[xxSurlyxx]

Im welding bicycle frames and get some cracking at the toe of the weld from time to time on the thin wall tube side. Seems like it only comes up from time to time. Crack forms immediately after weld is performed. Am I just cooking the thin wall tube? Im dabbing and forming the puddle about every second, manually "Pulsing." Seems like if I weld faster, I dont see the problem as much, but it doesnt give me the time to form a nice uniform puddle.

In the photos, the thin wall tube is on the left, thick on the right.

Machine setup:
Miller 210DX
200 amp peak
70% Balance
Freq: 120h
1/8" Zirconiated tungston
Gas lens with #7 cup

Material
7005 Aluminum 2mm wall tube welded to 4.5mm wall.
3/32" 5356 filler

[snoeproe]

Welds almost look to be concave shaped to me. Points to too much heat if that's the case.
Is that an unfilled crater at the end?
The cleaning action appears to look like undercut in spots also?
Beads need to be tightened up a bit between dips.

[LtBadd]

Are you sure it's 7005? Only a few of the 7XXX series are weldable.

Link

[cj737]

I would say that you're overheating the tube, but I would also ask is the frame "restrained" while you are welding it? Often those types of cracks immediately after a weld are an indication of stress. The fact that the entire toe tears out is concerning for sure.

[xxSurlyxx]

I guess I should have clarified a couple things.

This is a test joint of same thickness material. Not restrained in anyway.

110% certain this is 7005. We have these formed tubes custom made for us, and weld 100’s of these frames. We artificially age (heat treat) this material after welding to get them back to T6


The small crack (almost invisible) happens immediately after the weld is performed. The complete separation of tube from weld happens after intense beating on tube with a 5lb sledge. This is also pre-aging cycle so it’s known that it would break at HAZ.

Here’s a completel shot of the test joint and example of what the complete weld area looks like

[kiwi2wheels]

Is the 5356 filler from a new batch / brand, tagged/stamped ? Your tube is from the usual tube supplier ?

There are some comments here, re 5356. Not with the problems you're experiencing, but maybe try another lot ?

http://forum.weldingtipsandtricks.com/v ... 78&start=0

[Poland308]

Looks like you need a preheat. That’s a shrink crack. The two tubes are shrinking on opposite directions due to the different axis. And there shrinking faster than your weld can stretch.