Aircraft Sheet Metal Alloy Designations and Temper Codes

Walk into any aircraft sheet metal shop and you will encounter part numbers, material callouts, and stock labels bristling with numbers and letters like 2024-T3, 7075-T6, or 6061-T4. These are not arbitrary stamps — they are a tightly standardized language developed by the Aluminum Association and adopted throughout the aviation industry, including in FAA-accepted data and manufacturer structural repair manuals. For an Aviation Maintenance Technician (AMT) working on airframe sheet metal, reading these designations fluently is as fundamental as reading a torque value. Using the wrong alloy or wrong temper in a structural repair can compromise the load-carrying ability of the airframe in ways that are invisible to a visual inspection, making this knowledge a genuine safety issue.

This article unpacks the complete designation system for aluminum alloys and their temper codes as applied to aircraft sheet metal, explains why different alloys are chosen for different structural locations, and highlights the specific numbers and traps that appear on the FAA Airframe Knowledge Test.

The Aluminum Alloy Numbering System

Aircraft sheet metal is predominantly aluminum alloy, and the industry uses a four-digit numbering system to identify each alloy family. The first digit identifies the principal alloying element, the second digit (if non-zero) indicates a modification to the original alloy or impurity limits, and the last two digits identify the specific alloy within the series.

1xxx series — Pure aluminum (99% or higher): Excellent corrosion resistance and formability, but low strength. Rarely used structurally in aircraft; sometimes used for non-structural fairings or fuel lines.
2xxx series — Aluminum-Copper alloys: High strength, good machinability, and heat-treatable. The workhorse of aircraft primary structure. 2024 is the most widely used aircraft sheet alloy because of its excellent strength-to-weight ratio and fatigue resistance. However, it has relatively poor corrosion resistance and is almost always clad.
3xxx series — Aluminum-Manganese alloys: Moderate strength, good formability and corrosion resistance. Not heat-treatable. Used for fuel tanks, cowlings, and interior parts where strength demands are lower.
5xxx series — Aluminum-Magnesium alloys: Good strength through work hardening, excellent corrosion resistance (especially in marine environments), and good weldability. Used for fuel tanks and some skin panels.
6xxx series — Aluminum-Magnesium-Silicon alloys: Moderate strength, excellent corrosion resistance, and heat-treatable. 6061 is common in extruded shapes, tubing, and fittings but is also seen in sheet form for lightly loaded structures.
7xxx series — Aluminum-Zinc alloys: The highest-strength aluminum alloys. 7075 is used extensively in wing spars, skins, and other highly stressed components. It is more susceptible to stress-corrosion cracking than 2xxx alloys and must be handled and stored carefully.

What "Clad" Means

Many aircraft aluminum sheets are described as Alclad. This means the high-strength core alloy (such as 2024 or 7075) is bonded on each surface with a thin layer — typically 1.5% to 2.5% of total sheet thickness per side — of pure aluminum or a more corrosion-resistant alloy. The cladding provides galvanic (sacrificial) protection: the cladding is anodic relative to the core and corrodes preferentially, protecting the structural core. The tradeoff is a slight reduction in overall strength compared to bare (unclad) sheet of the same thickness, because the softer cladding material replaces some structural material. When repairing an Alclad sheet, the replacement material must also be Alclad unless the structural repair manual specifically permits bare sheet.

The Temper Designation System

The alloy number tells you what is in the metal; the temper code tells you what has been done to it after alloying — specifically how it has been heat-treated and/or mechanically worked. The temper is separated from the alloy number by a hyphen. Understanding tempers is critical because the same alloy in a different temper can have dramatically different strength, ductility, and formability.

Basic Temper Letters

F — As Fabricated: No special thermal or strain-hardening treatment after shaping. Properties are not controlled. Rarely specified for structural parts.
O — Annealed: The softest, most ductile condition. Produced by heating and slow cooling to relieve all work hardening or prior heat treatment. Used when maximum formability is needed, such as when bending tight-radius parts. Structural strength is at its lowest.
H — Strain Hardened (non-heat-treatable alloys): Work hardening is used to increase strength in alloys that cannot be strengthened by heat treatment (1xxx, 3xxx, 5xxx series). Sub-digits refine the degree of hardening.
T — Thermally Treated (heat-treatable alloys): Applies to alloys that can be significantly strengthened by solution heat treatment, quenching, and aging. This is the most important temper category for aircraft structural alloys like 2024 and 7075.
W — Solution Heat Treated (unstable temper): The alloy has been solution heat-treated but natural aging has not yet completed. This is an unstable, time-dependent condition. A sheet designated W is still changing its properties and must not be used structurally until it reaches a stable temper.

T-Temper Subdivisions

The number following the letter T is critical. Each digit represents a specific sequence of thermal and mechanical operations:

T3: Solution heat-treated, cold-worked, and naturally aged to a stable condition. 2024-T3 is the most common aircraft structural sheet. Cold working after quench improves strength and fatigue life.
T4: Solution heat-treated and naturally aged (no cold work). Slightly lower strength than T3 but better formability — useful when additional bending or forming will occur before the part is put in service.
T6: Solution heat-treated and artificially aged (elevated temperature aging). Higher strength than T4 because artificial aging drives precipitation hardening further. 7075-T6 is the highest-strength common aircraft sheet alloy. However, it is less formable and more brittle than T3 or T4.
T73: Solution heat-treated and overaged for improved stress-corrosion resistance. 7075-T73 has lower strength than 7075-T6 but is significantly more resistant to stress-corrosion cracking, making it preferable in highly stressed, corrosive environments.
T36: Solution heat-treated and cold-worked to a specific degree (approximately 6% reduction). Used for 2024 in some applications requiring slightly higher strength than T3.
T351, T451, T651: The added digit indicates stress relief by stretching after heat treatment. This reduces residual stresses in thick plate and sheet, improving dimensional stability during machining.

Why Alloy and Temper Selection Matters for Repairs

FAA Advisory Circulars and the aircraft manufacturer's Structural Repair Manual (SRM) specify exact alloy and temper for every structural repair. Substituting a higher-strength alloy is not automatically acceptable — a stiffer, stronger patch can actually create stress concentrations and transfer loads in ways the original design did not anticipate. Substituting a weaker alloy is obviously unsafe. Similarly, using an annealed (O) sheet where T3 is required dramatically reduces strength. The AMT must match the original specification precisely or follow approved engineering data for any substitution.

Additionally, heat-treating in the field is strictly controlled. Once a 2024-T3 sheet is annealed and re-formed, it cannot simply be re-heat-treated to T3 in a field shop without a properly calibrated furnace, quench tank, and documented process. Attempting to field-anneal and re-temper structural aluminum without approved facilities and data is a serious airworthiness violation.

Key Numbers and Rules

2024-T3 Alclad — most common aircraft skin and structural sheet alloy.
7075-T6 — highest common aircraft sheet strength; used for highly stressed skins and spars.
7075-T73 — overaged version of 7075 with better stress-corrosion resistance at some sacrifice in strength.
6061-T6 — common for fittings, brackets, and extrusions; moderate strength with good corrosion resistance.
Alclad cladding thickness: approximately 1.5% to 2.5% per side of total sheet thickness.
The O temper is the fully annealed, softest, most formable condition — lowest strength.
The W temper is unstable and time-dependent; do not use for structural parts in this condition.
Temper must match the repair data exactly — strength, ductility, and corrosion behavior all depend on it.

Common Test Traps

Confusing alloy series roles: The FAA test may ask which series is used for primary structure. The answer is the 2xxx and 7xxx series, not the 3xxx or 5xxx (which are lower-strength, non-heat-treatable).
Misidentifying the W temper: Students often think W means "worked" or "wrought." It means solution heat-treated but not yet aged — an unstable condition. Never use W-temper material as-is for structural work.
Assuming higher strength is always better: The test (and real repairs) require matching the original specification. A stronger alloy used as a patch may overstress surrounding structure by carrying more load than intended.
Forgetting that Alclad has lower strength than bare sheet: Because the soft cladding replaces some structural cross-section, Alclad sheet is slightly weaker than bare sheet of the same nominal thickness. This affects strength calculations.
Mixing up T3 and T4: Both are naturally aged, but T3 includes cold working after quench, giving it higher strength and better fatigue resistance. T4 is naturally aged without cold work and is more formable. For most aircraft skins, T3 is required.