Almost every enquiry about high-strength aluminium comes down to two decisions: which alloy series to use, and which temper to order. Getting either one wrong is expensive. A 7000 series alloy in the wrong temper can lose a large part of its ductility in service, and a 2000 series alloy chosen purely for strength will pit badly in a marine atmosphere. This guide covers the grades, the numbers and the limits, so the specification is written correctly the first time.
At Plus Metals we supply high-strength aerospace aluminium alloys as bar, rod, plate and sheet, supplied against the grade, temper and standard stated on the enquiry.
What "High-Strength" Actually Means
Commercial aluminium is soft and ductile, with a tensile strength near 40 MPa and a density of about 2.70 g/cm³. Strength is added in two fundamentally different ways, and the distinction decides everything that follows.
- Work hardening (strain hardening) — cold working deforms the crystal structure and makes the metal harder and stronger. The 5000 series (Al-Mg) is strengthened this way. It cannot be restored by heat treatment, and it does not gain strength with age.
- Precipitation hardening (heat treatment) — a supersaturated solid solution is heated, quenched, then aged so that fine precipitate particles block dislocation movement. The 2000, 6000 and 7000 series are strengthened this way. It allows very high strength together with good machinability, but only within a defined temper.
An aerospace 7000 series alloy reaches roughly 570 MPa in the T651 temper at a density of 2.81 g/cm³ — roughly fourteen times the strength of pure aluminium for a 4% weight penalty. That ratio is the entire reason the 2000 and 7000 series exist.
The Four Main Series at a Glance
| Series | Principal alloying elements | How it is strengthened | Strength level | Corrosion behaviour | Weldability |
|---|---|---|---|---|---|
| 2000 (Al-Cu) | Copper, with magnesium, manganese, sometimes nickel and titanium | Precipitation hardened | High | Poor unless clad or anodised | Limited |
| 5000 (Al-Mg) | Magnesium | Work hardened, not heat treatable | Moderate | Excellent — the marine standard | Good, but strength is lost in the weld |
| 6000 (Al-Mg-Si) | Magnesium and silicon | Precipitation hardened | Moderate to high | Good | Good |
| 7000 (Al-Zn-Mg-Cu) | Zinc, magnesium and copper | Precipitation hardened | Highest available | Poor; temper choice is critical | Poor |
The trade-off is consistent across the whole table. The alloys that give the highest strength are the ones that give up corrosion resistance and weldability. Aluminium has no chromium-rich passive film like stainless steel, so its corrosion behaviour is set mostly by how much copper and zinc it contains — and those are exactly the elements that make it strong.
2000 Series (Al-Cu): Structural and Forging Alloys
Copper is the primary strengthener in the 2000 series. It produces high tensile strength and excellent fatigue performance, but it also raises galvanic corrosion risk against other aluminium grades and steels, and the alloy needs protection from the atmosphere.
- 2011 — Al-Cu with a high free-machining addition. Chosen where machining volume dominates: fasteners, fittings and small components. Strength is moderate and corrosion resistance is poor, so it is normally used in non-exposed or plated parts.
- 2014 — Al-Cu for forged and machined parts. Higher strength than 2011, widely used for wheel and structural components and for general forging stock.
- 2017 — Al-Cu-Mg with better corrosion resistance than 2014 while retaining strength. Used for aerospace structures and as a core alloy under cladding.
- 2024 — Al-Cu-Mg with very high fatigue strength and good tensile strength. The classic aircraft structural alloy, but its corrosion resistance is poor, so it is normally supplied clad or anodised.
- 2219 — Al-Cu-Mn-Ti-V. One of the strongest 2000 series grades, with noticeably better corrosion resistance than 2024. Used for high-stress structural and rocket applications, frequently as the core of clad plate.
- 2618 — Al-Cu-Mg-Ni-Fe. Developed for elevated-temperature structural duty, retaining strength better than most aluminium at moderate heat while carrying excellent fatigue performance. Used in aircraft wing and fuselage structures.
Cladding: Why "Alclad" Is Part of the Grade
Cladding matters more than most buyers expect. Clad plate places a thin, corrosion-resistant layer of pure or 6000 series aluminium on each face of a high-strength 2000 or 7000 series core. The core supplies the strength, the skins supply the environmental protection, and the bond is metallurgical rather than mechanical.
Two practical points follow. First, when a drawing states "2024-T3 Alclad" or "7075-T6 Alclad", the cladding alloy and cladding thickness are part of the grade and must be quoted. Second, for two-sided 7075 cladding the nominal cladding thickness is about 4% of the gauge on sheet below 0.062 in (1.57 mm), and about 2.5% on gauges above it — so a clad order is incomplete without the side count and the cladding grade.
7000 Series (Al-Zn-Mg-Cu): The Highest-Strength Alloys
Zinc is the main strengthening element in the 7000 series, with magnesium and copper completing the precipitation reaction. These alloys deliver the highest tensile strengths commercially available, but they are also the most corrosion-sensitive and the least weldable of the major aerospace alloys.
- 7050 — Al-Zn-Cu with good fracture toughness and markedly better stress-corrosion cracking resistance than 7075. A common structural alternative where SCC is the governing concern.
- 7068 — among the highest-strength aluminium alloys produced, with exceptional tensile strength for weight. Aerospace structures where absolute strength governs.
- 7075 — the benchmark high-strength aluminium alloy and the highest-strength of the common screw-machining alloys. Excellent strength and fatigue, but poor corrosion resistance in the T6 condition and poor weldability. Usually supplied as Alclad or in a T73-type temper.
- 7175 — a higher-strength 7075 derivative used in aerospace structural and press-form applications, with good crack growth resistance.
- 7449 — a high-strength 7000 series grade developed for structural duties where strength and damage tolerance are balanced.
- 7475 — Al-Zn-Cu combining high strength with better fracture toughness and improved SCC resistance than 7075. Used for aircraft primary structures.
A useful substitution rule follows from the temper data below: because 7075-T73 and 7075-T7351 have superior stress-corrosion cracking resistance, they are frequently used as a direct replacement for 2024, 2014 and 2017 in critical applications where the 2000 series alloy would otherwise need protection.
7075 Chemical Composition
| Element | Minimum % | Maximum % |
|---|---|---|
| Aluminium | Balance | |
| Zinc | 5.10 | 6.10 |
| Magnesium | 2.10 | 2.90 |
| Copper | 1.20 | 2.00 |
| Chromium | 0.18 | 0.28 |
| Titanium | — | 0.20 |
| Iron | — | 0.50 |
| Silicon | — | 0.40 |
| Manganese | — | 0.30 |
| Other elements, each | — | 0.05 |
| Other elements, total | — | 0.15 |
The chromium addition, restricted to a narrow 0.18–0.28% band, is deliberate. It forms fine particles that inhibit grain-boundary attack, which is what gives the 7000 series better stress-corrosion cracking behaviour than earlier high-strength alloys. Titanium also forms fine particles that control grain structure during solution treatment.
7075 Temper Data
The following are typical values for rod and bar, from a 0.500 in (12.7 mm) diameter specimen. They are useful for comparing tempers but they are not specification minimums — see the sheet and plate table below for those.
| Property | 7075-O | 7075-T6 / T651 | 7075-T73 / T7351 |
|---|---|---|---|
| Tensile strength, ultimate | 228 MPa (33 ksi) | 572 MPa (83 ksi) | 503 MPa (73 ksi) |
| Yield strength, 0.2% offset | 103 MPa (15 ksi) | 503 MPa (73 ksi) | 434 MPa (63 ksi) |
| Elongation in 4D | 17% | 11% | 13% |
| Brinell hardness (500 kg, 10 mm) | 60 | 150 | 135 |
| Ultimate shearing strength | 152 MPa (22 ksi) | 331 MPa (48 ksi) | 303 MPa (44 ksi) |
| Fatigue endurance limit, 5 × 10⁸ cycles | 71 MPa (10.3 ksi) | 158 MPa (23 ksi) | 158 MPa (23 ksi) |
| Modulus of elasticity | 71.0 GPa | 71.0 GPa | 71.0 GPa |
Why Sheet and Plate Numbers Are Lower
This is the single most common source of confusion on an aluminium enquiry. The values above are rod and bar; sheet and plate carry lower minimums, and the minimum falls as thickness increases. The figures below are the standard minimum long-transverse mechanical properties for sheet and plate.
| Temper | Thickness (in / mm) | Tensile (ksi / MPa) | Yield (ksi / MPa) | Elongation |
|---|---|---|---|---|
| O | 0.015–2.00 / 0.38–50.8 | 40 max (276) | 21 max (145) | 9–10% |
| T6 (sheet) | 0.008–0.249 / 0.20–6.32 | 74–78 (510–538) | 63–69 (434–476) | 5–8% |
| T651 (plate) | 0.250–4.000 / 6.35–101.6 | 78–67 (538–462) | 67–54 (462–372) | 9–3% |
| T76 (sheet) | 0.125–0.249 / 3.18–6.32 | 73 (503) | 62 (427) | 8% |
| T7651 (plate) | 0.250–1.000 / 6.35–25.4 | 72–71 (496–490) | 61–60 (421–414) | 8–6% |
| T73 (sheet) | 0.040–0.249 / 1.02–6.32 | 67 (462) | 56 (386) | 8% |
| T7351 (plate) | 0.250–4.000 / 6.35–101.6 | 69–61 (476–421) | 57–48 (393–331) | 7–6% |
Two consequences matter commercially. A 25 mm 7075-T651 plate is not permitted to deliver the 572 MPa typical bar figure — the standard minimum is 538 MPa at 6.35 mm and falls towards 462 MPa at 101.6 mm. And elongation falls from 9% to 3% over the same thickness range, which affects forming and any fracture-tolerant design.
Typical physical constants for 7075: density 2.81 g/cm³, elastic modulus 71.7 GPa, melting range approximately 477–635 °C, coefficient of thermal expansion about 23.5 µm/m·°C, thermal conductivity about 130 W/m·K.
Temper Selection: The Part Most Often Missed
The alloy tells you what an alloy can do. The temper tells you what this delivery will do. Ordering "7075" without a temper is an incomplete specification — O, T6 and T73 differ by more than a factor of two in strength.
- O — annealed, fully soft. Low strength, high ductility, corrosion resistant and easily formed. Used as the starting state when the customer intends to solution-treat and age the part themselves.
- T4 — solution heat-treated and naturally aged. Strength and ductility reasonably balanced; the common condition for parts that will be formed.
- T6 — solution heat-treated and artificially aged. Peak strength, but the lowest ductility of the common tempers and the highest SCC susceptibility. Typically produced by homogenising the cast at about 450 °C, quenching, then ageing at about 120 °C.
- T73 / T7351 — over-aged. Strength roughly 12% below T6, but substantially better SCC resistance and slightly higher elongation. This is the SCC-resistant temper.
- T76 / T7651 — over-aged with a different ageing cycle. This is the exfoliation-resistant temper, preferred for sheet where exfoliation is the concern.
Within the 7000 series, resistance to general corrosion attack, SCC and exfoliation improves significantly in the over-aged T7-type tempers compared with the peak-strength T6 condition. Where the environment is corrosive and the load is sustained, T73 or T7351 is normally the correct specification.
Temper suffixes are dimensional or processing information rather than property claims. T51 denotes the specific stress-relieving treatment applied to plate or bar after ageing. The double-asterisk convention (T651**, T6**) means the standard minimum is not met by the standard product and the actual agreed value is stated on the certificate.
Corrosion Behaviour and Marine Exposure
The generalisation that "aluminium is corrosion resistant" is what gets 7075-T6 specified into marine hardware and then rejected. Stress-corrosion cracking (SCC) is the failure mode to design against: it occurs when a susceptible temper is held under sustained tensile load in a corrosive atmosphere, and the component does not need to look corroded first. Long-term marine exposure data for 7075 show tensile strength falling only around 3% over twelve months while elongation falls by roughly 58% — the material becomes brittle and crack-sensitive long before any visible damage appears.
Note also that 7075 sheet and plate products are not supplied with guaranteed minimum fracture toughness values. Where damage tolerance governs the design, that has to be established by the design authority rather than inferred from tensile figures.
Three mitigations, in order of preference:
- Order a T73 or T7351 temper instead of T6, or T76/T7651 where exfoliation is the concern.
- Use clad material (Alclad), which isolates the high-strength core from the electrolyte.
- Anodise or apply a protective coating system, and isolate galvanic couples. Aluminium is anodic to steel, so in a dissimilar-metal joint the aluminium corrodes at the interface.
For continuous immersion in seawater, a 5000 series alloy is normally the correct engineering answer rather than a high-strength 2xxx or 7xxx grade. High-strength grades belong in aerospace, defence and load-bearing structures where strength-to-weight governs and the environment is controlled or protected.
Fabrication Notes
- Machining — 7075 machines well in the annealed and aged conditions, and little or no warpage occurs during ageing. Use sharp tooling, positive rake and abundant coolant. Rigid workholding matters because the modulus is only about one third of steel, so the material deflects rather than cuts.
- Welding — the 7000 series is rated poorly weldable. It machines well but has poor resistance weldability and the weld metal loses strength. Design should minimise welds in primary 7xxx structure, and heat-affected zone properties must be verified against the governing specification.
- Forming — formability is directly related to temper strength and ductility. The material forms well in O, F and W tempers and is more difficult in the heat-treated condition; for complex parts the normal practice is to form annealed sheet and heat treat after forming.
- Anodising and brazing — 7075 has only a fair anodising response and is rated poorly for brazing, so both processes need qualification rather than assumption.
- Notch sensitivity — 7075 exhibits some degree of notch sensitivity, so stress concentrations deserve more attention than the nominal tensile figures would suggest.
Applicable Standards
| Product form | Typical ASTM / AMS specification |
|---|---|
| Sheet and plate | ASTM B209; AMS 4027, AMS 4028 |
| Extruded bar, rod and profiles | ASTM B221 |
| Drawn rod and bar | ASTM B211 |
| Clad sheet and plate | ASTM B547 |
| Forging stock | ASTM B247 |
| Common AMS bar / forging specs for 7075 | AMS 4122, AMS 4123, AMS 4124, AMS 4166–4169, AMS 4187 |
AMS specifications control the aerospace route in addition to ASTM. They carry tighter chemistry tolerances, mandatory mechanical properties and defined product forms, and they must be stated explicitly — material certified only to ASTM B209 is not automatically acceptable to an aerospace buyer or its first-tier suppliers. Note that B209 covers sheet and plate while B221 and B211 cover extruded and drawn bar respectively; specifying B221 for a plate order is a common error. On export, the equivalent EN designation is also commonly requested.
Typical Applications by Grade
- Aerospace structures — upper and lower wing skins, spars, ribs and stringers in 2024, 7075, 7050, 7475 and 2219, frequently as clad material.
- Landing gear and undercarriage — forged and machined high-strength grades.
- Fasteners and fittings — 2011, 2219, 7075 and 7050.
- Missile and launch vehicle structures — 2219, 2618 and 7075.
- Elevated-temperature structural duty — 2618 and 7050.
- High-performance automotive and motorsport — 2000 and 7000 series for chassis, suspension and wheel components.
The demand driver is straightforward: for aerospace and road vehicles, fuel burn scales directly with mass, so a 4% weight saving buys far more than a 4% strength gain would. That is why 2000 and 7000 series aluminium remains the backbone of airframe and missile structures despite being less corrosion resistant and less weldable than the lower-strength alternatives.
Frequently Asked Questions
Q1. What is the difference between 2000 and 7000 series aluminium?
Both are high-strength precipitation-hardened series, but they are strengthened by different elements. The 2000 series uses copper as the primary strengthener (2024, 2219, 2618) and generally gives better weldability and slightly higher absolute strength. The 7000 series uses zinc with magnesium and copper (7075, 7050, 7475) and gives better fracture toughness and stress-corrosion cracking resistance in the T7 tempers, but has poorer corrosion resistance and weldability. 2000 series is more common in fuselage and missile structures; 7000 series dominates wing spars, ribs and stringers.
Q2. Which 7075 temper should I order?
It depends on the environment and the load. T6 / T651 for peak strength in a benign or protected environment. T73 / T7351 for sustained loads in corrosive or marine service — it sacrifices about 12% of tensile strength for substantially better SCC resistance. T76 / T7651 where exfoliation rather than SCC governs. T4 when the part will be formed before ageing, and O when you intend to heat treat it yourself.
Q3. Why are the minimum strengths for 7075 sheet and plate lower than for bar?
They are different products with different specifications. The 572 MPa figure for T6/T651 is a typical value for rod and bar from a 12.7 mm specimen, not a specification minimum. Sheet and plate are governed by separate ASTM B209 minimums that fall as thickness increases — T651 plate runs from 538 MPa at 6.35 mm down to 462 MPa at 101.6 mm, with elongation dropping from 9% to 3% over the same range. Always state the product form when you need a specific guaranteed value.
Q4. Can 7075 be used in marine environments?
Only with care, and never in the T6 temper under sustained load. Marine exposure data show 7075 losing roughly 58% of its elongation over twelve months even though tensile strength falls only about 3% — it becomes crack-sensitive without looking corroded. Specify T73/T7351, or use Alclad, or specify a 5000 series alloy. For continuous seawater immersion a 5000 series alloy is the correct engineering answer, not a high-strength 2xxx or 7xxx grade.
Q5. What information should I include in an enquiry for high-strength aluminium?
Five items determine the correct material: the alloy and temper (e.g. 7075-T651, not just "7075"), the product form (bar, rod, plate or sheet) since the applicable standard and minimums differ, the dimensions, the governing specification (ASTM B209 / B221 / B211, AMS number, or EN designation), and the service environment including whether stress-corrosion cracking or marine exposure governs. Adding cladding grade and side count completes a clad order.
Why Choose Plus Metals for High-Strength Aluminium?
- Grade and temper stated on the certificate
Material is supplied against the alloy, temper, product form and standard written on your enquiry — not a nearest-equivalent substitution. - Form-correct sourcing
Bar, rod, plate and sheet are certified to the specification that actually governs each form, so a plate order is not shipped against an extrusion spec. - Clad material supplied as specified
Alclad orders are quoted with the cladding alloy and side count stated, not left to interpretation at inspection. - Documentation
Material Test Certificates under EN 10204 3.1 as standard, with chemical analysis and mechanical results reported against the specified standard. - Export experience
Aluminium alloys shipped to Europe, the Middle East and Asia with the paperwork, packing and labelling required for import clearance.
Order High-Strength Aluminium Alloys from Plus Metals
Send us the alloy, temper, form, dimensions, standard and service conditions, and we will confirm availability and the governing certification. Standard stock grades and custom-cut sizes are both quoted against the same specification discipline.
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