I was in a subcontractor's shop recently and was witness to welding on some pieces that were plasma cut and not ground and cleaned well afterwards.  I had heard of the "weld foam", but had never witnessed it before.  I did not realize the problem existed so badly.  We only have a very small plasma cutter here and use it mostly for splitting W shapes into Ts and similar work.  We only use oxy-fuel to cut our pieces in our shop.  We are considering a plasma bed in a near future shop expansion so this will be a subject I will follow with interest.

Darren,

as I saw, only little, but very expressive reaction - coming from Doug - on your topic.

Doug mentioned somewhat important in his reply, namely: "...that were plasma cut and not ground and cleaned well afterwards...".

Reason for the weld pools "foaming" may be the nitrogen being resolved in the plasma-cut caused dross on the bevels/flanks. As far as air (78% nitrogen-content) or nitrogen containing plasma-cutting-gas is used for cutting, the dross' nitrogen-content may extend 15 times the nitrogen content of the base material.

Due to reactions with the liquid melting pool it may generate also "foaming" depending on what base- and/or filler-material, shielding-gas,...

Therefore it should be recommended to "grind and clean" the edges, bevels or flanks after cutting with air or nitrogen-containing plasma-cutting gases prior to welding. It is interesting to see how the nitrogen works. Depending on how much nitrogen is being resolved in the dross, the hardness of the dross varies. I am sure you have already tried to use a file for removing the dross and you have seen how hard it really is. I have seen cases where the file has been damaged by the "nitrified" dross layer.

Solution can also be to use different plasma-cutting-gases (Argon/Hydrogen,...) to reduce or avoid the the content of nitrogen within the dross. As far as I know, the cutting-torch has to be equipped differentially considering the higher amount of kinetic energy of those mentioned plasma-gases, i.e. higher thermal impact on the torch-parts.

Will be interesting to read what other colleagues on the forum have experienced and may recommend regarding your topic.

Regards,
Stephan