6061-T651 Plate: Why It Still Moves When You Machine It

6061-T651 plate is the default for machined aluminum parts, and the "51" is the reason. It still isn't magic.

If 6061-T651 Is Stress Relieved, Why Did My Part Still Bow?

Because stress relieved isn't the same as stress free. Stretching takes out most of the quench stress, but if you hog most of a thick plate away from one side, what's left can still pull the part out of flat.

That's the complaint in an r/Machinists thread titled "Stress in 6061-T651 Plate." The poster kept seeing warped parts after heavy material removal, and sending the plate out for extra annealing hadn't helped. One reply described 41"-long, 1.75"-thick parts bowing up to .020" in the middle. Another said to do most of the roughing, leave the part on the bench overnight, then finish. A third wanted sharp tools with no wiper or honed edge, kept cool. Someone else had good luck with precision ground cast plate, which stayed far flatter under heavy removal but is more brittle.

What the 51 actually means

NORAMARK's 6061-T6 vs T651 comparison explains it well. T651 is stretched to a 1 to 3% permanent set after quenching and before aging, and that stretch evens out the quench stress. Alumeco's temper table narrows it to 1.5% to 3% for plate. Strength minimums are the same as T6: 290 MPa tensile, 240 MPa yield for plate.

NORAMARK's table says T651 "stays flat" when you machine one side away. The thread above is a reminder that's the usual case, not a promise for every geometry. NORAMARK also says most plate 6.35 mm (0.25 in) and thicker is already sold as T651. For machined plate parts it suggests writing "6061-T651" or "6061-T6/T651" on the drawing, and T6 for sheet, extrusion or rod parts.

Keeping long parts flat in 6061-T651 plate

Rough, release, finish. Take most of the material off, unclamp it, let it relax, then come back for light finishing passes. Flip it and work both faces if the design allows, so you balance the stress you remove. We went further into this in why a thin plate bows after milling one side.

Clamping counts too. If you squeeze a bowed part flat to finish it, it'll spring back the minute it's unclamped. Fixturing thin walls and thin sheet lifting mid-cut get into the details.

If you need something long and flat, say how flat. A flatness callout tells the shop to plan for it.

At DigiForge

Aluminum 6061 is on our Materials page, and our CNC capability page lists "ISO 2768-m standard, down to ±0.01 mm". Put flatness on the drawing for long or heavily pocketed parts. Cast plate isn't on our stock list and the plate temper isn't listed there either, so if either matters to you, ask us when you quote.