7079-T6
Solution heat-treated, quenched and artificially aged to peak strength. The temper named in AMS 4138. High strength, but the short-transverse SCC threshold is low.
Aerospace · Legacy forging alloy
7079 Aerospace Aluminium Forgings
7079 is a high-strength Al-Zn-Mg-Cu forging alloy that was used for heavy aircraft forgings because it hardens evenly through thick sections. It is now a legacy alloy: its poor resistance to stress-corrosion cracking led designers to replace it with 7075-T73, 7175-T74 and 7050-T74. Metline supplies 7079 forgings where a legacy drawing still requires them, and helps buyers document an approved substitute where it does not.
- Alloy
- 7079 (Al-Zn-Mg-Cu-Mn)
- Tempers
- T6, T652
- Spec
- AMS 4138 (noncurrent)
- Status
- Legacy; not for new designs
What 7079 is, and why it is now a legacy alloy
7079 contains less zinc and more magnesium than 7075, plus a manganese addition. That balance made it much less sensitive to quench rate, so thick forgings reached nearly full strength right through the section. For large landing-gear parts, wing fittings and bulkheads made before modern alloys existed, that was a real advantage.
The weakness showed up in service. In the peak-aged T6 temper, 7079 is highly susceptible to stress-corrosion cracking (SCC), especially when residual or assembly stresses act in the short-transverse direction of a forging. Cracking in fittings and gear components led airframe makers and military agencies to stop specifying the alloy. Research in the 1970s showed that newer alloys such as 7050 combined strength, toughness and SCC resistance far better than 7075-T6 or 7079-T6, and the SAE forging specification for 7079-T6, AMS 4138, is now listed as noncurrent.
Today 7079 is ordered mainly for maintenance, repair and spares for older aircraft and equipment whose type design still names it, or for reverse-engineering programmes that need the original material to match a test or qualification. For any new part, a designer should choose a modern over-aged 7xxx forging instead.
7079 chemical composition (wt %)
Limits are the registered Aluminum Association values for 7079 (UNS A97079), as used in AMS 4138.
| Element | Minimum | Maximum | Effect |
|---|---|---|---|
| Silicon (Si) | — | 0.30 | Impurity |
| Iron (Fe) | — | 0.40 | Impurity |
| Copper (Cu) | 0.40 | 0.80 | Adds strength; lower than in 7075 |
| Manganese (Mn) | 0.10 | 0.30 | Reduces quench sensitivity together with Cr |
| Magnesium (Mg) | 2.9 | 3.7 | Main strengthening partner of zinc |
| Chromium (Cr) | 0.10 | 0.25 | Grain control |
| Zinc (Zn) | 3.8 | 4.8 | Main strengthening element |
| Titanium (Ti) | — | 0.10 | Grain refiner |
| Others, each / total | — | 0.05 / 0.15 | |
| Aluminium (Al) | Remainder |
The nominal chemistry, 4.3 Zn - 3.3 Mg - 0.6 Cu - 0.2 Mn - 0.18 Cr, appears in the AMS title and is a useful check that a certificate refers to 7079 and not to 7075 (5.6 Zn - 2.5 Mg - 1.6 Cu).
Mechanical and physical properties
7079-T6 reaches strength close to 7075-T6. Its real advantage was that thick sections kept that strength, while 7075 forgings lost more in the centre. Properties in forgings depend on grain direction: longitudinal (L) is strongest, short transverse (ST) is weakest and is the direction where SCC starts.
| Property | 7079-T6 (typical) | 7075-T6 (typical, for comparison) | 7075-T73 (typical) |
|---|---|---|---|
| Tensile strength | 538 MPa / 78 ksi | 572 MPa / 83 ksi | 505 MPa / 73 ksi |
| Yield strength (0.2 %) | 469 MPa / 68 ksi | 503 MPa / 73 ksi | 435 MPa / 63 ksi |
| Elongation | 14 % | 11 % | 13 % |
| Brinell hardness | 145 HB | 150 HB | 135 HB |
| SCC resistance, ST direction | Low | Low | Very high |
Typical values from ASM data for wrought products. Specified minimums depend on forging type, thickness and test direction and are set by the purchase specification.
Physical properties (7079)
- Density
- About 2.74 g/cm³ (0.099 lb/in³)
- Modulus of elasticity
- About 71 GPa
- Melting range
- About 480 – 640 °C
- Quench sensitivity
- Low compared with 7075, so thick sections harden more evenly
7079 tempers and the stress-corrosion problem
7079-T652
Solution treated, stress-relieved by a controlled cold compression (typically 1 – 5 % permanent set) and aged. Compression reduces residual quench stresses, which lowers the SCC risk in machined parts but does not remove it.
7079-T6511 (extrusions)
A stretched temper for extruded bar and shapes, not forgings. Sometimes seen on legacy material lists alongside forgings.
Why over-ageing matters
The T73 and T74 tempers developed for 7075, 7175 and 7050 give up a little strength to raise the SCC threshold sharply. No widely used over-aged temper was established for 7079, which is one reason the alloy was dropped.
SAE ARP 823 sets out practices that reduce SCC in heat-treatable forgings: keep machining from cutting across the grain flow in highly stressed areas, avoid interference fits and clamp-up stress in the short-transverse direction, shot peen critical surfaces, and protect the part with anodising, primer and sealant. These measures are essential on any 7079 part kept in service.
7079 forging types we can supply
| Forging type | Typical use | Notes |
|---|---|---|
| Closed-die (impression) forgings | Fittings, brackets, gear components | Grain flow follows the part contour; made to a customer die or drawing |
| Open-die (hand) forgings | Blocks and blanks for machined parts | Simple shapes machined to final geometry |
| Forged bar and block | Machining stock for repair parts | Rectangular or round sections |
| Rolled rings | Rings and flanges | Seamless rings with circumferential grain |
Size ranges for 7079 depend on what the chosen forge and the specification allow, and are confirmed per enquiry. Quench-sensitivity data favour 7079 in thick sections, but the SCC risk rises with section thickness too, so very heavy legacy parts deserve careful review.
Testing and documentation for 7079 forgings
Typical requirements
- Mechanical tests
- Tensile tests in L, LT and ST directions from prolongations or sacrificial forgings
- Ultrasonic inspection
- To AMS-STD-2154 (formerly MIL-STD-2154), class per drawing
- Heat treatment
- Performed and recorded to AMS 2772 or the customer process specification
- Grain flow
- Macro-etch of a sample forging to confirm flow follows the part contour
- Hardness and conductivity
- Checked on each heat-treat lot as a process control
- Certification
- EN 10204 3.1 certificate with chemistry, heat-treat lot and test results
Modern alternatives to 7079 forgings
| Alloy and temper | Strength vs 7079-T6 | SCC resistance | Main specification |
|---|---|---|---|
| 7075-T73 / T7352 | About 5 – 10 % lower | Very high | AMS 4141 (T73 die forgings), AMS 4147 (T7352) |
| 7175-T74 / T7452 | Similar | High | AMS 4149 (T74), AMS 4179 (T7452) |
| 7050-T74 / T7452 | Similar or higher in thick sections | High | AMS 4107, AMS 4108 |
| 7085-T7452 | Higher in very thick sections | High | Customer-specific specifications |
7050 is the closest modern counterpart: like 7079 it is designed for low quench sensitivity, so thick forgings keep their strength, but it also offers good fracture toughness and SCC resistance in the T74 condition. Any change of material on an aircraft part needs approval from the type-certificate holder or the responsible engineering authority.
Where 7079 forgings were used
- Landing gear: cylinders, trunnions and links on older aircraft
- Wing and fuselage fittings: spar fittings, attach lugs and heavy bulkhead sections
- Flight controls: bellcranks, actuator brackets and control arms
- Helicopter and defence equipment: fittings and housings on legacy platforms
A typical current order is a short batch of closed-die fittings to replace parts on an older aircraft, delivered with full test results and heat-treat records so the operator can show the replacement matches the original material.
How to order 7079 forgings
- SpecifyAlloy and temper, forging type, drawing or envelope dimensions, quantity and the governing specification with its revision letter (AMS 4138 or the legacy drawing specification). Add any customer or prime-contractor material specification that sits on top of the AMS.
- ConfirmWe confirm the specification can be met, the tolerance class, any extra tests (ultrasonic class, conductivity, fracture toughness, grain flow) and the delivery terms (EXW, FOB, CIF or DAP).
- InspectMaterial is checked for dimensions, surface condition and marking; tests named on the order are carried out or arranged with an independent laboratory.
- DocumentEach lot ships with a mill test certificate to EN 10204 3.1 giving chemistry, mechanical results and heat-treatment lot, traceable to the cast number.
- Pack and shipExport packing in sealed, moisture-protected wooden cases or on pallets, shipped by sea or air to your port or door.
References and standards
- SAE AMS 4138 – Forgings, 4.3Zn - 3.3Mg - 0.60Cu - 0.20Mn - 0.18Cr (7079-T6)
- SAE ARP 823 – Minimizing Stress-Corrosion Cracking in Wrought Heat-Treatable Aluminium Alloy Products
- SAE AMS 2772 – Heat Treatment of Aluminium Alloy Raw Materials
- AMS-STD-2154 – Inspection, Ultrasonic, Wrought Metals
- The Aluminum Association – International Alloy Designations and Chemical Composition Limits (Teal Sheets)
- ASM Handbook, Vol. 2 – Properties and Selection: Nonferrous Alloys and Special-Purpose Materials
7079 aerospace aluminium (aluminum) forgings: FAQ
Is 7079 aluminium still produced?
Only in small quantities for legacy requirements. Its forging specification, AMS 4138, is noncurrent, and most mills no longer cast the alloy routinely. Availability should be confirmed per enquiry.
Why was 7079 replaced?
In the T6 temper it is very susceptible to stress-corrosion cracking in the short-transverse direction. Over-aged 7075-T73, 7175-T74 and 7050-T74 forgings offer similar or slightly lower strength with far better SCC resistance.
What is the best substitute for a 7079-T6 forging?
For thick forgings, 7050-T74 or T7452 is usually the closest match because it also has low quench sensitivity. 7175-T74 and 7075-T73 suit thinner parts. The substitution must be approved by the responsible design authority.
What does T652 mean on a 7079 forging?
Solution heat-treated, stress-relieved by controlled compression and then artificially aged. The compression lowers residual quench stresses, which reduces distortion when machining and lowers SCC risk.
Is 7079 weldable?
No. Like other high-strength 7xxx alloys, it cracks and loses strength when fusion welded. Parts are joined mechanically.
Can Metline ship 7079 forgings internationally?
Yes. We supply aerospace forgings worldwide from Mumbai with EN 10204 3.1 certificates, test reports and export packing.
Replacing a 7079 forging?
Send the drawing and specification. We will confirm whether 7079 can be supplied and suggest a documented modern alternative.
