Aluminum, Copper, Titanium and Other Alloys
Aluminum tempers, copper and brass, titanium, magnesium and zinc: properties, strength-to-weight and thermal expansion.
Why non-ferrous metals
Non-ferrous metals contain little or no iron. They are chosen for low weight (aluminum, magnesium, titanium), electrical and thermal conductivity (copper, aluminum), corrosion resistance (titanium, copper alloys, aluminum), bearing and wear properties (bronzes), or ease of casting and machining (zinc, brass, aluminum). They are generally more expensive per kilogram than steel but cheaper per part where weight or machining time matters.
Properties side by side
| Material | Density (kg/m³) | E (GPa) | Yield (MPa) | CTE (10⁻⁶/K) | k (W/m·K) | Specific strength σy ÷ ρ (kN·m/kg) | Specific stiffness E ÷ ρ (MN·m/kg) |
|---|---|---|---|---|---|---|---|
| 4140 Q&T | 7 850 | 210 | 900 | 12.3 | 42 | 115 | 26.8 |
| Mild steel S355 | 7 850 | 210 | 355 | 11.7 | 50 | 45 | 26.8 |
| 6061-T6 | 2 700 | 69 | 276 | 23.6 | 167 | 102 | 25.6 |
| 7075-T6 | 2 810 | 72 | 503 | 23.4 | 130 | 179 | 25.6 |
| Ti-6Al-4V | 4 430 | 114 | 880 | 8.6 | 7 | 199 | 25.7 |
| Brass C360 | 8 500 | 97 | 310 | 20.5 | 115 | 36 | 11.4 |
| Copper C110 | 8 940 | 117 | 70 | 17.0 | 390 | 8 | 13.1 |
| AZ31 magnesium | 1 770 | 45 | 200 | 26.0 | 96 | 113 | 25.4 |
Note that the specific stiffness E ÷ ρ is almost the same for steel, aluminum, titanium and magnesium (about 25 MN·m/kg): going to a lighter metal gives no stiffness advantage for the same section; the saving comes from the fact that a lighter material can be made thicker (see the bracket example). The specific strength differs more: titanium and high-strength aluminum beat mild steel by 2 to 3 times (see the chart on Material Selection, Testing and Failure).
Aluminum alloys and tempers
| Series | Main alloy | Strength | Typical use |
|---|---|---|---|
| 1xxx | pure aluminum (99+ %) | low (30 to 150 MPa) | electrical conductors, chemical equipment, foil |
| 3xxx | manganese | low to medium | cans, roofing, heat exchangers |
| 5xxx | magnesium | medium; non-heat-treatable | marine, tanks, welded structures (5083, 5052) |
| 6xxx | magnesium and silicon | medium to high; heat treatable | extrusions, frames, machined parts (6061, 6063) |
| 7xxx | zinc (with Mg, Cu) | highest (500 to 600 MPa) | aerospace, high-strength parts (7075); poor weldability, stress corrosion |
| 2xxx | copper | high; heat treatable | aircraft structures (2024); poor corrosion resistance |
| Cast alloys (A356, 380) | silicon | medium | castings, die-cast housings |
The letter and number after the alloy give the temper (the condition): O annealed (soft), H strain hardened (5xxx, 3xxx), T heat treated: T4 solution treated and naturally aged, T6 solution treated and artificially aged (the usual strong condition), T651 T6 with stress relief by stretching (flat plate: stable when machined). 6061-O has a yield of about 55 MPa, while 6061-T6 has 276 MPa: welding 6061-T6 locally returns it to nearly the annealed state, so the strength of a welded joint is about 40 percent of the parent (the heat-affected zone governs the design).
Worked example: steel or aluminum bracket
Worked example: a bracket on the press, 10 mm of steel or its equivalent in aluminum
- The bracket is a plate loaded in bending, so its stiffness goes as E t³. For equal stiffness the aluminum plate must be thicker by the cube root of the modulus ratio: tAl = 10 × (210 ÷ 69)1/3 = 14.5 mm.
- Mass: 1.45 × 2 700 ÷ 7 850 = 50 percent of the steel plate: a 100 × 100 mm plate of 10 mm steel weighs 0.79 kg, the stiffness-equivalent aluminum plate 0.39 kg (the 10 mm aluminum plate itself is 0.27 kg but 33 percent as stiff).
- If instead the bracket is limited by strength (yield 355 MPa for S355 against 276 MPa for 6061-T6) the thickness ratio is √(355 ÷ 276) = 1.13 and the mass is 39 percent of steel.
- But the aluminum is much worse for fatigue and for deflection under a fixed load if the section cannot grow, so check against both limits, and the cost per kilogram (typically 2 to 3 times steel).
Copper, brass and bronze
Copper has the best electrical and thermal conductivity of the engineering metals (the standard of 100 percent IACS = 58 MS/m), is easy to join and resists many forms of corrosion. It is soft (70 MPa yield annealed) and expensive, so it is alloyed for strength:
- Brass (copper and zinc, C360 free-cutting): the machining reference material, cheap to produce, for fittings, valves and instrument parts. Dezincification occurs in some waters (use inhibited brass).
- Bronze (copper and tin, aluminum or silicon): bearings, bushings, gears and marine hardware; good wear and corrosion resistance.
- Beryllium copper: springs and non-sparking tools, strong (to 1 200 MPa) with good conductivity; beryllium dust is toxic when machined or ground.
- Copper-nickel: seawater piping, condenser tubes.
| Conductor | Conductivity | In S/m |
|---|---|---|
| Copper (annealed) | 100.0 % IACS | 58.0 MS/m |
| Silver | 105.0 % IACS | 60.9 MS/m |
| Aluminum 1350 | 61.0 % IACS | 35.4 MS/m |
| Aluminum 6061-T6 | 43.0 % IACS | 24.9 MS/m |
| Brass C360 | 26.0 % IACS | 15.1 MS/m |
| Phosphor bronze | 15.0 % IACS | 8.7 MS/m |
| Stainless 304 | 2.5 % IACS | 1.5 MS/m |
Aluminum conducts only 61 percent as well as copper by volume, but weighs a third as much: a bus bar of equal resistance needs 1.74 times the cross-section but only 53 percent of the mass of copper. That is why overhead lines and large feeders use aluminum (see Cable Sizing and Voltage Drop).
Titanium, magnesium and zinc
- Titanium (Ti-6Al-4V, 880 MPa yield, 4 430 kg/m³): high specific strength, outstanding corrosion resistance (seawater, body fluids) and a low thermal expansion, but expensive, hard to machine (low conductivity, work hardens, galls) and poorly conductive: aerospace, medical implants, chemical plant. A 10 mm 100 × 100 mm plate weighs 0.44 kg.
- Magnesium (AZ31, AZ91): the lightest structural metal (1 770 kg/m³), good for housings and aerospace castings; flammable as chips and powder, corrodes in wet conditions, needs coatings.
- Zinc die-casting alloys (Zamak): cheap, thin-wall, accurate castings of small parts (locks, gears, housings); low creep strength and a low melting point (385 °C).
- Tin, lead, solders and bearing metals: low-melting alloys; lead and cadmium need handling controls (RoHS).
Thermal expansion and joining
Different metals expand differently (see the CTE column). An aluminum housing on a steel rod that warms by 80 K grows relative to the steel by (23.6 − 11.7) × 10⁻⁶ × 80 × 45 mm = 0.043 mm on a 45 mm diameter: enough to loosen an interference fit or bind a close clearance (see Temperature, Coatings and Fit Problems). Joining: welding aluminum needs AC TIG or MIG with an oxide-removing arc and a cleaned surface; titanium needs argon shielding on both sides; copper needs high preheat; and dissimilar metals in the presence of moisture form a galvanic cell (Corrosion and Protection), so isolate aluminum from steel fasteners with sealant, washers or coatings.
Related
- Previous: Steels and Heat Treatment. Next: Welding Processes and Weld Design.
- Material Properties Table.
- Steel and Plate Weight Calculator.
- Materials and Processes: the whole guide.
FAQ
What does T6 mean?
Solution heat treated and artificially aged to the strongest practical condition for that alloy. 6061-T6 is the most common structural aluminum extrusion.
Is aluminum always lighter overall?
Not always: it is a third of the density but needs thicker sections for the same stiffness, and joints, fasteners and fatigue limits can erase the saving. The gain is real where weight really counts.
Can I weld 7075?
Not by conventional fusion welding: it cracks. Use fasteners, adhesive or friction stir welding, or choose 6061 or 5083.
Why does my brass part crack months after bending?
Season cracking: residual stress plus ammonia traces. Stress relieve brass at 250 to 300 °C after forming, and avoid ammonia.
References
- ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International; Aluminum Association, Aluminum Standards and Data.
- Davis, J. R. (ed.), Aluminum and Aluminum Alloys, ASM Specialty Handbook; Lutjering, G., Williams, J. C., Titanium, Springer.
- ASTM B209 (aluminum sheet), B221 (extrusions), B265 (titanium), B16 and B36 (brass); Machinery’s Handbook, Industrial Press.
Last reviewed: October 2026. Material properties are typical handbook values; use the supplier’s data for design.