Aluminum Alloys — Engineering Reference
See also (Tier 3 family index): Aluminum Alloys Family Index
1. At a glance
Aluminum is the second most-used structural metal on Earth — global primary production sat at ~70 million tonnes in 2024 (IAI / World Aluminium), supplemented by ~35 Mt of recycled aluminum. Production is still 25–30× smaller than steel by tonnage, but on a per-volume basis aluminum has displaced steel in entire industries (aerospace structure, beverage packaging, transmission conductors, automotive closures) wherever weight, corrosion, or thermal/electrical conductivity matter more than first-cost.
Why engineers reach for aluminum. At ρ = 2.70 g/cm³, aluminum is roughly 1/3 the density of steel (7.85 g/cm³) and 1/3 its stiffness (E ≈ 70 GPa vs 200 GPa). It carries a self-passivating native oxide (Al₂O₃, ~2–5 nm) that gives it good atmospheric corrosion resistance without any coating, anodises beautifully into a durable hard surface, and is infinitely recyclable at ~5 % of the embodied energy of primary smelting — the recycled fraction now exceeds 75 % of all aluminum in circulation.
Two big families.
| Family | Form | Designation | Examples |
|---|---|---|---|
| Wrought | Rolled sheet/plate, extrusions, drawn rod/tube, forgings | 4-digit AA number (1xxx–8xxx) + temper | 1100-O, 2024-T3, 3003-H14, 5052-H32, 6061-T6, 6063-T5, 7075-T6 |
| Cast | Sand, permanent-mold, die, investment, lost-foam | 3-digit + decimal (1xx.x – 9xx.x) | 319.0, A356.0-T6, 380.0, 535.0, 713.0 |
The series digit encodes the principal alloying addition, which controls the strengthening mechanism and processing behaviour. Tempers describe the heat-treatment or strain-hardening state, and matter as much as the alloy itself: 6061-O and 6061-T6 differ in σ_y by >5×.
Where it sits in the design stack. Aluminum is the default first pick for:
- aerospace skin and primary structure (2xxx, 7xxx, increasingly Al-Li 8090/2099),
- transportation closure panels, crash structure, body-in-white (6xxx with growing 5xxx use),
- consumer packaging (3003 / 3004 cans, 8011 foil),
- architectural facades and fenestration (6063 extrusion, anodised),
- machine frames, robotic links, optical breadboards (6061-T6 plate and extrusion),
- electrical conductors (1350 transmission, 6101/6201 bus and overhead).
Steel still wins on cost-per-strength and stiffness-bound deflection problems; CFRP, magnesium, and titanium win at the upper end of weight-critical aerospace. Aluminum lives in the broad middle where the strength-to-density ratio and corrosion resistance pay for the 3–5× material-cost premium over mild steel.
2. First principles
2.1 Crystal structure and the basics
Pure aluminum is face-centred-cubic (FCC) at all temperatures from 4 K up to its melting point (660.32 °C). FCC delivers four critical engineering consequences:
- 12 active slip systems — full plastic ductility from cryogenic to elevated temperatures. No ductile-to-brittle transition; aluminum stays tough in liquid-helium service (the reason cryogenic LNG tanks and rocket propellant tanks favour 5083 and 2219 over carbon steel).
- High symmetry, low intrinsic Peierls stress — pure Al σ_y ≈ 7–11 MPa annealed. All useful aluminum strengths come from deliberate strengthening mechanisms layered onto a soft matrix.
- No allotropic transformation — unlike steel, there is no austenite-ferrite equivalent. Heat-treatment metallurgy works through precipitation and recrystallisation, not phase transformation of the matrix.
- High thermal/electrical conductivity — pure FCC Al has 237 W/m·K and 65 % IACS conductivity; alloying additions degrade both.
Density 2.70 g/cm³, atomic mass 26.98 g/mol, Young’s modulus E ≈ 69–71 GPa (essentially invariant across alloys — like steel’s 200 GPa, aluminum stiffness is set by the matrix and cannot be heat-treated upward). Poisson’s ratio ν ≈ 0.33. Shear modulus G ≈ 26 GPa.
2.2 Strengthening mechanisms
| Mechanism | Mechanism in atoms | Strength gain | Strengthens which series |
|---|---|---|---|
| Solid-solution | Foreign atoms (Mg, Mn, Si, Cu) distort the lattice and impede dislocation motion | +30 to +200 MPa | 3xxx (Mn), 5xxx (Mg) — non-heat-treatable |
| Strain hardening (cold work) | Dislocation density rises with deformation; tangles block further slip | +50 to +200 MPa, scales as ε^n | 1xxx, 3xxx, 5xxx (“H” tempers) |
| Precipitation (age) hardening | Coherent or semi-coherent intermetallic precipitates (GP zones → θ″ → θ′ → θ) block dislocations | +200 to +500 MPa | 2xxx (Al₂Cu), 6xxx (Mg₂Si), 7xxx (MgZn₂), 8xxx Al-Li (Al₃Li) — heat-treatable |
| Grain refinement (Hall-Petch) | Smaller grains = more boundaries = more obstacles; σ_y = σ₀ + k·d^(−1/2) | +30 to +100 MPa | Marginal in Al (k ~3 MPa·mm^½, far less than steel’s 19) |
| Dispersion / fibre | Insoluble ceramic particles (Al-MMC) or Li-bearing intermetallics | +variable | 8xxx Al-Li, MMC composites |
Why 1xxx, 3xxx, 5xxx are “non-heat-treatable”: their primary alloying elements (Mn, Mg) remain in solid solution and do not form a temperature-controllable precipitation sequence in the engineering range. You strengthen them only by cold work — the H tempers. Welding always softens an H-tempered alloy back to near-annealed strength in the heat-affected zone, full stop.
Why 2xxx, 6xxx, 7xxx are “heat-treatable”: the Cu, Mg-Si, or Mg-Zn additions exceed solubility limits at room temperature but dissolve into a single-phase α-Al solid solution at ~480–540 °C. Quench to retain a supersaturated solid solution, then age (room temperature → T4, or 120–195 °C → T6) to grow nanoscale precipitates that pin dislocations.
2.3 The Al-Cu age-hardening story (and why it was discovered)
Age hardening was discovered accidentally by Alfred Wilm in 1906 during research on Duralumin (early 2017-type Al-Cu-Mg-Mn alloy). Wilm quenched samples on Friday, found them surprisingly soft, came back Monday after the weekend, and discovered they had hardened spontaneously at room temperature. That overnight transformation is the classic Guinier-Preston (GP) zone formation:
- Supersaturated solid solution (immediately after quench): all Cu atoms randomly dissolved in FCC Al matrix.
- GP zones (hours at room T): Cu atoms cluster into coherent disc-shaped plates ~5 nm wide on {100} planes. Maximum coherency strain — this is the strengthening peak in 2xxx-T4.
- θ″ then θ′ semi-coherent precipitates (artificial aging at 175–195 °C): grow to ~50 nm, partial coherency, peak hardness for T6.
- θ (Al₂Cu) incoherent precipitate (overaging): equilibrium phase, large, no longer pins dislocations. Strength drops back toward annealed.
The 6xxx system uses Mg₂Si (β″ → β′ → β) and the 7xxx system uses MgZn₂ (η″ → η′ → η). Same kinetic story, different chemistry. Time-temperature-precipitation (TTP) diagrams describe each.
2.4 The native oxide layer
Aluminum forms a continuous, adherent, electrically insulating α-Al₂O₃ / γ-Al₂O₃ film 2–5 nm thick within milliseconds of exposure to air. It self-heals: scratch it and it regrows within seconds. Three engineering consequences:
- Atmospheric corrosion resistance without coatings (the basis of mill-finish architectural use).
- Welding requires removing the oxide first — Al₂O₃ melts at 2050 °C while Al melts at 660 °C. AC TIG and DC-EP cleaning, or chemical/mechanical pre-cleaning, are mandatory.
- Anodising thickens and densifies the oxide to 5–100 μm, giving very hard, abrasion-resistant, dyeable, electrically-insulating surfaces.
3. Practical math / design equations
3.1 Density-normalised strength (specific strength)
The figure of merit aluminum was invented for:
σ_y / ρ (specific yield strength)
| Material | σ_y (MPa) | ρ (g/cm³) | σ_y / ρ (kN·m/kg) |
|---|---|---|---|
| Mild steel A36 | 250 | 7.85 | 32 |
| AISI 4340 Q&T | 1280 | 7.85 | 163 |
| 6061-T6 | 276 | 2.70 | 102 |
| 7075-T6 | 503 | 2.81 | 179 |
| Ti-6Al-4V | 880 | 4.43 | 199 |
| CFRP UD T700/epoxy | 2500 (fibre dir.) | 1.55 | 1610 |
7075-T6 narrowly beats 4340 Q&T on specific yield despite being one-third the density — this is the aerospace argument for aluminum. Specific stiffness (E/ρ) is roughly equal across all metals (~25 GPa / g·cm⁻³) — the reason stiffness-driven designs do not favour any metal over another by weight, and the reason CFRP dominates stiffness-critical aerospace structure.
3.2 Anodising thickness and growth rate
Anodising grows oxide by converting aluminum to Al₂O₃ in an acid electrolyte (typically 15–20 % H₂SO₄ at 18–22 °C for Type II, or chilled H₂SO₄ at −5 to +5 °C for Type III hard anodise).
Mass of oxide formed: m = (I · t · M) / (n · F)
where I = current (A), t = time (s), M = molar mass of Al₂O₃ (101.96 g/mol), n = 6 (electrons per Al₂O₃ unit), F = 96485 C/mol.
Practical growth rate: ~0.5 μm/min at 1.5 A/dm² in Type II; ~1 μm/min at 3 A/dm² in Type III. Final thickness:
| Type | Process | Typical thickness | Hardness | Use |
|---|---|---|---|---|
| Type I | Chromic acid | 2–8 μm | low | Aerospace fatigue-sensitive parts (no acid attack) |
| Type II | Sulphuric acid | 8–25 μm | ~250 HV | Decorative, architectural, marine |
| Type III (“hard”) | Cold sulphuric | 25–100 μm | 400–600 HV | Wear surfaces, pistons, hydraulic cylinders, gun mounts |
~1/3 of the oxide thickness grows outward, 2/3 inward — so a 50 μm hard-anodise on a 25.000 mm shaft yields a finished diameter of ~25.033 mm (the part grows). Allow for this in tolerance stack-ups: machinists routinely undersize by half the planned coating thickness times two.
3.3 Heat-treatment cycles (heat-treatable wrought)
Generic age-hardening recipe:
- Solution heat treatment (SHT) — hold at the single-phase α-Al solvus, ~5 °C below the solidus, long enough to dissolve all precipitates (15–60 min for thin sheet, hours for thick plate).
- 6061: 530 ± 5 °C, hold 30 min/inch of section
- 7075: 470 ± 5 °C, hold 30 min/inch of section
- 2024: 495 ± 3 °C, hold 30 min/inch (narrowest SHT window — incipient melting at 502 °C)
- Quench — transfer to room-T water in < 15 s (sheet) to suppress grain-boundary precipitation. Slower quenches cause quench sensitivity: 7075 thick plate Q-quenched in glycol-water 30 °C delivers ~85 % of fast-water properties.
- Aging — either natural (room T, days to weeks → T4) or artificial (oven, hours → T6/T651).
- 6061-T6: 175 °C × 8 hr
- 7075-T6: 120 °C × 24 hr (slower because MgZn₂ kinetics are more temperature-sensitive)
- 2024-T4: hold 4 days room T (commercial standard)
- 7075-T73 (overaged for SCC resistance): 107 °C × 8 hr + 163 °C × 8 hr; σ_y drops 10–15 % but stress-corrosion lifetime jumps 100×.
Pre-stretch (the “51” suffix): T651 = T6 plus 1.5–3 % permanent stretch after quench to relieve residual quench stress and dimensionally stabilise plate for downstream machining. Without it, large machined pockets in T6 plate warp on stress-relieving.
3.4 Worked example — 6061-T6 plate in tension
Problem. A 6061-T6 aluminum plate 200 mm × 100 mm × 6 mm is loaded in tension by 25 kN axial along the long axis. Compute stress, strain, elongation, and factor of safety on yield. Compare deflection if the same load were applied to a steel plate of equal cross-section.
Step 1. Cross-section and stress.
A = 100 mm × 6 mm = 600 mm² = 6.00 × 10⁻⁴ m² σ = F / A = 25,000 N / 6.00 × 10⁻⁴ m² = 41.7 MPa (6.04 ksi)
Step 2. Compare to yield.
σ_y (6061-T6) = 276 MPa (per ASTM B557 / AMS 4027) Factor of safety FS = σ_y / σ = 276 / 41.7 = 6.6 Elastic, with comfortable margin. The plate is nowhere near yielding.
Step 3. Strain and elongation.
ε = σ / E = 41.7 MPa / 69,000 MPa = 6.04 × 10⁻⁴ ΔL = ε · L = 6.04 × 10⁻⁴ × 200 mm = 0.121 mm (0.0048 in)
Step 4. Steel comparison (A36). Same A and load → σ unchanged (41.7 MPa).
ε_steel = 41.7 / 200,000 = 2.085 × 10⁻⁴ ΔL_steel = 2.085 × 10⁻⁴ × 200 = 0.0417 mm
Insight. At equal cross-section, the aluminum plate deflects ~2.9× more than steel under the same load — exactly the ratio of E_steel/E_Al. Aluminum is not lighter at equal stiffness; for a deflection-bound design (machine frames, robot links where dynamic resonance matters), you size up the section. To match steel stiffness you need ~3× the cross-sectional area, which at 1/3 density gives equal mass — so aluminum’s weight advantage evaporates in deflection-critical designs. It re-emerges only when the loading is strength- or fatigue- limited.
Step 5. Density-mass check.
Plate volume V = 0.200 × 0.100 × 0.006 = 1.20 × 10⁻⁴ m³ m_Al = 1.20 × 10⁻⁴ × 2700 = 0.324 kg m_steel = 1.20 × 10⁻⁴ × 7850 = 0.942 kg Aluminum plate weighs 34 % of the steel plate. For a strength-limited tension member, this is the entire engineering argument.
3.5 Knockdown factors that bite Al designers
| Effect | Magnitude | When it matters |
|---|---|---|
| Welded HAZ in 6061-T6 → -O state | σ_y drops from 276 → 55 MPa, 80 % loss | Any weld in heat-treatable alloy. Aluminum Design Manual (ADM) tabulates F_yw for HAZ. |
| Fatigue endurance — no endurance limit | S-N keeps falling | Any cyclic load. Quote fatigue strength at a specific cycle count (typ. 5 × 10⁸). |
| Elevated temperature (T > 100 °C continuous) | 6061-T6 σ_y drops ~30 % at 200 °C | Any service above ~150 °C — overaging accelerates. |
| Cryogenic (T < 0 °C) | σ_y rises 5–15 %, ductility also rises | LNG tanks, aerospace propellant tanks (5083 / 2219 favoured). |
| Stress-corrosion cracking in 7xxx-T6 | Threshold K_ISCC ~ 5 MPa√m in chloride | Marine 7075-T6 in saltwater — always specify T73 or T76 instead. |
4. Reference data — common aluminum grades
| Alloy + temper | Series basis | σ_y (MPa) | σ_u (MPa) | Elong. (%) | Hardness | Cost tier | Notes |
|---|---|---|---|---|---|---|---|
| 1100-O | Pure | 35 | 90 | 35 | 23 HB | $ | Electrical, foil, food handling |
| 1100-H14 | Pure | 110 | 124 | 9 | 32 HB | $ | Cold-rolled sheet metal |
| 1350-H19 | Pure | 165 | 185 | 1.5 | 35 HB | $ | Electrical conductor, 61 % IACS |
| 2024-T3 | Al-Cu | 345 | 485 | 18 | 120 HB | $$$ | Aircraft skin (clad standard) |
| 2024-T4 | Al-Cu | 325 | 470 | 19 | 120 HB | $$$ | Natural-aged, formable |
| 2024-T351 | Al-Cu | 325 | 470 | 19 | 120 HB | $$$ | Plate, stretched, stress-relieved |
| 2219-T87 | Al-Cu (weldable) | 393 | 476 | 10 | 130 HB | $$$$ | Cryogenic, weldable Al-Cu (rocket tanks) |
| 3003-O | Al-Mn | 40 | 110 | 30 | 28 HB | $ | Annealed cans |
| 3003-H14 | Al-Mn | 145 | 150 | 8 | 40 HB | $ | Sheet ducting, can stock |
| 3004-H34 | Al-Mn-Mg | 200 | 240 | 9 | 63 HB | $ | Beverage can body |
| 4032-T6 | Al-Si | 315 | 380 | 9 | 120 HB | $$ | Forged pistons (low CTE) |
| 5052-O | Al-Mg | 90 | 195 | 25 | 47 HB | $$ | Annealed for forming |
| 5052-H32 | Al-Mg | 195 | 230 | 12 | 60 HB | $$ | Marine sheet, fuel tanks |
| 5083-H116 | Al-Mg | 215 | 305 | 16 | 80 HB | $$ | Marine plate, cryogenic |
| 5086-H116 | Al-Mg | 207 | 290 | 12 | 78 HB | $$ | Marine, pressure vessel |
| 5454-H32 | Al-Mg | 205 | 270 | 10 | 73 HB | $$ | Hot-service Mg (< 65 °C avoid Mg₅Al₈) |
| 6061-O | Al-Mg-Si | 55 | 125 | 25 | 30 HB | $ | Annealed, freely formable |
| 6061-T4 | Al-Mg-Si | 145 | 240 | 22 | 65 HB | $ | Natural-aged, weldable, formable |
| 6061-T6 | Al-Mg-Si | 276 | 310 | 12 | 95 HB | $$ | Workhorse: machine parts, structure |
| 6061-T651 | Al-Mg-Si | 276 | 310 | 12 | 95 HB | $$ | Stress-relieved plate, machining stable |
| 6063-T5 | Al-Mg-Si | 145 | 185 | 12 | 60 HB | $$ | Architectural extrusion, anodises clean |
| 6063-T6 | Al-Mg-Si | 215 | 240 | 12 | 73 HB | $$ | Structural extrusion |
| 6082-T6 | Al-Mg-Si | 250 | 295 | 11 | 95 HB | $$ | European structural extrusion |
| 6262-T9 | Al-Mg-Si-Pb-Bi | 380 | 400 | 10 | 120 HB | $$ | Free-machining (Pb replacement coming) |
| 7075-T6 | Al-Zn-Mg-Cu | 503 | 572 | 11 | 150 HB | $$$ | Aerospace high-strength |
| 7075-T651 | Al-Zn-Mg-Cu | 503 | 572 | 11 | 150 HB | $$$ | Stretched plate |
| 7075-T73 | Al-Zn-Mg-Cu | 435 | 505 | 13 | 135 HB | $$$ | Overaged for SCC resistance |
| 7050-T7451 | Al-Zn-Mg-Cu | 469 | 524 | 11 | 142 HB | $$$$ | Thick aerospace plate, low quench sensitivity |
| 7068-T6 | Al-Zn-Mg-Cu | 683 | 710 | 8 | 175 HB | $$$$$ | Highest-strength commercial Al |
| 8090-T8 | Al-Li-Cu-Mg | 415 | 480 | 5 | 130 HB | $$$$$ | Al-Li, ~10 % lighter than 2024 |
| 2099-T8 | Al-Li-Cu | 480 | 530 | 7 | 140 HB | $$$$$ | 3rd-gen Al-Li, Airbus A350 lower wing |
| Cast: 319.0-F | Al-Si-Cu | 165 | 235 | 2 | 70 HB | $ | Sand/perm-mold general engine |
| Cast: A356.0-T6 | Al-Si-Mg | 207 | 262 | 5 | 75 HB | $$ | Wheels, suspension, structural |
| Cast: 357.0-T6 | Al-Si-Mg | 250 | 310 | 3 | 85 HB | $$ | Premium structural cast |
| Cast: 380.0-F | Al-Si-Cu | 165 | 325 | 3 | 80 HB | $ | Die-cast standard (engine cases, gearboxes) |
| Cast: 535.0-F | Al-Mg | 140 | 240 | 9 | 75 HB | $$ | Marine corrosion (no heat treat needed) |
| Cast: 713.0-T5 | Al-Zn | 175 | 240 | 5 | 75 HB | $$ | Self-aging cast |
All wrought tensile data per ASTM B557 / ASTM E8; hardness per ASTM E10 (Brinell, 500 kgf / 10 mm ball). Cast data per ASTM E8 on separately-cast test bars per ASTM B26 (sand), B108 (perm-mold), B85 (die). Stiffness E ≈ 68–72 GPa across all alloys above; alloying does not measurably change matrix modulus.
5m. Composition & microstructure
5m.1 The Aluminum Association designation system
Wrought (ANSI H35.1): four digits.
- First digit = principal alloying element family.
- Second digit = modification of the original alloy (0 = original; 1–9 = modified). Originally for impurity-control updates.
- Third/fourth digits = arbitrary alloy identifier within the family, except in 1xxx where it specifies purity above 99 % directly (e.g., 1100 = 99.00 % min Al; 1350 = 99.50 % min Al; 1199 = 99.99 % min “five-nines” foil-grade).
| Series | Principal alloy | Heat-treatable? | Strengthening | Welding |
|---|---|---|---|---|
| 1xxx | ≥ 99 % Al | No | Cold work | Excellent (1100/4043 filler) |
| 2xxx | Cu (3–6 %) | Yes | Precipitation | Poor (hot cracks); 2219 weldable exception |
| 3xxx | Mn (~1.2 %) | No | Solid solution + cold work | Good (4043/5356) |
| 4xxx | Si (5–13 %) | Some (4032) | Mostly cast/filler use | n/a — used AS filler |
| 5xxx | Mg (0.5–5 %) | No | Solid solution + cold work | Excellent (5356/5183 filler) |
| 6xxx | Mg + Si | Yes | Mg₂Si precipitation | Good (4043 if dilution OK; 5356 if higher Mg needed) |
| 7xxx | Zn + Mg (+Cu) | Yes | MgZn₂ precipitation | Poor (HAZ SCC, hot cracking); 7039 weldable exception |
| 8xxx | Other (Li, Fe, Sn) | Yes (Al-Li) | Al₃Li precipitation | Specialised (FSW preferred) |
Cast (ANSI H35.1): three digits + decimal.
- First digit = principal alloy element family (numbering differs from wrought).
- Decimal: .0 = casting; .1 / .2 = remelt ingot composition variants.
| Cast series | Principal alloy |
|---|---|
| 1xx.x | ≥ 99 % Al (foundry-grade pure) |
| 2xx.x | Cu |
| 3xx.x | Si + Cu and/or Mg (the workhorse family — 319, 356) |
| 4xx.x | Si only |
| 5xx.x | Mg |
| 6xx.x | (unused) |
| 7xx.x | Zn |
| 8xx.x | Sn |
| 9xx.x | Other |
Prefix letters (A356 vs 356) denote impurity-control modifications — A356 has tighter Fe than 356, which directly raises fatigue life because Fe-bearing intermetallics (β-Al₅FeSi platelets) are crack nucleators.
5m.2 Wrought 1xxx — commercial-purity aluminum
- 1100 (99.00 % Al, 0.05–0.20 % Cu) — the most-used pure grade. Soft, infinitely formable, weldable, conductive (59 % IACS). Sheet metal ducting, name-plates, chemical equipment liners, food handling. Strengthened only by H tempers; even H18 only gets to σ_y ≈ 150 MPa.
- 1350 (99.50 % Al) — electrical conductor grade with Fe and Si held below 0.50 % combined. 61 % IACS. Used in overhead transmission line (paired with steel core in ACSR), bus bars, and motor windings where Cu cost is prohibitive.
- 1199 (99.99 % Al, “four-nines”) — foil for capacitors and high-purity reflectors.
5m.3 Wrought 2xxx — Al-Cu (and Cu-Mg)
The original engineering aluminum alloys, dating to Wilm’s Duralumin in 1906–1909.
- 2017 (Duralumin) — original 4 % Cu, 0.5 % Mg, 0.5 % Mn alloy. Largely replaced by 2024.
- 2014-T6 — 4.4 % Cu, 0.5 % Mg, 0.8 % Mn, 0.8 % Si. σ_y 415 MPa. Used in heavy-duty forgings, military and structural where the better fatigue of 2024 isn’t required.
- 2024-T3 — 4.4 % Cu, 1.5 % Mg, 0.6 % Mn. The standard aircraft skin alloy for ~70 years. σ_y 345 MPa, σ_u 485 MPa, excellent fatigue, poor corrosion resistance. Always supplied Alclad (1.5–5 % thickness of pure 1230 cladding on each face) for aircraft skin: the pure-aluminum surface protects the underlying 2024 sacrificially in marine atmosphere.
- 2024-T4 — naturally aged for ~96 hr at room T after solution quench. More formable than T3 (which is solutionised, then cold-worked, then naturally aged).
- 2219-T87 — 6.3 % Cu, 0.3 % Mn, 0.1 % V, 0.1 % Zr. The weldable Al-Cu. Saturn V S-IC tank, Space Shuttle external tank, modern launch-vehicle propellant tanks. Cryogenic-toughness specification (CVN at 20 K). Wide use in launch and pressure-vessel industries.
- Alclad 2024 — composite sheet with 1230 (99.30 % Al) cladding bonded each side during rolling. Standard for aerospace skin.
Critical caution. 2xxx alloys exhibit serious galvanic corrosion to all common engineering substrates; they pit aggressively in marine atmospheres and exhibit intergranular corrosion if SHT’d improperly (slow quench causes Al₂Cu precipitation on grain boundaries, leaving Cu-depleted anodic zones).
5m.4 Wrought 3xxx — Al-Mn
- 3003 (1.2 % Mn, 0.12 % Cu) — sheet metal workhorse. Spinning, deep drawing, brake forming, fluid handling. Roof flashing, HVAC ducting, residential cladding sheet.
- 3004 (1.2 % Mn, 1.0 % Mg) — the aluminum beverage can body. Higher strength than 3003 from Mg solid-solution, still drawable into the deep-drawn can shape. Cans are 3004 body + 5182 lid.
- 3105 — light-duty sheet, painted siding, mobile-home roofs.
5m.5 Wrought 4xxx — Al-Si
Primarily filler wire and the rare structural alloy.
- 4032 (12 % Si, 1 % Mg, 1 % Cu, 1 % Ni) — forged automotive pistons. Eutectic Si gives very low CTE (matched to cast-iron cylinder) and high wear resistance. T6 temper.
- 4043 (5 % Si) — workhorse GMAW/GTAW filler wire for welding 6xxx (and 3xxx). Lower Si content avoids brittle Mg-Si phase at the dilution boundary.
- 4047 (12 % Si) — higher-silicon filler, brazing alloy, automotive heat-exchanger braze fin stock.
5m.6 Wrought 5xxx — Al-Mg
The “marine and pressure” family. Non-heat-treatable but among the strongest non-HT Al alloys; excellent corrosion in chlorides; excellent welds (5356, 5183, 5556 fillers retain near-base-metal strength).
- 5005 — low-Mg architectural alloy; bright anodises well.
- 5052 (2.5 % Mg, 0.25 % Cr) — bread-and-butter marine sheet, fuel tank sheet, sheet-metal stamping. H32 = strain-hardened + stabilised.
- 5083 (4.4 % Mg, 0.7 % Mn) — marine plate (boat hulls, LNG vessels, shipbuilding). H116/H321 stabilised tempers prevent sensitisation. One of two structural alloys (with 5454) qualified for cryogenic LNG service.
- 5086 (4.0 % Mg) — slightly lower-Mg cousin of 5083; pressure-vessel and marine plate. Similar mechanicals.
- 5182 — beverage can lid (the easy-open tab). 4.5 % Mg gives sufficient stiffness for the score-and-pop mechanism.
- 5454 (2.7 % Mg) — Mg held below 3 % to avoid β-Mg₅Al₈ grain-boundary precipitation that causes SCC in 5083 above 65 °C. Used in hot road tankers (oil, asphalt, hot food product).
Sensitisation in 5xxx (Mg > 3 %): long exposure 70–200 °C precipitates β-phase on grain boundaries, causing intergranular SCC in marine service. H116 and H321 are processing routes that randomise the dislocation structure to prevent β precipitation. Above ~65 °C continuous service, drop to 5454.
5m.7 Wrought 6xxx — Al-Mg-Si
The most-used aluminum family by tonnage, dominating extrusion (~80 % of all extruded Al is 6xxx).
- 6061 (1.0 % Mg, 0.6 % Si, 0.25 % Cu, 0.2 % Cr) — the universal aluminum. Extrudable, machinable, weldable, anodisable, mid-strength. Robot links, optical breadboards, machine bases, custom plate parts, 80/20-style T-slot framing, bicycle frames (mid-range), small aircraft (Cessna 172 wing skin still 6061 to this day).
- 6063 (0.7 % Mg, 0.4 % Si) — slightly lower alloying; the architectural extrusion alloy. Better surface finish post-extrusion, anodises uniformly. Window frames, curtain-wall mullions, door frames, modular display systems.
- 6082 (0.9 % Mg, 1.0 % Si, 0.7 % Mn) — Mn replaces 6061’s Cr; preferred European structural extrusion. Slightly higher σ_y than 6061. Mining, bridge sections, vehicle chassis.
- 6101 (0.6 % Mg, 0.5 % Si) — electrical bus-bar extrusion, 55 % IACS.
- 6201 — high-strength conductor (overhead transmission alloy), used in AAAC (all-aluminum-alloy conductor).
- 6262 (0.7 % Mg, 1.1 % Si, 0.5 % Pb, 0.5 % Bi) — free-machining. Pb addition causes chip-breaking. RoHS / REACH are slowly phasing Pb out; Bi-only and Sn variants emerging.
- 6005A / 6005 — extrusion-friendly heavy-section structural, common in rail cars and trailer beds.
5m.8 Wrought 7xxx — Al-Zn-Mg-Cu
Highest-strength commercial aluminum family.
- 7075 (5.6 % Zn, 2.5 % Mg, 1.6 % Cu, 0.23 % Cr) — aerospace high-strength. σ_y 503 MPa T6. Wing spars, fuselage frames, military aircraft (F-15 / F-22 forged bulkheads), bicycle frames (high-end), military rifle receivers. Not weldable (severe HAZ cracking + SCC). Cannot be plated without delayed cracking risk.
- 7050 (6.2 % Zn, 2.3 % Mg, 2.3 % Cu, 0.12 % Zr) — Zr replaces Cr; lower quench sensitivity than 7075, so thick plate (>75 mm) retains properties to centreline. T7451 standard for aerospace thick plate.
- 7068 (8.0 % Zn, 2.5 % Mg, 2.0 % Cu) — the highest-strength commercially-available wrought aluminum, σ_y ~700 MPa T6. Defence (gun mounts, missile bodies), upper-end aerospace.
- 7039 — Mg-Zn dilute alloy that is weldable. Limited use (armour plate, military vehicles).
- T73 / T76 / T7351 tempers: deliberately overaged from peak T6 to trade ~10–15 % yield strength for ~100× longer stress-corrosion lifetime. Mandatory for 7075 in chloride service.
5m.9 Wrought 8xxx — Al-Li and others
- 8011 — Al-Fe-Si, the household and packaging aluminum foil alloy. Excellent rollability to < 6 μm gauge.
- 8090 (2.4 % Li, 1.3 % Cu, 0.95 % Mg, 0.12 % Zr) — first-generation Al-Li. Each 1 % Li adds 3 % stiffness and reduces density by 3 %. ~10 % weight saving vs 2024. Limited acceptance due to anisotropy and processing cost.
- 2099 / 2199 — third-generation Al-Li (formally 2xxx but Li-bearing). Lower Li (0.6–1.8 %), better damage tolerance. Used in Airbus A350 lower wing skins, Bombardier C-Series fuselage.
5m.10 Cast alloys
- 319.0 — 6 % Si, 3.5 % Cu. Sand and permanent-mold general-purpose. Engine cylinder heads, oil pans.
- 356.0 / A356.0 / B356.0 — 7 % Si, 0.3 % Mg. The most-used Al casting alloy. Heat-treatable (T6, T61). Cast wheels (factory-stock alloy automotive wheels are almost entirely A356-T6), suspension control arms, structural castings.
- 357.0 / A357.0 — 7 % Si, 0.55 % Mg. Higher Mg = higher strength post-T6 (premium structural castings, aerospace).
- 380.0 — 8.5 % Si, 3.5 % Cu. The die-casting workhorse, by far the largest-tonnage cast Al alloy globally. Engine blocks, transmission housings, automotive structural castings, electronics enclosures.
- 383.0 / 384.0 — modified die-cast 380 for thinner sections, better fluidity.
- 535.0 (Almag-35) — 6.8 % Mg, no Si. Marine castings. Not heat-treatable. Excellent corrosion resistance, intermediate strength.
- 713.0 (Tenzaloy) — 7.5 % Zn, 0.4 % Mg. Self-aging — develops T5 properties at room T over weeks without artificial aging.
- A356 modified with Sr or Na modifier transforms the eutectic Si from coarse acicular flakes to fine fibrous — doubles elongation, used in all structural automotive castings.
5m.11 Temper designations (ANSI H35.1)
The temper code follows the alloy and a hyphen.
Basic tempers:
- F — as fabricated. No control on temper.
- O — annealed (lowest strength, highest ductility).
- H — strain-hardened (non-heat-treatable alloys).
- W — solution heat treated only (unstable; natural aging in progress). Rarely seen on a print.
- T — thermally treated to stable temper (heat-treatable alloys).
H tempers (strain hardening):
- H1x — strain-hardened only, no thermal treatment.
- H2x — strain-hardened then partially annealed.
- H3x — strain-hardened then stabilised (low-T heat treat to prevent age softening, important for 5xxx).
- The second digit (x) is the degree of hardening: 2 = 1/4-hard, 4 = 1/2-hard, 6 = 3/4-hard, 8 = full hard, 9 = extra hard.
- H321 / H116 — 5xxx-specific stabilised tempers that resist sensitisation in marine service.
T tempers (heat treatment):
- T1 — cooled from elevated-T shaping, naturally aged.
- T2 — cooled, cold-worked, naturally aged.
- T3 — solution treated, cold-worked, naturally aged. (E.g. 2024-T3 sheet — solution-quenched, rolled to size, naturally aged 96 hr.)
- T4 — solution treated, naturally aged.
- T5 — cooled from elevated-T shaping (extrusion), artificially aged. The 6063-T5 architectural extrusion temper — uses the heat of extrusion as the SHT.
- T6 — solution treated, artificially aged. The peak-strength condition for most heat-treatable alloys.
- T7 — solution treated, overaged/stabilised. Lower strength, better SCC resistance (T73 in 7075).
- T8 — solution treated, cold-worked, artificially aged. Higher σ_y than T6 from the strain-aging interaction.
- T9 — solution treated, artificially aged, cold-worked.
- T10 — cooled from shaping, cold-worked, artificially aged.
Suffix digits: Txx51, Txx52, Txx53, Txx54 — stress-relief by stretching, compressing, both, or thermal. T651 = T6 + 1.5–3 % stretched plate. Essential for large machined parts.
6m. Mechanical properties
Properties cited per ASTM B557-23 (tensile) and ASTM E10-23 (Brinell, 500 kgf / 10 mm ball) at room temperature 23 ± 2 °C unless noted. Industry standard fatigue data follows ASTM E466 (axial) and is typically reported at 5 × 10⁸ cycles (the conventional “fatigue strength” for aluminum since there is no true endurance limit).
6m.1 Tensile summary (longitudinal direction)
| Alloy temper | σ_y MPa (ksi) | σ_u MPa (ksi) | Elong. % in 50 mm | Brinell HB |
|---|---|---|---|---|
| 1100-O | 35 (5.0) | 90 (13) | 35 | 23 |
| 1100-H14 | 110 (16) | 124 (18) | 9 | 32 |
| 2024-T3 sheet | 345 (50) | 485 (70) | 18 | 120 |
| 2024-T4 sheet | 325 (47) | 470 (68) | 19 | 120 |
| 2219-T87 plate | 393 (57) | 476 (69) | 10 | 130 |
| 3003-H14 | 145 (21) | 150 (22) | 8 | 40 |
| 3004-H34 | 200 (29) | 240 (35) | 9 | 63 |
| 5052-H32 | 195 (28) | 230 (33) | 12 | 60 |
| 5083-H116 plate | 215 (31) | 305 (44) | 16 | 80 |
| 5454-H32 | 205 (30) | 270 (39) | 10 | 73 |
| 6061-T4 | 145 (21) | 240 (35) | 22 | 65 |
| 6061-T6 / T651 | 276 (40) | 310 (45) | 12 | 95 |
| 6063-T5 | 145 (21) | 185 (27) | 12 | 60 |
| 6063-T6 | 215 (31) | 240 (35) | 12 | 73 |
| 6082-T6 | 250 (36) | 295 (43) | 11 | 95 |
| 7075-T6 / T651 | 503 (73) | 572 (83) | 11 | 150 |
| 7075-T73 | 435 (63) | 505 (73) | 13 | 135 |
| 7050-T7451 plate | 469 (68) | 524 (76) | 11 | 142 |
| 7068-T6511 | 683 (99) | 710 (103) | 8 | 175 |
| A356.0-T6 cast | 207 (30) | 262 (38) | 5 | 75 |
| 357.0-T6 cast | 250 (36) | 310 (45) | 3 | 85 |
| 380.0-F die-cast | 165 (24) | 325 (47) | 3 | 80 |
6m.2 Elastic properties (essentially alloy-invariant)
- Young’s modulus E = 68–72 GPa (9.9–10.4 × 10⁶ psi)
- Shear modulus G = 25–26 GPa
- Poisson’s ratio ν = 0.33
- Bulk modulus K ≈ 76 GPa
Like steel, heat treatment and cold work do not change elastic stiffness. A deflection-bound design (machine frame natural frequency, robot end-effector positional repeatability) cannot be solved by going from 6061-T6 to 7075-T6 — the section moment of inertia must change.
Note that Al-Li alloys are the exception: each 1 % Li raises matrix stiffness by ~3 %. 8090 at 2.4 % Li reaches E ≈ 77 GPa — 11 % stiffer than commercial 2024.
6m.3 Fatigue
Aluminum has no endurance limit. S-N curves continue to decline at low stress amplitudes; engineering practice quotes fatigue strength at 5 × 10⁸ cycles (R = −1, rotating beam, smooth specimen).
| Alloy | σ_a at 5 × 10⁸ cycles | σ_a / σ_u |
|---|---|---|
| 1100-H14 | 35 MPa | 0.28 |
| 2024-T3 | 140 MPa | 0.29 |
| 5052-H32 | 110 MPa | 0.48 |
| 6061-T6 | 95 MPa | 0.31 |
| 7075-T6 | 160 MPa | 0.28 |
| A356.0-T6 cast | 90 MPa | 0.34 |
Aircraft design uses safe-life or damage-tolerance philosophy with fatigue crack growth da/dN per ASTM E647 rather than infinite-life criteria — exactly because aluminum will eventually fatigue out at any stress level.
6m.4 Fracture toughness (per ASTM E399 / E1820)
| Alloy | K_IC (MPa√m) | Comment |
|---|---|---|
| 2024-T351 | 26–37 (L-T) | Plate, longitudinal-transverse orientation |
| 2024-T351 | 18–26 (T-L) | Transverse-longitudinal, lower as always |
| 7075-T651 | 22–29 (L-T) | High-strength penalty |
| 7075-T7351 | 28–35 (L-T) | Overaging restores toughness |
| 7050-T7451 | 32–40 (L-T) | The aerospace-thick-plate sweet spot |
| 6061-T6 | 29 (L-T) |
Aluminum K_IC is generally an order of magnitude lower than steel (4340 ≈ 60–80 MPa√m, A36 ≈ 60). Combined with no endurance limit, this is why aircraft inspection and damage-tolerance regulation (FAR 25.571, CS 25.571) are so much more rigorous than for steel structures.
6m.5 Hardness conversions (approximate)
For wrought aluminum at HB 30–150 the conversion to Rockwell is:
- HB 60 ≈ HRF 60 (Rockwell F, 60 kgf / 1/16″ ball)
- HB 95 ≈ HRB 60 (Rockwell B, 100 kgf / 1/16″ ball)
- HB 150 ≈ HRB 80
Never use HRC on aluminum — the diamond cone penetrates fully through the indent zone into the bulk, returning meaningless values.
7m. Thermal / electrical / chemical properties
| Property | 1100 | 6061-T6 | 7075-T6 | A356.0 | 304 stainless (ref) |
|---|---|---|---|---|---|
| Density ρ (g/cm³) | 2.71 | 2.70 | 2.81 | 2.68 | 7.95 |
| Young’s modulus E (GPa) | 69 | 69 | 71 | 72 | 200 |
| Thermal conductivity k (W/m·K, 20 °C) | 222 | 167 | 130 | 151 | 16 |
| Specific heat c_p (J/kg·K) | 900 | 896 | 960 | 963 | 500 |
| Coefficient of thermal expansion α (10⁻⁶/°C, 20–100 °C) | 23.6 | 23.6 | 23.4 | 21.5 | 17.3 |
| Electrical resistivity (nΩ·m) | 28.2 | 39.9 | 51.5 | — | 720 |
| Electrical conductivity (% IACS) | 59 % | 40 % | 32 % | — | 2.4 % |
| Melting range (°C) | 643–657 | 582–652 | 477–635 | 555–615 | 1400–1450 |
7m.1 Thermal conductivity
Pure aluminum at 237 W/m·K is the second-best engineering thermal conductor by mass (copper 401 W/m·K beats it absolute, but copper is 3× denser; on a per-mass basis aluminum beats copper). This drives:
- Heat sinks — 6061/6063 extruded fin stock is the dominant electronics heat sink material.
- Heat exchangers — automotive radiator and AC condenser tube/fin are brazed Al (3003 tube, 4xxx braze, 6951 sheet).
- High-voltage transmission — ACSR (aluminum conductor steel reinforced) replaced Cu by ~1940 for everything above 5 kV.
Alloying degrades k: 7075 at 130 W/m·K is only 55 % of pure Al because Zn, Mg, Cu solute atoms scatter phonons. For heat-sink and electrical service always specify 1100, 1350, 6061, or 6063, never the high-strength 2xxx/7xxx.
7m.2 CTE — the dissimilar-metal joint headache
Al at 23.6 × 10⁻⁶ /°C is roughly 2× steel’s 12 × 10⁻⁶. An aluminum plate bolted to a steel frame at 20 °C, heated to 100 °C through 1 m, grows ~1.9 mm relative to the steel — about 12× a bolt-clearance fit. Engineering responses:
- Slotted holes in one mating part, or oversized clearance.
- Bushed pivots for hinge-like joints.
- Match-machine cold (bored at operating T after thermal soak) for precision systems.
- Composite mounts (PEEK, glass-epoxy isolators) at fastener locations to absorb shear strain.
- All-aluminum design to eliminate the differential. The reason aerospace structures are often monolithically aluminum despite higher cost than mixed-material — predictable thermal behaviour wins.
7m.3 Electrical resistivity
Pure aluminum 1350 = 28.2 nΩ·m, or 61.8 % IACS (International Annealed Copper Standard, 100 % IACS = 17.241 nΩ·m). On a per-mass basis aluminum beats copper for conductor service:
conductivity / mass = (1/ρ_electrical) / ρ_density Cu: (1/17.24) / 8.96 = 0.00648 Al-1350: (1/28.2) / 2.70 = 0.0131
Aluminum 1350 carries ~2× the current per unit mass as copper — the only reason transmission lines are aluminum, despite copper’s lower volumetric resistivity. The price for low cost and low weight is larger conductor diameter, which is fine for overhead and bus-bar but a problem in motor windings (slot fill).
Conductor-grade aluminum (1350, 6101, 6201) cold-flows under bolt pressure, causing connection loosening and resistive heating over time. The dominant historical failure mode of aluminum residential wiring (1965–1973 US construction) was loose binding screws at outlets — not conductor failure. Modern aluminum building wire (AA-8000 series, 8030/8176) and CO/ALR-rated devices have solved this, but the lesson is to use specified torque, anti-oxidant compound, and listed connectors.
7m.4 Corrosion behaviour
The native Al₂O₃ passive film survives in pH ~4–9. Outside that range (caustic cleaning, hydrochloric acid, hot concentrated phosphoric, mercury contact) it breaks down catastrophically.
Common corrosion modes:
- Pitting in chloride environments (seawater, road salt). 2xxx and 7xxx most susceptible; 5xxx and 3xxx most resistant. Pit propagation rate ~0.1–1 mm/year in 3.5 % NaCl.
- Galvanic corrosion — aluminum is anodic to almost every other engineering metal except magnesium and zinc. Steel, stainless, copper, brass, graphite, carbon-fibre composite — all drive aluminum to corrode at the joint. Mitigation: dielectric isolation (gasket, washer), avoid contact-with-CFRP cathodic area >> Al anodic area, prime + paint the assembly, use sacrificial Zn coating on Al.
- Filiform corrosion under paint films, especially in humid environments (~85 % RH). Causes the worm-like tracks on poorly-prepped painted aluminum.
- Intergranular corrosion in 2xxx and 7xxx with slow quench or unstable temper. Stainless-like sensitisation mechanism: Cu or Zn-Mg precipitates on grain boundaries → adjacent depletion → galvanic attack.
- Exfoliation in 7xxx (and to a lesser extent 2xxx) plate with elongated pancake grains. The corrosion attacks along grain boundaries parallel to rolling direction, jacking the surface up in layers. T7x tempers mitigate.
- Stress-corrosion cracking — 7075-T6 in marine atmospheres. Threshold K_ISCC ~5 MPa√m. Always specify T73 or T76 for 7xxx in chloride service.
- Mercury embrittlement — catastrophic and rapid. A drop of mercury (and many amalgams) penetrates aluminum grain boundaries and disintegrates the part within hours. Aluminum is forbidden in mercury-bearing applications.
7m.5 Melting and elevated-temperature
Pure Al melts at 660.32 °C, but alloys with Cu, Mg, Zn have lower eutectic melting points. Incipient melting (initial grain-boundary liquefaction) is a serious processing constraint:
- 2024 solidus ≈ 502 °C — overshoot SHT temperature by 5 °C and the alloy “burns” (incipient melt), losing properties permanently.
- 7075 solidus ≈ 477 °C — even tighter.
- 6061 solidus ≈ 582 °C — comparatively forgiving.
In service, continuous operation above ~150 °C overages the heat-treatable alloys — strength drops over weeks. 6061-T6 at 200 °C loses ~30 % yield strength in 100 hours. Above ~200 °C, switch to dispersion-strengthened or P/M alloys (Al-Ni-Fe-Mo “8009”), or step up to titanium.
8m. Processing & joining
8m.1 Casting
Five major processes, each with characteristic alloys and applications:
| Process | Mold | Alloys | Typical part |
|---|---|---|---|
| Sand casting | One-shot sand | 319, 356, 535, 713 | Pump housings, machinery castings, prototypes |
| Permanent mold | Reusable steel | 319, 356, A356 | Wheels, manifolds, structural |
| Die casting | Hardened H13 die | 380, 383, 384 | Engine cases, gearbox housings, electronics, the majority of cast Al by tonnage |
| Investment | Lost-wax ceramic | 356, A357 | Aerospace structural castings, complex geometry |
| Lost foam / lost-PMMA | Pattern in sand | 356, 319 | Engine blocks, complex undercuts |
| Squeeze / semi-solid (rheocast / thixocast) | Steel die | A356, 357 | Premium-strength structural castings (wheels, control arms) |
Die casting dominates volume. Sand and permanent-mold dominate where post-cast T6 heat treatment is required (die castings entrain too much gas to T6 well — they blister).
Modifier addition of 0.005–0.02 % Sr (or 0.005–0.01 % Na) to Al-Si alloys transforms the eutectic Si from coarse acicular flakes to fine fibrous morphology, raising elongation from ~3 % to ~10 % in A356-T6. Sr is the modern industrial standard (Na fades during holding).
Grain refinement by Al-5Ti-1B master alloy addition (5 kg/tonne) drops grain size from ~mm to ~100 μm, reducing porosity and improving fatigue.
8m.2 Hot extrusion
Aluminum extrusion is the dominant aluminum semi-finished product by part count (and one of the dominant manufacturing process advantages of aluminum over steel — steel extrusion is industrially marginal). Heated billet at 400–500 °C is pushed through a hardened H13 die under 100–700 MPa ram pressure. The 6063 architectural extrusion industry alone is a $30 B/yr global market.
Why aluminum extrudes well: the FCC matrix, low flow stress at 450 °C (~50 MPa), and pre-existing soft-anneal microstructure all conspire to allow complex hollow profiles (curtain-wall mullions, heat-sink fin arrays, structural 80/20-style T-slot, automotive crash-management beams) to be extruded in a single pass. The exit cools fast enough to act as a solution heat treatment — the press quench — so 6063 and 6005 extrusions go directly from press to artificial-age oven (T5 temper) without an explicit reheat-and-quench cycle. This is the cost basis of 6063 architecture.
7075 and 2024 are extrudable but require slow press speeds, frequent die changes, and post-extrusion SHT + quench — much higher cost.
8m.3 Rolling, drawing, forming
- Cold rolling — sheet and foil. 1100 / 3003 / 5052 / 6061 are all available in cold-rolled sheet. Brake-press bending is standard for sheet-metal fabrication.
- Deep drawing — 3004 beverage can, 5xxx and 6xxx for automotive closure panels.
- Stretch forming — large-radius aircraft skin panels (2024 sheet).
- Hydroforming — automotive structural tube (5xxx, 6xxx).
- Forging — 2014, 2024, 6061, 6082, 7075, 7050 commonly forged. Closed-die for aerospace fittings, suspension uprights, wheels (the strongest factory aftermarket wheels are forged 6061/A356 hybrids).
- Impact / cold extrusion — Cu, Al fire extinguisher and aerosol can bodies.
Springback is more severe in aluminum than in steel — at equal section moduli, aluminum’s lower stiffness means more elastic recovery on unloading. Account for it with empirical overbend, or 3D forming simulation.
8m.4 Welding
Aluminum welding requires removing the Al₂O₃ oxide layer (because oxide melts at 2050 °C while Al melts at 660 °C — oxide inclusions become weld defects). The standard process is AC-TIG or DC-EP, where the reverse-polarity portion of the cycle cathodically cleans the oxide off the work, or GMAW which uses inert-gas shielding and the spray-arc current to disrupt the oxide.
| Process | Use case | Filler |
|---|---|---|
| GTAW (TIG, AC) | Thin sheet, root passes, repair, aerospace | 4043 for 6xxx (lower Mg means less brittle Mg-Si interface); 5356 for 5xxx and heterogeneous 5xxx/6xxx joints; 4047 for brazing; 2319 for 2219 |
| GMAW (MIG, spray) | Production, semi-automatic, heavier sections | 4043 or 5356 |
| FCAW | n/a — flux-cored Al wire failed commercially; oxide and flux chemistry don’t cooperate | — |
| Friction stir welding (FSW) | Aerospace, marine, automotive — the high-quality joining method for 2xxx and 7xxx that fusion welding ruins. Solid-state, no melt, HAZ losses much smaller. SpaceX, Boeing, Airbus, Tesla, Ford F-150 use FSW. | None — it’s solid-state |
| Resistance spot welding | Automotive body — harder than steel RSW due to high conductivity (requires 3× steel current) and oxide breakdown | None |
| Electron beam / laser | Aerospace specialty, fillet welds in spar caps | 4043, 5356 |
Filler selection cheat sheet:
- 4043 (5 % Si) — for 6061, 6063, 6082, 3xxx. Lower Mg-Si interface brittleness; reddish anodise.
- 5356 (5 % Mg) — for 5083, 5086, 5052, and mixed 5xxx/6xxx. Anodises clean (white). Higher post-weld strength than 4043 on 5xxx.
- 5183, 5556 — high-Mg fillers for 5xxx where 5356 isn’t enough.
- 4047 (12 % Si) — brazing alloy, also auto heat-exchanger fillet.
- 2319 — for 2219 (Cu-bearing). Specialty.
Critical aluminum weld defects:
- Hot cracking in 2xxx — wide solidification range with Cu-rich eutectic on grain boundaries. Almost no general-purpose filler will weld 2024 reliably; 2219 is the workaround.
- Porosity — H₂ solubility in liquid Al is 20× higher than in solid Al. Even a trace of water on the joint generates H₂ porosity. Mandatory: clean dry parts, oxide-removed within ~30 min of welding, dry filler wire, proper shield-gas flow.
- HAZ softening in 6061-T6 from 276 → 55–80 MPa σ_y over a ~10 mm band. The reason aluminum welds are designed to Aluminum Design Manual allowable F_yw (the HAZ allowable), not base-metal F_y.
- Hot cracking in 6xxx when filler dilution drops Mg below 1.5 %. 4043 generally OK; use 5356 if low-Mg filler dilution would degrade properties.
8m.5 Machining
Aluminum is among the most machinable engineering metals: low tool wear, high cutting speeds, good surface finish. Some practical notes:
- Cutting speeds (carbide tooling): 6061-T6 at 300–600 m/min; 7075-T6 at 200–400 m/min. Up to 3000+ m/min with ceramic / PCD for finishing on dedicated high-speed-spindle machines.
- Chip control is the issue, not chip removal. Aluminum chips are long, gummy, ductile — they tangle, wrap, and re-cut. Free-machining 2011 (Pb) and 6262 (Pb-Bi) exist exclusively to fix this. Polycrystalline diamond (PCD) tooling, high-pressure coolant, sharp positive-rake geometry, and chipbreakers all help.
- Built-up edge (BUE) at low speeds with no coolant. Aluminum welds to the tool tip. Mitigation: keep speed up, use coolant, use sharp tools.
- Surface finish — Ra < 0.4 μm achievable with diamond turning; mirror finishes on 6061 are routine. Aerospace mirrors are diamond-turned 6061-T6 or 7075-T6.
- Pocket milling — 6061-T651 stress-relieved plate is the standard. Use of unstress-relieved plate causes machined parts to warp on subsequent thermal cycles or machining of the opposing side.
- Drilling and tapping — Al taps fast and clean but spring-back is a concern. Use 2× pitch chamfered tooling and a tapping fluid; cut-tap rather than form-tap for blind holes (form-tapping aluminum is tempting and causes thread tearout).
8m.6 Anodising and surface treatments
| Treatment | Spec | Use |
|---|---|---|
| Type I — chromic acid anodise | MIL-A-8625 Type I | Aerospace fatigue-sensitive parts. Thin (2–8 μm), no acid corrosion of the substrate. |
| Type II — sulphuric acid anodise | MIL-A-8625 Type II, ASTM B580 | Decorative, architectural, marine. 8–25 μm. Dyeable to virtually any colour. |
| Type III — hard anodise | MIL-A-8625 Type III, ASTM B580 Class A | Wear surfaces, pistons, gun mounts, hydraulic cylinders. 25–100 μm. ~50 HRC equivalent surface hardness. |
| Chromate conversion | MIL-DTL-5541 Type I (Cr⁶⁺) / Type II (Cr³⁺) | Pre-paint adhesion + light corrosion. Type I (yellow chromate) being phased out for environmental reasons. |
| Alodine 1200 / Iridite | (proprietary chromate) | Aerospace pre-prime treatment. |
| Phosphoric acid anodise (PAA) | BAC 5555 | Aerospace adhesive bonding pre-treatment (Boeing standard for structural bonding of skin/stringer). |
| Painting | Various | Always over chromate / Alodine / PAA primer — bare painted aluminum filiform-corrodes. |
| Powder coating | AAMA 2604 / 2605 | Architectural. AAMA 2605 is the 10-year-warranty grade. |
| Bright dipping | Phosphoric-nitric polish | Cosmetic mirror finish, often followed by clear anodise. |
| Electroless nickel | AMS 2404 | Uniform wear coating, especially over electroplated zincate strike for adhesion. Useful for solderable surfaces. |
Anodised aluminum surfaces are electrically insulating — a Type III hard anodise resists hundreds of volts. This is sometimes leveraged in tooling and fixturing, and sometimes a problem when you need a chassis ground (mask the contact areas before anodising).
9m. Applications & selection trade-offs
9m.1 Quick-pick by application
| Application | Standard pick | Why |
|---|---|---|
| Beverage / food packaging | 3003 (cans), 3004 (body), 5182 (lid), 8011 (foil) | Formable, cheap, food-safe, recyclable |
| Aircraft skin | 2024-T3 Alclad (legacy); 2099 or 8090 (modern) | High σ_y, fatigue, damage tolerance |
| Aircraft spar / bulkhead | 7075-T7351 forged or 7050-T7451 plate | Highest σ_y available |
| Cryogenic / launch tank | 2219-T87 (legacy), 2195 (modern Al-Li) | Weldable, retains toughness at 20 K |
| Marine hull | 5083-H116 or 5086-H116 | Saltwater corrosion + weldable + strength |
| Boat fittings (cast) | 535.0 or 356 anodised | Marine corrosion + light cast |
| Architectural extrusion | 6063-T5 / T6 (anodised) | Surface finish, dimensional stability |
| Structural extrusion (heavy) | 6082-T6 (Europe), 6005A-T61 (rail), 6061-T6 (US general) | Stronger 6xxx for load-bearing |
| Machine frame / robot link / optical breadboard | 6061-T651 plate | Workhorse cost/strength/machinability |
| Automotive wheel (factory) | A356.0-T6 cast (low-pressure permanent mold or squeeze) | Stiffness, fatigue, cost |
| Automotive wheel (forged premium) | 6061-T6 forged + heat-treated | Highest fatigue/strength |
| Automotive body closures | 6061, 6111 (hood/door); 5754 (inner panels); 5182 (inner) | Formability + dent resistance + post-paint-bake age (6xxx bakes to T6 in paint oven) |
| Engine block (cast) | 319 / 380 / 356 | Wear, machinability, dimensional stability |
| Piston (forged) | 4032-T6 or 2618 | Low CTE, hot strength |
| High-end bicycle frame | 6061-T6 (mid), 7005-T6 (mid-premium), 7075 (very rare — SCC risk) | Light, strong, weldable |
| Electronics heat sink | 6061 / 6063 extruded fin | Conductivity + extrudability |
| Electrical bus bar | 6101-T6 or 6061-T6 | Strength + conductivity |
| Overhead transmission line | 1350 (ACSR core wrap) or 6201 (AAAC) | Conductivity + mass cost |
| 80/20 / industrial framing | 6063-T6 or 6105-T5 extrusion | Cheap, anodised, T-slot extrusion-friendly |
| Aerospace forged fitting | 2014-T6, 7075-T73, 7050-T74 | Highest forged strength + corrosion margin |
| 3D printed Al (DMLS) | AlSi10Mg (Al-Si cast-equivalent), Scalmalloy (Al-Mg-Sc) | Castable composition; Sc for strength |
9m.2 Major trade-off discussions
6061-T6 vs 7075-T6. This is the most frequent “which aluminum” question.
| Factor | 6061-T6 | 7075-T6 | Verdict |
|---|---|---|---|
| σ_y | 276 MPa | 503 MPa | 7075 +82 % |
| σ_u | 310 MPa | 572 MPa | 7075 +85 % |
| K_IC | 29 MPa√m | 25 MPa√m | 6061 slightly better |
| Weldable? | Yes (with HAZ softening to ~80 MPa) | No | 6061 wins outright |
| Corrosion (marine) | Good | Poor (SCC in T6; specify T73) | 6061 wins outright |
| Machinable? | Excellent | Good (gummier, more BUE) | 6061 slightly better |
| Cost per kg | $$ | $$$ (~2.5–3.5×) | 6061 wins |
| Anodising | Excellent (clear and bright) | OK (grey/dark, less uniform) | 6061 better cosmetic |
Pick 7075 only when static or fatigue strength dominates and the assembly is fully mechanical (no welding, no marine atmosphere). For everything else, 6061 is the default.
6061 vs steel A36. For a tension member, equal-load equal-strength: σ_y_6061 / σ_y_A36 = 1.10 — about the same. Density ratio 2.70/7.85 = 0.34. The aluminum tension member weighs ~31 % of the steel member for the same allowable load (Al is 10 % stronger, 66 % lighter). For a compression member or beam, the comparison flips because aluminum’s lower E reduces buckling allowable: an aluminum column needs 2.5–3× the moment of inertia to match steel buckling capacity, often killing the weight advantage. Conclusion: aluminum wins on tension, ties or loses on compression and bending, unless mass cost dominates first cost (aerospace).
5052-H32 vs 6061-T4 for sheet-metal bending. Both are common. 5052 bends more cleanly with less spring-back and less risk of bend-line cracking — it’s the standard for tight-radius brake work on fuel tanks, enclosures, and aircraft fairings. 6061-T6 cracks at tight bend radii (specify > 2× material thickness for inner radius) and is best bent in the T4 (natural-aged, formable) condition, then post-bend artificially aged to T6. The aging step is awkward in a job shop, so for fabrication-then-finish, choose 5052; for higher-strength end-state, choose 6061-T6 and pay for the bend radius.
Cast A356-T6 vs wrought 6061-T6. Cast A356-T6 σ_y = 207 MPa, wrought 6061-T6 σ_y = 276 MPa. Cast elongation 5 %, wrought 12 %. Cast fatigue depends heavily on porosity / inclusion size. For complex geometry (wheels, brackets, manifolds) cast wins on tooling cost amortised over high volume; for low-volume structural, machine from 6061 plate.
10m. Failure modes
10m.1 Yielding and ductile overload
Ductile, well-behaved up-front yielding. Design FS 1.5–3 (static), higher for impact and uncertain loading. Welds: design to HAZ allowable (typically 60–80 MPa for 6061-T6), not base metal. Aluminum Design Manual (ADM) F_yw values per alloy/temper/welding-process combination are the authoritative tabulation.
10m.2 Fatigue — no endurance limit
S-N keeps falling. Design to a specific cycle target (10⁵ for landing-gear strut, 10⁹ for in-service drive shaft). Apply knockdown factors:
- Surface finish (aluminum particularly sensitive; as-cast surfaces lose ~50 % of polished fatigue strength)
- Stress concentration K_t and notch sensitivity q (aluminum q ≈ 0.6–0.8 for r ~ 1 mm)
- Mean stress (Goodman or Smith correction)
- Corrosion environment — fatigue + saltwater ≈ 1/3 the cycles to failure.
Aerospace uses damage-tolerance philosophy per FAR 25.571 / CS 25.571 — assume initial flaw size, compute crack-growth life via Paris law, inspection interval is half the predicted growth time. Paris constants (da/dN = C·ΔK^m) for common alloys:
| Alloy | C (mm/cycle / (MPa√m)^m) | m |
|---|---|---|
| 2024-T351 | 2.4 × 10⁻⁹ | 3.3 |
| 7075-T6 | 4.4 × 10⁻⁹ | 3.7 |
| 6061-T6 | 1.5 × 10⁻⁹ | 3.5 |
10m.3 Stress-corrosion cracking (SCC)
The signature 7xxx failure mode. T6 temper in chloride atmosphere → intergranular cracking under sustained tensile stress at K_ISCC ≥ 5 MPa√m. Catastrophic for unmaintained marine 7075. Mitigation: specify T73, T76, or T7651 — overaged tempers that drop σ_y ~10–15 % but raise SCC threshold 5–10×. Avoid residual tensile stress (shot peen to compressive); avoid 7xxx in any saltwater service unless overaged.
2xxx also susceptible at lower magnitude; 5xxx and 6xxx generally immune.
10m.4 Galvanic corrosion
Al is anodic to nearly everything (cathodic only to Mg, Zn, Cd). Stainless or carbon steel fastener in aluminum substrate → aluminum corrodes around the fastener. Mitigation:
- Dielectric isolation washer + sleeve.
- Aluminum or zinc-plated fasteners (cadmium plating works but is being phased out for environmental reasons).
- Avoid carbon-fibre composite in direct contact with aluminum — CFRP is strongly cathodic. The 787 and A350 use titanium fasteners and isolation layers wherever Al-CFRP contact is unavoidable.
- Sacrificial coating (Zn-rich primer, alclad layer).
10m.5 Filiform corrosion
Worm-track corrosion under organic coatings at ~85 % humidity. Mitigation: proper pretreatment (chromate / Alodine / PAA), thicker coating, edge sealing.
10m.6 Hot cracking in welds
Wide solidification range + low-melting eutectic + restraint → solidification cracks. 2xxx particularly prone. Filler choice is everything: use 2319 for 2219, never try to fusion weld 2024 / 7075 in a service-load joint. Use FSW where possible.
10m.7 Exfoliation
Lamellar corrosion along elongated pancake grains in 7xxx and 2xxx plate. Surface jacks up like delaminating plywood. T7x tempers solve it; visual inspection identifies it (raised lines on otherwise-painted surface).
10m.8 Yield collapse and elastic buckling
Aluminum’s E = 69 GPa versus steel’s 200 GPa makes buckling the binding constraint in most aluminum compression members. Slender 6061-T6 columns reach Euler buckling well before σ_y. Slenderness ratio (KL/r) limit for aluminum is ~80 vs steel’s ~200 before plastic-zone behaviour matters. Always check buckling before yield in aluminum design (Aluminum Design Manual Section B.5).
10m.9 Creep at elevated temperature
6061-T6 above 150 °C continuous service overages → strength loss not recoverable. Above 200 °C, aluminum is essentially not a structural material; switch to titanium or refractory dispersion-strengthened Al P/M alloys.
10m.10 Mercury embrittlement
Mercury and many of its amalgams penetrate aluminum grain boundaries rapidly. Catastrophic — a single drop can disintegrate an aluminum aircraft component within hours. Aluminum is forbidden in mercury-handling and many petrochemical processes.
11. Cross-references
[[Engineering/mechanics-of-materials]]— uses Al properties (E, σ_y, ρ) in stress/strain/buckling calculations[[Engineering/materials-steel]]— sibling material; standard side-by-side comparison[[Engineering/materials-composites]]— Al-MMC (e.g., 6061/SiC_p), the boundary between metal and composite[[Engineering/materials-selection]]— Ashby method, Al vs steel vs CFRP per E/ρ, σ_y/ρ[[Engineering/joining-welding]]— Al filler selection (4043 / 5356), FSW, oxide-removal procedures[[Engineering/machining]]— free-machining Al (2011, 6262), chip control, BUE avoidance[[Engineering/Tier3/surface-treatments]]— solution + age cycles, T-temper details[[Engineering/casting-forging-forming]]— sand, die, permanent-mold processes[[Engineering/fatigue-analysis]]— Paris-law Al constants, no-endurance-limit S-N[[Engineering/additive-manufacturing]]— AlSi10Mg in DMLS, Scalmalloy[[Engineering/Tier3/surface-treatments]]— galvanic series, SCC, exfoliation[[Engineering/aerodynamics]]— Al as aerospace primary structure (2xxx, 7xxx, 8xxx Al-Li)[[Robotics/manipulator-design]]— 6061-T6 the standard for serial robot links and end-effector frames[[Robotics/mobile-base-wheeled]]— 6061 / 5052 plate and extrusion for AMR chassis[[Languages/Tier3/construction-bim]]— STEP material assignment conventions[[Languages/Tier3/industrial-automation]]— AWS D1.2 Structural Aluminum welding code
12. Citations
- Davis, J. R. (ed.) Aluminum and Aluminum Alloys. ASM Specialty Handbook (ASM International, 1993, reprinted with updates). The single-volume canonical reference on engineering aluminum.
- Polmear, I. J., StJohn, D., Nie, J.-F., Qian, M. Light Alloys: Metallurgy of the Light Metals, 5th ed. (Butterworth-Heinemann, 2017). Definitive academic text on Al, Mg, Ti metallurgy.
- Hatch, J. E. (ed.) Aluminum: Properties and Physical Metallurgy. (Aluminum Association / ASM International, 1984, reprinted 2017). The standard engineering reference.
- ASM International. ASM Handbook Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. (1990, with online updates).
- ASM International. ASM Handbook Volume 4E: Heat Treating of Nonferrous Alloys. (2016).
- Kaufman, J. G. Introduction to Aluminum Alloys and Tempers. (ASM International, 2000).
- The Aluminum Association. Aluminum Standards & Data 2024 — Wrought Products. (AA, 2024).
- The Aluminum Association. Aluminum Standards & Data 2024 — Cast Products. (AA, 2024).
- The Aluminum Association / ANSI H35.1/H35.1(M)-2017 — American National Standard Alloy and Temper Designation Systems for Aluminum.
- The Aluminum Association. Aluminum Design Manual 2020 (the structural-design code for aluminum, equivalent role to AISC 360 in steel).
- ASTM B209 / B209M-21 — Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate.
- ASTM B221 / B221M-21 — Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes.
- ASTM B247 / B247M-20 — Standard Specification for Aluminum and Aluminum-Alloy Die Forgings, Hand Forgings, and Rolled Ring Forgings.
- ASTM B308 / B308M-22 — Standard Specification for Aluminum-Alloy 6061-T6 Standard Structural Profiles.
- ASTM B557 / B557M-23 — Standard Test Methods for Tension Testing Wrought and Cast Aluminum- and Magnesium-Alloy Products.
- ASTM E8 / E8M-22 — Standard Test Methods for Tension Testing of Metallic Materials (parent tensile test method).
- ASTM E10-23 — Standard Test Method for Brinell Hardness of Metallic Materials.
- ASTM E466-21 — Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests.
- ASTM E647-15e1 — Standard Test Method for Measurement of Fatigue Crack Growth Rates.
- SAE AMS 4027N — Aluminum Alloy, Sheet and Plate, 1.0Mg - 0.60Si - 0.28Cu - 0.20Cr (6061-T6 / T651). Aerospace material spec for 6061.
- SAE AMS 4045P — Aluminum Alloy, Sheet and Plate, 5.6Zn - 2.5Mg - 1.6Cu - 0.23Cr (7075-T6 / T651). Aerospace material spec for 7075.
- SAE AMS 4218 — Aluminum Alloy, Plate, 6.2Zn - 2.3Mg - 2.3Cu - 0.12Zr (7050-T7451).
- MIL-A-8625F / SAE AMS-A-8625 — Anodic Coatings for Aluminum and Aluminum Alloys. (Anodise types I/II/III.)
- MIL-DTL-5541F — Chemical Conversion Coatings on Aluminum and Aluminum Alloys.
- International Aluminium Institute. Statistical Reports 2024. https://international-aluminium.org/statistics
- Starke, E. A., Staley, J. T. Application of modern aluminum alloys to aircraft. Progress in Aerospace Sciences (1996) — foundational aerospace selection paper, still widely cited.
- Mishra, R. S., Ma, Z. Y. Friction stir welding and processing. Materials Science and Engineering R 50 (2005) 1–78 — definitive FSW review.