FCAW Welding

Buoyancy.

Dual shield electrodes (FCAW designed for use with shielding gas) invariably have a recommended gas mixture- usually C25 (25% CO2- 75% Argon) but not always. They are generally not as forgiving to different gas mixtures as a mild steel wire & are also often used in codework- shipbuilding etc.- where this could mean burning out the weld & rewelding is required. That's why it's important to read the label.

As far as using helium itself, FCAW is a hot & fast process to begin with & my guess is helium may tend to exaggerate this similar to its effect on TIG welding(but then, why not use acetylene??).

You can answer your question by looking up the chemical weight of each of the gasses used. A shielding gas must stay in place long enough to shield the weld (molten area) from the impurity's in the air.

Using helium as a shielding gas is a bad idea because of:

1) The gas is too light to be of any use practically speaking

2) Expense

3) It makes all the welders talk as if they were tiny ducks,..... productivity then falls as they all begin to laugh at each other