Welding Tips and Tricks

Hi All,

On inspecting a few welds recently I've spotted some cracks on the back side of some T-joints. These are on the inside of a bicycle headtube, 6061, approx 2mm wall thickness. The joint is only welded on one side (It's actually tube to tube rather than a T-joint).

When sanded, the cracks tend to disappear quite quickly. My theory is that it is due to the inside of the tubing not having any mechanical cleaning before being welded. Therefore there's oxide present and this layer is the one that is cracking.

I've searched the literature but can't find anything to backup or dispel my theory, does anyone here have experience in this area and might have a clue what I am seeing?

One word of advice is don't use sharpie marker to mark things. It's the bane of aluminum welds.

Can you post up some shots showing more overview? I can't tell there where the mechanical joints are. I assume you mean there's welds on the far side of the stress cracks. They sort of look to me like they were there before you welded, not caused by it. But it's hard to say from behind my keyboard.

I highly doubt that those cracks are cosmetic or the result of the oxides cracking. I am more willing to bet they are stress cracks https://www.wileymetal.com/hot-cracks-v ... m-welding/

Nothing cosmetic about that mate. Definitely a crack, all the way through, sanding is just smearing it over, i.e. hiding it, it is still there and that part will fail in service. It appears to be a typical vibration or stress crack. I guessing from shape that it is more mtb type bike they take a lot of vibs on the top tube. Vibrations move through a part and dont seem to move evenly through even slight defects or changes in metal thickness etc, is why weld sizes are important to follow. Id guess there was a tiny bit of undercut or a minute crack and it has worked on it for a bit. Lucky you found it that way. I wouldnt try to repair personally as you would need to re jig and change tt and headtube.
By the time you do that youd be better off building a whole new one.
Or replace frame. Hate to be the bad news bear about mate, sorry.

Would that happen to be the seat tube?


Seen that kind of cracking once and it was beside the seat tube weld. Turns out you can only put so long of a seat stem in or that kinda stuff happens.


But I'd also say that crack runs through the metal. Dye check would tell you for sure.

Sanding against the crack just puts a burr over the crack and makes it 'disappear'



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Definitely stress cracks. Seen lots of stuff on steel like that. Metal fatigue from twisting stresses and flex that add up over time. The real problem is only visible at a microscope level, there will be micro cracks under the surface that you can’t see. You can test for that with a manga flux setup. Once the base metal is at that point you need to replace the whole area around the cracks.
https://www.magnaflux.com/NA/EN/Product ... -8-Kit.htm

Just thinking out loud.
Seeing as though that bike is done. I wouldn’t fix it or ride it, it’s just costing money now. But if are confident that it hasn’t moved and you liked the frame set up measure all your configs, like bb height, seat tube angle, head tube angle etc. it makes buying a new one so much easier. Strip off components and cut frame through top tube and down tube so if it’s found at the tip it doesn’t get dodged and sold to some unsuspecting cyclist. First welds are always the best ones on ally bikes.
Oh One can never have too many spare bicycle wheels, bicycle trailer frames are easy But wheels ain’t cheap.

hard to tell from a single pic.

one of the issues with bikes is underdone engineering causing stress cracks due to excessive flex.
trying to keep them light weight, so material is undersized a tad, no extra reinforcement at the joins etc. aluminium does not handle vibration well and needs extra support otherwise you get fatigue cracks. some bike makes/models are well known for frame failures.

with this i will take a wild guess possibly lack of penetration and filler. there is a possibility of hot cracks. easy to tell on the inside that there is not full penetration.
one of the ways to reduce hot short is filler use.

Thanks for all the replies guys.

The only thing that makes me wonder about your answers suggesting it is stress related is that this is a brand new frame.

The crack appears directly in line with the rear side of the joint circled in the attached picture.

Yeah we know. Hence the not wanting to be bad news bear comment. It is obvious that it wasn't used much. I didn't know it was new........but...... a crack is a crack. What brand of bike. Some welding procedures can be different batch per batch. WPS are for the procedure, they don't make assumptions about the workshop, whether the welder is closer to the door on that day, climate control and temp of workshop etc, etc. could be a batch issue. Doesn't change anything though......other than warranty.

chiroshi wrote: ↑Thu Sep 30, 2021 3:04 am Thanks for all the replies guys.

The only thing that makes me wonder about your answers suggesting it is stress related is that this is a brand new frame.

The crack appears directly in line with the rear side of the joint circled in the attached picture.

around the big crack area is a lot of peppering. is that just a bit of dirt? or is that indicating poor quality metal in that part.
it would be interesting to know how that head tube is formed.

the other easily noticeable thing is lack of penetration. one thing that can cause cracking is lack of filler used and also if its marginal in heat. the classic circle at the bottom of a fillet weld where its not melting into the bottom of the root. afaik that can cause internal cracking.

welding does create stress in the material, so being new doesn't mean its not a stress crack.

New or not fact is you got a crack that appears like a stress crack.

Could've happened during cooling after welding. Could've happened during forming the tube.

Hell could of even have happened during shipping somehow.


I've definitely learned in my years that just cause something is new does not mean it has zero flaws.

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