1. Core Definitions & Temper Designation Logic

Both 6061-T6 and 6061-T651 are heat-treatable tempers that achieve peak strength through solution annealing and artificial aging, but their final processing steps set them apart:

1.1 Key Terminology & Standards

• Alloy Base: 6061 (Al-Mg-Si-Cu-Cr), compliant with ASTM B209 (plate/sheet), AMS 4027 (aerospace), EN 573-3, and GB/T 3880.

• Temper Nomenclature (per ASTM B267):

• T6: "Solution heat treated, then artificially aged" – The foundational high-strength temper for 6061, delivering maximum strength without additional stress relief.

• T651: "Solution heat treated, stress relieved by stretching, then artificially aged" – A derivative of T6 with a critical stress-relief step (0.5–3% elongation via stretching) to eliminate internal residual stresses from rolling or forging.

• Critical Distinction: T651 retains T6’s peak strength but offers superior dimensional stability, flatness, and resistance to warping during machining—making it ideal for large plates and precision components.

2. Chemical Composition (Shared by Both Tempers)

6061-T6 and 6061-T651 share identical alloy chemistry, as their differences stem from processing, not composition (mass fraction, %):

Core Alloy Advantage

The Mg₂Si intermetallic phase, formed during artificial aging, is responsible for both tempers’ high strength—precipitates pin dislocations in the aluminum matrix, balancing strength and ductility.

3. Heat Treatment Process: Key Differences

The critical distinction between T6 and T651 lies in their post-solution annealing steps, which directly impact internal stress and dimensional stability:

3.1 6061-T6 Heat Treatment

1. Solution Annealing: Heat to 530–540°C (986–1004°F) for 2–4 hours to dissolve alloying elements into the aluminum matrix.

2. Rapid Quenching: Water quench (cooling rate ≥20°C/s) to trap elements in supersaturated solid solution (prevents premature precipitation).

3. Artificial Aging: Heat to 160–175°C (320–347°F) for 8–12 hours to form fine, uniform Mg₂Si precipitates—achieving peak strength.

4. Final Form: No stress relief; may retain residual stresses from rolling/quenching.

3.2 6061-T651 Heat Treatment

1. Solution Annealing: Same as T6 (530–540°C for 2–4 hours).

2. Rapid Quenching: Same as T6 (water quench).

3. Stress Relief Stretching: Stretch the material by 0.5–3% elongation to eliminate residual stresses (critical step for large plates).

4. Artificial Aging: Same as T6 (160–175°C for 8–12 hours).

5. Final Form: Stress-free, flat, and dimensionally stable—resists warping during machining or welding.

4. Mechanical & Physical Properties Comparison

While T6 and T651 share nearly identical strength properties, T651 outperforms T6 in dimensional stability and flatness. Below are typical values for 6–50mm thick plate (ASTM B209 compliant):

4.1 Physical Properties (Shared by Both)

• Density: 2.70 g/cm³ (1/3 of steel)

• Melting Point: 580–650°C (1076–1172°F)

• Thermal Conductivity: 167 W/m·K (superior heat dissipation)

• Electrical Conductivity: 43% IACS (suitable for electronic components)

• Corrosion Resistance: Excellent (self-healing Al₂O₃ oxide layer); outperforms 2xxx/7xxx series alloys

5. Product Forms & Dimensional Range

Both tempers are available in similar product configurations, but T651 is preferred for large or precision components:

6. Key Applications: When to Choose T6 vs. T651

The choice between T6 and T651 hinges on dimensional stability requirements, component size, and processing needs:

6.1 6061-T6: Ideal For…

• Small-to-medium components (≤500mm in length/width) where flatness is not critical.

• Extruded parts (e.g., bicycle frames, hydraulic cylinders, electronic enclosures).

• Welded assemblies where post-weld stress relief is planned.

• Cost-sensitive projects (T6 is 2–3% cheaper than T651).

• Typical Use Cases: Sports equipment, automotive brackets, consumer electronics, general machinery parts.

6.2 6061-T651: Ideal For…

• Large plates (≥500mm) and structural components (e.g., aerospace wing ribs, building curtain walls).

• Precision machining (CNC milling, turning) where warping would ruin tolerances.

• High-precision tools, molds, and jigs/fixtures.

• Welded structures requiring minimal distortion (e.g., marine decks, industrial platforms).

• Typical Use Cases: Aerospace structural parts, large machine bases, architectural facades, high-precision mold plates.

7. Production Quality Control & Certifications

Both tempers undergo rigorous quality checks, with additional controls for T651’s stress relief and flatness:

7.1 Common Quality Tests

• Chemical Analysis: Spectrometric verification of alloy composition (ASTM E1086).

• Mechanical Testing: Tensile, bend, and hardness tests for each production batch.

• Non-Destructive Testing (NDT): Ultrasonic testing (UT) for internal defects; eddy current testing (ECT) for surface flaws.

• Corrosion Testing: Salt spray testing (ASTM B117) for 300+ hours (95%+ integrity rate).

7.2 T651-Specific Quality Controls

• Residual Stress Measurement: X-ray diffraction or hole-drilling method to confirm stress relief (<50 MPa residual stress).

• Flatness Inspection: Laser scanning to verify tolerance compliance (±0.2mm/m).

• Straightness Testing: For long plates to ensure no bowing or twisting.

7.3 Certifications

• Standard Certifications: EN 10204 3.1/3.2, ISO 9001.

• Aerospace-Grade Certifications: AMS 4027, AS9100 (for critical applications).

8. Pricing & Supply Chain Considerations

8.1 Price Comparison (2025 Q4 Data)

8.2 Key Pricing Drivers

• T651 Premium: Attributed to the additional stress relief stretching and flatness inspection steps.

• Order Quantity: Bulk orders (≥5 tons) reduce the premium to 1–2% for T651.

• Plate Size: Larger plates (≥100mm thick, ≥2000mm width) have a higher T651 premium (5–7%) due to more complex stress relief.

8.3 Supply Chain Details

• MOQ: 100kg (T6); 500kg (T651 for large plates).

• Delivery Lead Time: 3–7 days (stock T6); 7–14 days (stock T651); 15–30 days (custom sizes for both).

• Global Supply Hubs: China (Shanghai, Guangdong), USA (Texas, Indiana), Europe (Germany, Italy).

9. Selection Guide: T6 or T651?

Use this decision matrix to align the temper with your project’s core requirements:

10. Conclusion

Aluminum 6061-T6 and 6061-T651 are the workhorses of high-strength aluminum, offering identical peak strength but distinct advantages for targeted applications. 6061-T6 excels as a cost-effective solution for small-to-medium components where dimensional stability is not critical, while 6061-T651’s stress-relieved design makes it indispensable for large plates, precision machining, and structural parts requiring uncompromised flatness and stability.

Both tempers leverage 6061’s inherent benefits—lightweight, corrosion resistance, and versatility—making them the top choices across aerospace, construction, transportation, and industrial sectors. By prioritizing component size, machining needs, and flatness requirements, engineers and buyers can select the optimal temper to balance performance, cost, and reliability. Whether for a consumer product or a critical aerospace component, 6061-T6 and T651 deliver the high-strength, cost-effective solutions that define 6061’s status as the most versatile aluminum alloy in the world.