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Imagine a massive aircraft gliding smoothly across the sky, safely transporting hundreds of passengers to their destinations. Supporting this engineering marvel are not just precise designs but also the ingenious use of high-performance materials. What materials can withstand the demanding flight environment and ensure aircraft reliability? This article examines the crucial materials in modern aircraft manufacturing: steel, aluminum alloys, titanium alloys, and fiber-reinforced composites, revealing their unique roles and development trends in aviation.

Titanium Alloys: The High-Temperature Guardians

Titanium alloys hold a pivotal position in aviation due to their exceptional properties. They boast an outstanding strength-to-weight ratio, meaning they provide sufficient strength while minimizing structural weight. Additionally, titanium alloys demonstrate excellent fatigue strength and tensile strength ratios, along with high fatigue limits, enabling them to endure prolonged, high-intensity cyclic loads. Remarkably, certain titanium alloys maintain considerable strength even at temperatures reaching 400-500°C, making them ideal for high-temperature components like jet engine turbine blades.

However, titanium alloys aren't without drawbacks. In salt-laden environments, the combined effects of high temperature and stress significantly degrade their performance—a serious challenge for carrier-based aircraft engines. Their relatively high density also increases overall aircraft weight when used extensively. Most notably, titanium alloys are extremely costly, with material and manufacturing expenses approximately seven times higher than aluminum or steel. Consequently, they're typically reserved for performance-critical components like jet engines.

Steel: From Dominance to Niche Applications

Steel, an alloy of iron and carbon, dominated aircraft manufacturing in the 1930s as both primary and secondary structural material. While aluminum alloys eventually displaced steel as the primary material, steel retains its place in aviation due to its high strength, rigidity, and damage resistance. Components like landing gear pivot brackets, wing root attachments, and fasteners often use steel castings to meet demanding strength and rigidity requirements.

Steel's primary limitation is its high density, restricting widespread structural use. Nevertheless, it remains indispensable for high-strength, durability-critical applications.

Aluminum Alloys: The Lightweight Revolution

Pure aluminum's low strength and high ductility make it unsuitable for structural applications. However, alloying with elements like copper, magnesium, manganese, silicon, zinc, and lithium significantly enhances mechanical properties while maintaining low density—a crucial advantage for weight-conscious aviation. Post-World War II, aluminum alloys replaced steel as the primary aircraft structural material.

The aviation industry primarily uses four aluminum alloy series:

• Al-Cu (2000 series)
• Al-Mg (5000 series)
• Al-Mg-Si (6000 series)
• Al-Zn-Mg (7000 series)

Recently, aluminum-lithium (Al-Li, 8000 series) alloys have entered aerospace applications. These materials are widely used in fuselages, skins, and load-bearing components due to their extremely low density.

Selecting aluminum alloys involves balancing multiple factors: strength (yield and ultimate), ductility, manufacturability, corrosion resistance, surface treatment compatibility, fatigue strength, stress corrosion resistance, and crack propagation resistance. Achieving optimal performance balance is challenging due to complex alloying mechanisms involving microstructural and chemical processes.

Recently, fiber-reinforced composites have begun replacing aluminum alloys, first in secondary structures and now in primary structures like the Airbus A350 and Boeing 787 Dreamliner.

Fiber-Reinforced Composites: The Future of Aviation Materials

Composites combine two or more materials with significantly different physical or chemical properties to create superior performance characteristics. In aircraft manufacturing, fiber-reinforced composites are increasingly prevalent. These typically consist of high-strength fibers (glass or carbon) embedded in plastic or epoxy resin matrices that provide mechanical and chemical protection.

Fiber-reinforced materials are anisotropic—their properties depend on fiber orientation. Structural applications typically use multiple material layers with fibers aligned to primary load directions. These laminates are embedded in resin matrices to form cohesive structures capable of withstanding bending and shear stresses.

Early glass fiber-reinforced plastics (GRP) were used in helicopter rotor blades but saw limited fixed-wing aircraft application due to low stiffness. The 1960s introduced new materials like Kevlar (an aramid fiber) with glass-like strength but higher stiffness. While durable, Kevlar composites have poor compressive strength and manufacturing challenges, limiting them to secondary structures. Boron fiber composites were the first sufficiently strong and stiff for primary structures, but carbon fiber-reinforced plastics (CFRP) later replaced them due to similar performance at lower cost.

CFRP's Young's modulus is approximately triple GRP's, 1.5 times Kevlar's, and double aluminum's. Its strength triples aluminum's, matches GRP's, and slightly trails Kevlar's. However, CFRP is brittle—it doesn't plastically yield at stress concentrations. Impact damage reduces strength, sometimes invisibly. Epoxy matrices also absorb moisture long-term, degrading matrix-dependent properties like compressive strength, especially at elevated temperatures. Conversely, CFRP stiffness is less moisture-sensitive and more fatigue-resistant.

Replacing 40% aluminum structure with CFRP saves about 12% total weight. Today, composites comprise up to 50% of aircraft weight, covering most secondary and some primary structures. For example, the Airbus A350XWB uses CFRP extensively for wings, tail sections, and select fuselage components. Its material composition by structural weight percentage is:

• 52% fiber-reinforced composites
• 20% aluminum alloys
• 14% titanium alloys
• 7% steel
• 7% other materials

Common aerospace aluminum alloys include 7075, 6061, 6063, 2024, and 5052 aluminum.