Details depend on the aluminum alloy, but, yes, the heat of welding will leave a heat affected zone in the base material that has altered mechanical properties, generally for the worse. this MAY require post-weld heat treatment.

If the design allowed for the changes in mechanical properties from welding, the PWHT isn't needed. Larger structures in aluminum generally are designed such that this isn't an issue, usually by using material larger than would be needed if there wasn't a reduction in properties. Many smaller parts and assemblies, especially those where weight is cut to a minimum, do need PWHT.

 

From what I read on Al bike frames they a air heated and then go into a salt bath quench. Maybe some age hardening after that at a slightly elevated temperature. Not the the easiest thing to set up for one-offs.

 

thanks for the replies guys, really helpful,

any ideas how much weaker the effected material would be 50% 25% ?,

my brother is a mech engineer and could start sunning some designs through mech stress programs if you guys have even a rough idea,

it the part of the bike where the rear wheel attaches, in you opinion from your experience would it need PWHT,

Thanks again guys,
thanks again,

 

Depends on the alloy. Bikes tend to be made from either 6061, or 7005.
With the 6061-T6, I would guess more than an 80% loss of strength. Yeah, its pretty severe.
With 7005, if you keep the heat input down, full heat treatment after welding is not really necessary, just aging.

Still, what sort of mod are you thinking of. Welding may not be your only option.

 

i''m trying to do this sort of mod,

http://www.63xc.com/clausc/trackend.htm

but thats with a steel frame and he is brazing of course,

but i''m trying to get those on my aluminium frame,

 

The alloy and actual tubeset that your frame is made from will determine what's required in the way of PWHT. As said, typical alloys used for bicycles are 6061 and 7005 which have fairly different properties but there's also a lot of proprietary alloys used these days. There's often a sticker on the frame advertising what it's made of, either something like 7005-T4, 6061-T6 or something like EA6X (treat like 6061) in the case of proprietary alloys.

Compared to 6061, 7005 takes longer to anneal (i.e. is 'damaged' less by welding) and makes a better (and fast) recovery.

Frames made from 7005 are typically just aged after welding- cheaper ones naturally, higher end frames artifically by soaking at set temperatures (upto around 160C) for a set amount of time- in the region of 8- 10 hours. 6061 frames are typically put through both a solution heat treatment (soak at around 500C for a set time and quench) and artifical aging. It depends on who you speak to but about the best recovery 6061 will make on it's own is upto around 50%, for the weld and surrounding HAZ which contains both partially annealled and overaged areas

If you intend to turn your bike into a single speed jump bike you'd be better off selling it and buying a single speed jump bike imo. If it's a relatively basic 7005 tubeset then i wouldn't worry about it too much for sensible (boring) use