Aluminum 7075-T6

Have any of you guys actually seen a crack develop in a part you made from 7075-t6? I have read about Stress corrosion cracking on the internet but i am none the wiser (Can a crack be caused by stress alone. Can a crack be caused by corrosion over time alone) if someone has seen a problem in person and can give their opinion on their experience with using 7075-t6 that would be very helpful. Has any one any experience of working with 7175 Aluminum as an alternative to 7075. Thanks

TeaJunky said:

Can a crack be caused by stress alone.

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Yes.

Can a crack be caused by corrosion over time alone

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Yes.

SCC is a result of both though. It's impossible to root cause this without a more information: part geometry, environment it was installed in, a picture of the failure.

Thanks for the reply. I made an hydraulic valve block from 7075-t6 which will experience both heat and pressure. The original block did crack I do not know the alloy of the original block. I want to make some more of them but the possible cracking issue with 7075 has me concerned that I may not be using the correct alloy.

The question here (at least in my mind) is whether it's a design problem or a manufacturing defect. That's very hard to determine without the details I specified above.

If it's just a question of 'make it work' then switch away from aluminum to something stronger.

Perhaps not entirely relevant but years ago we used to make a "spreader bar" (for want of a better term) for use in
helicopter logging. All loads lifted externally with a helicopter are suspended by hooks that can be opened remotely
by the pilot. For safety reasons if there is ever a problem the pilot can simply "pickle" the hook and dump the load.
The spreaders served an additional function by carrying two additional hooks on the lower holes.

When the pilot picked a load out of the bush most of the load was on one hook while a smaller load was carried on the
second of the two. As the load was coming up the pilot could sense how heavy it was and if the chopper--Sikorsky
S61s in this case) couldn't manage the total weight it was a simple (and safe) matter to pickle off the smaller load
and continue with that turn. Since the choppers used a combination of lift and forward motion to pull a lift of logs up
pickling the entire load off would require the machine to circle around and start the cycle all over again. By jettisoning
only a portion of the load they didn't waste any time.

My point for this slightly long-winded explanation was to show the amount of stress placed on the spreaders by the
cycling of these loads. Maximum load for an S61 was (if i remember correctly) about 20,000 lbs. and, for obvious
reasons the goal was to maximize the number of logs pulled out of the bush on each turn.

The spreaders were made out of 1" thick 7075-T6 plate and I know that many of them stayed in service for many years
so in this application stress cracking didn't seem to be an issue...

Thank you for going to the trouble of explaining the back ground to you answer and is much appreciated.

If you're willing to take a little bit of a strength hit, you can use -T73 rather than -T6. T73 is a little bit weaker, but much better from an SCC standpoint. It's very easy to turn -T6 into -T73, you just bake it at 350 for a few hours.

Do a search for 7050 aluminum and its various tempers. I haven't done any comprehensive research on it in a while, but at the time I did (~20 years ago) I found data indicating it had almost the same UTS as 7075, but better SCC and high temperature properties.

Some other processes can help, including controlled shot blasting at stressed sections to leave a residual compressed skin zone. Compressed materials are much less likely to become points of crack initiation and growth. Use stainless steel shot beads to minimize free iron residue.

Testing would be key in this situation. I believe if a crack was to appear it would develop over time. This makes testing a bit more difficult. I am looking into the availability of the 7050 and the 7075-T73 options. I have found further info on changing the T6 temper to T73 (225f for 8 hours then 325f for 24 hours to be confirmed) My question on this is how do you know when you are at the temper you are looking for, is it all down to temperature and time? At the moment My preference would be to buy the material at the T73 Temper.

Look at MIL-H-6088 for the heat treat, and QQ-250/12 for the SCC tests

many metals are tested cause they can have internal hairline cracks which are not obvious to the eye.
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many metals are tempered or heat treated and although they are higher strength they will not bend much before cracking. many aluminum sheetmetal alloys if you try to make a 90 degree tight bend they will crack immediately or possible crack easily later under load.
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take for example a grade 8 steel bolt. once bent even 5 degrees usually if you try to bend back straight the bolt will crack suddenly under load. although metal had high strength it has little ductility or ability to bend or stretch with out cracking suddenly
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quite common on high strength metals. they tend to not give much warning by metal bending or stretching. they tend to just snap suddenly with no warning. thats why often parts are load tested at 400% load. that way part rated 2 tons if it experiences a 2 ton shock load it has enough extra strength usually to handle it instead of suddenly failing
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many parts require corner radiuses so load not concentrated in corners. it might only be .030" to .125" corner radiuses but it corners too sharp it can crack under load suddenly

I was talking to a supplier today and they are suggesting I use 7075-t651 cold work stress relieved and they also suggested I consider 2618 as an option. Maybe I am wrong here but I thought when you start making holes in a material you were stress relieving it anyway especially through holes. I am also looking at 7075-t7351. I would like to try and possibly make a good decision on this occasion. Thanks for all the advice and help.

Wouldn't hurt to get the material Ultrasonically tested prior to machining.

Anything that's critical we get UT tested. Ussually we get the material vendor to UT the material so we don't end up with flawed material if we buy and UT ourselves.