Stress reliving aluminum

Hi group. I have a bunch of 6061 parts to do for aerospace company and just wandering if I can get some guidance about stress reliving. Parts vary from 6" X 6" X 1.5" to 24" X 12" X 3" but after they are done there is almost nothing left. Full of pockets, ribs and holes. Wall thickness from 0.03" to 0.08". I want to complete finish it from 5 sides on 5 axis mill and than do the other side. From your experience will it work or I will get lots of deflection even after stress reliving? And another question is how do you do that? I put it in a furnace at 350 for 4 hours as I was told by one person but other people had different ideas. Someone told me to dip it in nitrogen. Thanks in advance for any help. Jerry

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The most effective method I've come up with is to rough machine, unclamp, reclamp (with shims if needed) and finish. Make sure the finished part comes from the center of the material, and it should be pretty straight. This keeps outside vendors from screwing up your stuff.
Later,
Charlie

I know what you mean, I've done it this way for almost 30 years but those parts ar to flimsy to hold if you rough it first. Some have a note that you can't handle them without restrained. 0.03" is a thickness of a cardboard. Jerry
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I found several recipes that were in the 325-350F range for anywhere from 8 to 18 hours. As thin as you are working with four hours may be enough. The question becomes do you have a setup piece or extra that you can experiment on. If not, a chat with a heat treat facility to discuss your options would be a good way to go. You might check sites like efunda.com and matweb.com or a google search to get an idea of the process and properties. I did a 8" X 13" five sides .06 wall last week on our VMC using 3-axis toolpaths. I packed the exterior with an oil based modeling clay to reduce the chatter. The center lost about . 005" thickness from bowing in. I got surprisingly good tolerances on the vertical walls with a minimum of deflection. Don't know if it will help with the 5-axis twists and turns. Luckily our customer is bonding it to a carbon fiber enclosure that has its own flex. That is some pretty thin stuff. Good luck! JL

Thanks JL. I will start with the smallest part to see if it moves after machining and than do one next size up if everything is ok. Sounds like a good idea about the clay. I may use it. After your 8 to 18 hours in the furnace do you let the material cool down inside or do you take it out? If you leave it inside the furnace do you open the door or let it cool down with the furnace? Jerry
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I found several recipes that were in the 325-350F range for anywhere from 8 to 18 hours. As thin as you are working with four hours may be enough. The question becomes do you have a setup piece or extra that you can experiment on. If not, a chat with a heat treat facility to discuss your options would be a good way to go. You might check sites like efunda.com and matweb.com or a google search to get an idea of the process and properties. I did a 8" X 13" five sides .06 wall last week on our VMC using 3-axis toolpaths. I packed the exterior with an oil based modeling clay to reduce the chatter. The center lost about . 005" thickness from bowing in. I got surprisingly good tolerances on the vertical walls with a minimum of deflection. Don't know if it will help with the 5-axis twists and turns. Luckily our customer is bonding it to a carbon fiber enclosure that has its own flex. That is some pretty thin stuff. Good luck! JL

I looked to Marks Engineering Handbook and a materials textbook which indicated in stress relieving and annealing "the rate of cooling is unimportant". The critical item seemed to be heating the material though and consistent, so with the thinner material it should transfer heat faster and the four hours you mentioned may be enough. There doesn't seem to be any drawback to leaving it longer at a lower temp. Above the 750F threshold you cross over into artificial aging and heat treat. If you take it step by step you should find out pretty quick what is going to work.
Just a side note. I have seen orange translucent aluminum splash out of a heat treat furnace where it was separating packets of tool steel at 1800F for about six hours. When I opened the door the waterfall that solidified at my feet caused me to count my toes. Nobody will believe me when I say that a coworker loaded the oven and I was the unfortunate helper, so I will leave it with check your settings for C and F, aluminum melts around 1100F and an air tight oven generally won't leak. JL
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Thanks for the warning. I will keep it in mind. Jerry
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Too bad you can't use cast jig plate.
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Cast jig plate? What is that? Jerry
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Jerry,
Along with our machining service, we manufacture a product called skywindow www.tricomachine.com/skywindow
One of the early problems we had was machining the mirror cell ( the part that holds the mirror) from 6061 aluminum. Having a very thin bottom and walls, it would warp all over the place.
The solution was going to a cast tooling and jig plate. This aluminum structure is extremely stable and can be used in applications where high strength is not as important. I believe we are using alcoa m-100.
It machines great, and stays put. It even anodizes well ( but you need an anodizer that knows what he is doing )
Check with your supplier about mold and tooling plate. There are some grades of cast aluminum used for molding that approach the tensil strength of steel, and can be welded as easliy as steel.
Hope this helps
Jim
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