6061-T6 vs 6061-T651 Thick Plate: How to Choose

What separates T6 from T651?

Both tempers start the same way: the rolled plate is solution heat-treated at about 530 °C, quenched in water and then artificially aged at about 160 – 175 °C to precipitate Mg₂Si. Quenching a thick plate cools the surface much faster than the core, which locks in compressive stress at the surfaces and tensile stress in the middle.

In T651, the plate is stretched permanently by 1.5 – 3 % between quenching and ageing (the "51" suffix). Stretching yields the whole cross-section and evens out those internal stresses, typically reducing them by around 80 – 90 %. In T6, the stretch is omitted, so the quench stresses stay in the plate.

Strength is not the reason to choose one over the other. Standards set the same minimums for both, and typical values are the same. What changes is machining behaviour: remove material from one face of a T6 plate and the remaining stresses rebalance, so the part bows, twists or springs when unclamped. A T651 plate machined the same way moves far less.

How 6061 plate properties change with thickness

Thick plate cools more slowly at mid-thickness during the quench, so less magnesium silicide stays in solution and the core reaches slightly lower strength after ageing. Standards account for this by lowering minimum elongation and, in the heaviest bands, tensile strength.

Summarised from ASTM B209 / AMS 4027; elongation limits vary slightly between editions. EN 485-2 follows a similar pattern and reduces minimums above 100 mm. The actual results are always on the mill certificate.

Typical properties of 6061-T6/T651 are about 310 MPa tensile, 276 MPa yield, 12 % elongation and 95 HB. Some suppliers list "T6 ≥ 290 MPa, T651 ≥ 310 MPa" as if T651 were stronger; that mixes specified minimums with typical values. Design to the minimums for your thickness band, whichever temper you buy.

6061 composition limits

Thick plate uses the same registered chemistry as all other 6061 products. Chromium matters more in thick plate than in sheet, because it limits recrystallisation and helps retain strength and toughness at mid-thickness.

When to order T6 and when to order T651

For the very heaviest sections, alternatives include forged and compressed 6061 block (temper T652, stress-relieved by compression) and cast tooling plate, which has no rolling or quench stresses at all but lower strength.

Thick 6061 plate sizes and weights

Thick 6061 plate is rolled in widths of 1000 – 2500 mm and lengths of 2000 – 6000 mm, and is usually sold either as full plates or saw-cut blocks. Common thicknesses are 30, 40, 50, 60, 80, 100, 120, 150 and 200 mm. Weight is 2.70 kg per m² per mm of thickness.

Plates above about 200 mm and stretched T651 above roughly 150 – 200 mm depend on individual mill capability; confirm availability with each enquiry.

Ultrasonic testing, flatness and tolerances

Thick plate can hide porosity, inclusions or centreline defects that only appear when a deep pocket is cut. For moulds, aerospace tooling and pressure-retaining parts, specify ultrasonic inspection up front.

• Ask for tensile specimens from mid-thickness (or T/4) when the core carries load
• State whether the plate is for structural use or tooling; it changes the testing that matters
• Specify a machining allowance on each face if surfaces must be fully cleaned up

Machining thick 6061 without distortion

1. Choose T651Start with stress-relieved plate wherever thickness allows.
2. Machine symmetricallyRemove material evenly from both faces, flipping the part between roughing passes.
3. Rough, rest, finishRough to within 1 – 2 mm, unclamp so the part can relax, then re-clamp lightly and finish.
4. Control heatUse flood coolant and sharp, high-rake carbide tools; heat input causes local movement.
5. Clamp gentlyExcess clamping force bends the plate; it springs back when released.

6061-T651 produces short, broken chips and a bright finish at high spindle speeds. It takes clear, coloured and hard anodising well, which makes it a common choice for fixtures and machine components.

Mistakes buyers make with thick 6061 plate

• Ordering T6 for a heavily machined part to save a little cost, then losing far more in scrapped or re-worked parts when they distort
• Designing to typical values rather than the specified minimum for the actual thickness band; above 100 mm the minimum tensile strength is lower
• Leaving out ultrasonic testing on mould and tooling plate, where a hidden void found halfway through machining wastes days of spindle time
• Using thick 6061 where flatness matters more than strength; cast tooling plate holds flatness better for vacuum tables and inspection bases
• Forgetting the machining allowance; saw-cut faces and rolled surfaces usually need a few millimetres of clean-up per side
• Mixing standards on one order, for example ASTM tolerances with EN property tables; state one governing specification

Where thick 6061 plate is used

Alternative grades for thick plate: 6082-T651 (the European equivalent, slightly stronger), 7075-T651 (about twice the strength, for high-load tooling), 5083 (best for welded and marine parts) and cast tooling plate (best flatness).

Related plate pages

Ordering thick 6061 plate from Metline

1. SpecifyTemper (T651 or T6), thickness, width and length or cut-block sizes, quantity and the standard (ASTM B209, AMS 4027, EN 485).
2. Add testingUltrasonic class, mid-thickness tensile tests, flatness or milling requirements.
3. QuoteWe confirm availability, processing and delivery terms (EXW, FOB, CIF or DAP).
4. DocumentMill test certificate to EN 10204 3.1; third-party inspection on request.
5. ShipPlates and blocks are packed on skids or in wooden cases with moisture protection for export.

6061-T6 and T651 aluminum plate: frequently asked questions