Source: Aluminum Vol. III, Fabrication and Finishing, Kent R. Van Horn,
Editor, American Society for Metals, Metals Park, OH, page 333, Table 7.
This is a book I purchased (a long time ago) when my business card displayed
the title "aluminum metallurgist."
The recommended solution heat treatment for .125 inch to .250 inch (3.175 mm
to 6.35 mm) thick extruded 6061-T6 is 980 deg F (527 deg C) for 55 to 65
minutes followed by rapid quench in room temperature water. Precipitation
heat treatment (artificial ageing) is subsequently done at 350 deg F (177
deg C) for 8 hours soak time. There is another higher temperature solution
treatment cycle at 1025 deg F (552 deg C) which has a longer ageing time of
12 hours at the same 350 deg F soak. Sheet and plate use the lower solution
treatment temperature and a 18 hour soak at 320 deg F (160 deg C) for
artificial ageing. To get from T6 to T651 the metal is stretched at least
1% (permanent set) for stress relief. The Europa hardware is probably cut
---From sheet or plate (if I remember correctly plate is sheet that is thicker
than .125 inch).
The precipitation hardening cycle, if extended for a much longer time, can
result in the "over aged" condition, but powder coating soak time should not
be long enough to "over age" the aluminum parts.
For reference, the annealing cycle (to get the metal dead soft) is to soak
at 775 deg F (413 deg C) for 2 to 3 hours; a partial anneal that will only
partially remove the effects of solution heat treatment and ageing is done
at 650 deg F (343 deg C). (This is on page 327, Table 4, Ibid)
So, although the powder coating temperature is indeed in the range of the
precipitation heat treatment soak temperature, the soak time should be much
less than what it would take to do any harm (I'm guessing here because I
don't know how long the powder coat soak time is).
If you are still concerned, have the powder coater process a sample (not
coated) through the soak cycle, e. g., a small square of comparable
thickness 6061 sheet. Check the Rockwell hardness before and after the
powder coat thermal cycle and compare the values. The real test for over
ageing is to measure the conductivity but you may have more difficulty
finding a lab to perform that test.
Best regards,
Rob Housman
A070
-----Original Message-----
From: owner-europa@post.aviators.net
Subject: Powder coating aluminum - help!
I believe we went over this previously here, but I want to be doubly sure.
I just sent out all the wing/flap hardware to be powder coated gloss white
(W19, W20, OR3, etc.)
My wife, who works for Midwest Express Airlines, says two of the guys on the
maintenance floor said that the powder coating process would screw up the
heat treat of the aluminum. Now, I remember looking up the age hardening
time and temps for aluminum, and from what I remember, it was quite a bit
hotter than what the powder coating oven runs at(which is about 375 to 400
deg. F).
Before I go and wreck a bunch of parts, I want to be doubly sure that I'll
be OK. Powder coat is far superior to paint in both protection and
durability, so I'd like to be able to use it.
Thanks for the info, gang.
Chris and Sue
A159
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