Pin holes same place

[jaysonjay]

Hey guys, i know its now quite the place for robot welders? But I rate the question pertains to welding more than the welder.
Ok so we running production on 2 galvanized work pieces welded in 3 places
The issue I keep getting is a pin hole that shows up randomly. But when it does its in the same place of the weld (more or less center)
2 days ago showed up 20 parts out of 800 but yesterday it showed up way more with no setting changes and same batch of parts, weld pen on first off looked good too
We shielding with CO2
1.2mm wire
Panasonic robot welder

Any advice would be greatly appreciated.

 

[Wozzaaah]

Welcome to the forum.
We do actually have someone on here who knows the absolute ins and outs of robot welding, @Robotstar5
Are you able to post some good quality photos showing the defect?
I'm assuming you mean the parts are galvanised afterwards, or is the zinc being removed from the weld areas before welding? Are the parts being cleaned pre-welding? Any chance some could be contaminated?

 

[jaysonjay]

Hey ok, so no we don’t clean up any of the material we are using a super active Panasonic robot. And the material is galvanized before the forming process and then welded as is, we clean the parts but we don’t remove the coating. I blocked out stamps on the parts etc as the parts are for a OEM vehicle manufacturer and I don’t want any issues “sharing” potential confedential things. You will see this part has a tac just to show which jig (jig2) it was welded on. But the holes show up randomly on different welds and different jigs but pretty much most times in the center of the weld. We don’t weld around the threaded part that is a “buy out”. And you will see on the weld with the hole that’s not a burn through on the bracket but bracket is bent round there. I hope that makes sense but I attached a pic of the bracket on its own. We also weld other galvanized parts with a second but the same robot and that weld is 360’ around and we hardly get holes on that weld ever keeping in mind that weld uses 3m of wire per weld.

 

[chrismac]

Looks like there is just too much galvanising for the weld to cope with, with there being a hole for the weld to penetrate into there is far more contaminants being introduced, the other 2 welds dont seem to have this issue and have a better appearance, the weld with the pore is under flush/undercut making me think the set up for that weld on the program could do with being adjusted to try and help eliminate the pore, but the galvanising probably has a huge impact on the finished weld and leading to your issue, welding it in a galvanised finish is obviously not the best approach, but your looking for a work around as this cant be changed.

 

[8ob]

Try a few by hand with a normal mig welder, that would help eliminate a technique issue vs material problem.

Bob

 

[Robotstar5]

Welcome to the forum. Once a robot is programmed it will follow the same path every time so barring any mechanical issues, wear etc. you can rule that out.
Production welding issues are normally caused by the parts themselves or their presentation to the robot. Things to check are, has there been any changes with the part supplier, in your case I would look at their galv process in case it has got thicker, thinner etc. maybe you have some parts from an older batch you can compare. If you have a CMM (measuring) department/lab it's worth accurately measuring some new components and comparing to old components as I've experienced suppliers running production tooling until it's worn out of spec.
Piece part presentation is also important so check your jigs/fixtures/tooling are still within spec.
I have done a lot of robot programming, but am not a welding expert and would normally fine tune the welds on the advice of an expert so I can't really advise on the finer points of your welds.

 

[jaysonjay]

So thanks for the feedback, so we have run out of parts, and are waiting for production, we will try some by hand and see what comes off. Also jigs and tooling and robots are all only a few months old, it’s a brand new project that has kicked off. We using a slightly smaller nozzle so we can get around the clamps etc, how far away will you guys recommend my nozzle be? It’s currently maybe about 15mm away.
@Robotstar5, I’m only getting into robot programming now. I’m a toolmaker by trade, so welding I can do to a point of needing to run a job but the fine nitty gritty stuff I don’t know either.

 

[Erie Fred]

if it's always the same spot, program to go back and put small weld over it.

What wire chemistry ?
FWIw I read somewhere for galvanized -s2 was better than the very common -s6.

 

[Jack Ryan]

I'm not familiar with robot welding at all but I hope you don't mind me making some comments anyway. There is not enough detail in the descriptions of the setup or the symptoms so my uneducated robot welding comments may be off on a tangent.

Are you using FCAW-G? If so, I think the nozzle is too close at 15mm.

What is a "slightly smaller" nozzle. Too small is too small and if you are using 15mm to compensate I'd look for a better solution.

As mentioned, a change of zinc coating thickness may require a change of parameters. A thicker coating will increase the risk of embrittlement (see Erie Fred's comment re S6) and porosity from zinc evaporation.

I don't know what the porosity looks like but if it is due to zinc evaporation, the weld needs to cool slower to give the zinc a chance to exit the weld pool before solidification.

On the other hand, it might be some other contamination.

Jack

 

[kaiser]

To me it looks as gas escaping the weld pool. I think the weld is not filling the void to the bottom, and hot air and fumes are escaping both side of the weld, and preventing the fusing of the weld in the middle.
I would try 3 things.
I: get the weld down to the bottom of the groove.
2: stop the weld in the middle for a sec or two, then continue
3: Increase the gas flow while welding, possibly weld in a box, which will have a layer of stagnant gas in it at all times. (CO2 is heavier than air and will linger)

Last suggestion is to try and weld the material first and plate it later.

Option one and two are free of extra cost, except for a bit of time.

Looks like a jack?

 

[jaysonjay]

Hey so I’ll attach a pic of the wire we using. Not sure of all the differences etc but that’s what we are using.
The zink plating is already part of the coil before forming so we can’t zink coat after. The full car body itself is dipped I think in paint when assembled.
A lot of the processes are pre defined by the OEM so we can’t at this point in time change things too much, that would’ve had to be discussed at quoting level if we felt a process couldn’t work.
I can’t double back on the weld because the hole is so infrequent and shows up across all 3 welds not only the one I sent a pic of but generally every time in the middle of the weld. The nozzle measures 16mm OD. I’ll send a pic of it too.
So we are currently in a ramp up phase before hitting full production, so my parts being pressed are not important to press production at the moment but as soon as I get some I’ll keep everyone in the loop. Does anyone think an argon shielding gas could help? Or are the shielding gasses only really going to help spatter?

 

[Jelly_Sheffield]

Does anyone think an argon shielding gas could help? Or are the shielding gasses only really going to help spatter?

If you're not already using a tri-mix* it will improve puddle wetting, which might help...

*(Argon, CO2, & Oxygen. The Galvanisers Association of Australia recommends a 92:5:3 mix, which is close to BOC "Argoshield Light" or Air Products "Ferromaxx 7")

But it looks like it's zinc fume escaping from underneath the puddle that's causing your issues.

In which case even if different gas visually fixes the problem I would still want to cut sections through the currently affected point, in a statistically representative number of test parts, to confirm it wasn't just allowing filler metal to trap that fume causing a hidden defect.