Pure aluminium is soft at about 120 MPa, but alloying it with copper (2xxx series, 4-5%), magnesium and silicon (6xxx) or zinc (7xxx, 5-8%) and then solution treating, quenching and ageing it gives more than 500 MPa at one third the density of steel. That strength comes from fine precipitate particles, so it is lost when the metal is welded, overheated or remelted. A settlement will meet aluminium mainly as scrap, because new metal costs about 15 kWh per kilogram against 0.27 kWh to remelt it.
Part of Advanced Alloys
Aluminium alloys are light structural metals in which small additions of copper, magnesium, silicon, zinc or manganese raise strength several times above the pure metal.
The Series Number Names The Main Addition
The starting point is the pure metal at about 120 MPa, which the heat-treatable alloys below lift past 500 MPa.
The Advanced Materials article tabulates three heat-treatable series. The others are standard designations.
| Series | Main addition | Behaviour | Advanced Materials article use |
|---|---|---|---|
| 1xxx | None (nearly pure) | Soft, very corrosion resistant, high conductivity | Not listed |
| 2xxx | Copper 4-5% | High strength, heat treatable | Aircraft structures |
| 3xxx, 5xxx | Manganese, magnesium | Strengthened by cold work, not by ageing | Not listed |
| 6xxx | Magnesium and silicon | Good strength, weldable, extrudable | General structural |
| 7xxx | Zinc 5-8% | Highest strength of the aluminium alloys | Critical structural, springs |
Only the heat-treatable series gain their strength from the sequence in Precipitation Hardening. Cast engine parts such as pistons are usually aluminium-silicon alloys, which flow well into moulds but are not usually aged to the same strength.
Strength Per Weight Is The Reason For All This
Worked example, strength divided by density: the Advanced Materials article gives a strong heat-treated alloy above 500 MPa and a density of 2.7 g/cm³ (from Aluminum Smelting), so 500 / 2.7 = 185 MPa per g/cm³. Mild steel at 345-483 MPa (the converted table in Mechanical Properties) and 7.8 g/cm³ gives 345 / 7.8 = 44 to 483 / 7.8 = 62. So the aluminium alloy carries roughly three to four times more load per unit of weight. The comparison favours aluminium less for stiffness, since aluminium is only about a third as stiff as steel, and for fatigue.
Scrap Is The Realistic Feedstock
Why Aluminum Matters puts primary smelting at roughly 14 kWh per kilogram against about 0.27 kWh to melt scrap, and The Advanced Materials article puts new metal at about 15 kWh per kilogram. Ingot Casting covers the melting: sort, dry completely, melt at 660 °C, skim, pour at 700-750 °C. The materials problem comes next.
- Mixed scrap gives an unknown alloy. Cans, window frames, engine blocks and cookware are different alloys. Melted together they give something of unknown composition, usually weaker and more brittle than any single input, and it will not respond predictably to ageing.
- Sort by origin, not appearance. Extruded window frames and structural sections are mostly 6xxx. Cans are a manganese or magnesium alloy. Pistons and engine castings are aluminium-silicon. Keep like with like.
- Use remelted metal as a casting alloy. Cast the unknown into a test bar and tensile test it before trusting it with a load.
- Alloy control is the hard part. Magnesium and zinc oxidise or vaporise on remelting, so the chemistry drifts with every melt.
Working Limits To Respect
- No colour warning. The Advanced Materials article notes aluminium melts at 660 °C with no visible change. Measure temperature.
- Strength falls fast with heat. An aged alloy softens in a hot fire, so do not use it near stoves or furnaces.
- Welding undoes it. The metal beside the weld is overheated and loses the fine precipitate. A welded strong alloy is as strong as its weakest strip.
- Galvanic attack. Aluminium in damp contact with steel, copper or bronze corrodes first. Isolate or coat the joint. See Failure Modes.
- Surface. Anodizing thickens the natural oxide layer for wear and corrosion resistance.
Moisture and sealed hollow scrap explode in molten aluminium
Any water, even a damp scrap piece, a rain-wet mould or a closed hollow section, flashes to steam under molten metal and throws it out of the crucible. Preheat and dry all scrap, tools and moulds, and never melt sealed containers such as pressurised cans or tubes. Rust on a steel crucible or mould can also react violently with molten aluminium, so use a clean, coated crucible. Wear face and leg protection and keep the area dry underfoot.
Common Mistakes
- Assuming strength survives a remelt — it does not, and the alloy may be different from the one you started with.
- Welding a heat-treated structure — the repair is the weak point.
- Comparing alloys by price of metal only — the stiffer, thicker steel part may be lighter overall for a given deflection.
What’s Next
- Learn next: Precipitation Hardening — how the strong tempers are produced
- Combine with: Tensile Testing — proving what a remelted batch can carry