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Material World
Alloy

Duralumin

Aluminium alloyed with copper and left to harden by itself — the discovery that made metal aircraft possible.

Construction material

Pure aluminium is light and too weak to build anything demanding out of. Duralumin is aluminium with about four per cent copper and a little magnesium and manganese, and it is roughly five times stronger at the same weight — which is the difference between a metal that is a curiosity and one you can make an aeroplane from.

It was found by accident. Alfred Wilm, in Germany around 1906, quenched a sample, measured it, and left it over a weekend; on Monday it was substantially harder than it had been on Friday. Nobody had seen a metal strengthen while sitting on a bench at room temperature.

Processing

The mechanism is age hardening, and it took another twenty years to explain. Quenching from high temperature traps copper in solution where it does not belong; over hours or days it begins to gather into clusters a few atoms across, and those clusters obstruct the movement of dislocations through the crystal. Obstructing dislocations is what hardness is.

The strengthening is therefore temporary in both directions. Left too long, or warmed, the clusters coarsen into proper precipitates and the alloy over-ages and softens. Held cold, the process stalls — which is why aircraft rivets were once kept in dry ice and had to be driven within an hour or two of coming out.

Duralumin's weakness is corrosion. The copper that provides the strength also makes it far less resistant than pure aluminium, which is why aircraft sheet is clad with a thin layer of the pure metal on each face — Alclad, a solution that gives up a little strength to protect the rest.

History

Duralumin went into Zeppelin airframes almost immediately and into aircraft structure from the 1920s, and it is the reason the all-metal monoplane displaced the wood-and-fabric biplane within a decade. Every aluminium alloy used in aerospace since is a descendant, and the 2000-series designation still describes the same aluminium-copper family.

The accidental discovery is also a standard example of a real scientific pattern: the effect was observed, exploited industrially, and used to build aircraft for two decades before anyone could say what was physically happening.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

  • Aluminium — element · 90–95% · the base, and what the alloy exists to make useful
  • Copper — element · 3–5% · the element that does the age hardening, and the one that ruins the corrosion resistance
  • Magnesium — element · 0–2% · under a per cent, and enough to change how fast and how far the alloy ages
  • Manganese — element · 0–1% · a fraction of a per cent, controlling grain structure

is used as

  • Structural engineering — application · aircraft structure, which it was developed for and dominated for half a century

is produced by

  • Alloying and melting — process · melted and cast, then solution treated and quenched — the strength comes from the heat treatment rather than from the melting

succeeded

  • Babbitt metal — alloy · in engine bearings, by way of aluminium-tin — the soft-matrix principle survived and the tin and lead did not

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Duralumin back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Duralumin → is composed of (the element that does the age hardening, and the one that ruins the corrosion resistance) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Duralumin → is composed of (the base, and what the alloy exists to make useful) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Duralumin → is composed of (under a per cent, and enough to change how fast and how far the alloy ages) → Magnesium → is produced by (from seawater and from salt-lake brine, precipitated as the hydroxide before reduction — the ocean is an effectively unlimited magnesium resource) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → Salt → is sourced from (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Duralumin → is composed of (a fraction of a per cent, controlling grain structure) → Manganese → is produced by (as ferromanganese, by the same route and for the same reason — steel wants the alloy, not the element) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Duralumin → is produced by (melted and cast, then solution treated and quenched — the strength comes from the heat treatment rather than from the melting) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Duralumin → is composed of (the base, and what the alloy exists to make useful) → Aluminium → is produced by (collects at the cathode while the carbon anodes are consumed) → Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt

These are the most distinct paths back. Duralumin can be traced through others besides.

Downstream — what it becomes