When selecting aluminum alloys for precision machining, structural fabrication, or deep stamping, understanding heat treatment tempers—specifically stress-relieved designations like T351 and T651—is critical. A common source of confusion in metallurgical specifications is applying heat-treatment temper designations to non-heat-treatable alloys, such as comparing 5000 series and 6000 series aluminum coils.
Aluminum alloys fall into two primary structural categories: non-heat-treatable alloys (5000 series) and heat-treatable alloys (6000 series). Heat-treatment tempers (T351 and T651) strictly apply to heat-treatable alloys, whereas strain-hardening tempers (H-series) govern non-heat-treatable aluminum.
Metallurgical Categorization: 5000 vs. 6000 Series Alloys
To select the right temper and alloy combination, it is essential to distinguish between solution heat treatment and strain hardening mechanisms.
| Metallurgical Property | 5000 Series Aluminum Coils (e.g., 5052, 5083, 5754) | 6000 Series Aluminum Coils (e.g., 6061, 6063, 6082) |
| Alloy Family | Aluminum-Magnesium (Al-Mg) | Aluminum-Magnesium-Silicon (Al-Mg-Si) |
| Strengthening Mechanism | Solid solution hardening & Strain hardening (H-tempers) | Precipitation hardening (Age hardening via T-tempers) |
| Heat Treatment Response | Non-Heat-Treatable (Does not respond to quench/age cycles) | Heat-Treatable (Responds to solutionizing & aging) |
| Standard Tempers | O (Annealed), H111, H32, H34, H116, H321 | O, T4, T6, T351, T651 |
| Primary Applications | Marine hulls, fuel tanks, pressure vessels, automotive body panels | Structural framing, aerospace components, machined block plates |
1. Decoding the Tempers: What Do T351 and T651 Mean?
In heat-treatable aluminum alloys, temper designations denote a specific sequence of solution heat treating, quenching, cold working, and artificial aging. The digits following the main temper code designate stress relief by controlled stretching.
[ Solution Heat Treatment & Quench ]
|
v
[ Controlled Stretching (1% - 3% Permanent Set) ] <-- Indicated by digit "51"
|
+-------------------+
| |
v v
[ Natural Aging ] [ Artificial Aging (Heat Oven) ]
| |
v v
**T351** **T651**
A. The T651 Temper (T6 + 51)
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Process Breakdown: Solution heat-treated → Stress-relieved by stretching (1%−3% permanent deformation) → Artificially aged (heated in an oven to form Mg 2Si precipitates).
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Key Characteristics: Delivers maximum yield and tensile strength alongside exceptionally low residual stress.
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Machining Advantage: When deep machining, slotting, or milling high-strength plate or thick coil stock, standard T6 aluminum can warp or pinch cutting tools due to locked-in thermal stress from quenching. T651eliminates internal stress gradients, ensuring high dimensional stability during aggressive CNC milling.
B. The T351 Temper (T3 + 51)
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Process Breakdown: Solution heat-treated → Cold worked by stretching (1%−3%) → Naturally aged at room temperature.
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Key Characteristics: Offers higher ductility and fracture toughness than artificially aged tempers, though with slightly lower yield strength than T651.
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Primary Alloys: Commonly applied to 2000 series (Al-Cu) and 7000 series (Al-Zn), as well as select 6000 series applications requiring high damage tolerance before final forming.
2. Temper Misconceptions in 5000 Series Aluminum Coils
A frequent error in engineering drawings is requesting a "5083-T651" or "5052-T351" coil. Because 5000 series alloys do not contain significant precipitation-hardening elements like silicon or copper, they cannot be temper-designated as T351 or T651.
Instead, 5000 series aluminum relies on work hardening(H-tempers)or specialized thermal stabilizing:
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H111 Temper: Mildly strain-hardened during final levelling or coil processing; offers excellent formability for bending and deep drawing.
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H111 Temper: Strain-hardened and thermally stabilized by low-temperature heating to prevent age-softening over time.
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H116/H321 Tempers: Specially processed high-magnesium alloys (>3.0% Mg, such as 5083 and 5086) engineered to resist intergranular corrosion and stress-corrosion cracking (SCC) in marine environments.
3. Machining and Dimensional Stability Comparison
When processing aluminum coils into flat sheets or precision components, internal residual stress dictates whether a part remains flat or warps after processing.
[ Standard Quenched Coil (T6) ] [ Stress-Relieved Coil (T651) ]
Unbalanced Residual Stress Balanced / Minimal Stress
+--------------+ +--------------+
|---- -> <- ---| (Internal tension) |--------------| (Flat & stable)
+--------------+ +--------------+
Result: Warping after milling Result: Maintains flatness after milling
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Standard $T6$ Coil / Sheet: Quenching rapid-cools the outer surface faster than the core, locking in severe internal compression and tension zones. As CNC machinery removes surface metal, these internal stresses re-balance, causing the finished part to bow or twist.
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Stretching (T651 / T351): Mechanical stretching pulls the metal past its yield point, redistributing and neutralizing internal strain profiles. Parts cut from T651 coil maintain flatness, tight tolerances, and predictable geometry.
Summary Checklist for Engineering Specifications
When specifying aluminum coils or sheet products for manufacturing:
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Specify 6000 Series in T651 for high-strength structural parts, aircraft fittings, or heavy CNC machining where dimensional stability and zero distortion are required.
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Specify 5000 Series in H32, H111, or H116 for corrosion resistance, marine hulls, pressure vessels, fuel tanks, and sheet metal bending where high formability and weldability are needed.
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Avoid Combining T-Tempers with 5000 Series: Never specify T351 or T651 for 5000 series alloys; use equivalent strain-hardened (H) designations instead.