My coworker has some questions about stress releiving aluminum. Neither of us are very knowlegable about the subject, so I thought I might ask here and learn something. Below is the situation. Can anyone confirm, or offer advice, or point us to a source for some further information.
I have a part where a large amount of aluminum is hogged out in rough machining, the part is then stress relieved, then the final machining is performed. By hogging out all this material roughly. Then performing a stress releive. then go back and perform final machining, we are trying to prevent the part from turing into a "potato chip".
I called out AMS 2770E stress relieve on the drawing, but the machine shop came back and said they thought the temperatures listed in the spec were too high.
They said that typically 6061-T6 is treated at about 177C (350F) for stress relieve. But the spec called for a temp at about 341C (645F). Soak for 2 hours min for .5 inch thickness (add .5 hour for every addl .5 inch part thickness. And then air cool.
Is there some science or metallurgical basis for the lower temperatures suggested by the shop? I tend to believe the shop, but I want to know what the lower temp stress relieve is based on.
ASM Metals Handbook, Vol. 2, 8th Edition talks about "Stress Relief Annealing" and "Full Annealing", the former being done at 650+/-15 degrees F. for "no appreciable holding time", and cooling to room temperature. No other "stress relief" treatments are addressed. This stress relief treatment is specifically aimed at removing the effects of strain hardening as in forming operations.
Your 645*F temp will overage the part far past the T6 condition and the strength will drop significantly. The 350*F temp is right at the aging temp for 6061 to produce a T6, so it should not overage or weaken the material. However, whether it will achieve any beneficial stress relief is something I can't hazard a guess toward. I've machined some fairly funky parts from solid aluminum over the years and have never seen a need to stress relieve any of them to maintain the geometry as long as sharp tools are used for the finishing cuts such that you avoid cold working the part by rubbing on it with the cutter.
Personally, I've found cobalt HSS end mills to be far better than carbide in situations like this, simply because HSS will take an edge thats impossible to reproduce on carbide. The cobalt mills will hold the edge longer than standard HSS since aluminum tends to be somewhat abrasive in its action on the cutter.
Try some without stress relieving. With aluminum any excursions into aging/annealing temps can result in a loss of strength.
I wrote a paper for a journal a few years ago on the warpage and distortion of aluminum. A mistake was made and even after some of the samples were aged at almost double the temperature (turning them into mush) many of them still warped significantly.)
Warpage is a funny thing, if you can figure out a way to predict it, youd be a very rich man [img]smile.gif[/img]
If you're going to do any heat treat after the machining, you should be using T4. The heat treat at 340-360°F (171-182°C) for 18 hours with an air cool. You get a T-6 temper.
Good point Nick.
In general I've always found the softer the aluminum is, the more it wants to push and smear around even with sharp cutters. The more that happens, the more the stock is cold worked. I guess that's one thing that makes 7075 so nice to machine. The fact that it stands right up there like a big boy and takes the cut, instead of wimping out and flowing around like bubble gum to avoid being cut. In my experience, 7075 is also the most stable for intricate part details.
2024 T351 should work good. ???
Think Snow Eh!