Machining Tight Tolerance 6061-T6

[lou4gehrig]

We are attempting to maching a 20" diameter dog dish shaped part out of 6061-T6 to a tolerance of +/- 0.0015. We have some bar stock that has been sectioned and we are hogging out the parts. There is alot of residual stress buildup during the hogging and the part is out of round when finished. We were considering doing a rough machine, stress relieve (375F for 2 hours).

Should we be doing more frequent stress relieving heat treatments? Or is this material inappropriate for such a tight toleranced part?

We will also have to recondition back to the T6 condition when finished. Any suggestions would be welcomed.

 

[kenvlach]

It would be preferable to use 6061-T651 tooling plate, available up to 6" thick from Alcoa and others.

Al 6061-T6, electroless nickel-plated, is used quite a bit for high-precision plastics molds. I've seen the same warping problem when an idiot MBA tried eliminating a post-rough-machining stress relief bake, as he thought it could be combined with a post-plating EN hardening heat treatment!

As you will be re-solutionizing, etc., you have more freedom re stress relieving temperatures.
I suggest a 400 ^oF stress relief after sectioning to eliminate the high residual stress profile in the radial direction of the solutionizing quench, followed (if possible) ASAP by a subzero treatment (aka 'cold stabilization'), e.g., dry ice-alcohol or liquid nitrogen. Your 'hogging out' will be more accurate.

After solutionizing & forced-air or polymer quenching,* ASAP do the subzero treatment (aka 'cold stabilization') before aging. For further stress relief, 'uphill quench' into boiling water. This subzero-boiling water cycle may be repeated up to 5 times. From ASM Handbook vol. 4 Heat Treatment.

T6 age, deoxidize to remove surface scale from the solutionizing heating, finish machine; then do a final, lower-T stress relief.

*"Some products [i.e., thin stuff] may be adequately quenched in room-temperature air blast." Footnote in the ASM Handbook for 6061 extrusions. Less residual stress than water quenching. Alternatively, quench in polymer solutions. Per SAE heat-treatment specification AMS-2770, 6061 up to 0.375" thick can be quenched in 25 vol% glycol, and up to 0.500" thick in 17-22 vol % glycol.