Non-ferrous Alloys

Properties of aluminium

• Low density (2.7 g/cm$^3$, one third of steel), FCC structure (ductile at all temperatures).
• Good electrical and thermal conductivity, high reflectivity.
• Natural Al$_2$O$_3$ film gives good corrosion resistance.
• Pure Al is weak (tensile strength about 90 MPa), so it is alloyed (Cu, Mg, Si, Zn, Mn) and strengthened by cold work or precipitation hardening.

Classification

Aluminium alloys
 |-- Wrought (rolled, extruded, forged)
 |     |-- Non-heat-treatable : 1xxx, 3xxx (Mn), 5xxx (Mg)
 |     `-- Heat-treatable     : 2xxx (Cu), 6xxx (Mg-Si), 7xxx (Zn)
 `-- Cast
       |-- Non-heat-treatable : Al-Si (LM6)
       `-- Heat-treatable     : Al-Cu, Al-Si-Mg

Special alloys

• Duralumin: 3.5-4.5% Cu, 0.5% Mg, 0.5-1% Mn, rest Al. Age hardened; used in aircraft structures and automobile parts.
• Y-alloy: 4% Cu, 2% Ni, 1.5% Mg, rest Al. Retains strength at high temperature; used in pistons, cylinder heads and aero engine parts.
• Al-Si (silumin): 10-13% Si; excellent castability for engine casting and housings.

Age hardening raises the strength of alloys whose solid solubility decreases with falling temperature, by forming very fine precipitates that block dislocations. Al-4% Cu is the classic example (duralumin).

Conditions for age hardening

The phase diagram must show a solvus line, i.e., the solute (Cu) is soluble at high temperature (up to 5.65% at 548 °C) but only about 0.2% at room temperature.

Heat-treatment steps

 T
 |   solution  ______
 |   treat.   |      | (500-550 C)
 |            |      | quench
 |            |      `--.   ageing
 |            |           `----_____ (130-190 C, hours)
 +------------------------------------> time

1. Solution treatment: heat to about 520-540 °C, a single-phase $\alpha$ region, and hold to dissolve all Cu.
2. Quenching in water to room temperature: Cu is trapped in a supersaturated solid solution.
3. Ageing: hold at room temperature (natural ageing) or 130-190 °C (artificial ageing). Cu atoms diffuse and cluster to form precipitates.

Precipitation sequence

Supersaturated $\alpha$ $\rightarrow$ GP zones (Cu-rich discs, coherent) $\rightarrow$ $\theta''$ $\rightarrow$ $\theta'$ (semi-coherent) $\rightarrow$ $\theta$ (CuAl$_2$, incoherent, equilibrium).

Why strength increases

Coherent GP zones and $\theta''$ distort the surrounding lattice. Dislocations must cut through the particles or bow around them, which needs a higher stress. Maximum hardness occurs with a fine, dense dispersion of $\theta''$/$\theta'$.

Over-ageing

Long time or high temperature lets precipitates coarsen into large, widely spaced incoherent $\theta$ particles. Dislocations can easily bypass them (Orowan looping), so hardness and strength fall.

Aluminium alloys are used where low weight, corrosion resistance and good conductivity matter more than the highest stiffness or lowest cost.

Advantages of aluminium alloys

• Light weight and high strength-to-weight ratio.
• Good corrosion resistance, easy to extrude and cast in complex shapes.
• Recyclable using only about 5% of the energy needed for primary production.
• Non-magnetic and non-sparking; retains toughness at low temperature (FCC).

Limitations

• Low modulus gives larger deflection for same section.
• Loses strength above about 150-200 °C; low melting point (660 °C).
• Galvanic corrosion when in contact with steel or copper.

Applications

Aircraft skin and frames (2024, 7075), engine pistons and blocks (Al-Si), transmission lines (1350), beverage cans (3004, 5182), window frames (6063), and automotive body panels.