Aerospace and defense platforms are often regarded as the earliest adopters of new materials and processes technologies. Materials test programs provide validations of material supplier property claims, as the physical tests are performed and data is analyzed. Resultant material “design allowables” are developed and compared to the performance envelopes for the intended application(s). The quality of simulation software and the developed knowledge base that design and strength engineering personnel use for analyzing materials in the intended product design and environments have improved dramatically in the last decade. This has primarily been based on the efficiency and availability of computing capacity for complex simulations.
As computing times have dropped for simulations, it has encouraged design and strength engineers to incorporate more material properties and environmental conditions into component level simulations. Simultaneously considering the impact of properties such as tensile modulus, transmissibility, Coefficient of Thermal Expansion (CTE), thermal conductivity, and degradation of mechanical properties at temperature have enabled the use of new materials. All of these steps help materials and design engineering teams de-risk first implementations of new materials, thereby enabling new technology insertion with manageable risk.
A sample sector that is ripe for materials development and insertion is in the Unmanned Autonomous Systems (UAS) space. Airborne UAS platforms, or drones, are now up against the physical limit in applications that require a metallic material. UAS systems are finding that the standard aerospace aluminum and magnesium alloys are not adequate to produce systems with optimal performance envelopes. These platforms demonstrate some of the most critical demands on system weight, including payloads, as they massively affect the performance envelope for the airframe. Specifically:
- Weight impacts payload capacity (sensors, weapons, etc.);
- That impacts aerodynamic designs (wing profiles, etc.);
- Which impacts drag;
- That impacts fuel consumption;
- Which impact range and mission capability;
- That impacts weight; 7. Etc.
This negative spiral is a continuum that is not unique (albeit with different system characteristics) to UAS platform designs. One way for the UAS providers to break the negative spiral is to consider low-density alloys with other unique properties to generate components with a multi-functional capability.
An example of a material system that is taking advantage of this advanced analytical capability is IBC Advanced Alloys’ investment cast Beralcast® 363 beryllium-aluminum alloy. The properties of this material can be seen in Figure 1. Most design engineers will look at the strength and immediately dismiss the material. However, when the strength values are considered with the low density (2.66 g/cc), the specific ultimate strength is 1.1× higher and the specific yield strength is 1.2× higher than A356.0-T6. Assuming packaging allows for the thickness of strength-critical features to increase, the design may be produced from either material as a weight-neutral design. The designer now has the opportunity to consider the value and impact of other properties.
If the platform being evaluated has optical imaging or precision positioning payloads for supporting reconnaissance and target designation mission profiles, minimizing transmission of propulsion and service vibrations through the airframe is critical. These systems require high structural stiffness and benefit from high modulus materials that increase the natural frequencies of the components to minimize error and image jitter. Beralcast® 363’s tensile modulus value of 202 GPa allows the designer to consider significantly reducing the section thicknesses compared to all traditional aluminum alloys with modulus values of approximately 72 GPa. The two-phase composite microstructure also has the benefit of inherent vibration damping capability that presents as having a lower transmissibility (Goutput/Ginput). These characteristics can be confirmed utilizing a composite FEA analysis accommodating both the static and dynamic responses to the component.
Beryllium-aluminum alloys also have interesting thermal stability characteristics that can be exploited for airborne UAS propulsion system applications. As the McLaren Formula 1 racing team exploited the use of beryllium-aluminum pistons, UAS platforms could gain performance advantages in a similar fashion. Stability of the beryllium-phase at temperature helps to maintain a low CTE that in turn lowers stresses occurring from the use of aluminum adjacent to dissimilar materials in assemblies that experience changes in temperature through the service life-cycle. Minimizing stress as a result of CTE mismatch stabilizes component interfaces and helps prevent distortion of the lower strength materials at temperature.
Strength at temperature is another major contributor to system performance that can be designed into the propulsion components at temperature. 6061-T6 for instance loses 42% of its room temperature yield strength (to 152MPa) when exposed to operating temperatures of 232°C for half of an hour. At 1000 hours of exposure to the same temperature the loss extends to a 74% reduction in yield strength to 68 MPa. This loss is a result of over-aging and significant coarsening of the precipitates that form during heat treatment.
Beralcast® 363 is not a heat-treatable alloy, and therefore not susceptible to this phenomena. Yield strength is 164 MPa at 232°C and further maintains 83 MPa at operational temperatures of 371°C. The modulus values are also stable at temperature (172 GPa at 232°C and 143 GPa at 371°C), and exceed 6061-T6’s room temperature stiffness. These unique capabilities can be evaluated with thermal stress and fatigue analysis that are now available to improve the propulsion system efficiency and life for the platforms.
Cost must also be addressed, and advances made in net-shape investment casting reduce the amount of material procured to the end-use shape with minimal machine stock for part-to-part interfaces or critical profile geometries. Rapid prototype invests (or patterns) are often used to produce prototype or short run products, therefore eliminating the need to procure a wax injection tool. Implementation costs and risk associated with evaluating the material for a specific application are further minimized.
UAS platforms are ripe with applications for beryllium bearing alloys. Implementation of these materials is permitting platform, payload and propulsion designs that are extremely lightweight with enhanced dynamic and thermal stability. As the applications for UAS vehicles continues to expand, so will the application base for Beralcast® alloys to enhance the system capability to support the warfighters in the field.
This article was written by Chris Huskamp, Vice President of Technical & Business Development, IBC Advanced Alloys Corp. (Wilmington, MA). For more information, Click Here .