How is 7075 aluminum heat treated?
By solution treating at about 480 °C (900 °F), quenching and aging: 24 hours at 120 °C (250 °F) gives T6, and a two-step over-age gives T73 or T7351 for stress-corrosion resistance at about 10-15% less strength.
7075 is a precipitation-hardening alloy: its strength comes from fine zinc-magnesium (MgZn2) precipitates grown by controlled aging. The cycle starts with solution treatment at about 465-480 °C (870-900 °F), long enough to dissolve the alloying elements (roughly 30-60 minutes for thin sections, longer for heavy ones), followed by a fast quench, normally in water below 40 °C (100 °F). The quench is the sensitive step: 7075 is quench-rate sensitive, and a slow quench on thick plate or a part with mixed sections gives lower strength and worse corrosion resistance in the slowly cooled regions. Freshly quenched 7075 is soft (W temper) and begins to age naturally within hours, so any forming or straightening is done immediately.
Aging sets the temper. T6 is a single-step artificial age of about 24 hours at 120 °C (250 °F), giving the peak strength of roughly 570 MPa (83 ksi) tensile and 505 MPa (73 ksi) yield with 11% elongation in plate. The price of peak strength is stress-corrosion cracking (SCC) sensitivity in the short-transverse grain direction and exfoliation corrosion in thick sections. T73 (or T7351 for stretched plate) over-ages in two steps, typically 6-8 hours at 105-110 °C (225-230 °F) then 24-30 hours at 160-165 °C (320-330 °F), trading about 10-15% of strength for a large gain in SCC resistance: T73 tensile is about 505 MPa (73 ksi) with 435 MPa (63 ksi) yield. T76 and T7651 sit between the two, aimed at exfoliation resistance in sheet and extrusions. The T651 / T7351 suffixes mean the plate was stretched 1.5-3% after quenching to relieve quench stress; specify them for anything that will be machined deeply.
Re-heat-treating is possible but needs care. A part that has been welded, formed in the O condition or over-aged can be solution treated again and re-aged, but each solution treatment coarsens the grain a little, and the quench distorts finished parts; it is done on raw stock or rough-machined blanks, not on finished machined parts. The reverse conversion, T6 to T73, is a common field fix for SCC concerns: age the T6 part further at about 160-175 °C (325-350 °F) without re-solution treating; the resulting condition is close to T73 and is often written T7X with the engineering allowance. Annealing (O) is a 2-3 hour soak at about 415 °C (775 °F) with a slow, controlled cool, done to make the alloy formable before a fresh solution treatment. All of this is covered by AMS 2770 for parts and AMS 2772 for raw material, which are the specs to cite.
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What to specify
- Temper on the drawing, not just the alloy: 7075-T6 / T651 for strength, T73 / T7351 for stress-corrosion and thick-section service, T76 / T7651 for exfoliation resistance
- Heat treatment per AMS 2770 (parts) or AMS 2772 (raw material), and the mechanical property spec (for example AMS 4045, AMS 4078, ASTM B209)
- Stress-relieved product (T651, T7351, T6511) for plate and bar that will be machined deeply
- Whether the part is heat treated before or after machining; if after, state the allowed distortion and the straightening limits
- Hardness or conductivity verification: eddy-current conductivity (about 30-36% IACS for T6, 38-42% IACS for T73) is the usual temper check, per ASTM E1004
- Quench and section thickness limits if the part has heavy sections or mixed thicknesses
Pitfalls
- Using T6 where the part carries sustained tension across the short-transverse grain; stress-corrosion cracks start at machined bores and fillets
- Slow quenching heavy sections or a full basket: lower strength and worse corrosion resistance in the interior, and no visible sign of it
- Forming after natural aging has begun: W-temper 7075 gets stiffer by the hour and cracks in a bend that was fine that morning
- Powder-coat or paint cures above about 150 °C (300 °F) and long soaks in service over 120 °C (250 °F) over-age T6 toward T73 strength
- Re-solution treating a finished part: it warps in the quench and the surface grows a coarse-grain layer that machines poorly
- Reading conductivity alone as proof of temper: pair it with hardness or a tensile coupon when the temper matters structurally
Frequently asked questions
- What is the difference between 7075-T6 and 7075-T73?
- T6 is peak-aged for maximum strength, about 570 MPa (83 ksi) tensile, but is prone to stress-corrosion cracking. T73 is over-aged in two steps, about 10-15% weaker, and much more resistant to stress corrosion and exfoliation. T651 and T7351 are the same tempers with stress relief by stretching.
- Can 7075 be re-heat-treated?
- Yes, from any condition: solution treat at about 480 °C (900 °F), quench and re-age. Do it on blanks, not finished parts, because the quench distorts. Converting T6 to a T73-like condition needs only an extra aging step at about 160-175 °C (325-350 °F).
- Can 7075 be annealed?
- Yes. Soak 2-3 hours at about 415 °C (775 °F) and cool slowly at no more than about 30 °C (50 °F) per hour to 230 °C (450 °F). The O temper is soft enough to form, and a fresh solution treatment and age bring it back to T6.
- How do I check the temper of a 7075 part?
- Eddy-current conductivity plus hardness is the standard non-destructive check: about 30-36% IACS and 150 HB for T6, 38-42% IACS for T73. A tensile coupon from the same heat is the only definitive check when strength is at stake.
