We have a 7075 Al pump housing that has currently undergone a T651 temper but requires a T751 treatment to alleviate stress corrosion cracking.

Is it necessary to re-stretch a 7075 aluminum part to relieve internal stresses after a T751 heat treatment (Solution heat treated and stretched to remove internal stresses then overaged/stabilized) when a T651 (Solution heat treated and stretched to remove internal stresses then artificially aged) heat treatment was performed just previously?

The part will only be held at minimum over-age temperatures to alleviate SCC in order to maintain most of the tensile and yield strength of the material.

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    $\begingroup$ Could you add some links to some of the specifications that you are referencing? Are the TXX1 numbers ASTM standards? aluminum types? A little background information will help you get the answer that you are looking for! $\endgroup$ – hazzey Feb 15 '15 at 19:32
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    $\begingroup$ By adding additional detail, you make it easier for someone who is familiar with treatment processes but not necessarily those processes to be able to answer your question satisfactorily. Likewise, the additional details make it easier for someone to refer your question to someone else they know who may be able to answer your question. $\endgroup$ – user16 Feb 15 '15 at 19:37
  • $\begingroup$ Okay, I forgot to put in we are dealing with 7075 Al (thank you hazzey!), it is now part of the question. I am not sure who came up with the designations for the heat treatment standards (I thought these were generally accepted terms), I will look into it further. $\endgroup$ – eatscrayons Feb 15 '15 at 19:44
  • $\begingroup$ How is that hazzey, GlenH7? $\endgroup$ – eatscrayons Feb 16 '15 at 15:05
  • $\begingroup$ @hazzey I believe the normative reference for the alloy and temper designations is the Aluminum Association's "Aluminum Standards and Data" book, last published in 2013. $\endgroup$ – Ethan48 Feb 17 '15 at 16:34

I am going to start out by saying that aluminum is a mess. There are too many designations and things that you can do to it after it solidifies.

To directly answer your question, what might necessitate an additional round of heat treating is directly affected by the desired final mechanical properties. Each of the designations that you listed is a complete heat treatment. Either one is chosen or the other is chosen, they are not done in a sequence other than what is already defined by the string of numbers.

From the 7075 chart on page 11 of this pdf the only difference between the -T6 and the -T7 is that the -T7 has a slightly better resistance to stress corrosion cracking. If this is what you are looking for, choose the -T7.

The reason that there are so many different alloys and heat treatments is that each combination produces a material with slightly different physical properties.

Properties of the two heat treatments that you listed:

Without very detailed requirements (and lots of experimenting), it is almost impossible to say which alloy and heat treatment of aluminum will be the best for a given situation. You can always go with the most common types.

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  • $\begingroup$ I don't think the question was about which heat treatment to use, but rather about if the stress-relief stretch needs to be repeated (that is are the internal stresses developed from the initial forming/rolling/extrusion or from the heat treatment process?) $\endgroup$ – Ethan48 Feb 17 '15 at 22:56
  • $\begingroup$ @Ethan48 The heat treat processes described by the numbers is a complete sequence. We don't know the desired end result, so we can't figure out which steps might be dropped. It seems easier to just pick the standard that already gives the desired results. $\endgroup$ – hazzey Feb 18 '15 at 2:11
  • $\begingroup$ Ethan48 is correct. The material has already had a T651 heat treatment performed but the specs call for a T751 which is exactly the same as the T651 temper but with an additional over-age at an elevated temperature (~25C higher than the T6 age). The question was, is a re-stretch needed after the over-age step in the T751 temper, my apologies if that was not clear. $\endgroup$ – eatscrayons Feb 20 '15 at 0:38
  • $\begingroup$ @hazzey, I edited the question to better fit within the parameters you outlined, hope this helps! $\endgroup$ – eatscrayons Feb 20 '15 at 1:23

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