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6061 heat treat to T6

6061 heat treat to T6

(OP)
what is the proper procedure?
we need a part which is constructed with a 90 degree elbow tube weld to a flange. so, we will select O (annealed) condition for both components (mainly the tube has to be O to be bent). after welding, we want to get it to T6 condition. should the part go thru solution stage first or just 350F precipitation?
thanks.

RE: 6061 heat treat to T6

To go straight to age hardening, you would want the W condition (solution heat treated) not the O condition (annealed). The W condition requires higher temperature than O condition, 960 deg F vs. 760 deg F for 6061 according to MIL-H-6088. The W condition is unstable and may not be suitable.

RE: 6061 heat treat to T6

(OP)
thanks.
CoryPad, if i understand correctly, you are saying that the part after welding must first solution heat treated to W. right?
we are afraid the deformation when subject to high temperature. is it acceptable to weld T6 parts and skip any post heat treatment if W components are not obtainable to be welded together ?

RE: 6061 heat treat to T6

For your first question, I am saying that you have to go from W to get T6. You cannot go from O to T6.

If parts with T6 heat treatment are welded, then the areas near the welds no longer have T6 properties. Whether or not that is acceptable would have to be answered by someone doing the engineering for these parts.

RE: 6061 heat treat to T6

It's been a while since I researched this but as I recall the W condition is short lived and will soon become T4 (aged naturally). So, unless you do the solution treating yourself, it is more likely you will get T4 from distributors. After bending you heat treat to T6. The tubing you get may be stamped T6, which means it is certified to get T6 properties after heat treatment.

If you heat T6 much above 400F for long you will get the O (annealed condition) which contains large grains. The W condition (right after solution treating) contains no or few tiny grains). It is the growth of many small tightly packed grains that resist yielding that causes a high yield strength).

If any of this is incorrect I would really appreciate any correction.

RE: 6061 heat treat to T6

The W condition contains large grains, but within the grains are very small Guinier-Preston zones. These become intermetallic particles upon aging, which are the source of high strength.

RE: 6061 heat treat to T6

DHO... CAUTION... You mentioned welding.

6061-O and -T6 have poor welding qualities, different reasons. It is desireable to weld 6061 in -T4 temper... then go to the -T62 temper for the assy... as follows [A or B].

A. In typical scenario fabricate all parts from -T4/-T42 temper material; weld them together [AWS D17.1 + ER4043], then age-harden the assembly to -T6, same as per -6088 or AMS2770. The base materials will be -T6 temper and the weld filler will be somewhere between -O and -T4 upon final HT. Makes a very nice/sound weld assy that is stress relieved.

B. When highest strength is required, fabricate all parts from -T4/-T42 temper material; weld them together [AWS D17.1 + ER4643], then solution HT and age-harden the assembly to -T62, same as per -6088 or AMS2770. The base material and will be -T62 temper and the weld filler will be close to -T42 upon final HT. IF proper techniques are employed, then welds are noticibly stronger/tougher.

NOTE. ER4043 produces high quality sound welds; however ER4643 produces generally higher strength welds, at a premium price and whigh higher quality welding techniques [preparation, etc] required.

NOTE. For critical weldments, it is wise to test welds [trust but verify the materials, process (and welder)]! Fabricate the parts, weld-them, HT them to finished temper [whatever] and then cut-out/mechanical test each section to ensure mechanical properties are acceptable. Or trust me: Rough weldment values can be found for 6061 and other alloys in Aluminum Association [AA] Aluminum Design Manual; or the AA Aluminum Standards and Data book; or the AA Welding Aluminum : theory and Practice book. I can't remember which one of these had the general weld strength tables.

NOTE. If severe forming is required then form only the part(s) that need it in the -O condition and then HT 'it' to -T42 before welding. I don't believe that 6061 has the same grain-growth problems when severely deformed in the -O condition that 2xxx and 7xxx alloys do. However, forming in the -W temper would, perhaps, eliminate quench distortion issues. Again, HT the Assy to -T6 after all is said and done.

Any welders/weld-engrs 'out there' who have additional comments... or might want to raise the BS flag on this post?

Gotta go.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

RE: 6061 heat treat to T6

For those who have access, Boeing Design Manual section BDM–4085 has the best break down of weld allowables for 6061, I have seen. It gives allowable for both the HAZ and the weld metal.

RE: 6061 heat treat to T6

(OP)
thank you wktaylor.
you saved my life, seems.
as i said i am afraid the (flange) deformation going thru solution h.t. (see attached)
correct me if i am wrong.
i can have tube in O, bend to elbow, solution h.t., W, age to T4.
weld to T4 flange "extension".
then 350F precipitation to T6.
i will have no deformation on flange.
thanks to all helping me.

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