosion of the manufacturing expertise and infrastructure in the United States and Europe.

In addition to public consensus standards, there is an effort to reduce the time and cost to qualify AM suppliers. SAE is supporting Wichita State University’s National Institute for Aviation Research (NIAR) effort to develop a pre-qualification framework. This effort leverages SAE AMS7032, Machine Qualification for Fusion-Based Metal Additive Manufacturing, as well as the SAE-affiliated NADCAP qualified manufacturers list (QML).

Public-private partnerships (PPPs), like America Makes, are another mechanism to facilitate technology and market development. But maybe it is time that Western societies start to revisit their impatient, investor-driven investment thesis concerning industrial materials and manufacturing R&D. An industrial base takes time to develop.

It is unlikely that industrial product development will ever return to the hurried pace of the 1940 to 1970 period. Somehow balancing market forces, though, likely could nudge additive manufacturing towards its commercialization tipping point. The reemergence of the space program with its clean-sheet designs, emphasis on novel materials, and prodigious budgets, might just help, too.

Note: This article represents the views of the author and not necessarily those of SAE International. Bill Bihlman is a consultant for SAE and has been actively involved as a volunteer for nearly a decade. Please visit www.sae.org for more information.

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