Aluminium Alloy FAQ

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01 · Does aluminium rust 

     Not the way iron does — but it does corrode. The instant fresh aluminium meets air, a film of aluminium oxide forms on the surface, only about 2–10 nanometres thick. It is dense, tightly bonded, and it rebuilds itself the moment it is scratched. That is why bare aluminium survives outdoors for decades without a drop of paint.

The film is stable between roughly pH 4 and pH 8.5. Push outside that window — strong acids, strong alkalis — or expose it to a chloride-rich environment such as coastal air or de-icing salt, and the film breaks down locally. The result is pitting corrosion and the familiar white powdery spots.

Bottom lineAluminium is not rust-proof, it is film-protected. Anodising, painting and cladding all exist to protect that film — not the metal underneath.

02 · Aluminium vs. aluminium alloy

    Pure aluminium — the 1xxx series, 99% aluminium or more — is soft, around 80–100 MPa in tensile strength. What it does have is the best electrical and thermal conductivity of the family, plus the best corrosion resistance.

Alloys are made by adding controlled amounts of copper, magnesium, silicon, zinc and manganese. Those foreign atoms distort the crystal lattice and pin dislocations in place, which is what lifts strength into the 200–600 MPa range.

Bottom lineWhen a supplier quotes "aluminium", it is almost always an alloy. Pure aluminium is reserved for cable, foil, heat exchangers and chemical tanks.

03 · The 1xxx–7xxx series

     Wrought aluminium alloys are grouped by their principal alloying element, which is the fastest way to predict how a grade will behave:

• 1xxx — pure aluminium (1050, 1060, 1100): best conductivity and corrosion resistance, lowest strength.
• 2xxx — Al-Cu (2017, 2024): high strength, poor corrosion resistance, mostly aerospace.
• 3xxx — Al-Mn (3003, 3004): formable general-purpose sheet.
• 4xxx — Al-Si (4032, 4043): low melting point, welding wire and pistons.
• 5xxx — Al-Mg (5052, 5083): marine grade, excellent weldability.
• 6xxx — Al-Mg-Si (6061, 6063): the all-round structural and extrusion grades.
• 7xxx — Al-Zn-Mg-Cu (7075, 7050): the highest strength available.

Bottom line5xxx and 6xxx weld well and resist corrosion. 2xxx and 7xxx buy strength at the expense of both.

04 · What O, H and T6 mean 

     The letters after the grade are the temper designation, and they matter as much as the grade itself:

• O — annealed: softest condition, easiest to form.
• H — strain hardened: H18 fully hard, H24 half hard, common on sheet and foil.
• T — thermally treated: T6 is solution heat treatment plus artificial ageing, the peak-strength condition for heat-treatable alloys.
• 6061 makes the point cleanly: about 125 MPa in the O temper, about 310 MPa in T6 — the same alloy, a factor of 2.5 apart.

Bottom lineNever order by grade alone. "6061" without a temper is an incomplete specification.

05 · 6061 or 6063?

     Both are 6xxx extrusion alloys, tuned for different outcomes.

    6061-T6 reaches roughly 310 MPa, machines cleanly and holds up in loaded structures: frames, brackets, jigs, mould plates, bicycle and automotive parts.

    6063-T6 is a little weaker (about 240 MPa) but extrudes into far more complex shapes with a smoother surface, and it anodises to a uniform colour. It is the standard for window and curtain-wall profiles, trim and heat sinks.

Bottom lineLoad-bearing and machined parts go to 6061. Visible extrusions and heat sinks go to 6063.

06 · Why 7075 is ultra-high-strength

     7075 is an Al-Zn-Mg-Cu alloy that responds strongly to heat treatment. 7075-T6 reaches about 570 MPa tensile — roughly twice the yield of ordinary mild steel at one third of the weight. That ratio is why it dominates aircraft structures, motorsport components and high-end tooling plate.

     The price of that strength is real: poor corrosion resistance (usually clad or coated in service), very poor weldability, and a material cost well above 6061.

Bottom lineSpecify 7075 only where strength per kilogram is the genuine constraint. Otherwise 6061 is cheaper and far easier to live with.

07 · Can aluminium be welded?

      Yes, but the answer depends entirely on the series. 5xxx and 6xxx weld well by MIG or TIG — the usual fillers are ER5356 for 5xxx and ER4043 for 6xxx, the latter being more crack resistant. 2xxx and 7xxx are generally not recommended for fusion welding: hot cracking and severe softening in the heat-affected zone.

      Three things catch people out:    

•  The oxide melts near 2050 °C while the base metal melts near 660 °C — remove it immediately before welding.
• Thermal conductivity is about three times steel, so more heat input is needed.
•  Hydrogen porosity is a constant risk if filler and parent metal are not clean and dry.

Bottom lineDesign for welding by choosing 5xxx or 6xxx, clean the oxide off, and match the filler to the parent alloy.

08 · What anodising really does

        Anodising is an electrolytic process that thickens the natural oxide rather than adding a coating. The film grows to roughly 5–25 µm for decorative work and 50 µm or more for hard anodising, reaching HV 300–600 — far harder than bare aluminium. Corrosion resistance improves by about an order of magnitude.

        The freshly formed layer is porous, which is exactly why it can absorb organic dyes or be electrolytically coloured before sealing. Two practical notes: because it is grown from the metal it never peels, and roughly half of it grows outward, so close-tolerance parts need a dimensional allowance.

Bottom lineAnodising is the default finishing route when you need wear resistance, colour stability and corrosion protection in one step.

09 · Aluminium vs. steel on strength

      Comparing tensile strength alone is misleading — the right measure is specific strength, strength divided by density. Aluminium is 2.7 g/cm³ against steel's 7.85, a factor of 2.9. On that basis 6061-T6 already beats ordinary Q235 structural steel, and 7075-T6 approaches high-strength alloy steel. That arithmetic is what drives "aluminium instead of steel" in cars, rail and aircraft.

      The real constraint is stiffness: aluminium's elastic modulus is about 70 GPa versus steel's 210 GPa. An aluminium part of identical geometry will deflect about three times as much.

Bottom lineAluminium wins on weight-for-weight strength, but sections must be thickened or ribbed to match steel rigidity.

10 · Recycled aluminium performance

      No — and this is aluminium's strongest sustainability argument. Remelting scrap takes only about 5% of the energy of primary smelting and roughly a twentieth of the emissions, and aluminium does not degrade through the loop: it can be recycled indefinitely without losing properties. Around 75% of all aluminium ever produced is still in service today.

      The practical difficulty is not metallurgy but sorting. Mixed scrap contaminates the chemistry, so keeping series separated is what preserves the value of recycled feedstock.

Bottom lineSpecifying recycled content costs nothing in performance. Ask your supplier about scrap segregation and melt control.