Welding 6061-T6 weakens it?



Heavier? I am assuming this IS NOT a skinny wheel road bike or you wouldn't be adding all this junk. With big heavy shocks on the front end and (I assume) a great disk on the rear, you are worrying about weight ;-?
Another thought - it is possible to determine approximate strengths of heat treated aluminum by hardness testing. See the URL below for additional details (note that is all one long URL)
http://books.google.co.th/books?id=3h9aWwcDFxAC&pg=PA99&lpg=PA99&dqidentifying+aluminum+%2Bt6&source=bl&ots=1x999fJ-2R&sig=XzyENmGsN41kczxupR7VpxJIUCM&hlth&ei=_G-1Ssz4OsGMkAWMxqjXCw&sa=X&oi=book_result&ct=result&resnum=9#v=onepage&qidentifying%20aluminum%20%2Bt6&f úlse
You might be able to determine, very closely, what you have by hardness testing.
Another thought - Sail boats often use 6061-T6 for spars and I doubt that a 50 ft. mast is re-heat treated after welding.
A final thought - Why go it alone? All of the bike magazines have question and answer columns. Why not write and see whether they have any idea about the feasibility of welding on a T6 frame? After all; guys here are into welding. Guys at the bike magazine are into bicycles...
Cheers,
Bruce (bruceinbangkokatgmaildotcom)
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mtb
small disc on rear (front does majority of braking, so large disc there)

Within reason. Just as an example, a block of 6061 if bolted on (due to added mass for bracing, fasteners, etc) is probably 1 lb more than if welded.
And no one thinks of that extra weight until you've got lots of time to think, pushing up that long hill, regardless the type of bike...

Good point. You're using your special "outside the box" gray matter...

Because the turnaround time for magazine Q&A is typically many weeks. But a good resource, nonetheless...
Thanks!
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Gone Fishin' wrote:
(...)

Have you asked on rec.bicycles.tech yet?
--Winston
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I'm still waiting for another sublime, transcendent flash of adequacy.

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wrote:

I used to run skinny wheel bikes and those guys were into worrying about grams. A Titanium seat post clamp bolt!

Tain't really - it is all welding :-)

I found a web site that supplies frame tubes and fittings to the trade. http://www.framebuilding.com /
Their aluminum tubes are:
Material (7005-T6) Al-Zn-Mg T6 treated : excellent TIG weldability. High corrosion resistance. Ap5 = 10%. Mechanical Characteristics: Rm = 420 MPa Rs = 380 MPa Suggested welding rod: 5836-5180-5183-5556 alloys.
Their drop-outs are 6000 0r 6061.
There is mention of aging of the tubes 4 - 5 days after welding..
In addition they list quite a number of frame builders. You might try e-mailing some of those guys asking whether they heat-treat after welding (I have heard that TREK does).
Cheers,
Bruce (bruceinbangkokatgmaildotcom)
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In most cases the people doing professional fab work will have a process they follow to meet code requirements. In this case, they probably have some type of clamp-on resistance heater they put around the weld, crank up the temp, hold it for a set time, quench it, crank up the temp again (lower this time), hold it for another set time, then shut off the heaters, allowing it to cool to ambient and reach its final temper.
In the case of the spar, they are probably happy with a T4 temper due to its greater flexibility. T6 is much stonger, but also more brittle.
Lincoln Electric has a nice, free, no-registration-needed web-page:
http://www.lincolnelectric.com/knowledge/articles/content/comistakes.asp
Go to the section titled, "Heat Treatable Alloys."
hth,
--
Tin Lizzie
"Elephant: A mouse built to government specifications."-Lazarus Long
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Are you sure your bike is 6061? Most aluminum bikes are made from 7005 aluminum. \It has identical tensile strength to 6016-T6, but it's strength is derived from it's chemistry, not from heat treat, so you can weld it all you like and lose no strength. 7005 was created to get around the necessity of re-heat-treating a bike frame after welding.
BTW if you weld 6061-T6, immediately post-weld the aluminum will have dropped to a T1 or T2 hardness. However, over the next few weeks the hardness will creep back up to about a T5. This is called Age hardening or Precipitation hardening and is the normal mechanism of hardening aluminum. Unfortunately to get back to a T6 condition you would have to have the frame re-heat-treated by a professional shop.
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