Manufacturing Technology For Aerospace Structural Materials Pdf

[Ben Hollinbeck]

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Materials used in aerospace must meet a rigorous set of criteria in terms of performance and safety. For example, jet engines bring together superalloys, titanium alloys and thermal barrier coatings, operating under high temperature and cyclic stress. Light alloys and polymer composites used in airframes must combine sufficient mechanical performance with manufacturability. Meanwhile, thermal protection systems on space shuttles must survive extreme cold in space and the heat of atmospheric re-entry. As such, aerospace is one of the most demanding applications in materials science and there is a constant need for improved materials and manufacturing processes.

This cross-journal Collection between Nature Communications, Communications Materials and Scientific Reports brings together the latest developments in alloys, ceramics, polymers and composites used in aviation and space applications. Topics of interest include, but are not limited to, the following:

We welcome the submission of all papers relevant to structural materials used in aerospace. Nature Communications and Communications Materials will publish research papers, Reviews and Perspectives, and Scientific Reports will publish research papers. All submissions will be subject to the same review process and editorial standards as regular submissions at the participating journals.

The segregation of elements in superalloys is known to influence their mechanical properties. Here, atomic-scale imaging and theoretical calculations reveal a mechanism by which segregation causes a yield strength anomaly, strengthening the superalloy.

The orientation of reinforcing fibers in composite materials is key to their performance, yet is hard to determine as fibers are buried within a sample. Here, an algorithm allows for the rapid determination of in-plane fiber orientation, based on microscopy images of adjacent regions.

Dynamic process of epitaxial microstructure forming during laser additive manufacturing is important for achieving single crystalline texture. Here, the authors perform in situ, real-time synchrotron Laue diffraction to capture the microstructural evolution in Ni-based single-crystal superalloy.

Coarsening of precipitates in medium and high temperatures causes reduction in strength of Al alloys. Here, the authors design an Al-Cu-Mg-Ag-Si-Sc alloy with multiple interface structures, showing an excellent combination of strength and heat resistance compared to conventional Al alloys.

Refractory high entropy alloys (RHEAs) have recently been developed in the context of high-temperature and severe environmental applications. Here the authors, by combining simulation and experiments, develop an irradiation resistant, thermally stable, and strong RHEA for nuclear application.

Hypersonic vehicles experience extreme temperatures, high heat fluxes, and aggressive oxidizing environments. Here, the authors highlight key materials design principles for critical vehicle areas and strategies for advancing laboratory-scale materials to flight-ready components.

Fiber-reinforced polymer composites have found widespread use in critical engineering applications. Here, the use of simulations to understand the mechanical durability of polymer composites across a range of length scales is reviewed, with a focus on molecular dynamics simulations.

Lightweighting design is an extensively explored and utilized concept in many industries, especially in aerospace applications, and is associated with the green aviation concept. The contribution of aviation to global warming phenomena and environmental pollution has led to ongoing efforts for the reduction of aviation emissions. Approaches to achieve this target include increasing energy efficiency. An effective way to increase energy efficiency and reduce fuel consumption is reducing the mass of aircraft, as a lower mass requires less lift force and thrust during flight. For example, for the Boeing 787, a 20% weight savings resulted in 10 to 12% improvement in fuel efficiency. In addition to reduction of carbon footprint, flight performance improvements such as better acceleration, higher structural strength and stiffness, and better safety performance could also be achieved by lightweight design.

Lightweighting optimization of a solar-powered unmanned aerial vehicle (UAV) is an example of using both clean energy and lightweight structures to achieve green aviation operation. Current solar-powered UAV designs face challenges such as insufficient energy density and wing stiffness. Lightweight design is essential for ultralight aviation, enabling longer flight duration.

Structural optimization is another effective way to achieve lightweighting, by distributing materials to reduce materials use, and enhance the structural performance such as higher strength and stiffness and better vibration performance. Conventional structural optimization methods are size, shape, and topology. Manufacturability is a crucial constraint in both material selection and structural optimization. The development of advanced manufacturing technologies such as additive manufacturing, foam metal, and advanced metal forming not only enable the application of advanced materials, but relax constraints, enhancing the flexibility of multiscale structural optimization.

Many examples of lightweight design have been successfully applied in the design of lightweight aircraft. Figure 1(a) illustrates the SAW Revo concept aircraft (produced by Orange Aircraft), which is an ultralight aerobatic airplane with carbon fiber-reinforced composite wings and a topologically optimized truss-like fuselage. The empty weight of this 6-meter-wingspan aircraft is 177 kg. Figure 1(b) shows a high-altitude, pseudo-satellite, solar-powered UAV from Airbus. The Zephyr 7 currently holds the world record for the longest absolute flight duration (336 hours, 22 minutes, 8 seconds) and highest flight altitude (21,562 m) for UAVs, partly from increased energy efficiency by lightweighting. Figure 1(c) shows a model of a future concept lightweight airplane for 2050 from Airbus, inspired by a bird skeleton. Figure 1(d) demonstrates a concept of a box wing aircraft where shape optimization is employed in the wing design. Structural efficiency could be increased by using a box wing structure; higher stiffness and lower induced drag force result from the box wing compared with conventional wing structures.

The selection of aerospace materials is crucial in aerospace component design since it affects many aspects of aircraft performance, from the design phase to disposal, including structural efficiency, flight performance, payload, energy consumption, safety and reliability, lifecycle cost, recyclability, and disposability. Critical requirements for aerospace structural materials include mechanical, physical, and chemical properties such as high strength, stiffness, fatigue durability, damage tolerance, low density, high thermal stability, high corrosion and oxide resistance, and commercial criteria such as cost, servicing, and manufacturability. Studies have indicated that the most effective way to improve structural efficiency is reducing density (around 3 to 5 times more effective compared with increasing stiffness or strength), i.e. using lightweight materials.

Aluminum Alloys. Although high-performance composites such as carbon fiber are receiving increasing interest, aluminum alloys still make up a significant proportion of aerospace structural weight. The relatively high specific strength and stiffness, good ductility and corrosion resistance, low price, and excellent manufacturability and reliability make advanced aluminum alloys a popular choice of lightweight materials in many aerospace structural applications, e.g. fuselage skin, upper and lower wing skins, and wing stringers. The development of heat-treatment technology provides high-strength aluminum alloys that remain competitive with advanced composites in many aerospace applications. Aluminum alloys can offer a wide range of material properties meeting diverse application requirements, by adjusting compositions and heat treatment methods.

Titanium Alloys. Titanium alloys have many advantages over other metals, such as high specific strength, heat resistance, cryogenic embrittlement resistance, and low thermal expansion. These advantages make titanium alloys an excellent alternative to steels and aluminum alloys in airframe and engine applications; however, the poor manufacturability and high cost (usually about 8 times higher than commercial aluminum alloys) result in the restriction of titanium alloys being used extensively. Hence, titanium alloys are used where high strength is required but limited space is available, as well as where high corrosion resistance is required. The current applications of titanium alloys in aerospace are mainly in airframe and engine components, overall comprising 7% and 36% of the weight, respectively.

High-Strength Steel. Steel is the most commonly used structural material in many industry applications due to good manufacturability and availability, extremely high strength and stiffness in the form of high-strength steels, good dimensional properties at high temperatures as well as the lowest cost among commercial aerospace materials. But high density and other disadvantages, such as relatively high susceptibility to corrosion and embrittlement, restrict the application of high-strength steels in aerospace components and systems. Steel normally accounts for approximately 5% to 15% of structural weight of commercial airplanes, with the percentage steadily decreasing. Despite the limitations, high-strength steels are still the choice for safety-critical components where extremely high strength and stiffness are required. The major applications for high-strength steels in aerospace are gearing, bearings, and undercarriage applications.

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