2.1 Aluminium & Aluminium Alloy Classifications

2.1 Aluminium & Aluminium Alloy Classifications

Aluminium and its alloys form the structural backbone of modern civil and military transport aircraft. Combining low density, excellent thermal and electrical conductivity, high fracture toughness, and superior workability, aluminium alloys account for roughly 60% to 70% of the structural airframe weight of conventional commercial transport aircraft.

For the licensed aircraft maintenance engineer (EASA Part-66 Category B1 and B2), understanding the exact metallurgical classifications, elemental compositions, stress-strain characteristics, and corrosion-prevention requirements of non-ferrous alloys is critical for ensuring airworthiness during structural inspections, modifications, and sheet metal repairs.

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1. Physical Characteristics of Pure Aluminium

Commercially pure aluminium is an element with an atomic number of 13 and a face-centered cubic (FCC) crystal lattice. The FCC structure provides numerous slip planes, making pure aluminium exceptionally ductile, malleable, and easy to form cold.

Primary Physical Properties of Pure Aluminium:

• Density: Approximately $2.70 \text{ g/cm}^3$ (or $0.098 \text{ lb/in}^3$), which is approximately one-third the density of structural steel ($7.85 \text{ g/cm}^3$).
• Melting Point: $660^\circ\text{C}$ ($1220^\circ\text{F}$).
• Thermal Conductivity: High, approximately 50% to 60% of pure copper.
• Electrical Conductivity: Approximately 65% of the International Annealed Copper Standard (IACS) by volume, but over 200% by weight.
• Magnetic Permeability: Non-magnetic.
• Corrosion Behavior: When exposed to atmospheric oxygen, pure aluminium spontaneously forms an instantaneous, microscopically thin ($2$ to $10 \text{ nm}$), tenacious, self-healing oxide film of alumina ($\text{Al}_2\text{O}_3$). This layer passivates the metal against further oxidation in neutral pH environments ($4.5 \text{ to } 8.5$).

Mechanical Limitations:

In its unalloyed state (e.g., 1100 alloy), pure aluminium possesses very low tensile strength (ultimate tensile strength of only $90 \text{ MPa}$ or $13 \text{ ksi}$). Consequently, it is unsuited for primary load-bearing aircraft structures and is restricted to non-structural fairings, oil tanks, electrical busbars, and sacrificial cladding.

To achieve the high yield and fatigue strengths demanded by primary structures, aluminium is alloyed with copper, magnesium, zinc, manganese, and silicon, followed by precise thermal processing.

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2. Wrought Aluminium Alloy Classification (4-Digit System)

Wrought aluminium products—materials formed into sheets, plates, extrusions, forgings, and drawn tubes through mechanical working—are categorized using the universal four-digit index developed by the Aluminium Association (ANSI H35.1) and adopted under European Standards (EN 573-3).

Structure of the 4-Digit Index:

1. First Digit ($X\text{xxx}$): Identifies the major alloying group (primary alloying element added to the aluminium matrix).
2. Second Digit ($\text{x}X\text{xx}$): Indicates specific alloy modifications. A zero (0) indicates the original, unmodified alloy. Numbers 1 through 9 indicate successive compositional modifications of the original alloy (e.g., tighter limits on impurities such as iron or silicon).
3. Third and Fourth Digits ($\text{xx}XX$):
   • In the 1xxx series (pure aluminium), the last two digits indicate the hundredths of 1% of pure aluminium content above $99.00%$. For example, 1050 indicates a minimum of $99.50%$ pure aluminium; 1100 indicates a minimum of $99.00%$.
   • In the 2xxx through 8xxx series, the last two digits have no numerical significance; they serve simply as arbitrary identifying numbers to distinguish different alloys in the group (e.g., 2014, 2024, 7075).

4-Digit Wrought Identifier Example: 2 0 2 4
                                    │ │ └── Identified specific alloy (24)
                                    │ └──── Original alloy modification (0 = original)
                                    └────── Major Alloying Element: Copper (2xxx)

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Comprehensive Table of Wrought Aluminium Alloy Series

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3. Cast Aluminium Alloy Designation System

Aluminium casting alloys are poured directly into sand molds, permanent metal dies, or investment shells to manufacture complex geometric components such as bellcranks, pump housings, valve bodies, and structural brackets where machining from solid billet is uneconomic.

Designation Format: $xxx.x$

Cast aluminium uses a three-digit number followed by a decimal point:

• First Digit ($X\text{xx}.x$): Designates the principal alloying group:
  • 1xx.x: Unalloyed aluminium ($99.00%$ minimum).
  • 2xx.x: Copper ($\text{Cu}$).
  • 3xx.x: Silicon ($\text{Si}$) with Copper ($\text{Cu}$) and/or Magnesium ($\text{Mg}$).
  • 4xx.x: Silicon ($\text{Si}$).
  • 5xx.x: Magnesium ($\text{Mg}$).
  • 7xx.x: Zinc ($\text{Zn}$).
  • 8xx.x: Tin ($\text{Sn}$).
• Second and Third Digits ($\text{x}XX.x$): Identify the specific alloy composition within the series (e.g., 356).
• Decimal Digit ($\text{xxx}.X$): Indicates the product form:
  • 0 (e.g., 356.0): Designates a final shaped casting.
  • 1 (e.g., 356.1): Designates an ingot manufactured to standard compositional limits for remelting.
  • 2 (e.g., 356.2): Designates an ingot manufactured to tighter, premium compositional limits on impurities.
• Prefix Letter: A capitalized letter prefix (e.g., A356.0, B356.0) indicates a modification of the basic alloy. For instance, A356.0 possesses a substantially lower maximum iron allowance ($0.20%$) compared to original 356.0 ($0.60%$), delivering significantly higher fracture toughness and elongation.

Prominence of the 3xx.x Series:

The 3xx.x series (Al-Si-Cu/Mg) is by far the most widely utilized in aerospace. Silicon provides fluidity and low solidification shrinkage, preventing hot-tearing, while copper and magnesium enable precipitation hardening through subsequent heat treatment.

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4. Comparative Engineering Analysis: 2024 vs 7075

The two most prominent high-strength structural alloys in aircraft construction are 2024 and 7075. Each satisfies distinct structural demands based on its governing failure mechanisms under flight loads.

Head-to-Head Mechanical Property Comparison

Structural Allocation on the Aircraft Wing:

• Lower Wing Skin & Spar Caps (Tension Dominant): During positive $1\text{G}$ level flight, aerodynamic lift forces bend the wings upward. This places the lower wing skin in continuous cyclic tension. Under tensile loading, fatigue crack initiation, fatigue crack propagation, and fracture toughness govern airframe life. Because 2024-T3 has high fracture toughness and a remarkably slow crack propagation rate, it ensures that fatigue cracks remain sub-critical and detectable during routine NDT visual and eddy-current checks before catastrophic failure occurs.
• Upper Wing Skin & Spar Caps (Compression Dominant): The same upward wing bending subjects the upper wing skin to severe compressive stresses. Under compression, the primary failure mode is elastic or inelastic skin buckling and crippling. Buckling resistance is a direct function of elastic modulus and compressive yield strength. Fatigue cracking cannot propagate under continuous compressive stresses. Therefore, 7075-T6 (or advanced 7050/7150), with its massive $503 \text{ MPa}$ yield strength, is chosen to resist buckling while minimizing skin thickness and airframe weight.

Exam Trap: Never substitute 7075-T6 for 2024-T3 in a tension application simply because 7075 has higher tensile strength. Under cyclic tensile stresses, 7075's lower fracture toughness and higher fatigue crack growth rate will lead to rapid crack propagation and catastrophic structural failure.

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5. Clad Aluminium Alloys (Alclad)

High-strength 2xxx and 7xxx alloys suffer from poor corrosion resistance. In 2024, copper-rich intermetallic precipitates ($\text{CuAl}_2$) create localized electrochemical micro-cells that accelerate intergranular and pitting attack when exposed to moisture and salt air.

To overcome this vulnerability, the aviation industry developed Alclad (clad aluminium sheet).

Physical Architecture of Alclad:

Alclad consists of a high-strength core alloy (such as 2024 or 7075) metallurgically bonded via hot rolling between two thin surface layers of commercially pure aluminium (typically 1230 or 1100 series, $\ge 99.3%$ pure) or an aluminium-zinc alloy (such as 7072 for 7075 core materials).

• Cladding Thickness Specification:
  • For sheet thicknesses less than $0.063 \text{ in}$ ($1.6 \text{ mm}$): The cladding layer represents $5%$ of the total composite sheet thickness on each side ($10%$ total cladding).
  • For sheet thicknesses $0.063 \text{ in}$ ($1.6 \text{ mm}$) and greater: The cladding layer represents $2.5%$ of the total sheet thickness on each side ($5%$ total cladding).

┌─────────────────────────────────────────────────────────┐ ▲ 2.5% to 5% Cladding (Pure Al)
│         SACRIFICIAL CLADDING LAYER (ANODE)              │ │
├─────────────────────────────────────────────────────────┤ ▼
│                                                         │ ▲
│                                                         │ │
│               HIGH-STRENGTH CORE ALLOY                  │ │ 90% to 95% Core Alloy (2024 / 7075)
│               (e.g., 2024-T3 / 7075-T6)                 │ │ (Protected Cathode)
│                                                         │ │
│                                                         │ ▼
├─────────────────────────────────────────────────────────┤ ▲
│         SACRIFICIAL CLADDING LAYER (ANODE)              │ │ 2.5% to 5% Cladding (Pure Al)
└─────────────────────────────────────────────────────────┘ ▼

The Galvanic Protection Mechanism:

Alclad does not rely merely on physical encapsulation. Its true efficacy stems from cathodic (galvanic) protection:

1. In the electrochemical galvanic series in aerated seawater, commercially pure aluminium exhibits an electrode potential of approximately $-0.83 \text{ V}$ relative to a Saturated Calomel Electrode (SCE).
2. Copper-bearing alloy 2024 exhibits an electrode potential of approximately $-0.68 \text{ V}$ (SCE).
3. Pure aluminium is more electronegative (more anodic) than the core alloy. When electrolyte (water, condensation, salt spray) bridges the surface, the pure aluminium cladding becomes the sacrificial anode, while the high-strength core acts as the protected cathode.
4. Even when deep scratches, gouges, drilled fastener holes, or sheared sheet edges penetrate the cladding and expose the core, galvanic current flows from the surrounding cladding to the core, electrochemically shielding the core from pitting or intergranular attack.

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Workshop Handling and Maintenance Rules for Alclad:

Because the cladding layer is remarkably thin (often only $0.001 \text{ to } 0.003 \text{ in}$ or $0.025 \text{ to } 0.075 \text{ mm}$ thick), maintenance personnel must adhere to rigorous workshop constraints:

1. Abrasive Tool Restrictions:
   • NEVER use steel wire brushes, carbon steel wool, emery cloth, or rotary wire wheels on Alclad skins. Steel wool embeds microscopic carbon steel particles into the soft aluminium cladding, setting up severe galvanic couples that cause rapid pitting corrosion.
   • Only non-metallic abrasive pads (e.g., Scotch-Brite maroon/gray), aluminium oxide abrasive paper, or brass/nylon brushes are permitted for cleaning.
2. Scratch and Defect Limits:
   • Structural repair manuals (SRM) define rigid limits for scratch depths on Alclad skins. Scratches that penetrate through the cladding into the core alloy concentrate cyclic stresses and can initiate fatigue cracks.
   • Any surface scratch exceeding the cladding thickness requires blend-out within allowable SRM aerodynamic flushness limits, followed by immediate chemical conversion coating.
3. Polishing Hazards (Cladding Burn-Through):
   • Frequent buffing and mechanical polishing of unpainted executive aircraft fuselage skins slowly erodes the pure aluminium layer. Once the cladding is polished away, the bare 2024 core is exposed. This manifests as gray or dark circular halos ("burn-through"). The sacrificial protection is permanently destroyed in that zone, necessitating repainting or chemical treatment.
4. Chemical Restoration:
   • If cladding is breached or countersunk during rivet installation, technicians must restore surface passivity by applying a chemical conversion coating (MIL-DTL-5541 Class 1A, such as Alodine 1200 or Bonderite) followed by an epoxy chromate or non-chromate primer prior to final assembly.

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6. Practical Maintenance Scenario & Exam Traps

Maintenance Scenario:

An aircraft maintenance technician is performing a scheduled structural inspection on an Airbus A320 lower fuselage bilge panel. A small dent with a sharp scratch is noted adjacent to a lap joint. An optical micrometer measures the scratch depth at $0.004 \text{ in}$ ($0.10 \text{ mm}$). The sheet is $0.050 \text{ in}$ ($1.27 \text{ mm}$) 2024-T3 Alclad.

Analysis:

• Because the sheet thickness is less than $0.063 \text{ in}$, the cladding thickness is $5%$ per side: $0.050 \times 0.05 = 0.0025 \text{ in}$ ($0.0635 \text{ mm}$).
• The measured scratch depth ($0.004 \text{ in}$) exceeds the $0.0025 \text{ in}$ cladding layer, confirming that the scratch has penetrated into the 2024 structural core.
• Correct Maintenance Action: Consult the Structural Repair Manual (SRM Chapter 51). The technician must evaluate the remaining thickness against allowable stress limits, blend out the scratch with 400-grit aluminium oxide paper using a 1:20 or 1:50 taper ratio, verify no residual crack indications using dye penetrant or high-frequency eddy current (HFEC), apply Alodine 1200 chemical conversion coating, and seal with epoxy primer.

Exam Traps:

1. Wrought vs Cast Designations: Wrought alloys use a 4-digit system with no decimal point (e.g., 2024). Cast alloys use a 3-digit system with a decimal point (e.g., 356.0). A question showing 356.0 always refers to a casting, never a wrought plate.
2. Alclad Thickness Trap: Students often assume cladding is 10% to 20% of the total sheet. Remember: it is only $2.5%$ to $5%$ per side.
3. Tension vs Compression Misconception: Do not assume 7075-T6 is universally superior to 2024-T3 because of its higher yield strength. In cyclic tension, 7075 is inferior due to lower fracture toughness ($K_{IC}$) and rapid fatigue crack growth rates.