Compare 6061 T6 vs T651 aluminum alloys for strength machinability and stability to choose the best temper for your project needs.

Understanding Aluminum Temper Designations: The Basics of 6061

If you’ve ever worked with aluminum alloys, you’ve probably come across terms like 6061-T6 or 6061-T651. But what do these temper designations actually mean? It all starts with aluminum’s unique heat treatment process, which shapes its mechanical properties.

At the core, 6061 aluminum undergoes solution heat treatment, where the alloy is heated to dissolve soluble phases. This is followed by quenching, a rapid cooling step that “freezes” the structure in place. Next, the aluminum is subjected to artificial aging (or precipitation hardening), which carefully precipitates strengthening particles to boost its strength.

Why does this matter? Because these temper steps balance strength and ductility—too little heat treatment, and the alloy is soft but easy to shape. Too much, and it becomes strong but brittle. The temper letter and number after 6061 (like T6 or T651) tell you how exactly the metal was treated, affecting its final properties.

Specifically, the transition from plain 6061 alloy to T6 temper means the aluminum is solution heat-treated and artificially aged to peak strength. From there, T651 temper includes an extra step: it’s stress-relieved by stretching (or straightening) after the T6 process, which improves dimensional stability—critical for precision parts, especially in plate forms.

In short, temper designations are a shorthand for aluminum’s treatment history—and understanding them is key to choosing the right material for your project.

What is 6061-T6 Aluminum?

6061-T6 aluminum is one of the most common tempers you’ll come across in the U.S. It’s made by taking 6061 aluminum alloy through a standard heat treatment process: solution heat treating, then quenching, and finally artificial aging. This treatment boosts its strength while keeping a decent amount of ductility, making it a solid all-around choice.

Key Properties:

• Mechanical: Tensile strength typically around 42,000 psi, yield strength near 35,000 psi, and elongation about 12%.
• Physical: Density stays consistent at about 2.7 g/cm³, with good thermal and electrical conductivity. Corrosion resistance is strong, especially against atmospheric exposure.

Machinability and Weldability:

• Machinability is excellent; 6061-T6 works well with most cutting tools and holds tight tolerances.
• Weldability is good but needs care — heat from welding can reduce strength in the heat-affected zone, so post-weld heat treatment might be required for critical parts.

Pros and Cons:

• Pros: High strength-to-weight ratio, easy to machine, weld, and form; widely available and cost-effective.
• Contras: Can lose some strength after welding without proper treatment; not the best for applications needing very high dimensional stability.

Quick Fact:

6061-T6 is a go-to for general fabrication—from bike frames to structural components—thanks to its balanced strength and workability.

What is 6061-T651 Aluminum?

6061-T651 aluminum starts with the same T6 heat treatment process—solution heat treated, quenched, then artificially aged to boost strength. But here’s the difference: for plates thicker than 0.25 inches, T651 undergoes an extra 1–3% cold stretching. This step relieves internal stresses and includes straightening to improve flatness and dimensional stability.

Propiedades clave

• Mechanical: Like T6, it offers strong tensile (about 45 ksi) and yield strength (around 40 ksi), but you’ll notice slightly better elongation due to the stress relief.
• Dimensional Stability: Thanks to the cold stretch, T651 resists warping and distortion during machining or welding much better than T6.
• Physical: Density, corrosion resistance, and conductivity remain consistent with 6061 alloy norms.

Machinability & Weldability

T651 maintains good machinability, often smoother due to less internal stress. Welding works well too, with reduced risk of distortion or cracking compared to T6, making it excellent for precision components.

Pros y Contras

Pros:

• Improved dimensional stability for vital applications
• Better straightness in thick plates
• Less chance of warping during machining or heat exposure

Contras:

• Typically carries a small cost premium over T6 due to additional processing
• Slightly less common in some markets, which might affect availability

Quick Fact

That “51” suffix in T651 denotes this stress-relief step and is specifically referenced in ASTM B209 standards for aluminum sheet and plate. If you need a plate that’s flat, stable, and reliable under machining or heat, T651 is often the way to go.

Head-to-Head Comparison: 6061-T6 vs. T651

Let’s break down the key differences between 6061-T6 and 6061-T651 with a quick side-by-side view:

Propiedad	6061-T6	6061-T651
Resistencia a la tracción	~45,000 psi	~45,000 psi (same)
Yield Strength	~40,000 psi	Slightly higher (due to stretching)
Elongation (%)	~12%	~10-12% (similar but more consistent)
Densidad	2.7 g/cm³	2.7 g/cm³ (no difference)
Modulus of Elasticity	~10,000 ksi	~10,000 ksi (same)
Dimensional Stability	Standard	Improved (stress relieved via stretching)

Performance Differences

• Strength & Yield: Both tempers deliver strong performance, but T651 has a slight edge in yield strength because of the extra stress relieving step.
• Durabilidad: T651 tends to hold shape better over time, especially for thicker plates or parts under sustained loads.
• Mecanibilidad: Both are easy to machine, but T651 can reduce warping or distortion during complex machining or welding.
• Formability: 6061-T6 can be slightly easier to form, but T651 provides better dimensional control.
• Heat Effects: T651’s stress relief makes it less prone to distortion from heat cycles in welding or processing.

Cost and Availability

• 6061-T6: Widely available, more cost-effective, and common for general fabrication needs.
• 6061-T651: Comes at a premium due to extra processing and is usually in plate form following ASTM B209 standards, mainly for precision parts needing tight dimensional specs.

Quick tip: For visual learners, a stress-strain curve infographic comparing T6 and T651 would clearly show similarities in strength and slight differences in elongation and modulus — reinforcing the data here.

In , 6061-T651 is your go-to when you need stronger dimensional stability and reliability in thicker or critical parts, while 6061-T6 suits general projects where cost and availability matter most.

Applications: Where T6 Shines and T651 Takes Over

6061-T6 aluminum is a go-to for many general fabrication needs. You’ll often find it in automotive frames, bike parts, and structural beams where good strength and machinability are essential. It’s reliable, widely available, and ideal for projects that don’t require extreme dimensional precision.

On the other hand, 6061-T651 steps in where tighter dimensional stability and stress relief are critical. This makes it perfect for aerospace fittings, marine hardware, and precision molds. The slight cold stretching and straightening in T651 help prevent warping and distortion during machining or use, which is why it’s favored in industries that demand exact tolerances.

Industry highlights:

• Aeroespacial: T651 offers the stability needed for aircraft components.
• Marine: Corrosion resistance and dimensional control make T651 ideal for marine hardware.
• Automoción: T6 remains popular for structural parts and frames due to its cost-effectiveness.

Consejo profesional: For enhanced performance, hybrid T6511 extrusions are gaining ground—they combine the strength of T651 with improved formability and finish, useful in complex aerospace or precision engineering jobs.

Choosing T6 or T651 depends on your project’s precision, load demands, and budget. Both have strong places in the US manufacturing scene.

Factors to Consider When Choosing Between T6 and T651

Picking between 6061-T6 and T651 comes down to what your project really needs. Here are the key points to keep in mind:

• Load and Stress Requirements: If your project faces heavy or cyclic loads, T651’s stress-relieved process helps reduce distortion and cracking. For lighter-duty or general use, T6 often works fine.
• Machining Complexity: T651 usually offers better dimensional stability during machining, especially for plates over 0.25 inches thick. That means tighter tolerances and fewer adjustments. T6 is easier on the wallet but may need extra care in tight specs.
• Budget Considerations: T651 typically costs more due to added cold-stretching and straightening steps. Factor in if the extra cost matches your performance needs.
• Environmental Conditions: Both tempers have good corrosion resistance, but T651’s stress-relieved nature can improve long-term durability in harsh environments.
• Testing and Standards Compliance: Make sure your material meets industry specs like AMS-QQ-A-250 and ASTM B209, especially for aerospace or critical applications. Stress-relieved plates often pass tougher nondestructive stress tests.
• Errores comunes a evitar:
  • Using T6 when high dimensional stability is needed, causing warping.
  • Overpaying for T651 when T6’s properties suffice.
  • Ignoring how heat or welding might impact temper performance.
• Sustainability Notes: Both 6061-T6 and T651 are recyclable and commonly sourced from low-carbon aluminum suppliers—good news if eco-friendly material sourcing matters for your business.

In short, weigh your project’s mechanical demands, precision, cost limits, and environment before deciding between these popular 6061 aluminum tempers.

Sourcing Quality 6061-T6 and T651 from Vast

When you need reliable 6061-T6 sheets or 6061-T651 plates, Vast has you covered. We stock a wide range of thicknesses to fit your project needs, whether you’re working on light fabrication or demanding structural applications. Our inventory includes standard T6 heat-treated sheets and stress-relieved T651 plates, meeting strict ASTM B209 standards for quality and consistency.

What sets us apart is our value-added support. You get detailed property sheets with every order, so you know exactly what you’re working with. Plus, we offer volume discounts that keep your costs down if you’re buying in bulk. Need extra help? Our engineering team is ready to assist with technical questions or optimize your material choices, making your sourcing process smooth and efficient.

Ready to get started? Contact us today for quotes or samples to see firsthand why Vast is a trusted supplier of 6061 aluminum tempers in the U.S. market.