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On Fri, 11 Feb 2011 20:59:34 -0500, "Ed Huntress"
wrote:
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On Fri, 11 Feb 2011 22:49:30 GMT, (dan) wrote:
Karl Townsend wrote in
rec.crafts.metalworking on Thu, 10 Feb 2011 18:01:49 -0600:
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?
Or, am I better off with plan "A"? Don't anneal.
My vote is don't anneal. Hard spots in 7075 aren't like hard spots in
steels.
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. g 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 Ed - like I said, I was not familiar with the 70 series alloys
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.
--
Ed Huntress