Aerospace materials: aluminium, titanium alloys and composites

Why it matters

The choice of material decides the weight, the cost, the fatigue and corrosion behaviour, and the inspection burden of an airframe for its whole 25–30 year life. Each part is loaded differently: a lower wing skin is fatigue-critical in tension, an upper skin is buckling-critical in compression, a landing gear is strength-critical, a pylon is hot. Selecting the right aluminium temper, titanium alloy or composite layup for each job is a core skill of a structures engineer.

Key ideas

Selection by performance index. Light structures are compared per unit mass, not per unit volume. The right index depends on how the part is loaded:

• Tie or stiffness-limited bar: minimise mass for given stiffness → maximise E/ρ (specific stiffness).
• Strength-limited tie: maximise σ_y/ρ (specific strength).
• Column (Euler buckling, given length and load): maximise E^½/ρ.
• Thin panel buckling (given width and load): maximise E^⅓/ρ. Most structural metals (aluminium, titanium, steel) have almost the same E/ρ, about 25 MN·m/kg, so in stiffness they are equal by mass; lower-density materials win strongly in buckling-critical parts because of the fractional exponents. That is why thin-walled aircraft structure is made of aluminium or composite rather than steel.

Aluminium alloys. Density about 2.7–2.8 Mg/m³, E about 70–73 GPa.

• 2xxx (Al-Cu), e.g. 2024-T3: moderate strength, high fracture toughness, slow fatigue crack growth. Used for tension-dominated, fatigue-critical parts: lower wing skins, fuselage skins.
• 7xxx (Al-Zn-Mg-Cu), e.g. 7075-T6, 7050-T7451: high strength, lower toughness; overaged T7 tempers trade a little strength for stress-corrosion resistance. Used for compression-dominated parts: upper wing skins and stringers, spar caps, frames.
• Al-Li alloys (e.g. 2050, 2099): lithium lowers density and raises E, improving both specific stiffness and fatigue performance.
• Corrosion: high-strength alloys are not inherently corrosion-resistant; sheet is clad with pure aluminium, parts are anodised, primed, sealed and painted, and 7xxx in the T6 temper can suffer stress-corrosion and exfoliation.
• Limitation: strength falls above about 120–150 °C.

Titanium alloys. Ti-6Al-4V is the workhorse: density about 4.4 Mg/m³, E about 114 GPa, high strength and fatigue strength, excellent corrosion resistance, useful to around 350–400 °C. Used for engine pylons, firewalls, heavily loaded fittings, landing gear beams, and fasteners and fittings in carbon composite structures, because it is galvanically compatible with carbon and has a similar low thermal expansion. Drawbacks: cost, difficult machining (low conductivity, galling), and contamination-sensitive welding.

Steels. Ultra-high-strength low-alloy steels (e.g. 300M, about 1.9 GPa ultimate) are used where size is constrained and loads are very high: landing gear, flap tracks, some fittings. They are dense, notch- and hydrogen-embrittlement-sensitive, and need corrosion protection.

Composites. Carbon fibre reinforced polymer (CFRP, density about 1.55–1.6 Mg/m³) has a unidirectional modulus of about 130–180 GPa and very high fatigue resistance in the fibre direction, no corrosion, and allows tailored stiffness. Weaknesses: low transverse and through-thickness strength, sensitivity to impact damage that may be barely visible (so compression-after-impact allowables govern), moisture and temperature effects on the matrix, lightning-strike protection needs, galvanic corrosion of aluminium in contact with carbon, and difficult repair and recycling. Glass fibre (GFRP) is used for radomes and fairings (radio-transparent), aramid for impact and ballistic protection. Sandwich panels (thin skins on honeycomb or foam core) give very high bending stiffness per unit mass for floors, control surfaces and fairings.

Other selection factors. Fatigue and crack-growth behaviour, fracture toughness, corrosion and stress-corrosion, temperature range, manufacturability and repair, cost and availability, and certification data. Property values vary with temper, product form and direction; design uses statistically based allowables (e.g. A- or B-basis values from the material data handbooks), not typical values.

Formulas

Specific stiffness = E / ρ

• E: Young's modulus (Pa), ρ: density (kg/m³); units N·m/kg (often quoted in MN·m/kg).

Specific strength = σ_y / ρ

• σ_y: yield (or ultimate) strength (Pa).

m ∝ ρ / E (stiffness-limited tie), m ∝ ρ / σ_y (strength-limited tie)

• Mass m of a member of given length for a given stiffness or load.

m ∝ ρ / E^½ (column), m ∝ ρ / E^⅓ (plate buckling)

• Mass for a given buckling load, length and width fixed.

m = ρ·V

• Mass (kg) from density (kg/m³) and volume (m³).

Worked examples

Example 1 (standard): specific properties of three metals. Given (typical values for illustration): 7075-T6: E = 72 GPa, ρ = 2810 kg/m³, σ_y = 505 MPa; Ti-6Al-4V: E = 114 GPa, ρ = 4430 kg/m³, σ_y = 880 MPa; 300M steel: E = 200 GPa, ρ = 7830 kg/m³, σ_y = 1590 MPa.

1. 7075: E/ρ = 72 × 10⁹/2810 = 25.6 MN·m/kg; σ_y/ρ = 505 × 10⁶/2810 = 180 kN·m/kg.
2. Ti-6Al-4V: E/ρ = 114 × 10⁹/4430 = 25.7 MN·m/kg; σ_y/ρ = 880 × 10⁶/4430 = 199 kN·m/kg.
3. 300M: E/ρ = 200 × 10⁹/7830 = 25.5 MN·m/kg; σ_y/ρ = 1590 × 10⁶/7830 = 203 kN·m/kg. Answer: specific stiffness ≈ 25.6 MN·m/kg for all three; specific strength 180, 199 and 203 kN·m/kg. For a strength-limited tie steel and titanium are slightly lighter than aluminium; for stiffness the three are equal by mass.

Example 2 (GATE level): buckling-critical skin, aluminium versus CFRP. Given: a compression skin panel of fixed width and load, designed against plate buckling. Aluminium: E = 72 GPa, ρ = 2810 kg/m³. Quasi-isotropic CFRP: E = 55 GPa, ρ = 1600 kg/m³ (illustrative).

1. For plate buckling the mass index is ρ/E^⅓.
2. Aluminium: E^⅓ = 72^⅓ = 4.160 (GPa^⅓); ρ/E^⅓ = 2810/4.160 = 675.5.
3. CFRP: 55^⅓ = 3.803; ρ/E^⅓ = 1600/3.803 = 420.7.
4. Mass ratio: m_CFRP/m_Al = 420.7/675.5 = 0.623.
5. For comparison, as a stiffness-limited tie: (1600/55)/(2810/72) = 29.09/39.03 = 0.745. Answer: the CFRP panel weighs about 62% of the aluminium one (a 38% saving) when buckling governs, against about 75% for a stiffness-limited tie. Real savings are smaller because impact-damage allowables and joints add weight back.

Common mistakes

• Comparing materials by strength or stiffness alone instead of per unit mass with the correct index.
• Thinking steel is "stiffer" in a weight sense; its E/ρ is the same as aluminium's.
• Calling high-strength aluminium alloys corrosion-resistant without protection.
• Using 7075-T6 for a fatigue-critical tension skin, or 2024-T3 where compression strength governs.
• Using typical properties for design instead of statistically based allowables.
• Fastening aluminium directly to carbon composite without isolation (galvanic corrosion).

For GATE AE

Expect conceptual questions on which alloy or material suits which part and why, on composite advantages and drawbacks, and on specific stiffness and strength. Short numericals compute E/ρ, σ/ρ or mass ratios with given data. Practise the performance indices for ties, columns and plates.

Quick check

1. Which aluminium alloy is preferred for a lower wing skin, and why?
2. What is E/ρ for an alloy with E = 110 GPa and ρ = 4400 kg/m³?
3. Why are titanium fasteners used in carbon composite structure?
4. Which index should be maximised for a plate that fails by buckling? Answers: 1. 2024-T3 (or a damage-tolerant Al-Li), for slow crack growth and high toughness in tension; 2. 25 MN·m/kg; 3. galvanic compatibility and matching thermal expansion; 4. E^⅓/ρ.