Choosing Alloys for Aerospace Fasteners

## Why Fasteners Matter in Aerospace A commercial aircraft contains 2-3 million fasteners. Each one is a potential fatigue initiation site and critical load path. ## Primary Fastener Alloys ### Alloy Steel (4340, H-11) 4340 QT to 160-180 ksi (1100-1240 MPa) is the workhorse structural fastener steel. Cadmium or zinc-nickel plated for corrosion protection. Steel fasteners in aluminum structure require isolation to prevent galvanic corrosion. ### A-286 Iron-Base Superalloy Precipitation-hardened, 895 MPa UTS, service to 650 degrees C. Used for turbine engine case bolts and exhaust system fasteners. ### Titanium Ti-6Al-4V 40% weight savings over steel at comparable strength (1100 MPa STA). Compatible with aluminum and composite structure without galvanic isolation. Thread lubricant mandatory to prevent galling. ### Inconel 718 Highest-strength common aerospace fastener alloy: 1380 MPa UTS aged. Used for critical engine mount bolts and high-temperature applications. ### Aluminum Rivets (2024-T4, 7075-T73) 2024-T4 icebox rivets are the most common fastener by quantity in airframe construction. ## Selection by Application | Application | Primary Alloy | Reason | |-------------|--------------|--------| | Wing skin-to-spar | Ti-6Al-4V | Composite compatibility | | Fuselage lap joints | 2024-T4 rivets | Cost, speed | | Engine case | Inconel 718 | Temperature + strength | | Landing gear | 4340 (260 ksi) | Extreme strength | Every aerospace fastener is manufactured to a military or NAS specification. Substitution without engineering approval is a safety-of-flight concern.