I'm preparing to machine several AL 7075 T6 forgings...
"The Kid" (my son) tells me I should anneal before machining because there will be hard brittle spots. Easy enough to do at the anneal temp of 775 (not sure how long). But, then I haven't got T6 grade 7075 AL anymore. How would I get it back to T6 grade?
Hi, Karl. 'Just stopped in to see if anyone talks about metalworking here anymore.
Stick with Plan A. If you have hard spots, your 7075 sucks, but you're better off living with them if you do. If you want to anneal, it's 775 deg F for 2 -3 hours.
Forget natural aging. Unlike 2024, 7075 has a hardening curve that runs out for years. It's very unstable throughout its life in that state. To get a true T6, it requires a fairly tricky two-stage heat-treat: solution heat-treatment and quenching, followed by precipitation heat-treatment (artificial aging).
Unless you want to get into all of that, you won't see T6 again after you anneal the stuff.
We once had a job machining some annealed aluminum, I don't remember the alloy, and it was terrible gummy and hard to get a finish on so the boss sent the whole lot out for heat treating, we machined it and sent it back out for annealing.
We have parts cast from 356 aluminum alloy and heat treated at the foundry. We machine them after heat treating. Never thought of doing it any other way. The heat treatment can induce some distortion some times (not often) so that would be bad if one was going to anneal, machine and then heat treat.
T6 is age hardened. If you anneal, for instance, 6061T6 sheet to work it, and anneal it again to take the working stress out, and leave it sit for a couple months it is back to very close to T6 strength again.
Not sure how 7075 forgings will behave, but with the T6 designation I suspect it would be somewhat similar.
Not really similar, Clare. 7075 age hardens, but over years, not weeks. And it will never reach T6.
The peak tensile strength of 6061 that's naturally age-hardened is roughly the same as artificially aged 6061 T4 -- around 20 ksi yield. It achieves that with natural age hardening after a couple of weeks. After eight years, it climbs only to 22 ksi. For comparison, artificially aged 6061 T6 has a yield strength of roughly 40 ksi.
7075, on the other hand, reaches around 60 ksi -- after 15 years. It's
62 ksi or so after 25 years. But it reaches 45 ksi in around two months. It just keeps climbing, in a steady rise, but the material is fairly unstable all along the way. Artificially aged 7075 at the T6 temper is around 70 ksi yield.
They don't use a number for naturally aged 7075. They call it "W" temper, and to properly designate it, says the ASM, you include the time of aging after the "W." For example, W 10h means the temper after 10 hours of naturally aging. These designations are mostly of interest to metallurgists and research engineers.
As a practical matter, natural age hardening doesn't work out very well for
7075 and some of the other 7xxx series. It can work fairly well for 6061, and even better for 2024 (40 ksi after one hour; 42 ksi after 25 years).
I'll bet this info is available somewhere around the Web, but my primary source is the non-ferrous edition of ASM's _Metals Handbook_.
Thanks, Paul. I'll try to keep my eyes open for questions to which I may be able to contribute. This one happens to lie right in the heart of what I researched and studied when I was materials editor at _American Machinist_.
It's a tricky material in several ways. There are one or two tempers above T6, but they're not straightforward heat treatments. They're special treatments intended to prevent exfoliation and stress corrosion. They aren't harder or stronger than T6, just more complicated.
We used to machine some of it at the shop I was involved with in the mid-70s. It was used for some aerospace research models at Princeton University, which was our main customer. It was not as nice to machine as
2024 T4, which is, IMO, the sweetest grade of aluminum for turning or milling.