Help! Alu Frame Cracked In Two! 6065 7005 Tig Weld!

SniperGaulois

SniperGaulois

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Jan 31, 2008
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Rhone-Alpes
After 3000 miles and very many curbs and mountains, the bike did this:
GOrE8vn.jpg


I forced the bracket out wide when i fitted the motor. Do you think i should leave the 2mm gap in the crack to have more welding space and to account for the width of the motor? should i weld in the same alignment?
If i don't align the crack when i weld it, it will be stronger?

I want to go to a garage to have it TIG welded with an extra triangle as a reinforcement. I have a trashed ALU bike to salvage some alu as a triangle, maybe i should add a tube joint instead of a flat one.

It's a centurion backfire LRS 1, the versions 2 and 3 changed to a stronger layout:
11wUUn8.jpg
 
It cracked where it did because the material next to a weld in aluminum has it's alloy precipitate out the things that made it strong in the HAZ (heat affected zone).

If you have a shop weld it for you, and then don't re-heat-treat your frame, it will have 1/4 to 1/7th the strength it had before.

If you do re-heat-treat it, it still won't be as good as before, but perhaps only 15-30% weaker.

Aluminum is not so great of a metal to be welding to make structural parts.
 
additionally, the stresses that caused it to fail there were being placed on the whole rear structure. The opposite side is probably full of micro stress cracks, and will fail too.

The only good way to save the bike is to replace the swing arm. you might find one used on ebay, but the best plan would be to create one out of steel and prevent this from happening again.
 
Are there a lot of bikes out there where the makers skip the heat treatment process altogether?

I thought that it cracked beside the weld because the entire bike had not been heat treated.
T6 hardening should return all the metal to a uniform state, and it's rare to see bikes with T6 6061 Alu specification, perhaps that's because heat treatment changes alignment and costs a lot to get straight, i figure a lot of makers don't bother and send out flimsy bikes?!

I'll have to tig weld it for the moment, i'll have to distribute the join over a 7x larger zone. I can weld on a hand sized piece of aluminium on either side, and i can wrap a cupful of carbon fiber epoxy around a snaggable form. maybe carbon fiber is better.
 
liveforphysics said:
Aluminum is not so great of a metal to be welding to make structural parts.
Click to expand...
For 30 years I worked for a company that built remotely controlled under water vehicles (ROVs). Back in the 1970's some of the frames were steel but since then I can not recall a single work class ROV that did not have an aluminum frame. We did use SST fasteners.
 
These days aluminium is very different, they can hydroform tubes that start with in one shape like a D and end up in another, 7005 alledgedly is recognizeable because they use less welds on it, where as 6061 is more weldable, there's a video on YT of a weld professional guiding through an aluminium dropout weld repair. ROV has less mechanical wear from knocks than a bike going down mountains.

In the bike industry they are supposed to heat the frame to 480 for 6 hours after a weld stress which gives it plasticity and isotropy/homogeneity in order to straighten it up in the workshop, then they go to 890 degrees and drop the frame in glycol(antifreeze) water mix. do they do that with ROV's?

I'm tempted to make a hitch point on the bar zone and wrap 1-2 centimeters of carbon fiber around the problem zone. I'd be more confident with a fking huge clump of carbon fiber holding my weight than a technical tedious weld. if i wrap 100 grams of carbon fiber on there, it should be equivalent to 1/4 of kilogram of steel. Fancy my chances??? mad or what! It will buy me some time until spring? i'll test the epoxy edhesion first and perhaps buy an aluminium primer epoxy for the first layer of carbon fiber.
 
there is no need to heat treat 7005/7020 aluminium. it will regain almost full strenght within a month (full strenght after 3mo at room temperature). i invested a lot of time searching for aluminium welding when i built my kona mid drive frame. t6 means heat treated, and is needed for 60xx alloy to regain strength. other than the welds of steel frames, the 7xxx weld itself is the weakest part.
my advise: look for some guys who know their business and just reweld the frame. it will be stronger than before as you took out the additional stress you added by widening the dropout.
 
Hey thanks that's very good information. So 7005 bikes are air cooled and fixable. you have me learning about 7075 bonded aluminium frames that they made in the 90's. I'll see if i can find a welder that has worked with 7005, It's difficult to know if it isn't 6061 because it's an old bike.

To layer carbon fiber around it, it's similar to a lasagne dish, it just takes many days wrapping wet strands of 3k carbon fiber until the bike looks has an an alien nest area on it :) carbon fiber is similar to cooking. it's fun.
 
you CAN heat treat 7xxx alloy as well, and it will gain some better characteristics. t4, t6 and t7, depending on your needs and application.
but for us, who don't have heat treating ovens big enough for bike frames, self re-organizing of the alloy structure of 7xxx is perfect.
unfortunately this doesn't work for 6xxx alloy. this will be noticeably weaker, so it has tk built stronger (something where your triangles will be handy).
 
Can you believe!!! I uploaded that photo of the white LRS 2 with the stronger frame than the LRS 1, on this forum, and then it vanished!!! because there was a sales page online for one at 280 euros. Did a forum user in Germany get that frame? it was a bit 4 hours drive for me but for 280 instead of 1500 dollars i was very tempted!! who bought it?

So, I just got some carbon fiber because every strand can hold over 7 kilos and i have laid about 160 strands * 7 = at least one ton of carbon fiber on the crack. It's cheaper, takes less time to set, no uncertainty of which alloy, i have carbon and epoxy left over. There's almost 20 layers of carbon fiber around it.

In 6 hours it will be sticky like a cold freezer and i'll be able to tack down all the loose strands, then i'll bake it for a day, then i'll add a couple of layers of epoxy to protect the fibers.

I made some errors: 1/Forgot to fill the inside of the tube with fiber and through the 3mm crack. 2/I shoud have filled the triangle one with an inset of fiber to help support the stuff i wrapped around it.

i could have made it 10% stronger if i had written a list of what order to do the layers. crazy experiment!!!

Only worry is the other side. As LiveForPhysics pointed out, the derailer side has the same defect and is probably tired. and being the derailer side, there's no space for a carbon wrapping on it. if you are in europe i can highly recommend the quality of this guy's stuff. http://www.ebay.fr/itm/271987646427
 

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