The CCWJ (Canadian Center for Welding + Joining) have some of the most facinating high speed shots of welding pocesses in action. Using a high quality, high rate camera, they offer us the privilege to see what we can't with hour naked eyes.
Here are some GMAW (MAG/135) clips showing the different metal transfer modes on carbon steel using various power sources and gaz mixtures. We're starting with the 3 fundamental modes with an 85% argon/15% CO2 mix and straight current.
Short circuit transfer :
WIth a very low current density, we can clearly see how the wire tip heats up and strats to melt and then make contact with the weld pool which short cutts the eletrical circuit until the wire is released in a sparks explosion. That's how spatters occurs. The resulting variations of current from the short cuts are smoothed with the help of the inductance in order to reduce spatter emissions.
Globular transfer :
With a higher current density, the ball of liquid metal at the end of the wire detach before it reach the weld pool surface. Therefore no short cuts occurs in the circuit, still the current density is not high enogh to spray the drops in the pool with high velocity and spatters still occur as some drops doesn't reach its target. With less variation in the current due to the fact that there's no contact between work piece and electrode (wire), a lower inductance can be used than with a short arc, the current requires less smoothing.
Spray Arc :
With even higher current density, now the drops are smaller, literally sprayed straight into the pool in a quick series of droplets which helps for a smooth arc and little spattering as the curent flows nicely through the spray. Inductance in this case can be reduced to minimum value.
Now we have some more advanced transfers that are only available with certain advanced modern machines :
Pulsed spray arc :
We've seen earlier that we need high current density to cleanly spray metal into the puddle, to archive this with less overall power for position welding or thinner materials that are hardly welded with a spray arc, an hot impulse alternate with a cold one. When peak current is applied a drop is sprayed and right after that curent drops to cool things down. Drops are sprayed less frequently than with a classical spay arc regime reducing the amount of metal transferred and overall heat, but has its limits because it can't go too low in power or the drops would not detach effectively and it is not best suited for open root passes, or really thin materials, it can effectively replace globular, straight current spray arc or even short arc but in its higher range.
Cold short arc transfer :
These sorts have different names depending on the machine manufacturer. In this exemple it's the : STT from Lincoln Electric. It stands for Surface Tension Transfer but other manufacturers have similar modes. You'll usually find cold, or Root in the naming as it is a very stable, low heat input, low fumes, very maniable mode. Its principle is to start with a background current, then lower it when the wire hits the puddle to gently allow the ball to wet into the puddle, then, a peak is applied to help the wire to detach, when that happens curent is immediately lowered again to lower spatter emission and overall heat. High current is reapplied again to regain a bit of arc length, heat up the pool and the work piece for good fusion and it gently comes back to background the current we had at the start of the cycle. It's very similar with short arc transfer in its use but with a little more control, less overall heat input, less spatters and less fumes emission.