Considerations when selecting metal alloys for aerospace and defense

When the Wright brothers lifted off near Kitty Hawk, North Carolina, in 1903, their Flyer weighed just 750 pounds, including the pilot, engine, and fluids, and was constructed primarily from spruce and ash wood covered in muslin fabric. About a century later, aircraft such as the F-35 Lightning II started achieving supersonic speeds while operating in extreme thermal environments that would have incinerated those early materials in seconds.

Technological advancements have allowed researchers and engineers to develop materials that can be simultaneously lighter, stronger, and more heat-resistant. Today’s advanced metal alloys can shave thousands of pounds off commercial airframes or withstand turbine temperatures exceeding 2000° F. Such innovations enable aircraft that fly farther, faster, and more efficiently than ever before while meeting increasingly stringent performance and safety demands. In this article, we cover some prominent metallic materials and considerations for selecting them.

Metal alloys used in aerospace and defense

Advanced aluminum, titanium, and superalloys enhance performance and enable new designs that meet modern demands and challenges. Though considered a staple of aerospace and defense designs, metal alloys come with tradeoffs and challenges around manufacturing, cost, environmental durability, design integration, and certification. First, let’s take a look at the general advantages and typical applications of aluminum-lithium alloys, titanium alloys, and nickel superalloys.

Aluminum-lithium alloys

Aluminum-lithium (Al-Li) alloys are the pinnacle of lightweight metallic structural materials, reducing part density while increasing strength. Third-generation Al-Li alloys, such as 2195, 2050, and 2060, contain 0.8 to 2.5% lithium, as well as additions of copper, magnesium, and silver, to optimize strength and fracture toughness.

Engineers primarily use these alloys in the structures of commercial aircraft, military platforms, and space systems for fuselage skins, wing and tail structures, and lower wing covers. They also use such alloys in airframes to meet strict weight and performance requirements. The defense sector increasingly uses these materials in uncrewed aerial vehicles (UAVs) and reconnaissance aircraft to increase endurance.

In addition to its strength-to-weight ratio, Al-Li alloys resist corrosion and fatigue and are compatible with conventional manufacturing processes, including machining, rolling, and welding. This makes them attractive for developing new designs and upgrading and replacing heavier legacy materials. In space applications, Al-Li alloys such as 2195 are used in cryogenic fuel tanks and structural components for launch vehicles, where weight reduction can significantly lower launch costs.

Titanium alloys

Titanium (Ti) alloys have evolved beyond the ubiquitous Ti-6Al-4V with specialized compositions optimized for extreme operating conditions, such as high temperatures, heavy loads, or chemically aggressive environments.

For example, high-temperature alloys such as Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) are used in jet engine components, including compressor disks, blades, and casings. These advanced alloys maintain mechanical integrity and exhibit exceptional resistance to creep (gradual deformation under stress at high temperatures), making them suitable for the elevated temperature areas of commercial and military engines. Similarly, Ti-5Al-5V-5Mo-3Cr (Ti-5553) is a high-strength alloy used in landing gear, wing-box structures, and other heavily loaded components that endure considerable stress.

In defense systems, beta titanium alloys such as Ti-15V-3Cr-3Sn-3Al and Beta-21S are used for their hardenability, cold forming properties, and corrosion resistance. These materials are often selected for airframe fasteners, actuators, and hydraulic system components to balance weight savings with fatigue life and manufacturability. Additionally, titanium’s natural resistance to saltwater corrosion and high strength make it a strategic material for submarine hulls, missile skins, and naval exhaust systems.

With additive manufacturing advancements, custom formulations — often based on Ti-6Al-4V — are used to 3D print complex parts, such as lightweight brackets, trusses, and UAV structures. These alloys are developed for improved printability, dimensional stability, and post-processing performance.

Nickel superalloys

Nickel-based superalloys are another set of materials with exceptional strength, thermal stability, and corrosion resistance at elevated temperatures. These alloys have complex microstructures with carefully balanced compositions of chromium, cobalt, molybdenum, and aluminum. They are often used in the hottest sections of aircraft engines and military systems because they retain their mechanical properties in such environments. Beyond mechanical performance, nickel superalloys are compatible with additive manufacturing and advanced casting methods, enabling complex geometries, weight optimization, and part consolidation.

Inconel 718, Inconel 625, René 41, and Hastelloy X are widely used in jet engines, including turbine blades, disks, casings, combustion chambers, and exhaust systems. These components can withstand temperatures exceeding 1,832° F (1,000° C) while maintaining their shape, strength, and resistance to oxidation and creep deformation.

In the defense sector, these materials are also used in missile propulsion systems, hypersonic vehicles, naval gas turbines, and heat shields, where withstanding high heat and corrosive conditions is crucial. Some nickel superalloys are also used in nuclear propulsion systems for submarines and in armor applications that require reliable mechanical performance at elevated temperatures.

SpaceX uses proprietary SX300 and SX500 nickel superalloys for its Raptor engine manifolds. The company posted images of the Raptor evolution on X (formerly Twitter) in 2024, as shown below.

Performance stats of previous versions:

Raptor 1 (sea level variant)
Thrust: 185tf
Specific impulse: 350s
Engine mass: 2080kg
Engine + vehicle-side commodities and hardware mass: 3630kg

Raptor 2 (sea level variant)
Thrust: 230tf
Specific impulse: 347s
Engine mass: 1630kg
Engine… pic.twitter.com/8PgQeALOx4

— SpaceX (@SpaceX) August 3, 2024

Considerations for selecting metal alloys in aerospace and defense applications

Here are key tradeoffs engineers must consider early in the design phase when selecting Al-Li alloys, titanium alloys, and nickel superalloys for aviation, space, and defense systems.

Manufacturing considerations

• Anisotropic behavior and forming challenges: Some alloys, including Al-Li alloys, exhibit anisotropic mechanical properties, meaning their properties vary depending on the direction of the grain structure. This makes forming challenging and requires careful design consideration for multi-axial loading conditions.
• Specialized processing requirements: Advanced metallic alloys often require specialized processing techniques. For instance, titanium alloys may need hot working or superplastic forming as they are not easily cold-formed. Nickel-based superalloys may require specialized methods, such as vacuum induction melting and vacuum arc remelting, to maintain purity and prevent oxidation.
• Machining difficulties and tool wear issues: Many advanced alloys can be difficult to machine due to high strength, work-hardening tendencies, and resistance to heat. Machining titanium, for example, requires specialized tools and slow feed rates due to its low thermal conductivity, which can lead to rapid tool wear. Nickel superalloys generate high cutting forces and cause rapid tool wear, necessitating specialized tooling and cooling systems.
• Welding challenges: Welding advanced alloys can present problems. For instance, Al-Li alloys can experience porosity and hot cracking unless methods such as friction stir welding are used. Titanium requires inert gas shielding (argon) or a vacuum chamber to prevent oxidation. Also, welding nickel-based superalloys can lead to cracking, porosity, and grain boundary embrittlement, often requiring post-weld heat treatments or controlled environments.
• Complex heat treatment requirements: Achieving optimal properties in advanced alloys often involves complex heat treatments. For titanium alloys, precise control of cooling rates and aging temperatures is crucial for strength-toughness combinations.

Cost and economic realities

• Raw material costs: Advanced alloys are significantly more expensive than conventional materials. For example, Al-Li alloys can have three to five times higher raw material costs than conventional aluminum. Strategic elements, such as rhenium, found in superalloys currently cost more than $3,000/kg. Titanium extraction is an energy-intensive process, making it more costly than extracting aluminum or steel. Nickel-based superalloys often contain expensive and rare alloying elements, such as cobalt, rhenium, tantalum, and molybdenum.
• Total cost of ownership: Beyond raw material, the total cost of ownership, including fabrication, processing, and lifecycle costs, must be considered. For example, the use of Al-Li alloys is often reserved for applications where weight savings justify the added expense.
• Supply chain complexity and limited supplier bases: Advanced alloys and critical materials can have limited global supply chains, making them vulnerable to geopolitical and economic disruptions.
• Qualification timelines: Long qualification timelines contribute to overall costs.

Environmental and durability concerns

• Corrosion behavior differences: Alloys can have unique corrosion characteristics. Some Al-Li grades are susceptible to exfoliation corrosion and environmental-assisted cracking if not properly treated. Protective coatings or surface treatments are often required.
• Hot corrosion, oxidation, and thermal fatigue: Superalloys are designed for high temperatures but can still suffer from corrosion, oxidation, and thermal fatigue, particularly in hot sections. Even with resistance, protective oxide layers can break down under very high temperatures or aggressive chemical environments.
• Hydrogen embrittlement risks: During manufacturing and operation, hydrogen atoms can contaminate high-strength alloys, degrading their properties and risking failure. In addition to specialized production and testing methods, as well as protective coatings, selecting alloys with combinations of nickel, chromium, and molybdenum, for instance, can mitigate risks.
• Fretting fatigue at joint interfaces: This is a well-documented phenomenon in advanced alloys, particularly at bolted and clamped connections where contact stresses can initiate fatigue cracks below yield strength levels.
• Microstructural degradation: Over time, thermal cycling can lead to microstructural degradation, grain coarsening, or phase instability in superalloys, reducing mechanical performance.
• Cryogenic and upper thermal limits: Some titanium alloys can become brittle at cryogenic temperatures, while high-performance alloys have upper thermal limits beyond which they suffer from creep or microstructural degradation.

Design and integration challenges

• Material property variability: Engineers must account for material property variability using statistical design approaches. This is an area where simulation and machine learning can help.
• Dissimilar material joining complications: Joining different materials, such as Al-Li with carbon fiber composites or titanium with aluminum, can lead to delamination, galvanic corrosion, or thermal mismatch.
• Limited repair and maintenance capabilities: Advanced alloys often require specialized facilities, equipment, and expertise for repair, which may limit field repair capabilities and necessitate component removal for specialized treatment.
• Enhanced NDE requirements: Enhanced non-destructive examination (NDE) and inspection techniques are often required to ensure material integrity throughout manufacturing.
• Over-engineering and springback: For critical components made from materials such as titanium, there can be a tendency to over-engineer parts or apply excessive safety factors, potentially counteracting weight savings. Titanium’s springback behavior and high stiffness can also cause issues in assembly and precision fits.

Certification and qualification hurdles

• Extended qualification timelines: Introducing new advanced alloys, especially for regulated platforms, requires extensive material testing and can take five to 10 years for full qualification.
• Large test matrices: Statistical validation requires large test matrices to demonstrate mechanical performance, fatigue life, fracture toughness, and environmental durability across a wide range of conditions.
• Long-term durability assessment: Assessing long-term durability is a significant challenge, especially when faced with accelerated timelines, increasing regulatory pressure, and new materials and manufacturing processes. Though simulation platforms can save time and costs before physical prototyping, physical testing and long-term data acquisition can still present hurdles.
• Enhanced quality control: Stringent quality control requirements are essential throughout the production process. Military, aerospace, and space applications must adhere to rigorous industry standards, such as FAA, AS, MIL, and NASA.

End-of-life management

• Recycling difficulties: Recycling superalloys is challenging due to their complex chemistries and contamination risks, which can make it economically unviable to reclaim expensive alloying elements.
• Ethical sourcing and sustainability: Some alloys may contain strategic or conflict minerals, raising concerns about ethical sourcing and sustainability. International regulations and industry initiatives actively address these concerns to ensure responsible mineral sourcing throughout the supply chain; however, these considerations should still be taken into account during the design process.

Summary

In summary, while advanced metallic alloys, including Al-Li, titanium, and nickel superalloys, offer compelling advantages in terms of weight savings and performance for aerospace and defense applications, their adoption is constrained by high costs, manufacturing complexities, durability concerns, supply chain sensitivities, and rigorous qualification processes. Successful implementation requires careful trade-off analysis and robust strategies from design to end-of-life.

Looking for more in-depth guidance on aerospace and defense applications? Download our Special Report: Design Tips For Aerospace & Defense OEM Engineers.

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Filed Under: Aerospace + defense, Materials • advanced

 

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