Aluminium Mountain Bike Weld

[Peter Bingham]

Hi There

I recently developed a hairline shear crack on a supporting brace on my aluminium mountain bike. The supporting brace is between the top and down tubes near the headset (where the handle bars are) and the crack is in the previous weld. As such, because it is not on a critical join the bike wont fall apart if it completely fails, I decided to keep riding the bike.

I got a friend with an Aluminium welder to weld over the top of the crack. In hindsight was this wise because I have since read that subsequent welds can weaken the aluminium tubing? Do I need to put the bike in an oven?

Thanks

Peter

 

[Shox Dr]

Not enough information Peter.

Grade of Aluminium?
and a Picture

 

[Peter Bingham]

Thanks for that. The frame is 6061 Aluminum, Fully Hydroformed. I will get details of the weld next week.

 

[FranV8]

Peter, I'm guessing from the downtube shape and short head tube that's a 29er and therefore not that old. It's obviously stressed there or it would not have cracked. Most frames come with a long warranty, can you go back to the manufacturer? Aluminium welds, particularly on alloys, tend to take the properties back to a basic aluminium, which is very poor strength.

 

[Rocketman]

It's probably to late now to send the frame back because it's been welded.

6061 should be heat treated after welding so the frame is now ultimately less strong than it was pre crack. Though saying that there may have been a slight crack always there since it was manufactured and it just opened up over time.

Laser welding gets over the problem somewhat as it doesn't put much heat into the job but you can only do relatively small repairs in it.

 

[Peter Bingham]

Frame is out of warranty. 26".
"less strong than it was pre crack". Take it you mean pre-repair-weld? Not what I wanted to hear! How do I go about heat treating it. I've probably access to an oven.

 

[Pete.]

I'd probably just ride it and make periodic checks. My lad had an alloy frame welded and the weldor said there was no guarantee. That was probably 5-6 years ago and it's still going strong.

 

[Rig Pig]

Check the front forks, a mate of mine kissed a kerbstone when his broke, left him with terrible face scars and broken teeth!

 

[Peter Bingham]

Forks are OK, at least on visual inspection. Think I'll just ride the thing and keep a close eye on the weld. Thanks guys.

 

[Rocketman]

Frame is out of warranty. 26".
"less strong than it was pre crack". Take it you mean pre-repair-weld? Not what I wanted to hear! How do I go about heat treating it. I've probably access to an oven.

It's isn't going to do the paint much good so would require a respray and then you have got to think was it welded up 100% good or you will be wasting your time.

 

[Peter Bingham]

It's isn't going to do the paint much good so would require a respray and then you have got to think was it welded up 100% good or you will be wasting your time.

Did you heat treat the bike?

 

[hotrodder]

FWIW 6061 frames are virtually always T6 temper which is a bit more involved than just having access to an oven. First step is a 'solution anneal' which inolves heating the thing fairly close to it's solidus temp and holding it there to take the alloying elements into a solid solution before quenching it to 'freeze' things that way. After that it's baked again but at much lower temps for a long time, this is the precipitation heat treatment aka artificial aging

Solution temp for 6061 is ~ 525 - 530°C, aging temps/times can vary some but 180°C for 8 hours is fairly typical

Welding a 6xxx-T6 alloy results in a complex HAZ that contains both annealed and 'overaged' zones and the only way to 'fix' the overaging is to start over with a solution anneal. It's not gonna just fall apart if it's left 'as welded', even if the welding is below par. It may crack again fairly quickly or it could last for eons...

 

[Peter Bingham]

Thanks hotrodder. That's what I was wanting (and hoping) to hear. Decision made. Thanks again all.

 

[whm_fab]

for what its worth, already knowing what hotrodder has posted, and explicitly told people who asked me to weld frames, they insisted so i did them anyway - not one has come back. Sometimes you have to give the scientific facts a backstep and take it at face value and look at the odds.
they were all DH bikes by the way, mostly rear stays and the like that get hammered

 

[bod]

This is exactly why I love this forum, hardly any armchair posters, just people who can give practical advice.
Aluminium's weird stuff, I've done loads of research on it and the way it hardens over time still seems strange. My girlfriend's granddad's a metallurgist, if I speak to him soon I'll pick his brains.
I've no idea why alu frames have fallen from favour, they're pretty bloody good really.