Aluminum T-Tempers

The temper designation (T6, T651, T73, etc.) is not optional — it determines whether your part warps on the machine table, cracks in service, or costs 2x what it should. Most engineers just write "6061-T6" and hope for the best. This page tells you when that's wrong.

T6 vs T651 — Which Do You Need?

Start here. The temper you specify depends on your part geometry and service environment, not on what the supplier stocks.

Understanding T-Temper Designations

The Aluminum Association designation system uses prefixes (F, O, H, T, W) followed by digits. For CNC machining, you mainly deal with F, O, and the T-series.

Why T651 Matters for CNC Machining

This is the single most important section on this page. If you machine aluminum plate, you need to understand why T651 exists and when T6 will cause problems.

The distortion problem

When aluminum is solution-treated and quenched (the first steps of T6), the rapid cooling locks in thermal gradients. The outside of the plate cools and contracts faster than the inside. This creates residual stresses — the internal layers are in tension, the surface layers are in compression (or vice versa, depending on geometry).

As long as the plate is intact, these forces balance out. The plate appears flat. But when you start machining — especially removing material from one side — you break that balance. The remaining stressed material pulls the part out of shape.

How T651 fixes this

Between quenching and aging, the T651 process applies a controlled permanent stretch of 1–3% to the plate. This plastic deformation exceeds the yield point and allows the residual stresses to redistribute and partially cancel out. The result is a plate with the same mechanical properties as T6, but with residual stresses reduced by roughly 80–90%.

The stretching is done by the mill (Alcoa, Aleris, Southwest Aluminum, etc.) on the full-size plate before it's cut to size. You can't convert T6 to T651 in your shop — it has to be ordered that way.

6061-T6 vs 6061-T651

The most common aluminum alloy. The temper difference is subtle in the spec sheet but significant on the machine.

7075-T6 vs 7075-T651

Same mechanical property story as 6061 — identical on paper. But 7075 has an additional risk that makes temper selection even more critical.

T73: Over-Aging for SCC Resistance

T73 is a two-stage overaging process specifically developed for 7xxx series alloys. The first aging step is at a lower temperature than T6, the second at a higher temperature. This produces a coarser, more evenly distributed precipitate structure that is far less susceptible to stress corrosion cracking.

When to specify T73

The tradeoff

T73 costs more and gives less strength. For 7075, the numbers are:

• Tensile drops from 572 MPa to 503 MPa (minus 12%)
• Yield drops from 503 MPa to 434 MPa (minus 14%)
• Cost increases 15–25% over T6
• Lead time may increase (less common stock item)

If your design can accommodate the lower strength (redesign with slightly thicker sections), T73 is the safer choice. If the part is non-structural or used indoors, T6 is fine and cheaper.

O-Temper and Annealed

O-temper is the fully annealed condition. The material is heated to ~410–430°C (depending on alloy) and slow-cooled. This dissolves all precipitates and produces the softest, most ductile state.

When to use O-temper

Re-heat-treating after forming

After forming O-temper material, you can restore full T6 properties by solution treating and artificial aging. The process:

1. Solution treat: Heat to 530°C (6061) or 480°C (7075), hold 1 hour. Dissolves alloying elements into solid solution.
2. Quench: Rapid water quench within 10 seconds of removal from furnace. Traps elements in supersaturated solid solution.
3. Age: 6061 at 175°C for 8 hours. 7075 at 120°C for 24 hours. This produces the precipitation hardening.

If you need T651 after forming, specify T351 as the starting temper instead of O-temper. T351 is T4 + stress relieved. It's formable enough for moderate bends (not as soft as O) and can be aged to T6 after forming.

T4: Natural Aging

T4 means the alloy has been solution-treated and then allowed to age at room temperature. It reaches ~75–80% of T6 strength after 4–5 days and continues to slowly strengthen for months.

2024-T4 vs 2024-T3

When to specify T4

T4 is used almost exclusively with 2xxx series alloys (2024, 2014) for applications where fatigue resistance and fracture toughness matter more than absolute strength. T6 would give higher static strength but worse fatigue performance. The naturally aged microstructure has a coarser precipitate that resists crack propagation better than the fine precipitates in T6.

If you're machining 2024 parts, use T351 plate for the same distortion reasons as 6061-T651.

Practical Procurement Guide

What to specify on drawings

Always include the full alloy-temper designation. The correct format is ALLOY-TEMPER. Examples:

• 6061-T651 — aluminum plate for CNC machining (most common)
• 7075-T651 — high-strength plate, dry environment
• 7075-T7351 — high-strength plate, humid/marine environment
• 6061-T6 — bar stock, tube, thin sheet, extrusions
• 2024-T351 — 2024 plate for machined parts
• 6061-O — for forming/bending, to be re-heat-treated

Availability in China

Common stock sizes (China domestic market)

Cost comparison (approximate, China market, 2026)

Common Mistakes