Weld Termination Fish Eye

By TimGary (****)

Date 04-01-2011 13:19

Fish Eye = A single hole porosity left in the crater at the end of a stitch weld. (see attached picture)

Data = Carbon steel (A572 and A36) base metal, GMAW solid wire ER70S-3 in Pulse-Spray and Spray transfer fillet welds, 90/10 Argon/CO2 shielding gas, both manual and robotic

I'm working in a fab shop that makes thousands of these welds every day, and the fish eyes are a nuisance.
We use four different methods of avoiding them, during manual welding, while maintaing the requirement that every weld termination crater has to be filled to the full weld size (no shallow craters allowed):
1. When you reach the end of the weld hesitate in place long enough to fill the crater (this leaves a fish eye most of the time).
2. When you reach the end of the weld, back up toward the start of the weld just enough to leave a filled crater (this leaves a fish eye some of the time)
3. When you get to the end of the weld, break the arc and then hit the trigger again in order to spot fill the crater (this rarely leaves a fish eye but is often not cosmetic)
4. Before starting the weld, place a tack at its termination point and then weld into the tack (this leaves a filled crater and rarely results in a fish eye and is the favored method)
These steps are all well and good, when we can get the welders to use them, and minimize the trouble during manual welding.

However, robotic welding leaves an unacceptable fish eye half of the time. The acceptance criteria is that the hole must close, but even this leaves a non-cosmetic dimple where the fish eye was. We've adjusted and adjusted parameters in the sense of lengthening fill times at the end of the weld, all while dropping amperage and voltage, and every conceivable combination thereof. Short of reprogramming every weld to have the robot weld into a tack, as we do manually, I'm so far unable to find a cure.

Surely, we're not the only ones with this trouble, though perhaps most are not as anal as we are about rejecting it.
Can anyone offer some good advice, insight, or even better, a sure-fire cure?

Thanks in advance for your consideration,
Tim Gary

By jwright650 (*****)

Date 04-01-2011 14:00

Tim,
I'm not sure how much programming control you have, but is it feasible to add some extra wirefeed speed right at the very end before the arc is extinguished?

The fish eye...isn't that created by withdrawing the wire from the puddle too quickly?

By TimGary (****)

Date 04-01-2011 14:11

Thanks for the quick reply John.

While making the weld, especially with high Volt/Amp Spray welds, you can clearly see a large crater at the leading edge of the puddle and at the bottom of the arc. This crater is about 1/8" wide and deep. When you break the arc, the hole collapes on itself and hopefully closes while the puddle is still liquid and before it freezes. A fish eye results when this hole doesn't close all the way.
I see your point and it sounds like a good idea. I think we can add that adjustment to our expieriments.

Thanks for the input,
Tim

By jwright650 (*****)

Date 04-01-2011 14:17

Right!...the voltage spike at the end causes the burn back issue, so I'm wondering if adding a burst of wire feed at the very end will help offset that.

By jwright650 (*****)

Date 04-01-2011 15:04

What Ed Craig had to say on the subject:
copied from his site below

E-mail. Oct 2008: I am emailing you because I have come to a questionable snag with my pulsed MIG equipment. I have the equipment set in the spray mode. I am welding on 5/16” carbon steel material, my settings are set to spray transfer (29 volts 500 wire speed in/min).

When making a 3/16” fillet weld with the 0.035 (1mm) wire I have noticed that at the end of the weld, the weld flattens out and has what I have been taught to refer to as a “fish eye” ( I am not sure if this is the right term for this problem ).

The attached photo will show you what I am referring to. When coming to the end of my weld I back over the weld about ¼” instead of just stopping. I don’t pull my nozzle away before I let the trigger go, so I don’t think this issue is caused due to the length of the stick out. My gas is set to 35cfh argon/CO2 mix.

Could you please advise what may be causing this poor finish is this just cosmetic or an issue that needs to be addressed? If this is an issue that needs to be addressed could you please explain the proper procedure for fixing. These parts are under extreme vibrations and some stresses Vertical / Horizontal and Lateral. Thank you.T Eason.

Ed's Reply. Two things going on here.

[1] First the weld picture indicates poor side wall fusion. As you are using good spray parameters the lack of fusion is likely a result that the weld surface was wire brushed and the mill scale has been left. If you are concerned about fatique properties you don't MIG weld over mill scale. Grind the weld area before welding, I am sure you will see a difference in the weld appearence. As for the crater and crater hole.

[2] A fish eye is typically a pore evident in a failed weld and the bright shiny appearence in the pore indicates the presence of hydrogen, so you dont have a fish eye. You do have a pulsed power source that has a built in defect. This is a a commom classic issue with pulsed equipment in which the machine controlled end parameters or burn back parameters are set too high, (more evidence that pulsed equipment manufactures don't correctly test the equipment they build.) I see this defect all the time in pulsed equipment in robot cells. At the end of the weld, the high voltage spike applied for the burn back causes a suck back effect in the arc leaving that classic hole in the crater. In many instances if you examine with magnification you will find shrinkage cracks around that hole and with your fatigue concerns, this defect has to be ground out and the crater filled in. My MIG process control training resources deal with this issue and provide process solutions, however you would be well served to send the power source back to the company who manufactured it. It's ironic that this defect would not occur on a lower cost traditional CV power source.-end quote from Ed Craig