Aerospace Aluminum Alloys and Tempers

The short list engineers actually use

Six aluminum alloys cover most of what reaches a machine shop from launch, spacecraft and aircraft programs. The temper is not a detail: the same alloy moves between the best and worst stress corrosion categories depending on how it was aged.

The SCC column comes straight from Tables I, II and III of NASA MSFC-STD-3029. The 2024 weldability note comes from published alloy data stating that fusion welding causes solidification defects and that the alloy is joined by friction welding. Treat the 7xxx weldability notes as general practice: most 7075 and 7050 structure is machined from plate or forged, then fastened.

Published strengths, with where each number comes from

The numbers below are a mix of typical values and specification minimums, labeled as such. Do not design to the typical columns. For analysis use MMPDS allowables or the procurement spec minimum for the exact product form and thickness on your drawing.

Two patterns are worth remembering. First, overaging 7075 from T6 to T73 costs roughly 10 ksi of yield in exchange for moving from the lowest to the highest SCC category. Second, thick plate loses strength with thickness, as the 7050 bands show, which is why a stress analyst will ask for the plate thickness the part is cut from, not the finished part thickness. Published density is about 2.70 g/cc for 6061, 2.78 g/cc for 2024 and 2.81 g/cc for 7075; modulus is about 10 million psi for 6061 and 10.4 million psi (71.7 GPa) for 7075.

Temper designations that matter on a drawing

The letters and digits after the alloy tell the supplier which stock to buy and the processor what to do. Common aerospace tempers:

• T3 and T351: solution treated, cold worked and naturally aged. The 51 suffix means stress relieved by controlled stretching, which matters for machining distortion in plate.
• T4: solution treated and naturally aged, often the as-formed state before artificial aging.
• T6 and T651: solution treated and artificially aged to peak strength; T651 is the stretched plate version.
• T73, T7351: overaged for maximum SCC resistance in 7xxx alloys.
• T74 (formerly T736), T7451: intermediate overage in 7050, more strength than T73 with moderate SCC rating.
• T76, T7651: overaged mainly for exfoliation resistance; moderate SCC rating.
• T8x: solution treated, cold worked and artificially aged, used for 2024 and 2219 products.

MSFC-STD-3029 adds that mechanically stress relieved tempers (TX5X or TX5XX) should be specified where possible, and that sheet, unmachined extrusions and unmachined plate are the most resistant forms. Machining a deep pocket into thick plate exposes short transverse grain, which is exactly where SCC starts.

Stress corrosion: the short transverse problem

SCC in 7xxx and 2xxx alloys runs along grain boundaries, so it depends on load direction relative to the elongated grains of rolled or forged stock. Loads across the thickness of plate, the short transverse direction, are the dangerous case. MSFC-STD-3029 illustrates the typical sources: interference-fit bushings, tapered pipe threads, clamp-up across a joggle or mismatch, and residual quench stress in thick shapes.

Practical rules we see engineers apply:

• Use a Table I temper (T73 class for 7075) for any fitting with a press-fit bushing, tapered thread or bolted clamp-up across the thickness.
• Call out grain direction on forgings and on plate parts with lugs, and orient lug holes so the load runs longitudinal where possible.
• Avoid shimming large gaps by bolt preload; it puts sustained tension into the part.
• Do not rely on anodize or primer to fix a susceptible temper; the standard is explicit that coatings do not change the rating.

2219, 2024 and cast A356 in more detail

2219 is an aluminum-copper alloy chosen because it can be fusion welded while still responding to heat treatment, and because it holds toughness at cryogenic temperature. That combination is why it has a long history in welded propellant tankage. Its T6 and T8 tempers sit in the highly resistant SCC group, while T3 and T4 do not, so a part that is welded and then left in an intermediate state deserves a look. Expect weld procedures and filler selection to be covered by a welding spec such as AWS D17.1 on the drawing.

2024 remains the fatigue alloy for tension-dominated skins. It has poor general corrosion resistance and is often supplied as Alclad, with a thin pure aluminum or Al-1Zn cladding, which protects the core but can reduce fatigue strength. Do not specify bare 2024-T3 for parts that will see salt air without a finish system.

A356 is the workhorse aluminum-silicon-magnesium casting alloy, normally heat treated to T6. MSFC-STD-3029 rates 356.0 and A356.0 castings highly resistant in all tempers. The usual buying issues are porosity class, radiographic inspection grade and whether the casting is premium quality with separately cast or integral test bars. Spell those out on the drawing, because a casting cert without test bar data says little about the part.

Machining, distortion and plate inspection

High-strength aluminum machines fast, so the problems are usually distortion and inspection rather than tool life.

• Buy stress-relieved plate. T651, T7351 and T7451 plate is stretched after quench. Thick plate in a non-stretched temper will move as material is removed.
• Balance material removal. Rough both sides, let the part relax, then finish. Leave stock for a skim pass on datum faces.
• Specify ultrasonic inspection of thick plate when internal defects matter. Aerospace plate is commonly sold ultrasonically tested to AMS 2154 and ASTM B594.
• Watch heat after machining. Re-heat-treating a machined 7xxx part to restore temper will cause distortion and must be done to a heat treat spec, usually by a Nadcap-accredited processor.

For welded 6061, published data shows the heat affected zone drops to roughly T4 properties, a loss of around 40 percent, and the Aluminum Design Manual recommends 24 ksi design strength next to the weld without post-weld heat treatment. Size welded GSE accordingly.

Anodize and conversion coating response

MIL-PRF-8625 (formerly MIL-A-8625) covers anodic coatings: Type I chromic acid, Type II sulfuric acid, and Type III hard anodize, with variants IB, IC and IIB. Type II coatings run roughly 1.8 to 25 microns thick and Type III is thicker than 25 microns. Oxide grows about half into the surface and half out, so a 50 micron hard anodize adds about 25 microns per surface. Hold machined dimensions with that in mind and note whether dimensions apply before or after finish.

Alloy matters. High-copper 2xxx alloys and high-silicon castings such as A356 are harder to anodize to a light or uniform color, so ask the processor for a sample or first-article coupon in the actual alloy when appearance or coating thickness is critical. 6061 gives the most predictable result. For electrical bonding or paint base, chemical conversion coating to MIL-DTL-5541 is the usual alternative. See surface finishing for process controls and inspection.

Callouts and certs to require

A complete aluminum callout lists alloy, temper, product form and procurement spec, for example 7075-T7351 plate to the applicable AMS or AMS-QQ-A spec, plus grain direction if it matters and the finish spec and class. Require a mill cert with heat or lot number, chemistry and tensile results by the spec, and confirm the temper on the cert matches the drawing. Mixed T6 and T73 stock looks identical on the rack, which is why conductivity checks and hardness checks are common receiving tests for 7xxx. The certs and traceability page covers what a good cert pack contains.

Get aluminum parts quoted

Send drawings for machined aluminum parts through a CNC quote request. We flow down your alloy, temper, grain direction and finish requirements to qualified shops, check that the cert temper matches the drawing, and deliver the part with its certificate package. Related reading: materials hub, titanium alloys and stainless and specialty steels.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Titanium Alloys for Aerospace Hardware
• Stainless and Specialty Steels for Aerospace Parts
• Material Certs, Specialty Metals and Lot Traceability

Sources

• NASA MSFC-STD-3029A, stress corrosion ratings (Tables I-B, II-C, III-B)
• ASM aerospace specification metals data sheet, 7075-T6
• 7075 aluminium alloy, tempers and properties
• 6061 aluminium alloy, minimum and typical properties, weld HAZ
• 2024 aluminium alloy, tempers, Alclad and weldability
• Alro 7050-T7451 plate data (AMS 4050, AMS 2154 ultrasonic)
• Anodizing, MIL-A-8625 types and coating growth