Non-ferrous alloys: aluminium, copper and titanium

Why it matters

When weight, electrical or thermal conductivity, corrosion resistance or biocompatibility matters more than raw cost, engineers turn to aluminium, copper and titanium alloys. Aircraft structures, engine pistons, bus bars, heat exchangers, condenser tubes, bearings, compressor blades and hip implants all depend on choosing the right non-ferrous alloy and the right temper.

Key ideas

Aluminium and its alloys. Density 2.70 g/cm³ (about one-third of steel), E ≈ 70 GPa, FCC (ductile, no ductile-to-brittle transition), melting point 660 °C, high electrical and thermal conductivity, and a thin protective Al₂O₃ film that gives good atmospheric corrosion resistance. Pure aluminium is weak, so it is alloyed and strengthened.

• Wrought series (Aluminum Association): 1xxx (≥ 99 % Al; conductors, foil), 2xxx (Cu; aircraft skins, e.g. 2024), 3xxx (Mn; cans, cookware), 4xxx (Si; welding wire), 5xxx (Mg; marine, weldable, e.g. 5083), 6xxx (Mg + Si; extrusions, e.g. 6061, 6063), 7xxx (Zn + Mg + Cu; highest strength, e.g. 7075).
• Heat-treatable: 2xxx, 6xxx, 7xxx (strengthened by precipitation hardening). Non-heat-treatable: 1xxx, 3xxx, 5xxx (strengthened only by cold work and solid solution).
• Temper codes: O = annealed; H = strain-hardened; T4 = solution-treated and naturally aged; T6 = solution-treated and artificially aged.
• Cast alloys: mostly Al–Si (near the eutectic at about 12.6 % Si) for fluidity; Na or Sr modification refines the coarse, brittle eutectic silicon. Pistons, cylinder heads, housings.

Precipitation (age) hardening, for example of Al–4 % Cu:

1. Solution treatment: heat into the single-phase α field (about 500–550 °C) so all Cu dissolves.
2. Quench to room temperature, giving a supersaturated solid solution.
3. Ageing: at room temperature (natural) or 120–200 °C (artificial); Cu clusters into GP zones and then coherent θ″ and θ′ precipitates, whose strain fields block dislocations. Peak hardness comes at an optimum time; longer ageing gives coarse equilibrium θ (CuAl₂) and overageing (softening). The requirements are a solvus line with solubility falling sharply with temperature and the ability to form fine coherent precipitates. Rivets of 2xxx alloy are kept refrigerated after quenching to delay natural ageing until they are driven.

Copper and its alloys. Density 8.96 g/cm³, FCC, excellent ductility, the best electrical conductivity after silver (annealed pure Cu defines 100 % IACS), excellent thermal conductivity and good corrosion resistance. Electrolytic tough-pitch and oxygen-free copper are used for conductors.

• Brass (Cu–Zn): up to about 36 % Zn is single-phase α (FCC), very ductile for cold work: 70/30 cartridge brass for deep drawing. 60/40 Muntz metal is α + β (BCC β), stronger and hot-workable; adding about 2–3 % Pb gives free-machining brass. Admiralty and naval brasses add Sn for seawater. Brasses with more than about 15 % Zn can suffer dezincification and season cracking (stress-corrosion cracking in ammonia).
• Bronze: Cu–Sn (phosphor bronze for springs and bearings), aluminium bronze (Cu–Al, strong and corrosion-resistant), silicon bronze. Tin bronzes are good bearing and casting alloys.
• Cupronickel (Cu–10 to 30 % Ni): seawater condenser tubes and coins. Cu–Ni is completely isomorphous.
• Beryllium copper (about 2 % Be): precipitation-hardened to steel-like strength; non-sparking tools, springs, connectors.

Titanium and its alloys. Density 4.5 g/cm³, high melting point (1668 °C), E ≈ 110–115 GPa, outstanding corrosion resistance (TiO₂ film) and biocompatibility. Titanium is allotropic: α (HCP) below 882 °C, β (BCC) above. Alloying elements are α stabilisers (Al, O, N) or β stabilisers (V, Mo).

• α alloys: weldable, good creep resistance.
• α + β alloys: Ti-6Al-4V, the workhorse (about half of all titanium used), heat-treatable; aircraft structures, fan and compressor blades, implants.
• β alloys: highest strength and formability, used for springs and fasteners. Drawbacks: high cost (Kroll process), poor machinability (low thermal conductivity, chemical reactivity with tools), galling, and reactivity with oxygen and nitrogen above about 500 °C, so welding needs inert shielding.

Other non-ferrous metals briefly: magnesium (1.74 g/cm³, lightest structural metal, HCP, poor cold formability), nickel-base superalloys (creep resistance to about 1000 °C, turbine blades), zinc (die casting, galvanising), lead and tin (solders, bearings, babbitt).

Formulas

Specific strength = σy / ρ and Specific stiffness = E / ρ σy, E in Pa; ρ in kg/m³; results in J/kg (equivalently N·m/kg). Compare materials for weight-critical members of the same strength or stiffness.

R = ρe·L / A Electrical resistance (Ω) of a conductor: ρe = resistivity (Ω·m), L = length (m), A = cross-sectional area (m²).

m = ρ·A·L Conductor mass (kg).

Worked examples

Example 1 (standard): specific strength and stiffness. Given: Ti-6Al-4V: σy = 880 MPa, E = 114 GPa, ρ = 4430 kg/m³. Al 7075-T6: σy = 505 MPa, E = 71 GPa, ρ = 2810 kg/m³. Q&T steel: σy = 1000 MPa, E = 210 GPa, ρ = 7850 kg/m³.

1. Specific strength: Ti = 880 × 10⁶ / 4430 = 199 kJ/kg; Al = 505 × 10⁶ / 2810 = 180 kJ/kg; steel = 10⁹ / 7850 = 127 kJ/kg.
2. Specific stiffness: Ti = 25.7, Al = 25.3, steel = 26.8 MJ/kg — nearly equal. Light alloys win on strength per weight, not stiffness per weight.

Example 2 (GATE level): aluminium versus copper conductor. Given: two conductors of equal length and equal resistance. Resistivity: Cu 1.72 × 10⁻⁸ Ω·m, Al 2.82 × 10⁻⁸ Ω·m. Density: Cu 8960, Al 2700 kg/m³. Find the mass ratio m_Al / m_Cu.

1. Equal R and L → A_Al / A_Cu = ρe,Al / ρe,Cu = 2.82 / 1.72 = 1.640.
2. m_Al / m_Cu = (A_Al / A_Cu) × (ρ_Al / ρ_Cu) = 1.640 × 2700 / 8960.
3. Mass ratio = 0.494: an aluminium line weighs about half as much, which is why overhead transmission lines use aluminium (with a steel core for strength).

Example 3 (lever rule in age hardening). Al–4 wt % Cu at 200 °C lies in α + θ. Taking α ≈ 0.5 % Cu and θ (CuAl₂) ≈ 53 % Cu (from the diagram), equilibrium θ = (4 − 0.5)/(53 − 0.5) = 6.7 wt %. Fine dispersion of this small fraction is what gives the hardening.

Common mistakes

• Saying aluminium is stiffer per weight than steel; specific stiffness is about the same.
• Ageing too long (overageing) or at too high a temperature, and expecting higher strength.
• Treating 5xxx or 3xxx alloys as heat-treatable.
• Confusing brass (Cu–Zn) with bronze (Cu–Sn and others).
• Assuming titanium is easy to machine and weld like steel.
• Forgetting that α-brass is single-phase FCC and 60/40 brass contains BCC β.

For GATE PI

Expect one-mark questions on composition and use of common alloys (brasses, bronzes, Ti-6Al-4V, Al series), the precipitation-hardening sequence and overageing, temper designations, and why Al is used for overhead lines and Ti for aircraft and implants. Numericals compare specific strength or stiffness, or conductor mass for equal resistance.

Quick check

1. List the three steps of precipitation hardening.
2. What is 70/30 brass, and why is it good for deep drawing?
3. Which Al series are heat-treatable?
4. What is the crystal structure of titanium at room temperature?
5. What does T6 mean?

Answers: 1. Solution treatment, quench, ageing; 2. Cu–30 % Zn, single-phase FCC α, very ductile; 3. 2xxx, 6xxx, 7xxx; 4. HCP (α); 5. Solution-treated and artificially aged.