MIG "pimples"?

[LordBug]

Hey guys,

Sometimes when I'm on the MIG laying down tacks, the odd one or two will slowly erupt a blob of weld as it starts to cool, much like squeezing a pimple.
I'm going to cautiously wager a guess at the external skin cooling & contracting far quicker than the core, though I'm more interested in how I might avoid this happening.

Cheers

[rob streeper]

If you're using shielding gas your description sounds as if you may temporarily be losing effective coverage. Are you working in an area subject to wind?

[Karl Robbers]

Dead o the money about loss of gas shield. It can also be caused by gasses given off from paint or galvanising bubbling out of the weld before it solidifies.

[rob streeper]

Dead o the money about loss of gas shield. It can also be caused by gasses given off from paint or galvanising bubbling out of the weld before it solidifies.

I've never heard of it or seen it but is it possible that, when using flux cored wire, that a bubble or gap in the flux column may also cause this phenomenon?

[Grahame Collins]

Sometimes it is simple.
Consider also , the post arc gas cover. Some welders tend to forget this and rip the torch away from the completed bead the nanosecond that the weld arc is extinguished.

However I would recommend that the prep area is totally clean of any contaminants.Mill scale can retain oil and cause a problem.


I have even seen the cooling molten mig bead react to a different disc grinder wheel.It was thought that the wheel abrasive or binder reacted with the metal.When we changed the brand of wheel the problem went away.

Grahame

[LordBug]

The more I think about it, the more I'd point my finger at it being a contaminant.

With the job I perform, there is absolutely zero preparation of the metal. Mill scale, surface rust, two years of sitting in the weather rust, galv coating, paint, anti-spatter silicone stuff, wet and dry, just burn on through it all. It originally struck me with horror when my question of "What surface prep do we do?" when I first started was met with a blank face and "None", but I've come to appreciate how surprisingly well MIG performs on a range of surfaces (Enough to perform some form of welding test, I can't remember the specifics, just that there was a pressure test, cross-section check and maybe an x-ray, only one case of remaining impurity out of seven guys tests on some of the mildly rusted bar!). It also lets you appreciate the huge benefit of a properly prepared surface (penetration & flow).

I know when the shielding gas has been blown away/bottle has run empty, it's a very ugly brittle looking lattice of bubble-holes. Also happens a bit when the metal is wet, I'd presume that's due to the rapid vaporisation of the moisture.

Anyway, main clue to it being contamination is the piece that made me finally ask this question was a bracket I'd been making at home, I'd taken it in to fill in some gaps left by my stick attempts (I still have a ways to go with learning the finer points of stick welding), which must've had a bit of slag still in them. Hit them with the grinder, then a quick mig over one, nice pleasant puddle, repeat on the other, pimple! Apply more wire, pimple again!

Must admit, for the amount of tacks I lay down each day, I'm surprised at how rarely this happens.

[Karl Robbers]

With the job I perform, there is absolutely zero preparation of the metal. Mill scale, surface rust, two years of sitting in the weather rust, galv coating, paint, anti-spatter silicone stuff, wet and dry, just burn on through it all. It originally struck me with horror when my question of "What surface prep do we do?" when I first started was met with a blank face and "None", but I've come to appreciate how surprisingly well MIG performs on a range of surfaces (Enough to perform some form of welding test, I can't remember the specifics, just that there was a pressure test, cross-section check and maybe an x-ray, only one case of remaining impurity out of seven guys tests on some of the mildly rusted bar!). It also lets you appreciate the huge benefit of a properly prepared surface (penetration & flow).
Must admit, for the amount of tacks I lay down each day, I'm surprised at how rarely this happens.

What job exactly is this? Sounds a bit interesting.

[Grahame Collins]

Something I should have included in the last post was about the contaminants.

Basically its those sorts of materials with a hydrocarbon content reacting under the 6000°C temperature.

The reaction causes increasingly expanding gas bubble/s forming under the molten bead in very close proximity to the arc.

As the bead cools, the gas is still expanding causing the pimple/s to extrude upwards.

I know you were dealing with un-coated steel but I'll toss the following in about Duragal
The duragal crowd yell us its quite OK to weld thru the product but I am still a bit of a skeptic there and continue to scuff it off where I have to weld over it.

Overall shiny clean on steel is the way to go for me.I have picked up porosity before on work that was rusty and barely scuffed over the rusty pits.


Grahame

[Legion]

I have experienced exactly what you're describing recently with TIG. I was welding end caps on some round pipe so an outside corner joint essentially. I removed the galv off the outside of the pipe and off the end cap but not from inside the pipe. Exactly the same outgassing symptoms as I went around the cap. I had holes drilled to allow gas expansion so I figured it was the zinc.

One other time, joining galv plate on an outside corner, also failed to prep an area as well as I thought and got the same outgassing of zinc.

In both cases the welds were cosmetic only, so I overcame it by letting off the pedal heaps and just washing over the area adding filler as required. I found if I gave too much pedal I could keep going over the area and I'd fill in one hole and another would pop out right next to it. Until I just let the heat way down.