[Scuff]

Greetings All,

First let me say that all I know about welding/metal fab is what I see on American Chopper

I have an aluminum mountain bike frame that I am in the process of rebuilding. The frame is currently being stripped down to be powdercoated (it currently has a polished finish)

While inspecting the frame, I noticed a small gash on the downtube near the bottom bracket. Here's a few pics to better illustrate...

Pic #1
Pic #2

I sent these pics to the frame's manufacturer (Intense Cycles) who basically said that the gash does not appear to be a problem in terms of the structural integrity of the frame. The frame was built in 2005 and has taken quite a bit of abuse (it's a mountain bike after all)

What I really wanted to know is, what do you guys think is the best way to fix this gash (cosmetically) before I take it to the powdercoater? I was seriously considering just getting some bondo (HA HA!), but I figured I'd ask the experts!

[Geof]

Trying to weld it could be a bad idea unless you got an absolute wizard at TIG, and even then it might not be wise because the localized heating would induce stresses that could be less desirable than leaving it alone.

There is not much you can do other than try to file/polish it as smooth as possible; any filler you put in will probably get destroyed by the heat of baking in the powder coating.

[Scuff]

Cosmetics are not that big of a deal, in fact the gash will be hidden behind the drivetrain (chainrings, etc.) so once the bike is rebuilt you won't see it unless you really look.

I was worried more about the structural integrity of the frame, but according to Intesnse, I should just keep an eye on it.

Thanx again for your advice:cheers:

[ckirchen]

Talk to your powdercoater. My V10 has a dent that I was going to fill in before powdercoating and my powdercoater say he has some epoxy that I could use.

I didn't end up doing it (too much work...not enough riding time), but he did say it worked quite well for him in the past.

[Ark1]

You didn't say if you were going to do the welding or not. Guys that do this kind of welding know the in's and outs of doing this kind of repair.

If there is a hole in the frame somewhere to allow back purging this would be the first place to start. Yes you can weld it with TIG, back purge with argon and use a low alloy in the 3000 series that will flow out and fill the gash then file and sand to shape. There are some very low melting aluminium rods that can be torch melted as well. The problem with a sealed frame is pressure building and trapped inside the tube. As the area being welded isn't on a seam the heat will soften the weld area and the pressure will start to balloon the weld zone if the weld isn't done quickly. Back purging removes the oxygen from the back side of the weld and stops it from reacting with heat and dropping away.

Aluminium is used in fireworks as it burns and produces a lot of heat. It is a reactive metal, not so much as Magnesium but supply the heat with oxygen and it will burn.

You cannot fuse or weld 6061 alum. as it will crack at the weld zone. A filler must be used of lower alloy to joint the sections together.

Personally unless you are trying to sell this bike as mint I would just leave it alone and live with it.

TonyK.

[Sugdenr]

If that is 6061

DO NOT WELD IT

6061 bike frames are heat treated to the T6 condition. Untreated the 6061 has iro only 50% of its treated (design) strength in the HAZ.