Opinion: Engineered Materials Will Drive AAM and Aircraft Production

FAA building-block schema for part structural substantiation
FAA building-block schema for part structural substantiation.
Credit: FAA

Recently, Boeing CEO David Calhoun stated that the OEM’s future jetliner would be driven largely by the production system and not necessarily by new engine technology, a fundamental departure from previous product iterations. The same production strategy underpins the business model for advanced air mobility (AAM), a burgeoning market that has swiftly amassed billions of dollars from investors.

In both cases, material systems play a fundamental role—arguably, it is the material that dictates both the product design and manufacturing schema. Aircraft were traditionally made of homogeneous isotropic metals. Nowadays, “engineered” materials such as polymer and ceramic matrix composites play an increasingly important role. Unlike conventional alloys, their properties can be more easily optimized for a given application. These materials are extremely sensitive to manufacturing processes, though.

A common illustration is a composite part, whose mechanical properties can fundamentally vary with changes in cure cycle. Composites are considered process-intensive materials (PIM). Part pedigree is essential. Additive manufacturing (AM) is also process-intensive.

AM unlocks the potential of part design-and-build optimization. The basic process involves an energy source that melts a precursor (powder, wire, filament) in sequential layers to “grow” a part vertically. For aerospace, AM is used to create lightweight parts for newer designs or to replace obsolete parts for maintenance. In either case, qualified material is requisite. Thus, like composites, AM demands a controlled, fixed process.

The basic construct for qualifying materials remains unchanged and is outlined in the Code of Federal Regulations (CFR). In particular, 14 CFR 2x.603 (materials), 2x.605 (process) and 2x.613 (design values) help define the FAA “building-block” framework (see diagram). A portion of 2x.613 is represented by the base of the pyramid, comprising either statistically based company proprietary data or public handbook data from the Metallic Materials Properties Development and Standardization Handbook and the Composite Materials Handbook-17. The top of the pyramid constitutes 2x.305 (strength and deformation) and 2x.307 (proof of structure), which require final testing to corroborate major assemblies.

But there are challenges to applying this framework to PIMs. Mechanical properties for traditional metal preforms do not substantially change during fabrication, and so coupon tests are basically representative of the “bulk-material properties” of the larger structure. For PIMs, these coupon-level tests are more often used to validate fabrication process quality and stability.

AM involves four constituents: the machine, material/precursor, process control document (PCD) and process specifications (if available). The PCD delineates values for key process variables (such as laser power or scan strategy) and is often considered proprietary. It provides specificity concerning software, machine calibration, etc. This whole process is arduous. Clearly, there are items that can be standardized across organizations.

SAE International has a dedicated Aerospace Material Specifications AM working group, which has released 21 specifications, with at least 30 more under development. These documents cover foundational aspects of material and process for metals and polymers. Through its affiliate SAE-ITC, it recently launched a data consortium to provide material allowables for the handbooks. Without public datasets, manufacturers would have to undertake costly material-substantiation programs that could take years and millions of dollars.

AAM faces a similar dilemma. Due to a lack of publicly available data, the prevailing high-rate, low-cost production business case is at risk. The solution likely will involve out-of-autoclave quick-cure thermosets and discontinuous fiber thermoplastics using highly automated systems. These material systems are being qualified by authorities, with public materials databases to follow.

PIMs can be optimized for exceptional properties, but their qualification requires well-defined, robust material and process controls. It is crucial that empirical datasets and public standards be available to foster PIMs’ broader adoption, which in turn will enable next-generation aircraft and production systems.

Bill Bihlman is founder of Aerolytics, an aerospace management consultancy based in Lafayette, Indiana.