With its low density, high specific strength, corrosion resistance and recyclability, aluminum is the key material of lightweighting in automotive, structural design in aviation, and a wide range of applications from packaging to construction.
What this note covers
- Wrought and cast alloy series (1xxx–8xxx) and temper designations
- Precipitation hardening: solution treatment, quenching and aging
- Extrusion, rolling and the subtleties of aluminum casting
- Lightweighting strategies in automotive and aviation
Why Aluminium — and Where It Stops
At about one third the density of steel, with a self-healing oxide film and excellent formability, aluminium is the default choice when weight matters. The limits are equally definite: the elastic modulus is also about one third that of steel, so a like-for-like substitution deflects three times as much; and aluminium alloys show no true fatigue limit — the S–N curve keeps falling, so a design life must be stated, not assumed.
Alloy Series: What Strengthens What
Wrought alloys are designated by the EN 573 / AA four-digit system, where the first digit names the main alloying element. The practical division is between alloys strengthened by work hardening and those strengthened by precipitation (heat treatment).
| Series | Main element | Strengthening | Typical alloy | Where used |
|---|---|---|---|---|
| 1xxx | – (99%+ Al) | Work hardening | 1050 | Electrical, chemical, foil |
| 2xxx | Cu | Precipitation | 2024 | Aerospace structure (poor corrosion resistance) |
| 3xxx | Mn | Work hardening | 3003 | Sheet, heat exchangers, packaging |
| 5xxx | Mg | Work hardening | 5083 | Marine, pressure vessels, weldments |
| 6xxx | Mg + Si | Precipitation | 6061, 6082 | Extrusions, general structure — the balanced family |
| 7xxx | Zn (+Mg, Cu) | Precipitation | 7075 | Highest strength; sensitive to stress corrosion |
Casting alloys follow EN 1706; the Al–Si family (A356 / EN AC-42000 and relatives) dominates because silicon gives fluidity and low shrinkage. Cast alloys are not interchangeable with wrought alloys of similar chemistry — the solidification structure, not the analysis, governs the properties.
Temper Designations (EN 515)
The letter after the alloy number is not a formality; it often changes strength by a factor of three. F as fabricated, O annealed, H strain hardened (H1 strain hardened only, H2 partially annealed, H3 stabilised), T4 solution treated and naturally aged, T5 cooled from forming and artificially aged, T6 solution treated and artificially aged (peak strength), T7 over-aged for stress-corrosion resistance.
Precipitation Hardening: the Metallurgy of T6
Three steps: solution treatment dissolves the alloying elements into a single phase; quenching freezes them in a supersaturated solid solution; ageing then precipitates fine particles that obstruct dislocations. In 6xxx alloys the sequence runs from GP zones through the needle-shaped β″ phase — the strength peak — to the coarser β′ and equilibrium β (Mg2Si). Hold too long or too hot and the particles coarsen: strength falls again, which is over-ageing, deliberately used in T7 to buy corrosion resistance.
Melt Quality: Aluminium’s Two Enemies
Hydrogen. It is the only gas appreciably soluble in molten aluminium, and its solubility drops roughly twentyfold on solidification. Whatever the melt dissolved — from damp charge, damp refractory or humid air — must come out somewhere, and it comes out as porosity. Rotary degassing and dry materials are the answer; a reduced-pressure test gives a quick shop-floor measure.
Oxide bifilms. The surface oxide is protective in place, but turbulence folds it into the melt dry-face to dry-face. The folded film is not bonded: it behaves as a crack that was in the metal before any load was applied, and it is also the ready-made surface on which hydrogen porosity nucleates. Quiet transfer, filtration and non-turbulent gating are therefore quality operations, not refinements.
Grain refinement (Al–Ti–B master alloy) and modification of the Al–Si eutectic (strontium) complete the melt treatment: finer grains and fibrous rather than plate-like silicon mean better ductility.
Corrosion and Surface Treatment
The natural oxide protects in neutral environments; anodising thickens it to a controlled, hard and dyeable layer. The classic failure is galvanic: aluminium coupled to steel or, worse, to copper in a wet environment corrodes preferentially. Insulate the joint, choose the fastener material deliberately, and keep the area ratio in mind — a small aluminium part bolted with large steel fasteners is the worst case. In 5xxx alloys held warm, and in stressed 7xxx alloys, sensitisation and stress corrosion cracking need specific tempers rather than coatings.
Quality Control and Standards
- Alloy and temper verification: chemistry per EN 573, temper per EN 515, castings per EN 1706.
- Hardness or conductivity testing as a quick check of the ageing condition.
- Radiography and reduced-pressure testing for castings; macro and microstructure for grain refinement and eutectic modification.
- Coating control: anodic layer thickness and sealing quality.
The technical note below (A4 PDF, in Turkish) develops these subjects in depth with original diagrams prepared for this series.