Aerospace Fastener Selection: Titanium vs Inconel vs Steel Comparative Analysis
Executive summary: Aerospace fasteners are deceptively simple components, but the material selection drives the performance, the weight, and the cost in ways that ripple through the entire airframe. Titanium (Ti-6Al-4V for general structural bolting, Ti-6Al-4V ELI for fracture-critical, Beta-C for high-strength) is the dominant material for modern airframes, accounting for roughly 60 percent of structural fasteners by count. Inconel 718 takes over for high-temperature zones (engine mounts, exhaust fairings) where the temperature exceeds titanium’s limit of about 350 °C. A286 iron-nickel-chromium alloy and 17-4 PH stainless steel are used where the temperature is moderate but the cost is a primary driver, and where the magnetic signature of steel is acceptable. The selection framework reduces to four engineering questions: temperature, strength, weight, and corrosion environment. The procurement mistake is to default to the lowest-cost fastener that meets the static strength requirement, ignoring fatigue, galvanic compatibility, and temperature.
Fastener material families
The four dominant aerospace fastener material families occupy distinct positions in the temperature-strength-cost matrix.
Table 1: Fastener material family comparison
| Material | Typical UTS | Density | Max service temp | Relative cost | Primary use |
|---|---|---|---|---|---|
| Ti-6Al-4V (aged) | 1,100 MPa | 4.43 g/cm³ | About 315 °C | 5 to 10 × | General airframe structure |
| Ti-6Al-4V ELI (aged) | 1,100 MPa | 4.43 g/cm³ | About 315 °C | 8 to 14 × | Fracture-critical structure |
| Beta-C titanium (aged) | 1,400 MPa | 4.65 g/cm³ | About 315 °C | 15 to 25 × | High-strength structure |
| Inconel 718 (aged) | 1,380 MPa | 8.19 g/cm³ | About 700 °C | 10 to 18 × | Engine, exhaust, hot structure |
| A286 iron-nickel (aged) | 1,100 MPa | 7.94 g/cm³ | About 650 °C | 2 to 4 × | Moderate temperature, cost-driven |
| 17-4 PH stainless (aged) | 1,310 MPa | 7.78 g/cm³ | About 315 °C | 2 to 3 × | Corrosion-critical, moderate temperature |
| PH 13-8 Mo stainless (aged) | 1,520 MPa | 7.76 g/cm³ | About 315 °C | 4 to 6 × | High-strength, corrosion-critical |
| 300M steel (tempered) | 1,930 MPa | 7.87 g/cm³ | About 300 °C | 1.0 × | Landing gear (non-corrosive) |
The cost column is the most variable; actual prices depend on the fastener size, the head configuration, the thread form, the quantity, and the qualified supplier. A286 and 17-4 PH are the cost-driven choices; titanium is the volume-leading choice for structural airframe bolting; Inconel 718 is the temperature-driven choice; 300M is the landing gear workhorse where weight is less critical than ultimate strength. See the titanium grade selection for extreme stress and thermal environments guide for the high-stress application context.
The selection framework
The selection of an aerospace fastener material reduces to four engineering questions. Each has a default answer; the optimal answer may deviate when multiple requirements apply.
Question 1 — What is the service temperature? Below about 200 °C, titanium is acceptable and is the default for structural airframe bolting. Between 200 and 350 °C, titanium (with attention to creep) or A286 is acceptable. Above 350 °C, Inconel 718 or another nickel superalloy is required. The fastener temperature is the primary driver for engine and nacelle bolting.
Question 2 — What is the strength requirement? For UTS below about 1,100 MPa, titanium, A286, and 17-4 PH are all acceptable. For UTS above 1,100 MPa, titanium (aged to higher strength), Inconel 718, or PH 13-8 Mo is required. For landing gear (UTS above 1,800 MPa), 300M steel or a similar ultra-high-strength steel is the standard.
Question 3 — What is the weight sensitivity? For weight-critical structure (most of the airframe), titanium is preferred because of the density advantage. For non-weight-critical structure (landing gear interior, engine internal bolting where the weight is carried by the structure, not the fastener), steel is acceptable.
Question 4 — What is the corrosion environment? In marine environments, in chemical environments, and in any application where the fastener cannot be coated or where coating damage is likely, titanium or 17-4 PH is preferred over carbon steel. Steel fasteners require cadmium plating, zinc-nickel plating, or other protective coating for corrosion resistance; titanium does not.
Galvanic compatibility
Fasteners create galvanic cells with the materials they join. The fastener material and the structure material form a galvanic couple in the presence of moisture, and the less noble metal corrodes preferentially.
The galvanic series positions (from most active to most noble):
-
Magnesium, zinc, aluminum alloys (most active)
-
Carbon steel, low-alloy steel
-
Austenitic stainless steel (302, 304, 316) — passive
-
17-4 PH, 15-5 PH, PH 13-8 Mo (passive)
-
A286 (passive)
-
Lead, tin
-
Nickel alloys (Inconel 718, Waspaloy)
-
Titanium alloys (Ti-6Al-4V, Beta-C)
-
Graphite, noble metals (most noble)
The galvanic rule of thumb: avoid coupling metals more than 0.3 V apart in the galvanic series. Titanium coupled with aluminum alloy 7075-T6 is roughly 0.5 V apart and requires isolation. Titanium coupled with carbon steel is roughly 0.6 V apart and requires isolation or a coating. Titanium coupled with Inconel 718 is roughly 0.1 V apart and is acceptable without isolation. See the titanium vs aluminum structural components guide for the airframe galvanic context.
Head configurations and thread forms
The fastener material is one decision; the head configuration and the thread form are equally important for the application.
Head configurations. Protruding hex head (the standard), flush head (100° reduced head, used in aerodynamic surfaces), pan head (low profile, used in interior structure), and socket head (high strength, used in restricted access). The head configuration affects the tooling required for installation and the torque allowable on the fastener. The procurement specification should match the head configuration to the installation tooling and the torque procedure.
Thread forms. Unified National Coarse (UNC), Unified National Fine (UNF), and MJ (the metric aerospace thread form with controlled radius root). MJ threads are preferred for fatigue-critical aerospace fasteners because the radius root reduces the stress concentration at the thread root, improving fatigue life by 20 to 40 percent relative to UN threads with sharp roots.
Locking features. For vibration-critical applications, the fastener may include a prevailing-torque feature (a deformed thread or a nylon insert) to prevent self-loosening. The procurement specification should match the locking feature to the vibration environment.
Table 2: Aerospace fastener standard families
| Standard family | Scope | Material options | Application |
|---|---|---|---|
| MIL-F-5627 (legacy, use SAE) | General aerospace bolts | Various steels, titanium | Legacy military, transitioning to SAE |
| AS / SAE aerospace series | Standard aerospace fasteners | Steel, titanium, Inconel, A286 | Commercial and military aerospace |
| MS (military standard) | Military-spec fasteners | Various materials | Military aerospace and ground equipment |
| NAS (national aerospace standard) | NAS series bolts, screws | Steel, titanium, alloy 718 | Commercial aerospace, Boeing and Airbus common |
| EN / ASNA (European) | European aerospace fasteners | Steel, titanium, nickel alloys | Airbus and European aerospace |
| Customer proprietary (Boeing BAC, Airbus ABS) | OEM-specific fasteners | Per OEM specification | OEM internal programs |
| IFI / IFI-100 / IFI-104 (industrial) | Industrial inch and metric | Steel, stainless | Non-aerospace industrial applications |
Procurement rules for aerospace fastener selection
Rule 1 — Match the fastener material to the service temperature. Below 200 °C: titanium or 17-4 PH. Between 200 and 350 °C: titanium (with creep attention) or A286. Above 350 °C: Inconel 718. Above 600 °C: Waspaloy or nickel superalloys.
Rule 2 — Match the fastener strength to the joint requirement. For UTS below 1,100 MPa, titanium or 17-4 PH is acceptable. For UTS above 1,100 MPa, specify the appropriate aged condition. For landing gear, 300M steel is the standard.
Rule 3 — Specify the galvanic compatibility. The fastener material and the structure material must be galvanically compatible. When coupling metals more than 0.3 V apart, specify the isolation barrier or the fastener coating.
Rule 4 — Specify the thread form and the head configuration. The procurement specification should match the thread form to the fatigue requirement (MJ for high fatigue) and the head configuration to the installation tooling.
Rule 5 — Specify the qualified supplier. Aerospace fasteners are critical components; the supplier qualification is as rigorous as for the structure material. The qualified supplier list (QSL) for the prime contractor’s fastener part numbers should be followed.
Rule 6 — Engineer contradiction — the lowest-cost qualified fastener is not always the right fastener. A buyer who selects the cheapest fastener that meets the static strength requirement may be selecting a fastener with inadequate fatigue life, inadequate corrosion resistance, or inadequate galvanic compatibility. The selection framework should consider all four engineering questions, not only the static strength.
For the broader aerospace structural material selection context, see the titanium vs aluminum structural components guide. For the high-temperature titanium variants, see the extreme stress and thermal environments guide. To specify a fastener selection for an aerospace structural application, request a fastener review with the engineering team.