2.2 Heat Treatment & Temper Designations of Aluminium

2.2 Heat Treatment & Temper Designations of Aluminium

Heat treatment is a series of controlled heating and cooling operations applied to metals in their solid state to produce desired mechanical properties, such as increased tensile strength, higher hardness, improved ductility, or relief of internal residual stresses.

Aluminium alloys are divided into two fundamental metallurgical categories:

1. Non-Heat-Treatable Alloys (1xxx, 3xxx, 5xxx): These alloys cannot be strengthened by thermal cycles. Their initial mechanical strength is determined by solid-solution alloying, and any further strength increase is achieved exclusively through cold working (strain hardening).
2. Heat-Treatable Alloys (2xxx, 6xxx, 7xxx): These alloys contain elements whose solid solubility in aluminium increases significantly with temperature (notably copper, magnesium, zinc, and silicon). They can be substantially strengthened through precipitation hardening (also known as age hardening).

---

1. Basic Temper Designation System (ANSI H35.1 / EN 515)

The temper designation follows the 4-digit alloy identifier, separated by a hyphen (e.g., 2024-T3, 5052-H32). It indicates the sequence of mechanical and thermal treatments applied to the alloy.

The Five Basic Temper Designations:

• -F (As Fabricated): Applies to products shaped by cold working, hot working, or casting processes where no special control over thermal conditions or strain hardening is employed. Mechanical property limits are neither guaranteed nor specified.
• -O (Annealed): Applies to wrought products that have been thermally treated to achieve the lowest strength and highest ductility condition. This allows severe cold forming, deep drawing, and stamping operations without fracturing.
• -H (Strain Hardened): Applies to wrought non-heat-treatable products (1xxx, 3xxx, 5xxx) whose strength has been increased by cold work. The -H designation is always followed by two or three digits:
  • First Digit (Basic Operation):
    • -H1x: Strain hardened only.
    • -H2x: Strain hardened and partially annealed (softened back to target properties).
    • -H3x: Strain hardened and thermally stabilized (low-temperature bake to prevent spontaneous age softening in magnesium alloys such as 5052).
  • Second Digit (Degree of Hardness):
    • 2 = 1/4 Hard ($1/4\text{H}$)
    • 4 = 1/2 Hard ($1/2\text{H}$)
    • 6 = 3/4 Hard ($3/4\text{H}$)
    • 8 = Full Hard (Full hard, standard cold reduction)
    • 9 = Extra Hard (Tensile strength exceeds full hard by at least $15 \text{ MPa}$)
• -W (Solution Heat-Treated): An unstable temper that applies only to alloys that spontaneously age-harden at room temperature over weeks or months. This designation is incomplete unless the natural aging period is specified (e.g., 7075-W 1/2 hr).
• -T (Thermally Treated to Produce Stable Tempers): Applies to heat-treatable alloys that have undergone thermal treatment (with or without supplementary strain hardening) to produce stable tempers. The -T is always followed by one or more digits.

---

2. Subdivisions of the -T Temper System

The digits 1 through 10 following the -T denote specific sequences of solution heat treatment, cold work, and natural or artificial aging:

Core -T Tempers in Aircraft Maintenance:

• -T1: Cooled from an elevated temperature shaping process (e.g., extrusion) and naturally aged to a substantially stable condition.
• -T2: Cooled from an elevated temperature shaping process, cold worked, and naturally aged.
• -T3 (Solution treated, cold worked, and naturally aged): The alloy is solution heat-treated, quenched, mechanically cold worked (e.g., flattened or stretched to relieve internal stress), and allowed to naturally age to stable properties at room temperature. Classic example: 2024-T3. The cold-working step introduces dislocation networks that homogenize and accelerate precipitate nucleation.
• -T4 (Solution treated and naturally aged): The alloy is solution heat-treated, quenched, and naturally aged to a substantially stable condition at ambient temperature without any mechanical cold working. Example: 2024-T4 rivets.
• -T5: Cooled from an elevated temperature shaping process and artificially aged (baked in an oven).
• -T6 (Solution treated and artificially aged): The alloy is solution heat-treated, quenched, and precipitation heat-treated (artificially aged in an oven at elevated temperatures) to achieve peak tensile and yield strength. Examples: 6061-T6, 7075-T6.
• -T7 (Solution treated and stabilized / overaged): The alloy is solution heat-treated and artificially overaged past peak strength. Overaging coarsens precipitates, relieving internal micro-strains.
  • Critical Aerospace Sub-temper -T73: Specially developed for 7075 alloy (7075-T73). The alloy undergoes a precise two-stage artificial overaging cycle. While it sacrifices approximately $10%$ to $15%$ of peak tensile strength compared to -T6, it provides virtual immunity to stress corrosion cracking (SCC) and exfoliation corrosion in marine environments.
• -T8: Solution heat-treated, cold worked, and artificially aged. Example: 2024-T81.
• -T9: Solution heat-treated, artificially aged, and cold worked.
• -T10: Cooled from an elevated temperature shaping process, cold worked, and artificially aged.

Supplemental Digits for Stress Relief:

When extra digits are added (e.g., -T651, -T7351), they define mechanical stress relief:

• 51 (Stretched): Relieves residual quenching stresses by controlled permanent stretching ($1.5%$ to $3%$ permanent set). Essential for thick plates and extrusions to prevent violent warping during subsequent high-speed CNC machining.
• 52 (Compressed): Relieves quenching stresses by plastic compression (common for forgings).

---

3. The Solution Heat Treatment Process in Aircraft Maintenance

Solution heat treatment is the foundational thermal process for 2xxx, 6xxx, and 7xxx aluminium alloys. Its purpose is to heat the alloy to an elevated temperature to dissolve all soluble alloying constituents into a single, homogeneous solid solution, followed by rapid quenching to trap those elements in a supersaturated solid solution (SSSS).

Temperature (°C)
      ▲
500°C ┼─── SOAKING STAGE (490°C - 500°C) ───┐
      │   (All alloying elements dissolve)   │ Transfer Time (< 7-15s)
      │                                      │ [CRITICAL QUENCH DELAY]
      │                                      ▼
100°C ┼──────────────────────────────────── QUENCH IN COLD WATER (< 30°C)
      │                                      │
      │                                      ├─ Natural Aging (Room Temp) ──> -T4 / -T3
 20°C ┼──────────────────────────────────────┼─ Freezer Storage (-18°C)   ──> Icebox Retardation
      │                                      └─ Artificial Aging Oven     ──> -T6 / -T73
      └────────────────────────────────────────────────────────► Time

1. Heating Media:

• Molten Salt Baths: A eutectic mixture of sodium nitrate ($\text{NaNO}_3$) and potassium nitrate ($\text{KNO}_3$). Salt baths provide rapid, exceptionally uniform heat transfer and exclude air, preventing oxidation. Parts must be thoroughly rinsed in clean hot water following quenching to prevent corrosive salt residue accumulation.
• Air Furnaces: Recirculating hot-air furnaces. They require heavy internal circulation fans and baffles to maintain uniform temperature throughout the working zone without local hotspots.

2. Temperature Tolerances & The Eutectic Melting ("Burning") Catastrophe:

Solution heat treatment requires tight temperature control: typically $\pm 3^\circ\text{C}$ ($\pm 5^\circ\text{F}$).

• For 2024 alloy, the typical soaking temperature is $493^\circ\text{C}$ to $499^\circ\text{C}$ ($920^\circ\text{F}$ to $930^\circ\text{F}$).
• If the temperature drops below $490^\circ\text{C}$, the alloying constituents fail to dissolve fully, resulting in sub-standard mechanical properties after aging.
• The Eutectic Melting Trap ("Burning"): If the temperature exceeds the lower eutectic melting point of the alloy—which is $502^\circ\text{C}$ ($935^\circ\text{F}$) for 2024—the low-melting intermetallic compounds at the crystal grain boundaries begin to melt. This forms microscopic liquid films that vaporize or pull apart, leaving permanent voids, grain boundary separation, and severe brittleness. This condition is termed "burning." Burned aluminium cannot be salvaged by reheat treatment; it loses structural load-bearing capacity and must be unconditionally scrapped.

3. Soaking Times:

Soaking begins when the coldest part of the furnace charge reaches the minimum solution temperature. Soak time depends on the material thickness (gauge), the alloy, and the heating medium. In a salt bath, heat transfer is fast ($10$ to $25 \text{ minutes}$ for sheet); in an air furnace, soak times range from $25 \text{ minutes}$ for thin sheet up to several hours for thick forgings.

4. Quench Transfer Time (Quench Delay) — The Most Critical Window:

The elapsed time from the instant the furnace door opens (or the salt bath basket breaks the liquid surface) until the component is completely submerged in the cold water quench tank is known as the quench delay.

Industry and military specifications (such as AMS-2770 and MIL-H-6088) dictate maximum allowable quench delays:

• Sheet thickness $< 0.016 \text{ in}$ ($0.4 \text{ mm}$): $7 \text{ seconds}$ maximum.
• Sheet thickness $0.016 \text{ to } 0.031 \text{ in}$ ($0.4 \text{ to } 0.8 \text{ mm}$): $10 \text{ seconds}$ maximum.
• Sheet thickness $0.032 \text{ to } 0.063 \text{ in}$ ($0.8 \text{ to } 1.6 \text{ mm}$): $12 \text{ seconds}$ maximum.
• Sheet thickness $> 0.063 \text{ in}$ ($1.6 \text{ mm}$): $15 \text{ seconds}$ maximum.

The Metallurgical Danger of Excessive Quench Delay:

During the critical temperature zone from $400^\circ\text{C}$ down to $260^\circ\text{C}$ ($750^\circ\text{F}$ to $500^\circ\text{F}$), the driving force for precipitation is intense. If cooling is sluggish due to an excessive quench transfer delay:

1. Copper atoms have time to migrate out of the solid solution and concentrate at the grain boundaries, precipitating as coarse copper aluminide ($\text{CuAl}_2$) particles.
2. The crystal matrix immediately adjacent to the grain boundaries is stripped of copper, forming a narrow copper-depleted zone.
3. The copper-depleted zone has a much more negative (anodic) electrode potential ($-0.83 \text{ V}$) than the copper-rich core grains ($-0.68 \text{ V}$).
4. In the presence of moisture, an aggressive galvanic micro-cell is established: the grain boundaries corrode preferentially at high speed. This results in severe Intergranular Corrosion (IGC). An affected component can crack and disintegrate along its grain boundaries under normal flight loads, even though its surface appears pristine.

5. Quenching Bath Temperature:

The water quench bath must be maintained at temperatures below $30^\circ\text{C}$ ($85^\circ\text{F}$). If the water temperature exceeds $38^\circ\text{C}$ ($100^\circ\text{F}$), steam bubbles form on the metal surface, creating an insulating thermal blanket that slows the cooling rate and triggers the same grain boundary precipitation that causes intergranular corrosion.

---

4. Reheat Treatment Restrictions on Clad Aluminium

Clad aluminium alloys depend entirely on their pure aluminium surface layer for corrosion protection. During high-temperature solution heat treatment ($~500^\circ\text{C}$), copper and magnesium atoms from the high-strength core diffuse into the pure aluminium cladding layer.

Diffusion Mechanics and Reheat Limits:

• With each thermal cycle or prolonged soaking period, copper penetrates further toward the outer surface of the cladding.
• Once copper diffuses through to the exterior surface, the cladding's electrochemical potential drops to match the core. The sacrificial galvanic protection is permanently lost, and the cladding develops severe surface pitting.
• Aerospace Standard Restrictions (AMS-2770 / EASA SRM):
  • Sheet thickness $< 0.032 \text{ in}$ ($0.8 \text{ mm}$): Reheat treatment is strictly prohibited (0 reheat cycles permitted). Only the initial mill heat treatment is allowed.
  • Sheet thickness $0.032 \text{ in to } 0.063 \text{ in}$ ($0.8 \text{ to } 1.6 \text{ mm}$): Allowed a maximum of 1 reheat treatment.
  • Sheet thickness $> 0.063 \text{ in}$ ($1.6 \text{ mm}$): Allowed a maximum of 2 (rarely 3) reheat treatments.

---

5. Refrigeration & Retardation of Age Hardening ("Icebox Rivets")

Solid aircraft rivets are the standard fasteners for structural joints. Their ease of installation depends directly on their temper state.

The Aging Timeline of Quenched Rivets:

When an aluminium-copper alloy (such as 2017 or 2024) is quenched in cold water from its solutionizing temperature, it is initially soft, ductile, and in the unstable -W temper. At room temperature ($20^\circ\text{C} / 68^\circ\text{F}$), spontaneous precipitation hardening commences within 15 to 30 minutes. Within 2 hours, it hardens significantly; within 24 hours, it reaches roughly 90% of its final hardness; and within 4 days (96 hours), natural age hardening is complete (-T4 condition).

Rivet Alloy Types & Identification

Cryogenic Storage Principles ("Icebox Rivets"):

Because precipitation hardening is a diffusion-controlled atomic process, lowering the temperature dramatically reduces atomic mobility, retarding or halting the aging reaction:

• Storage at $-18^\circ\text{C}$ ($0^\circ\text{F}$): Retards age hardening for 7 to 14 days.
• Storage at $-40^\circ\text{C}$ (or on dry ice at $-78^\circ\text{C}$): Halts the aging reaction indefinitely.

Shop Floor Driving Constraints:

• Once removed from the freezer into room temperature, 2024-T4 (DD) rivets must be driven within 10 to 20 minutes.
• 2017-T4 (D) rivets age somewhat more slowly and may be driven within 1 hour of removal.
• Consequences of Exceeding Time Limits: If a technician attempts to drive a 2024 rivet that has thawed past 20 minutes, the advanced precipitation hardening makes the metal hard and brittle. Driving causes the manufactured head or formed shop head to develop severe radial cracks, distorts the rivet hole, and work-hardens the shank before the hole is properly filled.
• Scrap / Reheat Rule: Rivets that thaw and exceed their driving window must not be driven. They must be collected, returned for re-solution heat treatment (subject to alloy reheat cycle limits), or discarded.