Weld Porosity 5

[ajayputta]

Hi,

I needed help on understanding weld porosity.
What are the probable reasons for porosity to occur during MIG welding cold rolled sheets (2 mm thk).
Just to rule out the possibility of moisture, this problem creeps up even when the ambient temp during welding is as high as 35 deg C and the air is dry.

Thanks.

 

[MikeMilburn]

A few things that come to mind are:
1: Is there any wind or even a slight breeze to blow your gas away
2: oil or some other contimanent on the plate
3: incorrect wire, gas or combination of both
4: you could be drawing atmospheric air into the weld pool with the incorrect angle of the gun
Those are a few of the main ones that come to mind...Mike

 

[pipewelder1999]

1) Incorrect shielding gas flow rate. Too High or low. (related to mikemilburns #4)

2) If the material is sheared, check the edges for oil/grease. Not just the face.

3) The welder may be holding the nozzle too far from the weld.


Have a nice day

 

[Seldom]

Hi Ajayputta,
The previous two "Replyers" are correct in all their suggestions but I'd like to extend a couple of those suggestions.

A couple of the most often found conditions I've observed that have promoted porosity in in mig welds are as follow-

1. You mentioned 35 deg C as an ambient temperature. If your shop isn't extremely unique from others, this probably means some welders have fans blowing on themselves while welding. Air impingment is, as previously mentioned, an unacceptable condition while using the mig process. Several times I've observed where a welder was using too small of nozzle, he let it get crudded-up with spatter, and had a fan blowing on him while welding and couldn't figure out why he was getting porosity!

2. The second most common condition I've found is allowing the weld joints to become contaminated. This can be from dirty shears of rolls as already mentioned but it can fequently stem from oily gloves, rust, paint, heavy scale left on, or plain old dirt. Just because you've ground the joint to "bright metal" doesn't mean it can't become contaminated enough to cause porosity. Pneumatic tools are commonly overlooked culprits that often cause oil contamination to occur either by direct contact or from contaminated gloves. Also, many times weldments are tacked together then left for a period of time before welding. The "lay-down" area itself may allow for some kind of contamination to take place. The weldment was possibly nice and clean when tacked but by the time the welder gets it, it's become contaminated and everybody wonders why the welder has a porosity problem!

There are many causes and conditions that instigate or promote porosity with mig welds but usually a short investigation of the overall fabrication process will quickly disclose the culprit.
Good luck.

 

[ajayputta]

Hello MikeMilburn, Pipewelder1999 & Seldom,

Thanks verymuch for your valuable suggestions. The Air blown from the fan is particularly interesting. I'll carry out a process audit covering all your points.

Thanks again.

 

[ajayputta]

To be more specific on the porosity issue, the part concerned is a fuel tank. When the tank is inspected after fabrication no defects are visible. It is only after the tank is shot blasted for painting do the porosity become visible.
The slag ( I suppose !) covers the pores prior to shot blasting. The same also prevents leakages during leak testing.

Thanks