What 7075-T651 Aluminum Alloy Is
7075-T651 is a high-strength, zinc-based wrought aluminum alloy of the 7xxx series. It is chosen wherever a part must carry heavy static and cyclic loads at the lowest possible weight, which is why aircraft frames, gears and precision structural fittings are its classic applications. Zinc is the principal alloying element, supported by magnesium, copper and chromium, so that precipitation hardening pushes strength into a range normally associated with some structural steels while the material stays about one third as dense.
Plate, bar and extruded stock in this grade machine cleanly and hold tight tolerances. The trade-off is moderate corrosion resistance: the copper-rich chemistry makes the alloy less suitable than 5xxx or 6xxx grades for prolonged saltwater or chemical exposure unless the surface is protected by anodizing, painting or a conversion coating.
What the T651 Temper Actually Delivers
The T651 designation records a defined sequence of operations: solution heat treatment, controlled artificial aging to the T6 condition, and relief of internal stress by stretching. Stretching is the step that separates T651 from plain T6, because it removes a large share of residual quenching stress and therefore improves flatness when heavy cuts are taken from a thick plate.
Solution heat treatment dissolves the alloying elements into a single-phase structure.
Artificial aging precipitates fine particles that restrict dislocation movement and raise strength.
Stress relief by stretching improves dimensional stability during machining and reduces movement after forming.
The semi-smooth mill finish is uniform and dull, and it accepts anodizing or coating without extra preparation.
Chemical Composition
| Element | Content (%) | Role |
|---|---|---|
| Aluminum (Al) | Balance, approximately 90 | Matrix |
| Zinc (Zn) | 5.1 - 6.1 | Principal strengthening element |
| Magnesium (Mg) | 2.1 - 2.9 | Forms strengthening precipitates with zinc |
| Copper (Cu) | 1.2 - 2.0 | Raises strength and hardness |
| Chromium (Cr) | 0.18 - 0.28 | Refines grain structure and aids stress corrosion resistance |
| Iron (Fe) | 0.50 max | Residual element |
| Silicon (Si) | 0.40 max | Residual element |
| Manganese (Mn) | 0.30 max | Controls grain growth |
| Titanium (Ti) | 0.20 max | Grain refinement |
| Others, each | 0.05 max | Residual limits |
| Others, total | 0.15 max | Residual limits |
These limits correspond to the standard wrought 7075 chemistry for sheet and plate supplied to ASTM B209. Residual elements are controlled both individually and in total so that fracture toughness remains predictable from heat to heat.
Mechanical Properties
| Property | Imperial | Metric |
|---|---|---|
| Density | 0.102 lb/in³ | 2.81 g/cm³ |
| Ultimate tensile strength | 83,000 psi | 572 MPa |
| Yield strength, 0.2% offset | 73,000 psi | 503 MPa |
| Fatigue strength | 23,000 psi | 159 MPa |
| Shear strength | 48,000 psi | 331 MPa |
| Melting range | 890 - 1,175 °F | 477 - 635 °C |
| Hardness | 87 HRB | 150 HB |
| Temper | T651 | |
Two derived figures matter as much as the headline numbers. The first is the strength-to-weight ratio: 572 MPa at 2.81 g/cm³ means a component can be designed roughly three times lighter than an equivalent carbon steel part of the same strength class. The second is the yield ratio, the 0.2% offset yield strength divided by the tensile strength, which for this alloy is typically 0.85 - 0.88. A high yield ratio leaves little reserve between first yield and fracture, so designers size parts on yield and apply a generous safety factor against the fatigue limit of 159 MPa.
Where 7075-T651 Is Used
Aircraft and aerospace structures, wing fittings and critical load-bearing components
High-stress mechanical parts such as gears, shafts, sprockets and cams
Bicycle frames and components, climbing hardware and military equipment
Plastic mold tooling and industrial tooling where stiffness and wear resistance matter
Because the alloy is not intended for continuous immersion or hot service, designers normally pair it with a protective finish and keep operating temperature well below the aging temperature of the temper.
Frequently Asked Questions
Q: What is 7075-T651 aluminum used for?
Aerospace and aircraft structural components, high-stress gears and shafts, bicycle and climbing hardware, and plastic mold tooling. It suits parts that need maximum strength at minimum weight.
Q: Is 7075-T651 the same as 7075-T6?
The chemistry is identical; only the temper differs. T651 adds stress relief by stretching after aging, which improves dimensional stability and flatness for heavy machining.
Q: What is the density of 7075-T651?
About 2.81 g/cm³, or 0.102 lb/in³. That is roughly one third the density of carbon steel, which is the source of its strength-to-weight advantage.
Q: Can 7075-T651 be welded?
Fusion welding is not recommended because the copper-bearing 7xxx chemistry is prone to hot cracking and loses strength in the weld zone. Riveting, bolting and adhesive bonding are the usual joining methods.
Q: What is the yield ratio of 7075-T651?
Typically 0.85 - 0.88, calculated as the 0.2% offset yield strength divided by the ultimate tensile strength. It indicates limited ductility reserve, so fatigue and notch sensitivity need attention in design.
Q: How well does 7075-T651 resist corrosion?
Only moderately. It performs acceptably in normal indoor service but is not recommended for extended saltwater exposure. Anodizing, painting or a conversion coating is normally required.
