If the unprotected human eye could observe the laser welding process this is what you would see. The illustration shows the ends of two thick plates being joined together by welding – commonly referred to as a butt-weld. On the right an electric arc is melting the upper part of the heavy steel plate. The electrode is a wire, continuously fed to the melt pool in the narrow gap, a few tens of millimeters in size, between the plates. A fraction of a second later the melt is hit by a laser beam, moving in the trail of the arc and instantly stabilizing the weld pool and increasing the speed and penetration.
The hot plasma distributes the heat all the way down to the bottom of the plate, where the vapor pressure is so high that liquid steel from the arc flows around the laser beam to the solidification zone behind.
Depending on process parameters, e.g. movement of the work piece relative to the process heads, the melted steel will cool, eventually forming a trail, the socalled bead. The overall result is solidification and successful joining of the work pieces.
Movement of the work piece relative to the process head is one of a range of adjustment parameters to be considered - the power in the laser beam and in the electric arc, the distance between them, the travelling speed, the metal composition, the inert gas pressure, the type of process gas, the ventilation of fumes away from the optics, etc.
For example, if the laser power is set too high a vaporized metal plume can form above the weld, where it will absorb or scatter the energy coming from the laser, taking away heat from the bottom of the weld.
On the other hand if the speed is too high, droplets may form on the falling slope from the bowl to the weld bottom, and these can cause irregularities in the solidification zone.
However, once all parameters are correct the perfect weld can be repeated again and again.