Comprehensive Study Guide on Aluminium and Aluminium Alloys in Aviation

Overview of Aluminium in Aviation

Aluminium is a member of the "nonferrous" metal group. Nonferrous refers to all metals that utilize elements other than iron as their base or principal constituent.
This group includes various metals such as aluminium, titanium, copper, and magnesium, alongside alloyed metals like Monel.
Aluminium is characterized as a soft, lightweight, silver-colored, and ductile metal.
It possesses the chemical symbol $Al$ and an atomic number of $13$ .
It is the most abundant metal on Earth, making up approximately $8\%$ of the world by weight.
In modern aircraft construction, aluminium is one of the most widely used metals, exemplified by its use in structures like the Airbus A350 XWB fuselage.
It is vital to the aviation industry due to two primary factors:
- High strength-to-weight ratio.
- Comparative ease of fabrication.

Physical and Mechanical Properties of Aluminium

Weight and Density: Aluminium is lightweight with a Specific Gravity ( $SG$ ) of $2.7$ , which contributes to significant savings in weight and fuel consumption.
Melting Point: Aluminium melts at a relatively low temperature of $1,250\,^{\circ}\text{F}$ .
Magnetism: The metal is nonmagnetic.
Conductivity: It is an excellent conductor of electricity, possessing about $60\%$ of the conductivity of copper. When compared by weight rather than volume, aluminium is a better conductor than copper.
Corrosion Resistance: It exhibits good resistance to corrosion, a critical property for aircraft structures.
Toxicity: Aluminium is non-toxic.
Strength:
- Commercially pure aluminium has a tensile strength of approximately $13,000\,\text{psi}$ .
- Rolling or other cold-working processes can approximately double this strength.
- By alloying with other metals or utilizing heat-treating processes, tensile strength can be raised as high as $65,000\,\text{psi}$ , placing it within the strength range of structural steel.
Formability:
- Aluminium alloys are malleable and ductile, allowing them to be easily worked.
- They can be rolled into sheets as thin as $0.0017\,\text{inch}$ .
- They can be drawn into wire as fine as $0.004\,\text{inch}$ in diameter.
- Standard aircraft sheet stock typically ranges from $0.016$ to $0.096\,\text{inch}$ in thickness.
- Larger aircraft may utilize sheet stock as thick as $0.356\,\text{inch}$ .

General Classification of Aluminium Alloys

Aluminium is divided into two general classes:

Casting Alloys: These are suitable for casting in sand, permanent molds, or die castings.
Wrought Alloys: These are shaped by mechanical processes such as rolling, drawing, or forging. Wrought alloys are the most widely used in aircraft construction, appearing in components like stringers, bulkheads, skin, rivets, and extruded sections.

Casting Alloys

Casting alloys are further divided into two basic groups:
- Group 1: Physical properties are determined purely by alloying elements and cannot be changed after casting.
- Group 2: Alloying elements allow the casting to be heat-treated to produce desired physical properties.
Identification: Casting alloys are identified by an alloy number. A letter preceding the number indicates a slight variation in the composition of the original alloy intended to impart a specific quality.
- Example: Alloy $214$ becomes $A214$ when zinc is added to improve pouring qualities.
Temper Identification: For heat-treated castings, the letter $T$ followed by an alloying number indicates the treatment and composition.
- Example: Sand casting alloy $355$ can be designated as $355\text{-}T6$ , $355\text{-}T51$ , or $C355\text{-}T51$ .

Aluminium Casting Methods

There are three basic methods for producing aluminium alloy castings:

1. Sand Mold Castings

Produced by pouring molten metal into a sand mold and allowing it to solidify under the force of gravity.
The principal alloys used for sand casting are $112$ and $212$ , which have similar mechanical properties and are adaptable to many products.
A disadvantage of sand casting is porosity (small openings in the metal). This is caused by gas given off by the sand and the binder used to hold the sand together.

2. Permanent Mold Castings

Similar to sand casting but uses a metallic mold (usually cast iron).
Like sand casting, metal flows into the mold under gravity alone.
Advantages: Fewer openings/porosity than sand castings and a finer grain structure due to rapid cooling. These are superior to sand type castings.
Types:
- Permanent metal mold with metal cores.
- Semi-permanent types containing sand cores.
Common Alloys: $122$ , $A132$ , and $142$ are frequently used, primarily in internal combustion engines.

3. Die Castings

Produced by forcing molten metal under great pressure into a metallic die.
Used for relatively large production volumes of a specific part.
Aircraft die-castings are usually aluminium or magnesium alloys.
Magnesium is used when weight is the primary concern (it is lighter), while aluminium is used when higher strength is required.

Wrought Aluminium Classification and Indexing

Wrought aluminium and its alloys are categorized into two classes:

Non-Heat-Treatable Alloys: Mechanical properties are determined by the amount of cold work (strain hardening) introduced after final annealing. These properties are destroyed by subsequent heating and cannot be restored without further cold working.
Heat-Treatable Alloys: Mechanical properties are achieved by heating to a specific temperature, holding for a "soak" time to allow constituents to enter solid solution, and quenching.

Four-Digit Index System

1xxx Group: $99.00\%$ or higher pure aluminium.
- The last two digits indicate the hundredths of $1\%$ above the initial $99\%$ .
- Example: $1130$ contains $99.30\%$ pure aluminium.
- The second digit indicates control over impurities ( $0$ for no special control, $1$ through $9$ for consecutive controls).
  - $1100$ : $99.00\%$ pure with one control.
  - $1275$ : $99.75\%$ pure with two controls.
2xxx through 8xxx Groups: These identify alloys based on the major alloying element.
- 2xxx: Copper
- 3xxx: Manganese
- 4xxx: Silicon
- 5xxx: Magnesium
- 6xxx: Magnesium and Silicon
- 7xxx: Zinc
- 8xxx: Other elements
- 9xxx: Currently unused.
Digit Meanings for 2xxx-8xxx:
- First digit: Major alloying element.
- Second digit: Alloy modification ( $0$ for original alloy, $1$ through $9$ for modifications).
- Last two digits: Identify different alloys within the specific group.

Detailed Characteristics of Wrought Alloy Series

1000 Series (Non-Heat-Treatable)

Contains $99\%$ or higher aluminium.
High corrosion resistance and high thermal/electrical conductivity.
Low mechanical properties but excellent workability.
Major impurities are iron and silicon.

2000 Series (Heat-Treatable)

Principal alloying element: Copper (the prime hardening element).
Can undergo solution heat treatment and has properties equal to mild steel.
Magnesium is often added to increase natural aging and maximum strength.
Weakness: Poor corrosion resistance (especially if unclad), poor weldability, and poor extrudability.
Often clad with 6000 series or high purity alloy. Common aircraft alloy: $2024$ .

3000 Series (Non-Heat-Treatable)

Principal alloying element: Manganese (usually around $1.5\%$ ).
Good corrosion resistance and formability.
Popular alloy: $3003$ (moderate strength), commonly used for cooking utensils.

4000 Series (Non-Heat-Treatable)

Principal alloying element: Silicon.
Silicon lowers the melting temperature.
Primary use: Welding and brazing filler wire (e.g., alloy $4043$ ).

5000 Series (Non-Heat-Treatable)

Principal alloying element: Magnesium.
Features high tensile strength, good welding, and high sea-water corrosion resistance (if Mg > $3\%$ ).
Warning: Temperatures over $150\,^{\circ}\text{F}$ or excessive cold working increase susceptibility to corrosion.
Used for plates, sheets, boats, LNG tanks, and offshore structures (e.g., $5083$ , $5154$ ).

6000 Series (Heat-Treatable)

Principal elements: Silicon and Magnesium (forming magnesium silicide).
Medium strength, good forming, and high corrosion resistance in marine atmospheres.
Excellent extrudability; $80\%$ of global extrusion production uses this series.
Popular alloys: $6061$ and $6063$ .

7000 Series (Heat-Treatable)

Principal alloying element: Zinc.
When coupled with magnesium (and sometimes copper/chromium), it achieves the highest strength of all commercial aluminium alloys, exceeding structural steel.
Weakness: Poor extrudability and not weldable.
Typical alloy: $7075$ , used for high-strength aerospace applications.

Hardness and Temper Designations

Temper designations are separated from the alloy number by a dash (e.g., $7075\text{-}T6$ ).

Basic Temper Designations

F: As fabricated (no controlled cold working or thermal treatment).
O: Annealed, recrystallized (wrought products only).
H: Strain hardened (followed by one or more digits).
- H1: Strain hardened only.
- H2: Strain hardened and partially annealed.
- H3: Strain hardened and stabilized.
W: Solution heat treated, unstable temper.
T: Treated to produce stable tempers other than F, O, or H.

Specific Heat Treatment Sub-divisions (T-series)

T2: Annealed (cast products only).
T3: Solution heat treated then cold worked.
T4: Solution heat treated and naturally aged to a stable condition.
T5: Artificially aged only.
T6: Solution heat treated and then artificially aged.
T7: Solution heat treated and then stabilized.
T8: Solution heat treated, cold worked, and then artificially aged.
T9: Solution heat treated, artificially aged, then cold worked.
T10: Artificially aged and then cold worked.

Heat Treatment Processes

Heat treatment hardens aluminium alloys through four distinct steps:

Heating to a predetermined temperature.
Soaking at that temperature for a specified time.
Quenching rapidly to a relatively low temperature.
Aging (Precipitation Hardening), which occurs either spontaneously at room temperature or via low-temperature thermal treatment.

Technical Nuances of Heat Treatment

Solution Heat Treatment: Refers to the first three steps. Unclad alloys can be solution heat-treated repeatedly without harm.
Warping: The process may cause kinks, buckles, or twists. Straightening operations follow.
- If straightening significantly increases tensile/yield strength and slightly decreases elongation, it is designated $\text{-}T3$ .
- If properties are not materially affected, it is designated $\text{-}T4$ .
Aging Mechanism: Hardening is caused by the uniform distribution of finely dispersed submicroscopic precipitate particles. These act as "keys" or "locks" within the grain structure to resist internal slippage and distortion under load.
Natural vs. Artificial Aging:
- Natural Aging: Reaches full strength at room temperature after $4$ to $5$ days ( $90\%$ of strength usually achieved within $24\,\text{hours}$ ).
- Artificial Aging (Precipitation Thermal Treatment): Requires elevated temperatures. These alloys are often slightly over-aged to increase corrosion resistance, especially high-copper alloys susceptible to intergranular corrosion.

Additional Fabrication Processes

Alclad and Pureclad

Consists of an aluminium-alloy core coated with a layer of pure aluminium on each side, with the coating depth approximately $5.5\%$ per side.
Provides dual protection: physical barrier against corrosion and electrolytic (galvanic) protection if the surface is scratched.
Warning: Clad parts should be heated quickly during heat treatment to prevent the core constituents from diffusing into the cladding, which reduces corrosion resistance.

Annealing

Consists of heating to an elevated temperature, soaking based on metal mass, and cooling in still air.
Leaves metal in the best condition for cold working.
Mechanical Hardness: During prolonged forming, metal resists further working; multiple anneals may be required to prevent cracking.
Safety Note: Aluminium alloys should never be used in the annealed state for final structural parts or fittings.

Quotation

"You haven't seen a tree until you've seen its shadow from the sky." — Amelia Earhart