The Four Criteria That Decide Alloy Selection
An airframe aluminium alloy is never chosen on tensile strength alone. The selection is made against four criteria that interact: static strength for limit and ultimate load cases, damage tolerance for residual strength with an assumed crack, fatigue crack growth rate for inspection intervals, and corrosion resistance for the service environment. A grade that wins on strength can lose on toughness, which is why the high-strength 7xxx tempers used on wing tension surfaces are never the ones used on highly loaded fuselage skins.
Alloy Families and Where They Are Used
| Alloy and temper | Typical tensile range (MPa) | Typical 0.2% proof range (MPa) | Typical application |
|---|---|---|---|
| 2024-T3 sheet | 440 to 470 | 290 to 325 | Fuselage skin, lower wing skin |
| 2024-T351 plate | 440 to 470 | 290 to 325 | Machined frames and ribs |
| 7075-T6 plate | 540 to 572 | 470 to 503 | Upper wing skin, spar caps |
| 7075-T73 / T7351 | 470 to 505 | 400 to 435 | Parts exposed to sustained stress in chloride |
| 7050-T7451 plate | 510 to 540 | 440 to 470 | Thick machined bulkheads |
| Al-Li 2099-T83 extrusion | design data per approved handbook | design data per approved handbook | Weight-critical stiffeners |
The ranges above are the handbook values normally quoted for thin sheet and plate in the short transverse direction is ignored; a designer must take the minimum value for the governing direction and product form at the temperature of interest, and must confirm it against the approved allowables document for the airframe programme. Heat treatment of raw material and of finished parts follows AMS 2770 for wrought aluminium, and any deviation from the specified soak, quench delay or ageing practice invalidates the allowables.
Al-Li grades are grouped separately because lithium reduces density by about 3 percent per 1 percent addition and raises the elastic modulus; the benefit is a stiffer, lighter structure at a modest strength penalty. Their anisotropy and fracture behaviour differ from conventional 2xxx, so property values are taken only from qualification data for the specific product.
Damage Tolerance and Fatigue Requirements
Certification practice under 14 CFR 25.571 requires that the structure retain residual strength after failure or crack growth, and that inspectable crack growth be predictable. The working numbers are the plain-strain fracture toughness measured to ASTM E399, the fatigue crack growth rate measured to ASTM E647, and the constant-amplitude fatigue behaviour measured to ASTM E466 or equivalent. Multiple-site damage and widespread fatigue damage drive the choice of fastener spacing and lap joint design more than the base alloy does.
Corrosion, Stress Corrosion and Surface Protection
SCC susceptibility of 2xxx and 7xxx product forms is evaluated to ASTM G47, and exfoliation resistance to ASTM G34. As a rule, T3 and T351 tempers of 2024 have good fatigue and toughness but limited corrosion resistance, while T73 and T7451 tempers of 7075 accept a strength penalty of roughly 10 to 15 percent in exchange for resistance to stress corrosion cracking in the short transverse direction. Clad sheet, anodising and chromate-free conversion coatings are the usual surface protection routes; where chromates are excluded, zirconium-based pretreatments are the common substitute and require their own qualification.
Frequently Asked Questions
Q: Why is 2024-T3 preferred over 7075-T6 for fuselage skin?
A: 2024-T3 combines higher fracture toughness and much better fatigue crack growth resistance with acceptable strength, which supports damage-tolerant design. 7075-T6 is stronger but more sensitive to stress corrosion cracking and has lower toughness in the short transverse direction.
Q: What does the T73 temper change compared with T6?
A: The two-step overageing of 7075 to T73 and T7351 trades roughly 10 to 15 percent of tensile strength for markedly better resistance to stress corrosion cracking and exfoliation, verified by ASTM G47 and ASTM G34 testing.
Q: Which standards fix the design values for aerospace aluminum?
A: Static allowables come from the approved material properties handbook referenced by the airframe programme, base material is produced to AMS or ASTM specifications, and heat treatment follows AMS 2770. Test methods are ASTM E8 or ISO 6892-1 for tension, ASTM E399 for toughness and ASTM E647 for crack growth.
Q: How does thick plate behave differently from thin sheet?
A: Thick plate shows a through-thickness property gradient and lower short transverse properties. Design allowables for thick 7050-T7451 or 7075-T7351 must be taken for the short transverse direction if the part is machined from the centre of the plate.
Q: Can aluminium structures be repaired in service?
A: Yes, but repairs must restore the damage tolerance assumptions of the original design. Riveted or bonded repairs are qualified against the same crack growth and residual strength criteria, and heat-treatable 2xxx and 7xxx parts cannot be re-aged locally without affecting the surrounding allowables.
Q: Is Al-Li always the lighter choice?
A: Not automatically. The density saving is real at roughly 3 percent per percent of lithium, but higher anisotropy, tighter process control and higher material cost mean the benefit must be demonstrated for the specific part with qualified property data.
