Aluminium Alloys

Quick Stats

• Melting Point: $660^{\circ }C$ (Softens at $150-200^{\circ }C$)
• Crystal Structure: FCC (Face Centered Cubic) $\to$ No ductile-to-brittle transition (remains ductile at low temps)
• Density: $2.7{\text{ g/cm}}^3$ (vs $7.7$ for Fe) - It is light!
• Young’s Modulus (E): $70\text{ GPa}$ (vs $210\text{ GPa}$ for Fe) - It is 1/3 as stiff as steel.

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Why Aluminium? (Pros & Cons)

The Good

• Conductivity: High electrical conductivity ($62\%$ IASC). Good heat conductor.
• Formability: Easily formed by extrusion, forging, rolling. Can make intricate shapes due to FCC structure.
• Corrosion Resistance: Excellent due to a thin, strong oxide coating.
• Machinability: Good.

The Bad

• Fatigue Strength: Low. Be careful with cyclic loading.
• Cost: Sheet form is $\sim 4.5\times$ mild steel cost.
• Temperature: Low melting point limits high-temp applications.

Critical Exam Concept: SCC

Precipitation-hardened alloys can suffer from Stress-Corrosion Cracking (SCC) due to precipitate-free zones near grain boundaries. Do not forget this.

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Strengthening Mechanisms

How do we make soft aluminium strong?

• Solid Solution Hardening: Adding atoms that distort the lattice.
• Cold Work (Strain Hardening): Dislocation tangles.
• Age Hardening (Precipitation): The big one. Requires heat treatment.

${\sigma }_{ys}={\sigma }_o+k{d}^{-1/2}$ (Hall-Petch relation: Smaller grains = Higher strength)

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The Designation System (Wrought & Cast)

Memorize the Series Logic

The first digit tells you the principal alloying element.

Wrought Alloys (4 Digits: xxxx)

Series	Element	Treatable?	Main Characteristic
1xxx	Pure Al ($99\%+$)	No	Foil, electrical conductors. Weak.
2xxx	Copper (Cu)	YES	Aircraft. High strength, age-hardened. Not weldable.
3xxx	Manganese (Mn)	No	Drink cans. Ductile, good formability.
4xxx	Silicon (Si)	Some	Pistons. Forgeable, lowers melting point.
5xxx	Magnesium (Mg)	No	Marine. Best corrosion resistance. Weldable.
6xxx	Mg + Si	YES	Structural. Window frames. Good medium alloy, weldable.
7xxx	Zinc (Zn)	YES	High Stress Aircraft. Highest strength. Not weldable.
8xxx	Other (Li)	YES	Specialist (e.g., Al-Li for aerospace).

Cast Alloys use a decimal point (e.g., 3xx.x). Most common is Al-Si eutectic for automotive castings.

Practical Naming Examples

How to read the code on the exam:

Example 1: AA 7075-T6 (Common aircraft alloy)

• 7: Principal element is Zinc.
• 0: No major modification to the original alloy limits (0 = original).
• 75: Identifies the specific alloy in the 7xxx series.
• T6: Solution heat treated + Artificially Aged (Furnace).

Example 2: AA 5083-H116 (Marine/Shipbuilding)

• 5: Principal element is Magnesium.
• 0: Original alloy limits.
• 83: Specific alloy ID.
• H1: Cold worked only (Strain hardened).
• 16: Specific degree of hardness/processing.

Example 3: AA 1050 (Electrical wiring)

• 1: Pure Aluminium ($99.00\%$ min).
• 0: Original impurity limits.
• 50: Indicates purity of $99.50\%$. (Last two digits in 1xxx series = decimal percentage above 99%).

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Temper Designations (The Suffixes)

This tells you the history of the metal.

• F: As Fabricated (Raw output).
• O: Annealed (Softest, lowest strength).

H - Cold Worked (Strain Hardened)

Used for Non-Heat Treatable alloys (1xxx, 3xxx, 5xxx)

• H1: Cold worked only.
• H2: Cold worked + partially annealed.
• H3: Cold worked + stabilized.
• Second Digit: Indicates hardness (2 = 1/4 hard … 8 = Hard).

T - Heat Treated (The Important Ones)

Used for Heat Treatable alloys (2xxx, 6xxx, 7xxx)

Don't mix these up!

• T4: Solution Heat Treated + Natural Ageing (Room temp).
• T6: Solution Heat Treated + Artificial Ageing (Furnace).
• T3: Solution + Cold Work + Natural Ageing.

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Age Hardening (Precipitation Hardening)

Focus: 2xxx Series (Al-Cu)

This is the primary way we strengthen high-performance aluminium. It is a process over time:

1. Solid Solution Treatment: Cu is in solid solution.
2. GP Zones: Cu forms clusters. Strain fields created.
3. ${\theta }^{\prime \prime }$ Precipitates: Coherent precipitates. PEAK AGED condition. Maximum hardness.
4. ${\theta }^{\prime }$ and $\theta$ (CuAl${}_2$): Incoherent precipitates. Overaged. Hardness drops.

The Curve

The Yield Stress rises to a peak and then falls. Overaging (holding it in the oven too long) ruins the material strength.

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Applications Cheat Sheet

Application	Series	Why?
Cooking Foil	1xxx	Pure, ductile, corrosion resistant.
Drink Cans	3xxx	Deep drawing capability (ductile).
Airplane Fuselage	2xxx	High strength (but needs Alclad for corrosion).
Airplane Wing Structures	7xxx	Highest strength available.
Car Pistons	4xxx	Low thermal expansion, wear resistance (Si).
Window Frames	6xxx	Extrudable, good corrosion resistance.
Ship Hulls	5xxx	Saltwater corrosion resistance.