Bubbles in the weld?

[arcpaws]

I've been doing some welding on an old car. Sometimes it goes beautifully (so I must be doing something right...) but other times it is not satisfying:

Sometimes when I weld a spot, things look OK at first, and then suddenly the weld pool rises up in a lump, generally as it cools. If I grind it off, I find it is full of voids. It if is particularly bad, it just erupts and spatters while being welded.

Is this caused by crud on the old metal vapourising and making gas, or is there some other cause? Are there any tricks to help (more/less shield gas, more/less power...)?

Obviously cleaning the metal better might help, but sometimes this is very difficult when I can't get to the back easily. It seems to be worse lately - so I am wondering if I am just working on a bad section, or if I have got something wrong.

Thanks!

 

[backpurge]

It's normally down to contamination on the plate, you really need to get it as clean as you can, make sure you have sufficient shielding gas coverage as well

 

[rkde]

I had this once when I changed to another gas bottle, it was not enough gas or moving the torch away too quickly after the weld

 

[bluebobtail]

Hi is your gas shield/ shroud clean?
A clogged up shroud will reduce gas flow.

 

[Kiwi K]

Sounds like possible rust/paint contamination, are you welding into clean solid metal,
If so then as previously mentioned lack of gas

 

[arcpaws]

Thanks for suggestions! Gas flow seems good so I will assume it is the old rusty metal. New metal definitely works better. I will endeavor to clean more...

 

[arcpaws]

I made an interesting discovery yesterday. I managed to get the settings right to get a nice smooth weld on power setting 'A' (was using 'B'), and I found that I got a lot less bubbling and erupting slag on slightly dodgy bits, as well as the weld being more controlled and less likely to melt big holes.

Obviously, it's best to be working on clean shiny metal, and welding rust just doesn't work. However, sometimes I've had to work on tricky corners where I can't get access to clean them up as well as I would like (short of cutting the car up into little bits...).

I've been using the welder on 'B' because it was a bit unstable on 'A' (or so I thought). It helps that I now have Argoshield rather than CO2. Also, I've added a bank of capacitors which I can wire in series, individually or in parallel to tune the amount of capacitance, which seems to have a significant effect on the voltage. If I add more capacitance, I have to up the feed speed (and thus increase the current, I guess) or it vapourises the wire and makes a nice molten blob on the end of the torch which drips off onto my foot rather than going on to the work...